Super-soft waterborne polyurethane leather and preparation method thereof

Abstract

The invention relates to the technical field of synthetic leather, in particular to super-soft waterborne polyurethane leather and a preparation method thereof. The leather sequentially comprises a surface layer, a middle layer and a bottom layer from top to bottom, wherein the surface layer is composed of a PCL-PEG-PCL flexible film; the bottom layer is composed of base cloth and a PCL-PEG-PCL flexible film coated on the surface of the base cloth; and the middle layer is composed of modified waterborne polyurethane prepared from PCL-PEG-PCL flexible dihydric alcohol and aliphatic linear chain diisocyanate. The preparation method comprises the following steps: synthesizing PCL-PEG-PCL dihydric alcohol, preparing each layer, and performing condensation reaction on the middle layer and the surface / bottom layer to realize chemical curing. The leather is normally super-soft, the stress strength is remarkably improved, a full-water-based system is environment-friendly, the low-temperature stability and interlayer binding strength are excellent, and the leather is suitable for the field of high-end flexible materials such as furniture veneers, automobile interiors and shoe materials.

Description

Technical Field

[0001] This invention relates to the field of synthetic leather technology, and in particular to an ultra-soft waterborne polyurethane leather and its preparation method. Background Technology

[0002] With increasingly stringent global environmental regulations, the control of volatile organic compound (VOC) emissions has become a core constraint on the development of the synthetic leather industry. Traditional solvent-based synthetic leather, due to its high pollution and high emissions, is gradually being phased out by the market. Water-based synthetic leather, with its advantages of being solvent-free and environmentally friendly, has become the mainstream choice for furniture upholstery, automotive interiors, and footwear materials. At the same time, consumers' demands for the user experience of synthetic leather continue to upgrade. "Ultra-flexibility" has become a core performance indicator for high-end products. For example, car seats need to conform to the human body's curves for a long time, and baby products need to withstand repeated folding. This requires synthetic leather to not only have low hardness and high elongation at break under normal conditions, but also sufficient strength to avoid damage under stress, achieving the dual characteristics of "ultra-flexibility + enhanced stress resistance".

[0003] However, existing waterborne synthetic leather technology has consistently struggled to overcome the challenge of balancing flexibility and strength. Traditional waterborne polyurethane often involves reacting a single soft segment with aromatic or cyclic aliphatic isocyanates. Simply increasing the proportion of soft segments to enhance flexibility weakens intermolecular forces, resulting in insufficient tensile strength under normal conditions and susceptibility to permanent deformation under stress. On the other hand, adding crosslinking agents or rigid groups to increase strength increases the rigidity of the synthetic leather due to higher crosslinking density or a rise in the glass transition temperature of the soft segments, leading to a loss of the core ultra-soft feel. This contradiction of "softness leading to weakness, strength leading to rigidity" has become a key bottleneck restricting industry upgrades.

[0004] Furthermore, existing functional modification technologies suffer from significant limitations in adaptability to various application scenarios. To enhance strength, some solutions introduce rigid functional groups into diols. While this can improve mechanical properties to some extent, it significantly increases the glass transition temperature of the soft segment, causing the synthetic leather to become brittle at low temperatures and resulting in a substantial decrease in elongation at break. This makes it unsuitable for applications such as automotive interiors and outdoor products in cold regions. Simultaneously, traditional layered synthetic leathers often rely on physical bonding or adhesives to achieve interlayer adhesion. This not only results in insufficient bonding strength and a tendency for peeling but also may affect overall flexibility due to the rigidity of the adhesives, further limiting the application of water-based synthetic leather in high-end fields. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and to propose an ultra-soft waterborne polyurethane leather and its preparation method.

[0006] To achieve the above objectives, the present invention provides an ultra-soft water-based polyurethane leather, which comprises, from top to bottom, a top layer, a middle layer, and a bottom layer.

[0007] The surface layer is composed of a PCL-PEG-PCL flexible film;

[0008] The bottom layer consists of a base fabric and a PCL-PEG-PCL flexible film coated on the surface of the base fabric.

[0009] The intermediate layer is composed of modified waterborne polyurethane;

[0010] The PCL-PEG-PCL flexible film comprises the following raw materials in parts by weight: PCL-PEG-PCL flexible diol: 100 parts, antioxidant: 0.1-0.3 parts, ultraviolet absorber: 0.05-0.2 parts;

[0011] The modified waterborne polyurethane comprises the following raw materials in parts by weight: PCL-PEG-PCL flexible diol: 50-70 parts, aliphatic linear diisocyanate: 30-40 parts, 2,2-dimethylolpropionic acid: 5-8 parts, triethylamine: 3-5 parts, and deionized water: 100-140 parts.

[0012] The preparation method of the PCL-PEG-PCL flexible diol is as follows:

[0013] S1. Add 4-oxocyclohexanecarboxamide to dichloromethane, then add m-chloroperoxybenzoic acid, heat to 35-45℃, react for 18-24 h, cool to room temperature, filter, remove solvent from the filtrate by rotary evaporation to obtain crude product, recrystallize the crude product in diethyl ether to obtain functionalized caprolactone; the chemical reaction equation is as follows:

[0014] The product was characterized by ¹H NMR. In this step, 4-oxocyclohexane carboxamide reacts with m-chloroperoxybenzoic acid in dichloromethane via a Baeyer-Villiger oxidation reaction, ultimately generating a functionalized caprolactone with a carboxamide side chain. The peroxy acid first undergoes nucleophilic addition with the carbonyl group, and then a hydroxyl group on the ketone carbonyl group migrates with a pair of electrons to the oxygen atom directly bonded to the carbonyl carbon atom in the -OO- group, while simultaneously undergoing heterolytic cleavage of the OO bond. The reaction mechanism is as follows:

[0015] .

[0016] S2. Add 6-caprolactone, functionalized caprolactone, and PEG-400 to a reaction vessel. After dehydration, add stannous octoate, then purge three times with nitrogen. Heat to 120-140℃ and react for 8-12 hours. After cooling to room temperature, add to dichloromethane and, with stirring, add to diethyl ether at 0℃. The resulting precipitate, after filtration and drying, yields PCL-PEG-PCL flexible diol. The chemical reaction diagram is shown below:

[0017] In this step, PEG-400 is used as the dual-terminal hydroxyl initiator and stannous octoate as the catalyst to perform coordination ring-opening polymerization of 6-caprolactone and functionalized caprolactone to form a triblock diol. First, the hydroxyl groups at both ends of the PEG-400 molecule react with Sn in the stannous octoate catalyst. 3+ Coordination occurs, forming the "Sn-O-PEG" active intermediate, Sn 3+ The empty orbital of the hydroxyl oxygen atom combines with the lone pair electrons of the hydroxyl oxygen atom, lowering the OH bond energy, activating the reactivity of the hydroxyl group, and simultaneously establishing a bi-terminal initiation site. Subsequently, due to ring strain, the six-membered ring ester structure of 6-caprolactone and functionalized caprolactone allows the oxygen atom on the ring to move towards the Sn in the "Sn-O-PEG" reactive intermediate. 3+ Empty orbital coordination leads to enhanced polarity and breakage of the CO bond within the ring. The monomer unit after ring opening connects to the end of the "Sn-O-PEG" active chain through an insertion reaction, achieving chain growth. Since PEG-400 is a dual-end initiator, the active chains at both ends will grow synchronously during polymerization, forming segments with polycaprolactone as the main chain and functionalized caprolactone side chains, namely PCL-PEG-PCL flexible diol.

