Production process of waterproof and moisture-permeable multi-layer PU synthetic leather

Abstract

The application discloses a production process of waterproof and moisture-permeable multi-layer PU synthetic leather and belongs to the technical field of synthetic leather production. The structure comprises a bottom layer, a self-repairing intermediate layer and a surface layer from bottom to top. The preparation method comprises the following steps: firstly, performing surface modification on non-woven fabric, then coating water-based PU resin and entering a coagulation bath; secondly, synthesizing self-repairing water-based PU containing furan / maleimide, coating and hot-pressing to form an intermediate layer; finally, blending temperature-changing substances, cellulose nanocrystals and modified nano-enhancing agents to prepare a surface slurry, coating and hot-pressing, and then performing aging, washing and post-finishing to obtain a finished product. Through the synergistic design of multiple functions, the application constructs gradient moisture-permeable channels, a dynamic self-repairing network and a temperature and moisture sensitive response surface layer, solves the problems that existing synthetic leather is prone to peeling between layers, the performance is irreversibly attenuated after damage and cannot dynamically adapt to temperature and humidity changes, and the product has excellent waterproof and moisture-permeable, self-repairing and intelligent response performance and is suitable for outdoor shoes and clothes, protective equipment and other fields.

Description

Technical Field

[0001] This invention relates to the field of synthetic leather production technology, and in particular to a production process for waterproof and breathable multilayer PU synthetic leather. Background Technology

[0002] Polyurethane (PU) synthetic leather is widely used in footwear, apparel, bags, furniture, and other fields due to its appearance and feel resembling genuine leather and its excellent physical properties. Among these, PU synthetic leather, which combines waterproof and breathable properties, is a key material for high-value-added products such as outdoor sportswear and specialized protective equipment. However, achieving high-performance waterproof and breathable properties while ensuring durability in complex operating environments has always been a technological bottleneck in this field.

[0003] To improve the waterproof and breathable properties of PU synthetic leather, the industry generally adopts a technical approach that combines multi-layer composite structures with material modification. On a non-woven fabric substrate, a microporous bottom layer is formed through wet coagulation, followed by the sequential coating of a hydrophilic intermediate layer and a hydrophobic top layer. The intermediate layer incorporates pore-forming agents such as polyethylene glycol to create breathable channels, while the top layer is enhanced with nanomaterials and fluorine modification to improve its density and waterproofness. These methods, by optimizing the physical structure and process parameters, improve the initial waterproof and breathable properties of synthetic leather to a certain extent.

[0004] However, the existing solutions still have significant limitations. First, their performance improvement essentially relies on static physical structure and chemical modification. Once microcracks, interfacial peeling, or blockage of micropores occur during processing or use, their waterproof and breathable properties will irreversibly decline, lacking self-repair capabilities. Second, the existing hydrophilic / hydrophobic gradient structure is fixed and cannot respond to dynamic changes in body surface temperature and humidity caused by human activity. In low-temperature, low-humidity environments, its permeability may be too high, resulting in insufficient warmth retention; while in high-temperature, high-humidity environments, its fixed permeability rate may not be sufficient to quickly expel sweat.

[0005] Therefore, developing a novel waterproof and breathable PU synthetic leather production process that can self-repair microscopic damage, intelligently respond to environmental changes, and achieve a robust integrated composite interface has become an urgent technical problem to be solved in this field. Summary of the Invention

[0006] The purpose of this invention is to address the shortcomings of existing technologies by proposing a production process for waterproof and breathable multilayer PU synthetic leather.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] A manufacturing process for waterproof and breathable multilayer PU synthetic leather includes the following steps:

[0009] S1, Bottom Layer Preparation:

[0010] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidyl etheroxypropyltrimethoxysilane, baked for 2 minutes, coated with aqueous anionic polyurethane resin, and then placed in an N,N-dimethylformamide / water coagulation bath, washed with water, and dried.

[0011] Under the bombardment of high-energy plasma particles, the originally chemically inert polyester fiber surface undergoes two key changes: first, physical etching creates a micro-nano scale rough structure on the fiber surface, significantly increasing its specific surface area; second, in an oxygen-containing atmosphere, the high-energy particles break the polyester molecular chains and introduce a large number of oxygen-containing active groups, such as hydroxyl (-OH), carboxyl (-COOH) and carbonyl (C=O), which transforms the originally low surface energy and difficult-to-bond polyester surface into a highly reactive interface.

[0012] Subsequently, a padding treatment was performed using an aqueous solution of γ-aminopropyltriethoxysilane (KH-550) and γ-glycidoxypropyltrimethoxysilane (KH-560). The ethoxy (-OC2H5) and methoxy (-OCH3) groups at the ends of the KH-550 and KH-560 molecules hydrolyzed in water to generate highly reactive silanol groups (-Si-OH). Then, during the baking and curing stage, these silanol groups underwent a dehydration condensation reaction with the hydroxyl groups on the nonwoven fabric surface generated by plasma activation, forming a strong bond. The solid -Si-OC- covalent bonds, along with the condensation between adjacent silanol groups, form a stable siloxane network (-Si-O-Si-), thereby constructing a robust nanoscale interface layer with specific organic functional groups on the fiber surface. KH-550 provides primary amino groups (-NH2), and KH-560 provides highly reactive epoxy groups. Both groups can chemically react with functional groups (such as carboxyl groups and isocyanate groups) in waterborne polyurethane (PU) resins, providing crosslinking sites.

[0013] Waterborne PU resin typically exists stably in water in the form of carboxylate anions. Its molecular chain contains unreacted isocyanate groups (-NCO) or other polar groups. After the coating comes into contact with the substrate, the functional groups in the PU resin react chemically with the amino groups (from KH-550) and epoxy groups (from KH-560) at the end of the silane bridging layer. For example, the isocyanate groups react with primary amino groups to form urea bonds (-NH-CO-NH-), while the epoxy groups can undergo ring-opening reactions with carboxyl groups and other groups on the PU chain. This series of reactions forms a strong covalent bond interface between the PU coating and the nonwoven substrate, which greatly enhances the interlayer bonding force and effectively avoids the risk of delamination during use.

