Super-soft waterborne polyurethane leather and preparation method thereof

Abstract

The present application relates to the technical field of synthetic leather, in particular to a super-soft water-based polyurethane leather and a preparation method thereof.The leather comprises a surface layer, a middle layer and a bottom layer from top to bottom; the surface layer is composed of PCL-PEG-PCL flexible film; the bottom layer is composed of a 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 water-based polyurethane made of PCL-PEG-PCL flexible dihydric alcohol and aliphatic linear diisocyanate.The preparation steps include synthesis of PCL-PEG-PCL dihydric alcohol and preparation of each layer, and chemical curing is realized by condensation reaction of the middle layer with the surface layer and the bottom layer.The leather is super-soft in normal state, and the stress strength is significantly improved; the water-based system is environmentally friendly; the low-temperature stability and the interlayer bonding strength are excellent; and the leather is suitable for high-end flexible material fields such as furniture facing, automotive interior, shoe materials and the like.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of synthetic leather, and in particular to a super-soft water-based polyurethane leather and a preparation method thereof. BACKGROUND

[0002] With the increasingly stringent global environmental regulations, volatile organic compound (VOCs) emission control has become the core constraint for the development of the synthetic leather industry. Traditional solvent-based synthetic leather is gradually eliminated by the market due to its high pollution and high emission. Water-based synthetic leather has become the mainstream choice in the fields of furniture facing, automotive interior, shoe materials, etc. due to its advantages of no solvent pollution and environmental friendliness. At the same time, consumers' requirements for the use experience of synthetic leather continue to upgrade, and "super flexibility" has become a core performance indicator of high-end products. For example, automotive seats need to be in close contact with the human body curve for a long time, and baby products need to adapt to repeated folding, which requires synthetic leather not only to have low hardness and high elongation at break in normal state, but also to have sufficient strength to avoid damage under stress, realizing the dual characteristics of "super flexibility + stress enhancement".

[0003] However, the existing water-based synthetic leather technology has always been difficult to break through the balance between flexibility and strength. Traditional water-based polyurethane is mostly reacted with a single soft segment and aromatic or cyclic aliphatic isocyanate. If the proportion of soft segment is simply increased to improve flexibility, the intermolecular force will be weakened, the tensile strength in normal state will be insufficient, and permanent deformation will easily occur under stress. If crosslinking agents or rigid groups are added to improve strength, the crosslinking density or the glass transition temperature of the soft segment will increase, resulting in increased rigidity of the synthetic leather and loss of the core super-soft touch. This contradiction between "flexibility and weakness, rigidity and strength" has become a key bottleneck restricting the upgrading of the industry.

[0004] In addition, the existing functional modification technology also has obvious scene adaptation defects. In order to improve the strength, some schemes introduce rigid functional groups into diols, which can enhance the mechanical properties to a certain extent, but will significantly increase the glass transition temperature of the soft segment, resulting in brittle cracking of the synthetic leather in low temperature environment and a significant decrease in elongation at break, which cannot meet the use requirements of automotive interiors, outdoor products, etc. in cold regions. At the same time, traditional layered synthetic leather mostly relies on physical adhesion or adhesives to realize the combination between layers, which not only has insufficient adhesion and is prone to peeling problems, but also may affect the overall flexibility due to the rigidity of the adhesives, further limiting the application expansion of water-based synthetic leather in high-end fields. SUMMARY

[0005] The purpose of the present application is to solve the problems in the prior art and provide a super-soft water-based polyurethane leather and a preparation method thereof.

[0006] In order to achieve the above purpose, the present application provides a super-soft water-based polyurethane leather, which comprises a surface layer, an intermediate layer and a bottom layer from top to bottom.