[0018] Preferably, the base fabric refers to polyester knitted fabric.

[0019] Preferably, the antioxidant refers to antioxidant 1010 and the ultraviolet absorber refers to UV-531.

[0020] Preferably, the aliphatic linear diisocyanate has the following general structural formula:

[0021] , where n = 4 - 8.

[0022] Preferably, in S1, the molar ratio of 4-oxocyclohexanecarboxamide and m-chloroperoxybenzoic acid is 1:1-1.2.

[0023] Preferably, in S1, the weight ratio of 4-oxocyclohexanecarboxamide and dichloromethane is 1:8-12.

[0024] Preferably, the ratio of the total mass of 6-caprolactone and functionalized caprolactone to the mass of PEG-400 in S2 is 2-3:1.

[0025] Preferably, the molar ratio of 6-caprolactone and functionalized caprolactone in S2 is 1:0.15-0.25.

[0026] Preferably, in S2, PEG-400, stannous octoate, dichloromethane, and diethyl ether at 0°C are in a weight ratio of 1:0.15-0.2:25-35:40-60.

[0027] Preferably, the average molecular weight of the PCL-PEG-PCL flexible diol in S2 is 3000.

[0028] Furthermore, the present invention also provides a method for preparing ultra-soft waterborne polyurethane leather, comprising the following steps:

[0029] (1) After drying and dehydrating PCL-PEG-PCL flexible diol, it is heated to melt with antioxidant and UV absorber under stirring and then cast into a film by casting machine. After pre-curing at 60-70℃ for 10-15 min and cooling to room temperature, the surface layer is obtained.

[0030] (2) Soak the base fabric in sodium hydroxide solution for 10-20 min, wash it with deionized water until the washing solution is neutral, and dry it for later use; dry and dehydrate the PCL-PEG-PCL flexible diol, and heat it with antioxidant and UV absorber until it melts under stirring, and then apply it to the surface of the base fabric. Control the wet film thickness to be 0.3-0.5 mm, pre-cur it at 60-70℃ for 10-15 min, and then cool it to room temperature to form a PCL-PEG-PCL flexible film to obtain the bottom layer;

[0031] (3) After dehydrating the PCL-PEG-PCL flexible diol, add it to the reaction vessel and heat it to 65-75℃. Add aliphatic linear diisocyanate and stir the reaction under nitrogen protection for 2.5-3.5h. Then add 2,2-dimethylolpropionic acid and heat it to 80-90℃ and continue stirring the reaction for 1-1.5h to obtain the hydrophilic prepolymer.

[0032] (4) Cool the hydrophilic prepolymer to 40-45℃, add triethylamine, stir and neutralize for 15-20 min, then add deionized water under high speed stirring at 2000-2400 rpm, keep stirring for 15-25 min, filter through 300 mesh filter cloth and vacuum degas for 10-20 min to obtain modified waterborne polyurethane.

[0033] (5) Lay the bottom layer flat, apply modified waterborne polyurethane to its surface, control the wet film thickness to 0.2-0.4 mm to obtain the middle layer, then cover the surface of the middle layer with the top layer, apply 0.1-0.3 MPa pressure, first dry in a forced air at 60-70℃ for 15-20 min, then heat to 85-95℃ to cure for 30-40 min, and after naturally cooling to room temperature, place for 24 h to obtain ultra-soft waterborne polyurethane leather.

[0034] Preferably, the thickness of the cast film in (1) is 0.5-1 mm.

[0035] Preferably, the concentration of the sodium hydroxide solution in (2) is 5-10 wt%.

[0036] Preferably, the mechanism of action of the ultra-soft waterborne polyurethane leather in this invention is as follows:

[0037] The ultra-soft feel of this invention originates from the synergistic design of the three-layer structure and the inherent characteristics of the molecular chains. The outer layer is a pure PCL-PEG-PCL flexible film, and the inner layer is a composite structure with a PCL-PEG-PCL flexible film coated on the surface of the base fabric. The core flexibility of both comes from the block structure of PCL-PEG-PCL. The glass transition temperatures of the PEG-400 segment in the middle and the PCL segments at both ends are in an extremely low range. Under normal conditions, the molecular chains always maintain a highly elastic state, without the constraint and obstruction of rigid segments. These linear flexible segments are bound together only by weak van der Waals forces, allowing them to be freely rolled and slid, giving the outer layer an extremely soft feel and ensuring that the flexible film portion of the inner layer maintains a consistent soft texture. At the same time, the uniformly dispersed side chain carboxamide groups in the segments form weak NH…O hydrogen bonds, slightly adjusting the segment arrangement and avoiding a loose feel caused by excessive disorder of the molecular chains, making the softness more uniform and controllable. After being treated with a low-concentration sodium hydroxide solution, the base fabric of the bottom layer only forms a slightly rough surface structure, while the internal fiber structure remains intact, retaining its flexibility. This provides support without compromising overall softness. Although the modified waterborne polyurethane in the middle layer contains hard segments formed by aliphatic linear diisocyanates, these hard segments have no cyclic structure and are more flexible than traditional cyclic isocyanate hard segments. Furthermore, it has excellent compatibility with the PCL-PEG-PCL chain segment materials of the surface and bottom layers, allowing it to be evenly dispersed in the soft segment matrix without forming rigid aggregation areas. The three elements work together to maintain the overall Shore hardness of the leather in the ultra-soft range, providing a tactile feel that meets human needs.