[0014] Subsequently, the substrate carrying the wet-process PU slurry enters a coagulation bath of N,N-dimethylformamide (DMF) and water. This is the core of the wet-process PU film formation process. The solvent (DMF) in the slurry and the non-solvent (water) in the coagulation bath diffuse into each other. When water diffuses into the PU resin phase and the solvent diffuses out of the resin phase, the solubility of PU in the mixed solvent decreases sharply, causing liquid-liquid or solid-liquid phase separation in the homogeneous PU solution. PU molecular chains aggregate, solidify, and precipitate, forming a solid framework, while the solvent-rich phase surrounded by the PU framework forms pores. By precisely controlling the ratio of DMF to water and the temperature in the coagulation bath, the speed and degree of phase separation can be controlled, thereby forming a gradient porous structure that transitions from large-sized finger-like pores near the nonwoven fabric side to small-sized sponge-like pores near the air side. This gradient structure provides an ideal channel for the rapid transport of water vapor.

[0015] Finally, through washing and drying steps, the residual DMF solvent is completely removed, the microporous structure is stabilized, and a synthetic leather underlayer with a strong interface and efficient moisture-permeable channels is obtained.

[0016] S2. Preparation and coating of self-healing intermediate layer slurry:

[0017] Poly(1,4-butanediol adipate), isophorone diisocyanate and catalyst were prepolymerized for 2 hours, 2,2-dimethylolpropionic acid was added and reacted for 1 hour, furanol was added and reacted for 30 minutes, 4,4'-methylenebis(N-phenylmaleimide) was added, triethylamine was added, deionized water was added under high-speed shearing, and the mixture was coated onto the bottom surface, baked for 3 minutes and hot-pressed for 3 minutes.

[0018] Poly(1,4-butanediol adipate) diol and isophorone diisocyanate (IPDI) were prepolymerized for 2 hours under the action of a catalyst. The core mechanism is the stepwise addition polymerization reaction between isocyanate groups and alcohol hydroxyl groups. Polyester diol acts as the soft segment, providing the material with flexibility and elasticity; IPDI acts as the hard segment, and its cyclic structure endows the material with good mechanical strength and chemical stability. The catalyst accelerates the reaction, and the excess of isocyanate groups ensures that a prepolymer with isocyanate end groups is formed within 2 hours.

[0019] 2,2-Dimethylolpropionic acid, as a hydrophilic chain extender, has two hydroxyl groups on its molecule that react with the terminal -NCO groups of the prepolymer to extend the chain segment; at the same time, the carboxyl group of its side chain is introduced into the main chain of polyurethane. This carboxyl group is the key to the subsequent water-based conversion because it can be neutralized by alkali to form a salt, giving the polymer water-dispersibility.

[0020]

[0021] The hydroxyl group of furanol reacts with the remaining terminal -NCO groups in the system to firmly graft the furan ring onto the polyurethane molecular chain in the form of a side chain. The furan ring is a diene in the diene reaction, and its introduction lays the groundwork for the subsequent construction of a reversible crosslinking network.

[0022]

[0023] 4,4'-Methylenebis(N-phenylmaleimide) contains two maleimide groups, making it a dienophile in the diene reaction. At the synthesis temperature, the maleimide groups undergo a diene cycloaddition reaction with the furan groups already grafted onto the PU chain, thereby forming dynamically reversible covalent crosslinking points between two or more PU molecular chains. This step is the core of endowing the material with self-healing capabilities;

[0024] Triethylamine, acting as a neutralizing agent, reacts with the carboxyl groups introduced by 2,2-dimethylolpropionic acid to generate ammonium carboxylate. This process significantly enhances the hydrophilicity of the polymer. Subsequently, deionized water is added under high-speed shear. The powerful force of water forces the hydrophobic polymer backbone to disperse into the water, while the hydrophilic ionic groups arrange themselves on the outside and interact with the water, thereby forming a stable, nanoscale aqueous polyurethane dispersion.

[0025] -COOH+N(Et)3→-[COO - ][NH(Et)3 + ]

[0026] The purpose of baking for 3 minutes after slurry coating is to remove moisture, allowing PU particles to approach, deform, and fuse together to form a continuous and dense film. The subsequent 3-minute hot pressing is crucial. The heat and pressure provided by hot pressing have a dual effect: 1. Promoting physical film formation: forcing polymer molecular chains to further diffuse and entangle with each other, forming strong physical entanglements. 2. Activating and stabilizing chemical crosslinking: the energy provided by hot pressing causes some Diels-Alder bonds to undergo reversible breakage and then recombine in new locations. This process is equivalent to "annealing" the dynamic network, enabling it to form a thermodynamically more stable and uniform reversible crosslinked network after cooling to room temperature. This network allows the material to self-repair microcracks if local damage occurs during subsequent use by reheating to break the DA bonds at the damaged area, causing chain segment movement, and then cooling to re-crosslink.

[0027]

[0028] S3. Preparation and coating of surface slurry:

[0029] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, disperse ultrasonically, then mix with waterborne fluorocarbon modified polyurethane resin, add waterborne wetting and leveling agent, and stir at low speed for 2 hours.

[0030] Poly(N-isopropylacrylamide) is a thermosensitive polymer whose molecular chains bind to water molecules through hydrogen bonds in water. Cellulose nanocrystals, on the other hand, are rigid rod-shaped nanocrystals with a surface rich in hydroxyl groups, exhibiting strong hydrophilicity and the ability to form networks. The hydrophilic network formed by the abundant hydroxyl groups (-OH) on the surface of cellulose nanocrystals, along with the hydrogen bonds and physical entanglement between the poly(N-isopropylacrylamide) molecular chains, enables uniform dispersion of both, building the basic framework for temperature and humidity response. The poly(N-isopropylacrylamide) molecular chains undergo a hydrophilic-hydrophobic conformational transition around the low critical dissolution temperature (around 32°C), while cellulose nanocrystals, as a rigid hydrophilic reinforcing phase, can improve the mechanical strength and hydrophilicity control capability of subsequent coatings. One is responsible for temperature-sensitive shrinkage, and the other for moisture-sensitive expansion. Together, they provide the driving force for the dynamic opening and closing of micropores in the film after the slurry has cured.