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

[0008] The bottom layer is composed of base cloth and PCL-PEG-PCL flexible film coated on the surface of the base cloth;

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

[0010] The PCL-PEG-PCL flexible film comprises the following raw materials 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 by weight: PCL-PEG-PCL flexible diol: 50-70 parts, aliphatic linear diisocyanate: 30-40 parts, 2,2-dimethylol propionic acid: 5-8 parts, triethylamine: 3-5 parts, deionized water: 100-140 parts;

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

[0013] S1. 4-oxocyclohexane carboxamide is added to dichloromethane, and m-chloroperbenzoic acid is added, and the temperature is raised to 35-45℃, and the reaction is carried out for 18-24h, and then cooled to room temperature, filtered, and the filtrate is subjected to rotary evaporation to remove the solvent to obtain a crude product, which is recrystallized in diethyl ether to obtain a functionalized caprolactone; the chemical reaction equation is as follows:

[0014] The product is characterized by H NMR; in this step, 4-oxocyclohexane carboxamide and m-chloroperbenzoic acid undergo Baeyer-Villiger oxidation reaction in dichloromethane, and finally functionalized caprolactone with carboxamide side chain is generated; the peroxo acid first undergoes nucleophilic addition with the carbonyl group, and then one of the hydroxyl groups on the ketone carbonyl group migrates to the oxygen atom directly connected to the carbonyl carbon atom of the -O-O- group, accompanied by O-O bond heterolysis, and the reaction mechanism is as follows:

[0015] .

[0016] S2. 6-caprolactone, functionalized caprolactone and PEG-400 are added to a reaction vessel, dehydrated, then stannous octoate is added, and then replaced with nitrogen three times, and then heated to 120-140℃ for 8-12h, and then cooled to room temperature, and then added to dichloromethane, and then added to diethyl ether at 0℃ with stirring, and then the obtained precipitate is filtered and dried to obtain PCL-PEG-PCL flexible diol; the chemical reaction diagram is as follows:

[0017] ; in this step, PEG-400 is used as a double-end hydroxyl initiator, and stannous octoate is used as a catalyst to coordinate ring-opening polymerization of 6-caprolactone and functionalized caprolactone to form a triblock structure diol. First, the hydroxyl groups at both ends of the PEG-400 molecule coordinate with Sn 3+ in the catalyst stannous octoate to form a "Sn-O-PEG" active intermediate, and the empty orbital of Sn 3+ combines with the lone pair of electrons of the hydroxyl oxygen atom, reducing the O-H bond energy and activating the reaction activity of the hydroxyl group, while establishing a double-end initiation site; subsequently, the six-membered ring structure of 6-caprolactone and functionalized caprolactone is coordinated with Sn 3+ in the "Sn-O-PEG" active intermediate due to the ring strain, resulting in an increase in the polarity of the C-O bond in the ring and the rupture of the ring, and the monomer unit after ring-opening is connected to the end of the "Sn-O-PEG" active chain through an insertion reaction, realizing chain growth; because PEG-400 is a double-end initiator, the active chains at both ends will grow synchronously during polymerization, respectively forming a segment with poly-caprolactone as the main chain and containing a functionalized caprolactone side chain, i.e., a PCL-PEG-PCL flexible diol.

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

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

[0020] Preferably, the structural general formula of the aliphatic linear diisocyanate is:

[0021] wherein n = 4-8.

[0022] Preferably, the molar ratio of 4-oxocyclohexane carboxamide to meta-chloroperoxybenzoic acid in S1 is 1:1-1.2.

[0023] Preferably, the weight ratio of 4-oxocyclohexane carboxamide to dichloromethane in S1 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 to functionalized caprolactone in S2 is 1:0.15-0.25.

[0026] Preferably, the weight ratio of PEG-400, stannous octoate, dichloromethane, and ethyl ether at 0°C in S2 is 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] Further, the present application also provides a preparation method of the super-soft water-based polyurethane leather, comprising the following steps:

[0029] (1) After drying and dehydrating the PCL-PEG-PCL flexible diol, and adding antioxidants and ultraviolet absorbers, heat to melt under stirring, and then flow-cast into a film by a flow-casting machine. After pre-curing at 60-70℃ for 10-15 min and cooling to room temperature, a surface layer is obtained.

[0030] (2) After soaking the base fabric in a sodium hydroxide solution for 10-20 min, wash with deionized water until the washing liquid is neutral, and dry for standby use. After drying and dehydrating the PCL-PEG-PCL flexible diol, and adding antioxidants and ultraviolet absorbers, heat to melt under stirring, and then coat on the surface of the base fabric, controlling the wet film thickness to be 0.3-0.5 mm. After pre-curing at 60-70℃ for 10-15 min and cooling to room temperature, a PCL-PEG-PCL flexible film is formed, and a bottom layer is obtained.