[0038] When leather is subjected to external forces such as stretching and friction, the system achieves enhanced strength through segment orientation crystallization, hydrogen bond stabilization, and synergistic chemical bonding, without losing its flexibility and reversibility. Under external force, the PCL segments in the surface pure PCL-PEG-PCL film, the bottom coated PCL-PEG-PCL film, and the intermediate modified waterborne polyurethane simultaneously and rapidly extend and align along the direction of force. The linear structure of the PCL segments gives them excellent crystallization tendency. During the orientation process, the molecular chains are regularly arranged to form crystalline regions. These crystalline regions act as physical reinforcing nodes, significantly improving the tensile strength of the material. At the same time, the carboxylamide groups on the side chains of the PCL segments approach each other as the segments align, and the intermolecular hydrogen bonds formed by the NH…O bonds further fix the structure of the crystalline regions, preventing slippage of the crystalline regions under force, resulting in a significant increase in tensile strength compared to the normal state. The terminal isocyanate groups formed by the excess isocyanate in the intermediate layer of modified waterborne polyurethane undergo condensation reactions with the terminal hydroxyl groups on the surface of the surface and bottom PCL-PEG-PCL flexible films during the composite curing process, forming stable urethane covalent bonds. Under stress, these chemical bonds effectively disperse stress, avoiding damage caused by localized stress concentration. Meanwhile, the aliphatic linear hard segments can be oriented synchronously with the molecular chains, neither interfering with the crystallization process of PCL nor hindering stress dispersion, ensuring that the reinforced material still maintains a high elongation at break. After the external force disappears, the PCL crystalline region at room temperature gradually recovers its molecular chain thermal motion due to its low melting point, and the crystalline region slowly melts, with the crystallinity returning to normal levels. The hydrogen bonds of the side chain carboxylamide groups redisperse with the molecular chain coiling, and the weak hydrogen bonds spontaneously dissociate, returning the molecular chain to a disordered coiled state. The small number of covalent cross-links formed during the curing stage, due to their low density and moderate bond energy, do not hinder the recovery of chain segment flexibility, ultimately restoring the leather to its initial super-soft state and achieving a long-term reversible balance between flexibility and mechanical properties.

[0039] The robust bonding of the three-layer structure and the flexible support of the base fabric stem from the dual effects of chemical bonding and physical anchoring. During the preparation of the modified waterborne polyurethane middle layer, an excess of aliphatic linear diisocyanate is controlled to ensure residual terminal isocyanate groups at the ends of the molecular chains. During the composite curing process, these terminal isocyanate groups undergo a condensation reaction with the terminal hydroxyl groups on the surfaces of the pure PCL-PEG-PCL film on the surface and the PCL-PEG-PCL film coated on the bottom layer, forming stable urethane covalent bonds. This firmly connects the three layers into an integrated structure, avoiding the interlayer delamination problems caused by traditional physical bonding. Simultaneously, the middle layer is a fully waterborne emulsion with excellent wettability with the surface and bottom layers, and all three layers contain flexible PCL-PEG-PCL segments, ensuring good material compatibility and further enhancing interfacial bonding strength. After the base fabric in the bottom layer is soaked in a 5-10wt% sodium hydroxide solution, the surface grease and weakly bound impurities are removed, and a micro-rough structure is formed. When the PCL-PEG-PCL flexible film is applied to the surface of the base fabric, it can penetrate into the micropores of the base fabric surface and enhance the bonding force between the film and the base fabric through mechanical anchoring. The alkaline washing only acts on the surface of the base fabric without damaging the internal fiber structure and retaining the flexibility of the base fabric itself. This allows the bottom layer to have both support and not affect the overall super softness of the leather, achieving a synergy between support and softness.

[0040] The stable dispersion and efficient moisture removal during the curing process of the intermediate layer modified waterborne polyurethane ensure both product performance and environmental friendliness. During preparation, the carboxyl groups introduced by 2,2-dimethylolpropionic acid are neutralized by triethylamine to form hydrophilic carboxylate groups. Under high-speed stirring, the prepolymer is dispersed into nano-sized droplets, forming a stable emulsion with deionized water. This achieves the required viscosity for coating without the need for organic solvents, ensuring both workability and compliance with all-waterborne environmental requirements. The two-step "pre-drying and curing" process in the curing stage completely removes moisture. Even trace amounts of unevaporated moisture react with the excess terminal isocyanate groups remaining in the intermediate layer, generating urea bonds and releasing carbon dioxide. This carbon dioxide is expelled under pressure and blower, leaving no residue in the film layer. The generated urea bonds further enhance the film layer strength, achieving a dual effect of moisture removal and performance enhancement.

[0041] The leather maintains its excellent performance even at low temperatures, primarily due to the synergistic adaptation of soft segment characteristics, hard segment structure, and intermolecular forces. The PCL-PEG-PCL soft segment has an extremely low glass transition temperature, allowing the molecular chain to remain highly elastic even at low temperatures, preventing embrittlement. The selected aliphatic linear diisocyanate (n=4-8) has an acyclic structure and strong intramolecular rotational capacity, retaining a certain degree of flexibility at low temperatures and not affecting the movement of the overall chain segments. Simultaneously, the NH…O hydrogen bonds formed by the side-chain carboxamide groups remain stable at low temperatures, effectively stabilizing any crystalline regions that may form in the PCL chain and preventing brittleness due to low temperatures. Furthermore, the urethane covalent bonds between the three layers are structurally stable at low temperatures, preventing breakage or embrittlement and continuously ensuring interlayer bonding and stress dispersion capabilities. The fiber structure of the base fabric remains flexible at low temperatures and does not harden as the temperature drops. It works synergistically with the PCL-PEG-PCL film on the surface to ensure that the leather still has good tensile strength and elongation at break at low temperatures, making it suitable for the needs of automotive interiors, outdoor products and other scenarios in cold regions.

[0042] The beneficial effects of this invention are:

[0043] 1. This invention utilizes a molecularly designed PCL-PEG-PCL block flexible diol. Leveraging the low glass transition temperature of the PEG segment in the middle and the PCL segments at both ends, combined with the weak hydrogen bonds formed by the carboxylamide groups on the side chains, the molecular chains of the leather can freely curl and slide under normal conditions, resulting in an extremely soft feel. Under stress, the PCL segments crystallize along the direction of stress, and the hydrogen bonds further stabilize the crystalline region. Combined with the stress-dispersing effect of the urethane covalent bonds between the three layers, a significant increase in strength is achieved. After the external force disappears, the crystalline region melts, the hydrogen bonds dissociate, and the molecular chains return to their disordered curled state, balancing the "ultra-soft feel" with "high strength under stress," thus solving the industry pain point of traditional technologies where "softness leads to weakness, and strength leads to rigidity."

[0044] 2. In the curing stage of this invention, no additional curing agent is required. It relies on the mild self-crosslinking of the residual isocyanate groups and hydroxyl groups in the prepolymer with trace amounts of water, combined with the reversibility of side-chain hydrogen bonds, to form a low-crosslinking density structure. This structure avoids the increased rigidity caused by excessive crosslinking and stabilizes the film structure. During long-term use, the chain segments can recover flexibility through thermal motion and hydrogen bond reconstruction, making it less prone to embrittlement, deformation, or interlayer delamination, significantly extending the product's service life.