[0031] The high energy provided by ultrasonic treatment is used to break up the agglomerates of nanoparticles, allowing them to be uniformly dispersed in the aqueous network as single or primary particles. KH-570 silane coupling agent, having previously anchored the siloxane groups at one end to the nanoparticle surface through hydrolysis and condensation, while the methyl groups at the other end impart hydrophobicity to the particles, is then physically embedded within the hydrogel network. Simultaneously, its hydrophobic surface exhibits better compatibility with the subsequently added waterborne fluorocarbon modified polyurethane (FPU) resin. These nanoparticles serve as physical crosslinking points and reinforcing units, improving the surface's mechanical strength, abrasion resistance, and static water resistance during film formation.

[0032] FPU resin itself is an aqueous dispersion, and its particle surface is stabilized by hydrophilic ionic groups. When it is mixed with poly(N-isopropylacrylamide), cellulose nanocrystals and modified nano-reinforcing agents, a physical blending and reemulsification process occurs. There is a hydrophobic interaction between the hydrophobic FPU segments and the surface of the modified nano-reinforcing agents, while the hydrophilic layer on the surface of the FPU particles is compatible with the aqueous phase of poly(N-isopropylacrylamide)-cellulose nanocrystals. Under low-speed stirring, the components permeate each other to form a macroscopically uniform composite dispersion. FPU acts as a continuous film-forming matrix, which encapsulates and binds all functional components together and provides basic waterproofness, flexibility and adhesion after curing.

[0033] Adding a water-based wetting and leveling agent reduces the surface tension of the system, promotes the uniformity of the slurry spreading on the surface of the intermediate layer, and avoids defects such as pinholes and craters during the coating process. Low-speed stirring allows the components to fully integrate without destroying the already formed hydrogen bond network and dispersion system, ultimately resulting in a stable and uniform intelligent surface slurry, which provides a guarantee for the subsequent formation of temperature and humidity responsive biomimetic microporous structures through non-solvent phase separation.

[0034] Throughout the process, the strong hydrophilicity of cellulose nanocrystals enables them to sensitively sense changes in humidity and absorb water and swell. Together with the temperature-sensing poly(N-isopropylacrylamide) gel, they generate a stronger driving force, which expands the internal micropores and doubles the amount of moisture permeability. The barrier to liquid water is provided by the dense and hydrophobic fluorocarbon polyurethane continuous phase and the modified nano-reinforcing agent. Water cannot penetrate due to its extremely high surface tension. These components together achieve a balance between dynamic comfort and reliable protection.

[0035] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0036] The amount of slurry applied is precisely controlled by the cell volume of the micro-gravure roller. Combined with the linear pressure between the roller and the substrate, the slurry is evenly spread on the surface of the intermediate layer. This step ensures that the surface layer thickness is uniform and avoids differences in moisture permeability / waterproof performance caused by uneven coating, thus laying the foundation for the formation of subsequent functional structures.

[0037] The process employs a programmed hot-pressing method. In the low-temperature stage (80℃), the water in the slurry evaporates slowly, preventing the collapse of the microporous structure. In the medium-temperature stage (105℃), the hydrogen bonding and physical entanglement between the components are strengthened, while the migration and aggregation of FPU fluorocarbon segments to the surface are accelerated, forming a dense hydrophobic and waterproof membrane. In the low-temperature final stage (65℃), the internal stress of the coating is reduced, reducing the risk of cracking. The mechanical force of hot pressing can also promote molecular diffusion at the interface between the surface layer and the intermediate layer, allowing the active groups of the surface FPU to form weak interactions with the amino and hydroxyl groups of the intermediate PU, thereby enhancing the interlayer bonding force.

[0038] The curing process further stabilizes the hydrogen bond network within the coating, ensuring the fluorocarbon segments are fully and regularly arranged to guarantee long-term waterproof and responsive performance. It also eliminates residual stress within the coating, making the temperature and humidity response mechanism more stable and reliable. The core mechanism of the washing step is to remove any small molecule additives that may remain in the surface slurry, preventing them from clogging micropores. This process also unblocks the hydrophilic network channels constructed by poly(N-isopropylacrylamide)-cellulose nanocrystals, ensuring efficient moisture permeability. Hot air setting removes residual moisture from the coating using high-temperature hot air, while simultaneously further cross-linking and curing the surface PU molecular chains, improving surface mechanical strength and dimensional stability to prevent deformation during subsequent use. Finally, low-temperature texturing, achieved through mechanical pressing at low temperatures, creates a natural and uniform texture in the coating. This process also moderately disrupts some weak hydrogen bonds on the surface, increasing the connectivity of the microporous structure. This step balances the appearance simulation and softness of the synthetic leather, ultimately resulting in a multi-layered PU synthetic leather product that combines waterproof and breathable properties, temperature and humidity response, self-healing capabilities, and excellent tactile feel.

[0039] Preferably, the operation steps of the N,N-dimethylformamide / water coagulation bath are as follows:

[0040] The nonwoven fabric coated with water-based polyurethane slurry is immersed at a uniform speed of 1-3 m / min into a mixed coagulation bath consisting of N,N-dimethylformamide and water in a mass ratio of 3:7, and immersed for 5-15 min.

[0041] Preferably, the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol methanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:5-7:3-7:1-3.

[0042] Preferably, the mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent to waterborne fluorocarbon modified polyurethane resin is 10-20:5:3-5:0.5-1:70-90.