[0045] 3. This invention utilizes the condensation reaction between the isocyanate groups at the intermediate layer and the hydroxyl groups at the surface and bottom layers to form stable chemical covalent bonds, firmly connecting the three layers into an integrated structure. This solves the problem of interlayer delamination caused by traditional physical bonding or adhesive adhesion. After treatment with a low-concentration sodium hydroxide solution, the bottom substrate fabric develops a micro-roughened surface structure, removing impurities without damaging the internal fibers, thus enhancing the bond between the PCL-PEG-PCL flexible film and the substrate fabric through mechanical anchoring. The support of the substrate fabric and the flexibility of each layer work synergistically to ensure the overall structural stability of the leather without affecting its ultra-soft feel and bending flexibility.

[0046] 4. This invention uses an acyclic aliphatic linear diisocyanate, whose hard segments possess excellent internal rotational capability. Combined with the extremely low glass transition temperature of the PCL-PEG-PCL soft segments, this allows the leather to maintain high elasticity even at low temperatures. The hydrogen bonds of the side-chain carboxamide groups are stable at low temperatures, effectively preventing brittleness in the PCL crystalline region, and the covalent bond structure between the three layers will not break due to low temperatures. This characteristic allows the leather to maintain good strength and elongation at break in applications such as automotive interiors and outdoor products in cold regions, significantly expanding its application range. Attached Figure Description

[0047] Figure 1 Functionalized caprolactone prepared in Example 2 of this invention 1 H NMR spectrum. Detailed Implementation

[0048] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0049] Preparation Example 1: The specific preparation method of PCL-PEG-PCL flexible diol includes the following steps:

[0050] S1. Add 100g of 4-oxocyclohexanecarboxamide to 800g of dichloromethane, then add 122.22g of m-chloroperoxybenzoic acid, heat to 35℃, react for 18h, cool to room temperature, filter, remove solvent from the filtrate by rotary evaporation to obtain crude product, recrystallize the crude product in diethyl ether to obtain functionalized caprolactone;

[0051] S2. Add 100g PEG-400, 165.76g 6-caprolactone and 34.24g functionalized caprolactone to a reaction vessel. After removing water, add 15g stannous octoate, then purge with nitrogen three times. Heat to 120℃ and react for 8 hours. After cooling to room temperature, add 2.5kg dichloromethane and stir. Then add 4kg diethyl ether at 0℃. The resulting precipitate is filtered, dried, and the product with an average molecular weight of 3000 is collected to obtain PCL-PEG-PCL flexible diol.

[0052] Preparation Example 2: The specific preparation method of PCL-PEG-PCL flexible diol includes the following steps:

[0053] S1. Add 100g of 4-oxocyclohexanecarboxamide to 1kg of dichloromethane, then add 134.46g of m-chloroperoxybenzoic acid, heat to 40℃, react for 22h, cool to room temperature, filter, remove solvent from the filtrate by rotary evaporation to obtain crude product, recrystallize the crude product in diethyl ether to obtain functionalized caprolactone.

[0054] S2. Add 100g PEG-400, 196.02g 6-caprolactone and 53.98g functionalized caprolactone to a reaction vessel. After removing water, add 18g stannous octoate. Then, purge with nitrogen three times, heat to 130℃ and react for 10h. After cooling to room temperature, add 3kg dichloromethane and stir. Then add 5kg diethyl ether at 0℃. The precipitate obtained is filtered, dried and the product with an average molecular weight of 3000 is collected to obtain PCL-PEG-PCL flexible diol.

[0055] Preparation Example 3: The specific preparation method of PCL-PEG-PCL flexible diol includes the following steps:

[0056] S1. Add 100g of 4-oxocyclohexanecarboxamide to 1.2kg of dichloromethane, then add 146.68g of m-chloroperoxybenzoic acid, heat to 45℃, react for 24h, cool to room temperature, filter, remove solvent from the filtrate by rotary evaporation to obtain crude product, recrystallize the crude product in diethyl ether to obtain functionalized caprolactone;

[0057] S2. Add 100g PEG-400, 223.17g 6-caprolactone and 76.83g functionalized caprolactone to a reaction vessel. After removing water, add 20g stannous octoate, then purge with nitrogen three times. Heat to 140℃ and react for 12h. After cooling to room temperature, add 3.5kg dichloromethane and stir. Then add 6kg diethyl ether at 0℃. The resulting precipitate is filtered, dried, and the product with an average molecular weight of 3000 is collected to obtain PCL-PEG-PCL flexible diol.

[0058] Comparative Preparation Example 1: The difference between Comparative Preparation Example 1 and Preparation Example 2 is that step (1) is omitted, functionalized caprolactone is not added in step (2), and 196.02g of 6-caprolactone and 53.98g of functionalized caprolactone are replaced with 250g of 6-caprolactone.

[0059] Example 1: A specific method for preparing an ultra-soft waterborne polyurethane leather, comprising the following steps:

[0060] (1) After drying and dehydrating 1 kg of PCL-PEG-PCL flexible diol prepared in Preparation Example 1, 1 g of antioxidant 1010 and 0.5 g of UV absorber UV-531 were heated to melt under stirring and then cast into a film with a thickness of 0.5 mm through a casting machine. After pre-curing at 60°C for 10 min and cooling to room temperature, a surface layer was obtained.

[0061] (2) Soak the polyester knitted fabric in a 5wt% sodium hydroxide solution for 10 min, wash it with deionized water until the washing solution is neutral, and dry it for later use; dry and dehydrate 1 kg of the PCL-PEG-PCL flexible diol prepared in Preparation Example 1, and heat it with 1 g of antioxidant 1010 and 0.5 g of UV absorber UV-531 until it melts under stirring and apply it to the surface of the base fabric. Control the wet film thickness to 0.3 mm, pre-cur it at 60℃ for 10 min, and cool it to room temperature to form a PCL-PEG-PCL flexible film to obtain the bottom layer;

[0062] (3) After dehydrating 500g of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 1, add it to the reaction vessel and heat it to 65°C. Add 300g of hexamethylene diisocyanate and stir the reaction under nitrogen protection for 2.5h. Then add 50g of 2,2-dimethylolpropionic acid and heat it to 80°C and continue stirring the reaction for 1h to obtain the hydrophilic prepolymer.

[0063] (4) Cool the hydrophilic prepolymer to 40°C, add 30g of triethylamine, stir and neutralize for 15min, then add 1kg of deionized water under high-speed stirring at 2000rpm, keep stirring for 15min, filter through 300-mesh filter cloth and vacuum degas for 10min to obtain waterborne modified polyurethane.

[0064] (5) Lay the bottom layer flat, apply modified waterborne polyurethane to its surface, control the wet film thickness to 0.2 mm to obtain the middle layer, then cover the surface of the middle layer with the top layer, apply 0.1 MPa pressure, first dry at 60℃ for 15 min, then heat to 85℃ for 30 min, cool naturally to room temperature, and place for 24 h to obtain ultra-soft waterborne polyurethane leather.

[0065] Example 2: A specific method for preparing an ultra-soft waterborne polyurethane leather, comprising the following steps:

[0066] (1) 1 kg of PCL-PEG-PCL flexible diol prepared according to Preparation Example 2 was dried and dehydrated, and then heated to melt with 2 g of antioxidant 1010 and 1.2 g of UV absorber UV-531 under stirring. The mixture was then cast into a film with a thickness of 0.8 mm through a casting machine. After pre-curing at 65°C for 12 min and cooling to room temperature, a surface layer was obtained.

[0067] (2) Soak the polyester knitted fabric in an 8 wt% sodium hydroxide solution for 15 min, then wash it with deionized water until the washing solution is neutral, and dry it for later use; dry and dehydrate 1 kg of PCL-PEG-PCL flexible diol prepared according to Preparation Example 2, and heat it with 2 g of antioxidant 1010 and 1.2 g of UV absorber UV-531 until it melts under stirring and apply it to the surface of the base fabric. Control the wet film thickness to 0.4 mm, pre-cur it at 65°C for 12 min, and cool it to room temperature to form a PCL-PEG-PCL flexible film to obtain the bottom layer;

[0068] (3) After dehydrating 600g of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 2, add it to the reaction vessel and heat it to 70°C. Add 350g of hexamethyl diisocyanate and stir the reaction under nitrogen protection for 3h. Then add 60g of 2,2-dimethylolpropionic acid and heat it to 85°C and continue stirring the reaction for 1.2h to obtain the hydrophilic prepolymer.

[0069] (4) Cool the hydrophilic prepolymer to 43°C, add 40g of triethylamine, stir and neutralize for 18min, then add 1.2kg of deionized water under high-speed stirring at 2200rpm, keep stirring for 20min, filter through 300-mesh filter cloth and vacuum degas for 15min to obtain modified waterborne polyurethane.

[0070] (5) Lay the bottom layer flat, apply modified waterborne polyurethane to its surface, control the wet film thickness to 0.4 mm to obtain the middle layer, then cover the surface of the middle layer with the top layer, apply 0.2 MPa pressure, first dry at 65℃ for 18 min, then heat to 90℃ for 35 min to cure, and after naturally cooling to room temperature, place for 24 h to obtain ultra-soft waterborne polyurethane leather.

[0071] Example 3: A specific method for preparing an ultra-soft waterborne polyurethane leather, comprising the following steps:

[0072] (1) 1 kg of PCL-PEG-PCL flexible diol prepared according to Preparation Example 3 was dried and dehydrated, and then heated to melt with 3 g of antioxidant 1010 and 3 g of ultraviolet absorber UV-531 under stirring. The film was then cast into a film with a thickness of 1 mm through a casting machine. After pre-curing at 70°C for 15 min and cooling to room temperature, the surface layer was obtained.

[0073] (2) Soak the polyester knitted fabric in a 10wt% sodium hydroxide solution for 20 min, then wash it with deionized water until the washing solution is neutral, and dry it for later use; dry and dehydrate 1 kg of PCL-PEG-PCL flexible diol prepared according to Preparation Example 3, and heat it with 3 g of antioxidant 1010 and 2 g of UV absorber UV-531 until it melts under stirring and apply it to the surface of the base fabric. Control the wet film thickness to 0.5 mm, pre-cur it at 70℃ for 15 min, and cool it to room temperature to form a PCL-PEG-PCL flexible film to obtain the bottom layer;

[0074] (3) After dehydrating 700g of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 3, add it to the reaction vessel and heat it to 75°C. Add 400g of hexamethylene diisocyanate and stir the reaction under nitrogen protection for 3.5h. Then add 80g of 2,2-dimethylolpropionic acid and heat it to 90°C and continue stirring the reaction for 1.5h to obtain the hydrophilic prepolymer.

[0075] (4) Cool the hydrophilic prepolymer to 45°C, add 50g of triethylamine, stir and neutralize for 20min, then add 1.4kg of deionized water under high-speed stirring at 2400rpm, keep stirring for 25min, filter through 300-mesh filter cloth and vacuum degas for 20min to obtain modified waterborne polyurethane.

[0076] (5) Lay the bottom layer flat, apply modified waterborne polyurethane to its surface, control the wet film thickness to 0.5 mm to obtain the middle layer, then cover the surface of the middle layer with the top layer, apply 0.3 MPa pressure, first dry at 70℃ for 20 min, then heat to 95℃ for 40 min, cool naturally to room temperature, and place for 24 h to obtain ultra-soft waterborne polyurethane leather.

[0077] Example 4: The difference between Example 4 and Example 2 is that hexamethylene diisocyanate is replaced with isocyclohexaneimide, with the following chemical structural formula:

[0078] .

[0079] Example 5: The difference between Example 5 and Example 2 is that hexamethylene diisocyanate is replaced with 1,8-diisocyanooctane, with the following chemical structural formula:

[0080] .

[0081] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that the PCL-PEG-PCL flexible diol prepared according to Preparation Example 2 is replaced with the PCL-PEG-PCL flexible diol prepared according to Comparative Preparation Example 1.

[0082] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that the PCL-PEG-PCL flexible diol prepared according to Preparation Example 2 is replaced with PEG-400.

[0083] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that hexamethylene diisocyanate is replaced with 1,10-diisocyanodecane, with the following chemical structural formula:

[0084] .

[0085] Comparative Example 4: The difference between Comparative Example 4 and Example 2 is that hexamethylene diisocyanate is replaced with 1,12-diisocyanate tridecane, with the following chemical structural formula:

[0086] .

[0087] Comparative Example 5: The difference between Comparative Example 5 and Example 2 is that hexamethylene diisocyanate is replaced with 1,3-diisocyanopropane, with the following chemical structure:

[0088] .

[0089] Performance testing:

[0090] 1. Comparison of tensile strength under normal and stressed conditions: Following GB / T 1040.3-2006 "Determination of tensile properties of plastics - Part 3: Test conditions for films and sheets", the leather samples prepared in Examples 1-5 and Comparative Examples 1-5 were cut into dumbbell-shaped specimens (total length 75 mm, effective length 25 mm, width 6 mm). These specimens were placed in an environment of 23℃±2℃ and relative humidity 50%±5% for 24 h. Using a universal testing machine, a normal tensile test was first performed at a tensile speed of 500 mm / min, and the normal tensile strength and elongation at break were recorded. Subsequently, specimens from the same batch were taken, and a constant stress of 5 MPa was applied to the testing machine and held for 10 s. Immediately afterward, the tensile strength under stressed conditions was tested at the same tensile speed, and the tensile strength and elongation at break were calculated. The experimental results are shown in Table 1.