[0043] Preferably, the preparation steps of the modified nano-reinforcing agent are as follows:

[0044] Silica and graphene oxide were mixed at a mass ratio of 5:1 to 8:1 and added to an ethanol solution containing 3-5 wt% silane coupling agent KH-570. The mixture was ultrasonically dispersed and refluxed at 80-100℃ for 2-4 hours. After filtration and drying, the modified nano-reinforcing agent was obtained.

[0045] Compared with the prior art, the beneficial effects of the present invention are:

[0046] 1. By employing plasma activation and compounding silane coupling agents, combined with the covalent bonding between silanol groups and active groups of the substrate, and the chemical cross-linking mechanism between the coupling agent and PU, the problem of weak bonding and easy delamination between the traditional bottom layer and the substrate is solved. At the same time, a gradient microporous structure is constructed through the phase separation mechanism of N,N-dimethylformamide / water coagulation bath to achieve efficient moisture permeability of the bottom layer.

[0047] 2. By using furan and maleimide functionalization to modify PU, combined with the dynamic reversible cross-linking mechanism of diene reaction, the problem of irreversible degradation of waterproof and breathable properties after damage in existing synthetic leather is solved, enabling micro-cracks to self-repair after heating and extending the product's service life.

[0048] 3. By adopting a poly(N-isopropylacrylamide)-cellulose nanocrystal temperature and humidity responsive system and fluorocarbon PU composite method, combined with the synergistic mechanism of poly(N-isopropylacrylamide) temperature-sensitive conformational change and cellulose nanocrystal humidity-sensitive expansion, the problem of traditional fixed gradient structures being unable to respond to changes in environmental temperature and humidity is solved, and dynamic regulation of micropore expansion to increase moisture permeability at high temperature and high humidity and micropore contraction to strengthen waterproofing at low temperature is achieved.

[0049] 4. By adopting an integrated hot-pressing composite method of bottom layer-middle layer-top layer, combined with the molecular diffusion and weak interaction mechanism of functional groups between each layer, the problem of poor interface fusion of multi-layer structure is solved, the overall structure is coordinated, and the stable performance of waterproof, breathable, self-healing and intelligent response functions is guaranteed. Attached Figure Description

[0050] Figure 1 This is a schematic diagram of the structure of the waterproof and breathable multilayer PU synthetic leather produced by this invention. Detailed Implementation

[0051] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments.

[0052] Example 1: A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following steps:

[0053] S1, Bottom Layer Preparation:

[0054] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane in a mass ratio of 1:1. After baking for 2 minutes, aqueous anionic polyurethane resin was applied to the surface of the modified nonwoven fabric. The fabric was then immersed in a coagulation bath of N,N-dimethylformamide / water at a uniform speed of 2 m / min in a 3:7 mass ratio of N,N-dimethylformamide and water for 10 minutes. Finally, the fabric was washed and dried.

[0055] S2. Preparation and coating of self-healing intermediate layer slurry:

[0056] Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and dibutyltin dilaurate for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:6:3:2, then add triethylamine to neutralize the 2,2-dimethylolpropionic acid with a 1:1 molar ratio of carboxyl groups, add deionized water under high-speed shearing, coat the bottom surface, bake for 3 minutes, and hot press for 3 minutes;

[0057] S3. Preparation and coating of surface slurry:

[0058] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, ultrasonically disperse, then mix with waterborne fluorocarbon modified polyurethane resin WPU-F50, add waterborne wetting and leveling agent BYK-346. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 is 15:5:4:0.7:80, stir at low speed for 2 hours;

[0059] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0060] Example 2: A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following steps:

[0061] S1, Bottom Layer Preparation:

[0062] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane in a mass ratio of 1:1. After baking for 2 minutes, aqueous anionic polyurethane resin was applied to the surface of the modified nonwoven fabric. The fabric was then immersed in a coagulation bath of N,N-dimethylformamide / water at a uniform speed of 2 m / min in a 3:7 mass ratio of N,N-dimethylformamide and water for 10 minutes. Finally, the fabric was washed and dried.

[0063] S2. Preparation and coating of self-healing intermediate layer slurry:

[0064] Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and dibutyltin dilaurate for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:6:7:2, then add triethylamine to neutralize the 2,2-dimethylolpropionic acid with a 1:1 molar ratio of carboxyl groups, add deionized water under high-speed shearing, coat the bottom surface, bake for 3 minutes, and hot press for 3 minutes;

[0065] S3. Preparation and coating of surface slurry:

[0066] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, ultrasonically disperse, then mix with waterborne fluorocarbon modified polyurethane resin WPU-F50, add waterborne wetting and leveling agent BYK-346. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 is 15:5:4:0.7:80, stir at low speed for 2 hours;

[0067] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0068] Example 3: A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following steps:

[0069] S1, Bottom Layer Preparation:

[0070] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane in a mass ratio of 1:1. After baking for 2 minutes, aqueous anionic polyurethane resin was applied to the surface of the modified nonwoven fabric. The fabric was then immersed in a coagulation bath of N,N-dimethylformamide / water at a uniform speed of 2 m / min in a 3:7 mass ratio of N,N-dimethylformamide and water for 10 minutes. Finally, the fabric was washed and dried.

[0071] S2. Preparation and coating of self-healing intermediate layer slurry:

[0072] Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and dibutyltin dilaurate for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:6:5:2, then add triethylamine to neutralize the 2,2-dimethylolpropionic acid with a 1:1 molar ratio of carboxyl groups, add deionized water under high-speed shearing, coat the bottom surface, bake for 3 minutes, and hot press for 3 minutes;

[0073] S3. Preparation and coating of surface slurry:

[0074] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, ultrasonically disperse, then mix with waterborne fluorocarbon modified polyurethane resin, add waterborne wetting and leveling agent BYK-346. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 is 15:5:4:0.7:80, stir at low speed for 2 hours;

[0075] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0076] Example 4: A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following steps:

[0077] S1, Bottom Layer Preparation:

[0078] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane in a mass ratio of 1:1. After baking for 2 minutes, aqueous anionic polyurethane resin was applied to the surface of the modified nonwoven fabric. The fabric was then immersed in a coagulation bath of N,N-dimethylformamide / water at a uniform speed of 2 m / min in a 3:7 mass ratio of N,N-dimethylformamide and water for 10 minutes. Finally, the fabric was washed and dried.

[0079] S2. Preparation and coating of self-healing intermediate layer slurry:

[0080] Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and dibutyltin dilaurate for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:6:5:2, then add triethylamine to neutralize the 2,2-dimethylolpropionic acid with a 1:1 molar ratio of carboxyl groups, add deionized water under high-speed shearing, coat the bottom surface, bake for 3 minutes, and hot press for 3 minutes;

[0081] S3. Preparation and coating of surface slurry:

[0082] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, ultrasonically disperse, and then mix with waterborne fluorocarbon modified polyurethane resin WPU-F50. Add waterborne wetting and leveling agent BYK-346. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 is 10:5:4:0.7:80. Stir at low speed for 2 hours.

[0083] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0084] Example 5: A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following steps:

[0085] S1, Bottom Layer Preparation:

[0086] The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane in a mass ratio of 1:1. After baking for 2 minutes, aqueous anionic polyurethane resin was applied to the surface of the modified nonwoven fabric. The fabric was then immersed in a coagulation bath of N,N-dimethylformamide / water at a uniform speed of 2 m / min in a 3:7 mass ratio of N,N-dimethylformamide and water for 10 minutes. Finally, the fabric was washed and dried.

[0087] S2. Preparation and coating of self-healing intermediate layer slurry:

[0088] Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and dibutyltin dilaurate for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:6:5:2, then add triethylamine to neutralize the 2,2-dimethylolpropionic acid with a 1:1 molar ratio of carboxyl groups, add deionized water under high-speed shearing, coat the bottom surface, bake for 3 minutes, and hot press for 3 minutes;

[0089] S3. Preparation and coating of surface slurry:

[0090] S301. Mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, ultrasonically disperse, then mix with waterborne fluorocarbon modified polyurethane resin WPU-F50, add waterborne wetting and leveling agent BYK-346. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 is 20:5:4:0.7:80, stir at low speed for 2 hours;

[0091] S302. Apply the prepared slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. After coating, the non-woven fabric is first hot-pressed at 80℃ for 3 minutes, then hot-pressed at 105℃ for 5 minutes, and finally cooled to 65℃ and hot-pressed for 5 minutes. After curing, the fabric is washed, hot-air shaped, and low-temperature rubbing to obtain the final synthetic leather product.

[0092] The preparation steps of the modified nano-reinforcing agents in Examples 1-5 are as follows: silicon dioxide and graphene oxide are mixed at a mass ratio of 6:1, added to an ethanol solution containing 4 wt% silane coupling agent KH-570, ultrasonically dispersed, refluxed at 90°C for 3 h, filtered and dried to obtain the modified nano-reinforcing agent.

[0093] Comparative Example 1:

[0094] Compared with Example 3, the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol methanol and 4,4'-methylenebis(N-phenylmaleimide) in Comparative Example 1 was 100:40:6:9:2.

[0095] Comparative Example 2:

[0096] Compared with Example 3, the mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent and waterborne fluorocarbon modified polyurethane resin WPU-F50 in Comparative Example 1 was 25:5:4:0.7:80.

[0097] Comparative Example 3:

[0098] Compared to Example 3, the nonwoven fabric in Comparative Example 3 was not treated with silane reagent.

[0099] Comparative Example 4:

[0100] Compared to Example 3, in Comparative Example 4, the nonwoven fabric coated with waterborne polyurethane resin was subjected to a coagulation bath of N,N-dimethylformamide / water.

[0101] Comparative Example 5:

[0102] Compared with Example 3, triethylamine was not added during the preparation of the self-healing intermediate layer slurry in Comparative Example 5.

[0103] Comparative Example 6:

[0104] Compared to Example 3, no cellulose nanocrystals were added during the preparation of the surface slurry in Comparative Example 6.

[0105] Comparative Example 7:

[0106] Compared with Example 3, no modified nano-reinforcing agent was added during the preparation of the surface slurry in Comparative Example 7.

[0107] Comparative Example 8:

[0108] Compared to Example 3, the modified nano-reinforcing agent in Comparative Example 8 was not treated with silane during addition.

[0109] Comparative Example 9:

[0110] Compared with Example 3, in Comparative Example 9, the final hot pressing stage of the synthetic leather was not programmed hot pressing, but was directly hot pressed at 100°C for 10 minutes.

[0111] Performance testing:

[0112] GB / T 38612-2020 "Test Methods for Artificial Leather and Synthetic Leather: Determination of Tensile Load and Elongation at Break", ISO 2419:2012 "Leather—Physical Tests: Determination of Hydrolytic Resistance", QB / T 2711-2005 "Leather—Physical and Mechanical Tests: Determination of Tear Strength: Bilateral Tear", GB / T 12704.1-2009 "Textiles—Test Methods for Moisture Permeability of Fabrics—Part 1: Moisture Absorption Method", GB / T 12704.2-2009 "Textiles—Test Methods for Moisture Permeability of Fabrics—Part 2: Evaporation Method", GB / T 4744-2013 "Textiles—Test and Evaluation of Water Repellency—Hydrostatic Pressure Method", GB / T 2423.3-2016 "Environmental Testing—Part 2: Test Methods—Cab: Constant Damp Heat Test", GB / T The standard and testing methods of 32367-2015 "Determination of Volatile Organic Compound (VOC) Content in Rubber Footwear" are used to test the tensile load, elongation at break, abrasion resistance, tear strength, moisture permeability, moisture permeability stability, waterproofness, self-healing effect, and VOC content of this invention.

[0113] Table 1. Basic physicochemical properties of synthetic leather prepared from each group.