[0091] 2. Dynamic Step Stress-Strength Response Test: Based on the optimized test mode of GB / T 1040.3-2006, a universal testing machine with real-time stress-strain acquisition function was used to conduct a "step stress loading" test on dumbbell-shaped specimens of Examples 1-5 and Comparative Examples 1-5. The specimen pretreatment was the same as before. The tensile speed was set to 500 mm / min. First, the tensile strength was measured by loading at a rate of 1 MPa / min to 2 MPa and holding for 5 s. Then, the tensile strength was measured by loading to 5 MPa and holding for 10 s. Finally, the tensile strength was measured by loading to 8 MPa and holding for 10 s. The increase in tensile strength at different stress stages compared to the tensile strength under normal conditions was calculated. The tensile strength increase under different stresses was calculated as (tensile strength under different stresses - tensile strength under normal conditions) / tensile strength under normal conditions × 100%. The experimental results are shown in Table 1.

[0092] 3. Constant stress fatigue strength test: Referring to GB / T 12000.2-2016 "Plastics - Polyurethane Elastomers - Part 2: Requirements for Molding and Extrusion Materials", constant stress fatigue strength tests were performed on the dumbbell-shaped specimens of Examples 1-5 and Comparative Examples 1-5. The specimens were fixed on a universal testing machine, loaded with a constant stress of 5 MPa, and held for 100 hours. Real-time stress was recorded every 10 hours. After 100 hours, the load was removed, and the residual tensile strength of the specimens was tested. The strength retention rate (residual strength ÷ initial 5 MPa strength × 100%) was calculated. The experimental results are shown in Table 1.

[0093] 4. Low-Temperature Strength Test: Referring to GB / T 1040.3-2006, the test environment was adjusted to -45℃±2℃, and the dumbbell-shaped specimens of Examples 1-5 and Comparative Examples 1-5 were tested. The specimens were first placed in a -45℃ low-temperature chamber for 2 hours to ensure uniform temperature. Then, a stress of 5MPa was applied on a universal testing machine (maintaining -45℃) in the low-temperature chamber and held for 10 seconds. Subsequently, the tensile strength and elongation at break after being subjected to stress were tested at a speed of 500mm / min. The experimental results are shown in Table 1.

[0094] 5. Normal Shore hardness test: The test was performed in accordance with GB / T 2411-2008 "Determination of indentation hardness (Shore hardness) of plastics and hard rubber using a hardness tester". A Shore A hardness tester was used. 100mm×100mm samples were cut from the leather prepared in Examples 1-5 and Comparative Examples 1-5. The surface was ensured to be flat, without wrinkles or bubbles. After the environmental stress was eliminated by placing the sample in an environment of 23℃±2℃ and 50%±5% relative humidity for 24 hours, the hardness tester indenter was pressed vertically into the sample surface (indenter distance from edge ≥10mm, distance between adjacent test points ≥15mm). Six points were tested on each sample. The maximum and minimum values ​​were removed and the average was taken. The experimental results are shown in Table 1.

[0095] 6. Dynamic bending stiffness test: The test was conducted in accordance with QB / T 4463-2013 "Determination of bending properties of synthetic leather in physical and mechanical tests". A dynamic bending tester was used. The leather samples prepared in Examples 1-5 and Comparative Examples 1-5 were cut into strips of 150mm×25mm. After the environmental stress was eliminated by placing them in an environment of 23℃±2℃ and relative humidity of 50%±5% for 24h, the bending angle was set to ±45° and the bending rate was 30 times / min. The samples were bent continuously for 100 times. The maximum bending force at the 1st and 100th bending times was recorded and the rate of change of bending force was calculated. The experimental results are shown in Table 1.

[0096] Table 1 Performance Test Results

[0097]

[0098] Performance Analysis:

[0099] As can be seen from the experimental data in Table 1, the ultra-soft waterborne polyurethane leather prepared by the present invention in Examples 1-5 is significantly better than that in Comparative Examples 1-5 in terms of normal flexibility, strength under stress, long-term load stability and low-temperature adaptability. Among them, Example 2 has the best overall performance.

[0100] Example 2 achieves an optimal balance between tensile strength and elongation at break under normal conditions, primarily due to the microscopic interaction of molecular structure and the synergistic effect of the three layers. In the block structure of PCL-PEG-PCL flexible diol, the middle PEG segment and the two PCL segments at both ends are soft segments with low glass transition temperatures. The molecular chains are bound together only by weak van der Waals forces, allowing them to freely curl and slide, providing a flexible basis for elongation at break. At the same time, the weak NH…O hydrogen bonds formed by the carboxylamide groups on the side chains moderately regulate the chain segment arrangement, avoiding insufficient tensile strength caused by excessive molecular chain disorder. The middle layer forms an integrated structure with the surface and bottom layers through urethane covalent bonds. Under stress, the three layers work together to bear the load, resulting in more uniform stress distribution, thus providing both suitable tensile strength and excellent elongation at break under normal conditions. In comparison, Comparative Example 1, lacking functionalized caprolactone and missing side-chain carboxamide hydrogen bonds, exhibited weakened intermolecular forces, resulting in decreased tensile strength and elongation at break. Comparative Example 2, using pure PEG-400 instead of triblock diol, lacked the rigid support of PCL segments; although its elongation at break was acceptable, its tensile strength was significantly reduced, failing to meet practical requirements. Comparative Examples 3-4 used aliphatic linear diisocyanates with excessively long carbon chains; the hard segments tended to arrange themselves regularly, forming crystalline regions that restricted the sliding of the soft segments, leading to a decrease in elongation at break under normal conditions. Comparative Example 5 used short-chain isocyanates; the intermolecular forces in the hard segments were weak, resulting in significantly insufficient tensile strength under normal conditions.