[0114] Group Tensile load (MPa) Elongation at break (%) Tear strength (N / mm) Interlayer peel strength (N / mm) Example 1 12.8 325 4.8 4.2 Example 2 13.2 338 5.1 4.5 Example 3 14.5 356 5.6 4.8 Example 4 13.8 342 5.3 4.3 Example 5 14.2 350 5.5 4.6 Comparative Example 1 9.6 268 3.2 2.8 Comparative Example 2 10.3 285 3.5 3.1 Comparative Example 3 8.7 245 2.8 1.8 Comparative Example 4 7.2 210 2.3 1.5 Comparative Example 5 6.8 198 2.1 1.2 Comparative Example 6 11.5 302 4.2 3.8 Comparative Example 7 9.8 275 3.4 2.9 Comparative Example 8 8.5 238 2.7 1.7 Comparative Example 9 10.1 290 3.6 3.3

[0115] Table 2 Functional property data of synthetic leather prepared in each group

[0116] Group Moisture permeability (g / (m²·24h)) Moisture permeability stability (%) Hydrostatic pressure resistance (kPa) Self-repair efficiency (%) VOC content (g / kg) Example 1 1850 92 45 88 0.35 Example 2 1920 94 48 90 0.32 Example 3 2150 96 52 93 0.28 Example 4 1780 91 46 89 0.33 Example 5 2080 95 50 92 0.3 Comparative Example 1 1250 75 32 65 0.58 Comparative Example 2 1320 78 35 68 0.62 Comparative Example 3 1180 72 28 62 0.65 Comparative Example 4 650 68 25 58 0.72 Comparative Example 5 820 65 22 55 0.85 Comparative Example 6 1560 85 40 82 0.42 Comparative Example 7 1380 76 33 66 0.55 Comparative Example 8 1120 70 27 60 0.68 Comparative Example 9 1450 82 36 75 0.48

[0117] Data Analysis:

[0118] Figure 1 This is a schematic diagram of the structure of the waterproof and breathable multilayer PU synthetic leather produced by this invention. From top to bottom, it consists of a surface layer, a self-healing intermediate layer, and a bottom layer. The surface layer is a fluorocarbon PU layer containing a poly(N-isopropylacrylamide)-cellulose nanocrystal temperature and humidity responsive system, which can dynamically control micropores to achieve waterproof and breathable properties. The self-healing intermediate layer is a furan-maleimide functionalized PU layer, which achieves damage repair through reversible diene crosslinking. The bottom layer is a non-woven fabric-based PU microporous layer modified by plasma and silane, providing breathable channels and interlayer bonding. The three layers are composited through an integrated process to synergistically achieve comprehensive performance in terms of waterproof and breathable properties, self-healing, and intelligent response.

[0119] Compared to Example 3, the core difference between Example 1 and Example 3 lies in the amount of furanyl alcohol added in the self-healing intermediate layer; the amount of this substance added in Example 1 is lower. This reduces the number of furan rings grafted onto the polyurethane molecular chain, and consequently lowers the density of dynamic crosslinking points formed by the subsequent diene cycloaddition reaction with the maleimide group in 4,4'-methylenebis(N-phenylmaleimide). The integrity of the crosslinking network is slightly weaker. According to the data in Tables 1 and 2, the key indicators such as mechanical properties, moisture permeability, and self-healing efficiency of the synthetic leather in Example 1 are slightly lower than those in Example 3, showing a slight overall performance degradation.

[0120] The difference between Example 2 and Example 3 lies in the amount of furanol added in the self-healing intermediate layer, which is higher than in Example 1 but lower than excessive. The appropriately increased furanol increases the number of furan rings grafted onto the polyurethane molecular chain, improves the density of dynamic crosslinking points formed by the diene cycloaddition reaction, and makes the crosslinking network structure more complete, thus enhancing mechanical properties. Simultaneously, the fluorocarbon segments in the surface fluorocarbon modified polyurethane resin are more regularly arranged, and the hydrophobic and waterproof structure is denser, resulting in better hydrostatic pressure resistance and other waterproofing properties than in Example 1. Overall, the performance is close to that of Example 3. However, because the furanol content is not at the optimal ratio of Example 3, the double bonds in the free furanol are not as stable as those formed after the diene reaction, therefore the overall performance is still slightly inferior.

[0121] Example 3, as a standard example, achieves an optimal ratio of furanol to 4,4'-methylenebis(N-phenylmaleimide) in its self-healing intermediate layer. This results in a moderate density of the dynamic cross-linked network formed by the diene cycloaddition reaction, ensuring good mechanical strength without affecting the dynamic movement of molecular chains, thus achieving optimal self-healing efficiency. In the surface layer, the synergistic ratio of poly(N-isopropylacrylamide) to cellulose nanocrystals is optimal, resulting in the most significant effect of temperature and humidity response driving micropore opening and closing, and excellent moisture permeability. Simultaneously, the combination of silane modification in the bottom layer and the programmed hot-pressing process optimizes interlayer bonding, leading to the best performance across all indicators.

[0122] Compared to Example 3, Comparative Example 1 showed a significantly higher amount of furanyl alcohol added to the self-healing intermediate layer than the reasonable range. Excessive furanyl alcohol resulted in overly dense furan rings grafted onto the polyurethane molecular chains, leading to excessive cross-linking with maleimide groups. This resulted in an excessively dense and complex cross-linked network with an increased number of free double bonds. This not only restricted the movement of molecular chains, reducing the material's flexibility, mechanical properties, and stability, but also blocked the microporous channels formed in the bottom and surface layers, causing a substantial decrease in moisture permeability. Furthermore, excessive cross-linking disrupted the reversibility of dynamic cross-linking bonds, making it difficult for chain segments to move freely and recombine during the self-healing process, significantly reducing self-healing efficiency and resulting in a significant difference in performance compared to Example 3.

[0123] The amount of poly(N-isopropylacrylamide) added to the surface slurry differed between Example 4 and Example 3, with a lower amount added in Example 4. Poly(N-isopropylacrylamide) is a thermochromic gel; the reduced content decreased the proportion of thermosensitive components in the surface layer, lowered the density of the synergistic response network formed with cellulose nanocrystals, and resulted in insufficient power for thermosensitive micropore opening and closing, leading to a decrease in moisture permeability compared to Example 3. Simultaneously, the reduced amount of poly(N-isopropylacrylamide) also resulted in a slightly weaker interfacial interaction between the surface and intermediate layers, with slightly lower mechanical properties and interlayer peel strength compared to Example 3, exhibiting an overall slight decrease dominated by thermo-humidity responsiveness.

[0124] The difference between Example 5 and the standard Example 3 is that the amount of poly(N-isopropylacrylamide) added to the surface slurry is higher than that in Example 4, but does not exceed the optimal range. The appropriately increased amount of poly(N-isopropylacrylamide) increases the number of temperature-sensitive response sites, improves the synergistic effect with cellulose nanocrystals, and results in a more reasonable range of micropore expansion and contraction under temperature and humidity changes, with moisture permeability close to that of Example 3. Furthermore, the increased amount of poly(N-isopropylacrylamide) did not lead to surface embrittlement; its compatibility with fluorocarbon modified polyurethane (FPU) is good, and its mechanical properties and hydrostatic pressure resistance are only slightly lower than those of Example 3, demonstrating the positive benefit of increasing the amount of poly(N-isopropylacrylamide) within a reasonable range on the responsive performance.

[0125] Compared to Example 3, Comparative Example 2 showed an excessive amount of poly(N-isopropylacrylamide) added to the surface slurry, far exceeding the reasonable range. The excessive poly(N-isopropylacrylamide) resulted in an excessively high proportion of temperature-sensitive components in the surface system, exceeding the load-bearing capacity of the FPU film-forming matrix. This led to numerous microcracks within the surface layer and a significant decrease in mechanical properties. Simultaneously, the excessive poly(N-isopropylacrylamide) molecular chains became entangled, blocking the hydrophilic network channels constructed by cellulose nanocrystals, significantly reducing moisture permeability. Furthermore, the overly dense poly(N-isopropylacrylamide) segments also affected the surface migration and orderly arrangement of fluorocarbon segments in the FPU, reducing the density of the hydrophobic and waterproof structure and consequently lowering hydrostatic pressure resistance. All performance characteristics differed significantly from those of Example 3.

[0126] The key difference between Comparative Example 3 and Example 3 lies in the fact that the nonwoven fabric was not treated with an aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidoxypropyltrimethoxysilane, and the surface, activated only by low-temperature plasma, lacks a stable silane bridging layer. This results in the inability of the waterborne polyurethane resin to form strong -Si-OC- covalent bonds with the nonwoven fabric surface, and the interlayer bonding relies solely on physical adsorption. Consequently, the interlayer peel strength is significantly reduced, and the interfacial fusion between the bottom and middle layers is insufficient. The mechanical properties and moisture permeability stability are also compromised, and the overall structural synergy is disrupted.

[0127] Compared to Example 3, Comparative Example 4, the nonwoven fabric coated with waterborne polyurethane resin, did not undergo N,N-dimethylformamide / water coagulation bath treatment, and therefore could not form a finger-like / sponge-like composite gradient microporous structure through a non-solvent-induced phase separation mechanism. The bottom layer lacked efficient moisture-permeable channels, making it difficult for water vapor to be transported quickly, resulting in a significant reduction in moisture permeability. Simultaneously, without the support of a microporous structure, the bottom layer lacked sufficient mechanical strength and flexibility, and the interlayer bonding strength decreased due to the lack of physical anchoring effect from the micropores. Consequently, the core functions of waterproofing and moisture permeability could not be synergistically achieved.

[0128] The difference between Comparative Example 5 and Example 3 lies in the absence of triethylamine in the preparation of the self-healing intermediate layer slurry. The carboxyl groups introduced by 2,2-dimethylolpropionic acid cannot be neutralized to form hydrophilic ammonium carboxylate salts, resulting in insufficient hydrophilicity of the polyurethane molecular chains. Under high-speed shearing, it is difficult to disperse and form a stable aqueous polyurethane dispersion. After coating, the slurry results in uneven film formation, with defects such as pinholes and craters. This leads to an incomplete dynamic cross-linking network in the intermediate layer, significantly reducing self-healing efficiency, mechanical properties, and interlayer bonding strength. The VOC content is also higher due to uneven dispersion.

[0129] Compared to Example 3, Comparative Example 6 did not incorporate cellulose nanocrystals during the preparation of the surface slurry. It relied solely on poly(N-isopropylacrylamide) to achieve a temperature-sensitive response, lacking the rigid hydrophilic enhancement and synergistic effect of nanomicroscopy on moisture-sensitive expansion. The surface layer's temperature and moisture response driving force was insufficient, resulting in reduced flexibility and amplitude of micropore opening and closing, and decreased moisture permeability and stability. Simultaneously, the absence of cellulose nanocrystals weakened the surface layer's mechanical strength and structural stability, leading to a decrease in tear strength and abrasion resistance, and overall insufficient functional synergy.

[0130] The difference between Comparative Example 7 and Example 3 lies in the absence of modified nano-reinforcing agents during the preparation of the surface slurry. The surface layer relies solely on fluorocarbon modified polyurethane resin (FPU) to provide basic mechanical properties, lacking the physical cross-linking and reinforcing effects of nanoparticles. This results in a significant decrease in the tensile load, tear strength, and other mechanical properties of the surface layer, as well as insufficient abrasion resistance. Simultaneously, the absence of nano-reinforcing agents reduces the density of the surface hydrophobic network, slightly decreases hydrostatic pressure resistance, and weakens the stability of the moisture permeability channels due to the lack of particle support, leading to a reduction in moisture permeability stability.