[0101] Example 2 exhibits a significant strength enhancement under stress, primarily due to the dual guarantee of molecular chain synergy and chemical bonding. Under external force, the PCL segments in the surface, bottom, and intermediate layers synchronously extend and align along the direction of force. The linear structure of the PCL segments gives them excellent crystallization tendency. During orientation, the molecular chains arrange themselves regularly to form crystalline regions, which act as physical reinforcing nodes to improve tensile strength. Simultaneously, the carboxylamide groups on the side chains approach each other as the segments align, and the resulting NH…O intermolecular hydrogen bonds further fix the structure of the crystalline regions, preventing them from slipping under stress. The urethane covalent bonds in the intermediate layer and the surface and bottom layers effectively disperse stress, preventing localized stress concentration. Meanwhile, the aliphatic linear hard segments can align synchronously with the molecular chains, neither interfering with the PCL crystallization process nor hindering stress dispersion, resulting in a significant increase in strength under stress. Due to the lack of side-chain hydrogen bonds, the PCL segments in Comparative Example 1 were difficult to stabilize after orientation crystallization, and the crystalline region was prone to slippage, resulting in a lower strength increase after stress compared to Example 2. Comparative Example 2 lacked PCL segments and could not achieve strength improvement through force-induced crystallization, leading to a significant reduction in the strength increase after stress. In Comparative Examples 3-4, the long-chain isocyanate hard segment crystalline region competed for orientation space with the PCL crystalline region, interfering with the force-induced crystallization efficiency of PCL and causing a significant decrease in the strength increase after stress. In Comparative Example 5, the short-chain isocyanate could not provide effective support for the soft segments, and the PCL crystalline region was prone to breakage under stress, resulting in a lower strength increase than in Example 2.

[0102] Example 2 maintains a high strength retention rate even after long-term constant stress loading. This is due to the synergistic effect of the slightly self-crosslinked structure and the reversibility of hydrogen bonds. The small amount of -NCO groups remaining during the prepolymer preparation reacts with the terminal hydroxyl groups of the diol and trace amounts of water to form urethane and urea bonds with low crosslinking density, which only stabilize the film structure without increasing rigidity. At the same time, the side-chain carboxamide hydrogen bonds are reversible, and during long-term loading, they can slightly dissociate and reconstruct with stress fluctuations, avoiding irreversible damage to the crystalline region due to continuous stress, thereby maintaining a high strength retention rate. In Comparative Example 1, due to the absence of side-chain carboxamide hydrogen bonds, the PCL crystalline region was prone to slippage during long-term loading, resulting in a low strength retention rate. In Comparative Example 2, pure PEG-400 was used to replace the triblock diol, and the support was only provided by the weak intermolecular forces of PEG. After long-term loading, the chain segments were prone to permanent deformation, and the strength retention rate decreased significantly. In Comparative Examples 3 and 4, the long-chain isocyanate hard segment crystalline regions had high rigidity and were prone to cracking under long-term loading, leading to a decrease in strength retention rate. In Comparative Example 5, the short-chain isocyanate hard segment had weak intermolecular forces, and the hard segments were prone to detach from the soft segment matrix during loading, resulting in significant strength attenuation and a lower retention rate than in Example 2.

[0103] Example 2 demonstrates excellent strength and elongation at break even under low-temperature conditions, primarily due to the synergistic adaptation of soft segment characteristics, hard segment structure, and chemical bonding. The PCL-PEG-PCL soft segment has an extremely low glass transition temperature, far below the test temperature, allowing the molecular chain to maintain a highly elastic state at low temperatures without becoming brittle. The selected aliphatic linear diisocyanate (n=4-8) has an acyclic structure and strong internal rotational ability, retaining a certain degree of flexibility at low temperatures without affecting the overall chain segment movement. The NH…O hydrogen bonds formed by the carboxylamide groups in the side chains remain stable at low temperatures, effectively stabilizing any crystalline regions that may form in the PCL chain and preventing brittle fracture due to low temperatures. The urethane covalent bonds between the three layers are structurally stable at low temperatures, continuously ensuring interlayer bonding and stress dispersion capabilities. Due to the lack of side-chain hydrogen bonds, the PCL crystalline region in Comparative Example 1 is prone to slippage and brittle fracture at low temperatures, resulting in a decrease in both strength and elongation at break. In Comparative Example 2, which lacks PCL segments, the intermolecular forces of the pure PEG soft segments are further weakened at low temperatures, leading to a significant decrease in strength. In Comparative Examples 3 and 4, the rigidity of the long-chain isocyanate hard segments in the crystalline region increases further at low temperatures, making them prone to brittle fracture and resulting in a significant decrease in elongation at break. In Comparative Example 5, the short-chain isocyanate exhibits even weaker intermolecular forces at low temperatures, failing to provide effective support for the soft segments and exhibiting poor low-temperature stress performance.

[0104] The Shore hardness of Example 2 is in the ultra-flexible range, which microscopically stems from the synergistic regulation of molecular structure and interlayer interactions. The PEG segment in the middle of the PCL-PEG-PCL flexible diol and the PCL segments at both ends are low glass transition temperature soft segments. Under normal conditions, the molecular chains always maintain a highly elastic state, and the molecules are only bound by weak van der Waals forces, without the strong constraint of rigid segments. The NH…O hydrogen bonds formed by the amide groups on the side chains are weak interactions, which only slightly adjust the chain arrangement to avoid excessive disorder and do not increase the rigidity of the molecular chains. The aliphatic linear diisocyanate is a soft hard segment with an acyclic structure, which can be uniformly dispersed in the soft segment matrix and does not form rigid aggregation areas. Ultimately, this makes the leather surface less resistant to indentation and exhibits a low Shore hardness. Comparative Example 1, lacking functionalized caprolactone and missing side-chain carboxamide hydrogen bonds, relies solely on van der Waals forces for intermolecular bonding, increasing the disorder of chain segment arrangement. However, the PCL segments themselves still possess a certain degree of rigidity, resulting in a higher Shore hardness than Example 2. Comparative Example 2 uses pure PEG-400 instead of triblock diol, lacking the rigid support of PCL segments. The intermolecular forces in the soft segments are extremely weak, leading to a lower Shore hardness than Example 2, but insufficient film strength renders it impractical. Comparative Examples 3-4 employ long-chain isocyanates, where the excessively long carbon chains in the hard segments easily form regular arrangements to create crystalline regions. These crystalline regions exhibit high rigidity, significantly enhancing the surface's resistance to indentation, resulting in a Shore hardness far exceeding that of Example 2. Comparative Example 5, with its short-chain isocyanate, exhibits weak intermolecular forces in the hard segments, failing to effectively support the soft segments. The intermolecular constraints are weaker than in Example 2, resulting in a relatively higher Shore hardness.