[0131] Compared to Example 3, Comparative Example 8, which did not undergo treatment with silane coupling agent KH-570 during addition, lacked a hydrophobic modification layer on the surface of the nanoparticles, resulting in insufficient compatibility with the poly(N-isopropylacrylamide)-cellulose nanocrystal aqueous system and FPU. This led to easy agglomeration in the slurry, causing dispersion defects within the surface layer, fluctuations in mechanical properties, and an overall decline. Agglomerated particles also blocked some moisture permeability channels, reducing moisture permeability and affecting the regular arrangement of fluorocarbon segments. This impacted waterproofing performance and interfacial interactions during the self-healing process, resulting in a comprehensive decline in overall performance.

[0132] The difference between Comparative Example 9 and Example 3 lies in the fact that the surface hot-pressing stage did not employ programmed hot-pressing, but instead involved direct hot-pressing at a single temperature. The absence of the low-temperature stage resulted in excessively rapid evaporation of moisture in the slurry, making the microporous structure prone to collapse; insufficient cross-linking enhancement and fluorocarbon segment migration during the medium-temperature stage led to inadequate density of the surface waterproof membrane; and the lack of stress relief during the low-temperature finishing stage resulted in high internal stress within the coating. This resulted in insufficient interlayer molecular diffusion and functional group interaction, decreased interlayer bonding strength, poor dynamic cross-linking network stability, and lower self-healing efficiency, mechanical properties, and synergistic effect of moisture permeability and waterproofing compared to Example 3.

[0133] The comparison of all the above embodiments and comparative examples shows that the comprehensive performance of waterproof and breathable multilayer PU synthetic leather depends on the optimal ratio of key components in each layer and the synergy of processes: the silane modification and phase separation process of N,N-dimethylformamide / water coagulation bath in the bottom layer are the foundation for ensuring interlayer bonding and breathability channels; the reasonable ratio of furanol methanol and BMI in the middle layer determines the self-healing efficiency of the dynamic cross-linking network; the synergy of poly(N-isopropylacrylamide) and cellulose nanocrystals in the surface layer and the addition of modified nano-reinforcing agents optimize temperature and humidity response and mechanical properties; and programmed hot pressing is the key to achieving the integration of the multilayer structure. Excess or absence of any core component or omission of key processes will lead to a significant decline in mechanical, waterproof, breathable, and self-healing properties. Only precise control of each step can achieve functional synergy.

[0134] The above description is only a preferred embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any equivalent substitutions or modifications made by those skilled in the art within the scope of the technology disclosed in the present invention, based on the technical solution and inventive concept of the present invention, should be covered within the scope of protection of the present invention.

Claims

1. A production process for waterproof and breathable multilayer PU synthetic leather, comprising the following preparation steps: S1, Substrate Preparation: The microfiber polyester nonwoven fabric was subjected to single-sided treatment using a low-temperature plasma device. It was then impregnated with a 2wt% aqueous solution of γ-aminopropyltriethoxysilane and γ-glycidyl etheroxypropyltrimethoxysilane, baked for 2 minutes, and then coated with an aqueous anionic polyurethane resin onto the surface of the modified microfiber polyester nonwoven fabric. The fabric was then placed in a mixed coagulation bath composed of N,N-dimethylformamide and water, washed with water, and dried. S2. Preparation and coating of self-healing intermediate layer slurry: Prepolymerize poly(1,4-butanediol adipate), isophorone diisocyanate, and catalyst for 2 hours, add 2,2-dimethylolpropionic acid, react for 1 hour, add furanol, react for 30 minutes, add 4,4'-methylenebis(N-phenylmaleimide), the mass ratio of poly(1,4-butanediol adipate), isophorone diisocyanate, 2,2-dimethylolpropionic acid, furanol, and 4,4'-methylenebis(N-phenylmaleimide) is 100:40:5-7:3-7:1-3, add triethylamine, add deionized water under high-speed shearing, coat the substrate surface, bake, and hot press; S3. Preparation and coating of surface slurry: S301. Mix silica and graphene oxide at a mass ratio of 5:1-8:1, add to an ethanol solution containing 3-5wt% silane coupling agent KH-570, disperse ultrasonically, reflux at 80-100℃ for 2-4 hours, filter and dry to obtain modified nano-reinforcing agent. Then mix poly(N-isopropylacrylamide) and cellulose nanocrystals in deionized water, add modified nano-reinforcing agent, disperse ultrasonically, and then mix with waterborne fluorocarbon modified polyurethane resin. Add waterborne wetting and leveling agent. The mass ratio of poly(N-isopropylacrylamide), cellulose nanocrystals, modified nano-reinforcing agent, waterborne wetting and leveling agent to waterborne fluorocarbon modified polyurethane resin is 10-20:5:3-5:0.5-1:70-90. Stir at low speed for 2 hours to obtain surface slurry. S302. Apply the prepared surface slurry to the surface of the self-healing intermediate layer using a micro-gravure coating. First, hot press the non-woven fabric coated with the surface slurry at 80°C for 3 minutes, then hot press at 105°C for 5 minutes, and finally cool it down to 65°C and hot press for 5 minutes. Then, after curing, washing, hot air setting, and low-temperature texturing, the final synthetic leather product is obtained.

2. The production process of waterproof and moisture-permeable multi-layer PU synthetic leather according to claim 1, characterized in that, The operation steps for entering the mixed coagulation bath composed of N,N-dimethylformamide and water in S1 are as follows: Waterborne anionic polyurethane resin is applied to the surface of modified microfiber polyester nonwoven fabric, and then immersed in a mixed coagulation bath consisting of N,N-dimethylformamide and water at a mass ratio of 3:7 at a uniform speed of 1-3 m / min for 5-15 min.

3. The production process of waterproof and moisture-permeable multi-layer PU synthetic leather according to claim 1, characterized in that, Both 2,2-dimethylolpropionic acid and 4,4'-methylenebis(N-phenylmaleimide) were first dissolved in N-methylpyrrolidone and then added to the system in solution form. The baking and hot-pressing times were both 3 minutes.

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