[0105] Example 2 exhibits a low rate of change in dynamic bending force, demonstrating excellent flexibility and stability during repeated bending. This is likely due to the dual advantages of substrate-membrane interface synergy and molecular chain recovery. After the substrate is treated with sodium hydroxide solution, a micro-rough structure is formed on its surface. When the PCL-PEG-PCL flexible film is coated, it can penetrate deep into the micropores on the substrate surface, enhancing the bonding strength of the film-substrate interface through mechanical anchoring and preventing interface slippage during repeated bending. The linear structure of the PCL-PEG-PCL soft segments gives it good chain segment recovery. The side-chain amide hydrogen bonds can reversibly dissociate and reconstruct during bending. During bending, the chain segments are oriented along the direction of force, and can quickly return to their original position after the force is released, without rigid aggregation or permanent deformation. Comparative Example 1, lacking side-chain carboxamide hydrogen bonds, suffers from a lack of hydrogen bond stability after the PCL segments crystallize. During repeated bending, the crystalline regions easily slip, leading to increased local rigidity of the film. Furthermore, weak intermolecular constraints cause micro-displacement at the film-base fabric interface, resulting in a higher rate of change in bending force compared to Example 2. Comparative Example 2, using pure PEG-400 instead of diol, has low film strength. During repeated bending, the film easily extends slightly along the base fabric surface. Additionally, PEG has poor compatibility with the base fabric, resulting in low interfacial bonding and a higher rate of change in bending force. Comparative Examples 3-4 exhibit high rigidity in the hard segments of the long-chain isocyanate crystalline regions. During repeated bending, these regions easily crack, forming local rigidity sites. The hard segment crystallization interferes with the synergistic deformation of the film and base fabric, significantly increasing the bending force with each bend, with a rate of change far exceeding that of Example 2. Comparative Example 5 shows that the short-chain isocyanate hard segments cannot effectively disperse bending stress, and the soft segments are prone to permanent deformation. Furthermore, the weak interfacial bonding between the hard and soft segments results in a higher rate of change in bending force compared to Example 2.

[0106] 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. An ultra-soft water-based polyurethane leather, characterized in that, From top to bottom, it includes the top layer, the middle layer, and the bottom layer; The surface layer is composed of a PCL-PEG-PCL flexible film; The bottom layer consists of a base fabric and a PCL-PEG-PCL flexible film coated on the surface of the base fabric. The intermediate layer is composed of modified waterborne polyurethane; The PCL-PEG-PCL flexible film comprises the following raw materials in parts by weight: PCL-PEG-PCL flexible diol: 100 parts, antioxidant: 0.1-0.3 parts, ultraviolet absorber: 0.05-0.2 parts; The modified waterborne polyurethane comprises the following raw materials in parts by weight: PCL-PEG-PCL flexible diol: 50-70 parts, aliphatic linear diisocyanate: 30-40 parts, 2,2-dimethylolpropionic acid: 5-8 parts, triethylamine: 3-5 parts, and deionized water: 100-140 parts. The preparation method of the PCL-PEG-PCL flexible diol is as follows: S1. Add 4-oxocyclohexanecarboxamide to dichloromethane, then add m-chloroperoxybenzoic acid, heat to 35-45℃, react for 18-24h, cool to room temperature, filter, remove solvent from the filtrate by rotary evaporation to obtain crude product, recrystallize the crude product in diethyl ether to obtain functionalized caprolactone. S2. Add 6-caprolactone, functionalized caprolactone, and PEG-400 to a reaction vessel, remove water, add stannous octoate, then purge with nitrogen three times, heat to 120-140℃ and react for 8-12 hours, cool to room temperature, add to dichloromethane, stir, add to diethyl ether at 0℃, the precipitate is filtered and dried to obtain PCL-PEG-PCL flexible diol.

2. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, The base fabric refers to polyester knitted fabric.

3. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, The antioxidant refers to antioxidant 1010, and the ultraviolet absorber refers to UV-531.

4. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, The general structural formula of the aliphatic linear diisocyanate is: , where n = 4 - 8.

5. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, In S1, the molar ratio of 4-oxocyclohexane carboxamide and m-chloroperoxybenzoic acid is 1:1-1.2, and the weight ratio of 4-oxocyclohexane carboxamide and dichloromethane is 1:8-12.

6. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, The ratio of the total mass of 6-caprolactone and functionalized caprolactone to the mass of PEG-400 in S2 is 2-3:1, and the molar ratio of 6-caprolactone to functionalized caprolactone is 1:0.15-0.

25.

7. The ultra-soft waterborne polyurethane leather according to claim 1, characterized in that, In S2, PEG-400, stannous octoate, dichloromethane, and diethyl ether are present in a weight ratio of 1:0.15-0.2:25-35:40-60.

8. The method for preparing ultra-soft waterborne polyurethane leather according to any one of claims 1-7, characterized in that, Includes the following steps: (1) After drying and dehydrating PCL-PEG-PCL flexible diol, it is heated to melt with antioxidant and UV absorber under stirring and then cast into a film by casting machine. After pre-curing at 60-70℃ for 10-15 min and cooling to room temperature, the surface layer is obtained. (2) Soak the base fabric in sodium hydroxide solution for 10-20 min, wash it with deionized water until the washing solution is neutral, and dry it for later use; dry and dehydrate the PCL-PEG-PCL flexible diol, and heat it with antioxidant and UV absorber until it melts under stirring, and then apply it to the surface of the base fabric. Control the wet film thickness to be 0.3-0.5 mm, pre-cur it at 60-70℃ for 10-15 min, and then cool it to room temperature to form a PCL-PEG-PCL flexible film to obtain the bottom layer; (3) After dehydrating the PCL-PEG-PCL flexible diol, add it to the reaction vessel and heat it to 65-75℃. Add aliphatic linear diisocyanate and stir the reaction under nitrogen protection for 2.5-3.5h. Then add 2,2-dimethylolpropionic acid and heat it to 80-90℃ and continue stirring the reaction for 1-1.5h to obtain the hydrophilic prepolymer. (4) Cool the hydrophilic prepolymer to 40-45℃, add triethylamine, stir and neutralize for 15-20 min, then add deionized water under high speed stirring at 2000-2400 rpm, keep stirring for 15-25 min, filter through 300 mesh filter cloth and vacuum degas for 10-20 min to obtain modified waterborne polyurethane. (5) Lay the bottom layer flat, apply modified waterborne polyurethane to its surface, control the wet film thickness to 0.2-0.4 mm to obtain the middle layer, then cover the surface of the middle layer with the top layer, apply 0.1-0.3 MPa pressure, first dry in a forced air at 60-70℃ for 15-20 min, then heat to 85-95℃ to cure for 30-40 min, and after naturally cooling to room temperature, place for 24 h to obtain ultra-soft waterborne polyurethane leather.

9. The method for preparing ultra-soft waterborne polyurethane leather according to claim 8, characterized in that, The thickness of the cast film in (1) is 0.5-1 mm.

10. The method for preparing ultra-soft waterborne polyurethane leather according to claim 8, characterized in that, The concentration of sodium hydroxide solution in (2) is 5-10 wt%.

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