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

[0032] (4) After cooling the hydrophilic prepolymer to 40-45℃, add triethylamine, and neutralize by stirring for 15-20 min. Then add deionized water under high-speed stirring at 2000-2400 rpm, and keep stirring for 15-25 min. After filtering through a 300-mesh filter cloth, vacuum degassing for 10-20 min to obtain a modified water-based polyurethane.

[0033] (5) After laying the bottom layer, coat the modified water-based polyurethane on the surface thereof, controlling the wet film thickness to be 0.2-0.4 mm to obtain an intermediate layer. Then cover the surface layer on the surface of the intermediate layer, and apply a pressure of 0.1-0.3 MPa. First, dry at 60-70℃ for 15-20 min, and then heat to 85-95℃ for curing for 30-40 min. After natural cooling to room temperature, place for 24 h to obtain a super-soft water-based polyurethane leather.

[0034] Preferably, the thickness of the film formed by flow-casting 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 the super-soft waterborne polyurethane leather in the present application is as follows:

[0037] The super-soft touch of the present application is derived from the synergistic design of the three-layer structure and the inherent properties of the molecular chain. The surface layer is a pure PCL-PEG-PCL flexible film, and the bottom layer is a composite structure of the base cloth surface coated with a PCL-PEG-PCL flexible film. The core flexibility of both comes from the block structure of PCL-PEG-PCL. The glass transition temperature of the middle segment PEG-400 and the two end PCL segments are both in the extremely low interval. Under normal circumstances, the molecular chain always remains high-elasticity state without the constraint and obstruction of rigid segments. These linear flexible segments are only combined by weak van der Waals force and can freely curl and slide, giving the surface layer an extremely soft touch and also allowing the flexible film part of the bottom layer to maintain consistent softness. At the same time, the uniformly dispersed side chain carboxamide groups in the chain segment slightly adjust the chain segment arrangement by forming weak N-H…O hydrogen bonds, avoiding the loose touch caused by excessive disorder of the molecular chain, and making the softness more uniform and controllable. The base cloth of the bottom layer is treated with a low-concentration sodium hydroxide solution, only forming a micro-rough structure on the surface, without damaging the internal fiber structure, still retaining its own flexibility, providing support without interfering with the overall softness. The modified waterborne polyurethane in the middle layer contains aliphatic linear diisocyanate hard segments, but the hard segment has no ring structure and is more flexible than the traditional cyclic isocyanate hard segment. Moreover, it has excellent compatibility with the PCL-PEG-PCL segments of the surface layer and the bottom layer, can be uniformly dispersed in the soft segment matrix, and does not form rigid aggregation areas. The synergy of the three makes the overall leather hardness maintain in the super-soft interval, and the touch fits the human body needs.

[0038] When the leather is subjected to external forces such as stretching, friction, etc., the system realizes strength improvement through chain segment orientation crystallization, hydrogen bond stability and chemical bond cooperation, and does not lose flexibility reversibility. Under the action of external force, the PCL-PEG-PCL film on the surface layer, the PCL-PEG-PCL film coated on the bottom layer and the PCL chain segment in the middle layer modified waterborne polyurethane are stretched, oriented along the force direction, and the linear structure of the PCL chain segment makes it have excellent crystallization tendency. During the orientation process, the molecular chain is arranged regularly to form a crystalline region. These crystalline regions act as physical reinforcement nodes, significantly improving the tensile strength of the material. At the same time, the carboxamide groups on the side chain of the PCL chain segment approach each other with the orientation of the chain segment, forming N-H…O intermolecular hydrogen bonds that further fix the structure of the crystalline region, preventing the crystalline region from slipping under stress, and greatly improving the tensile strength compared to the normal state. The terminal isocyanate groups of the modified waterborne polyurethane in the middle layer are formed due to the excess of isocyanate, which have undergone condensation reaction with the terminal hydroxyl groups on the surface of the PCL-PEG-PCL flexible film in the surface layer and the bottom layer during the composite curing process, forming stable urethane covalent bonds. These chemical bonds can effectively disperse stress when stressed, avoiding damage caused by local stress concentration; and the aliphatic straight-chain hard segment can be oriented with the molecular chain, which does not interfere with the crystallization process of PCL and can also help disperse stress, ensuring that the enhanced leather still maintains a high elongation at break. After the external force disappears, the PCL crystalline region gradually recovers due to its low melting point, and the crystalline region slowly melts, and the crystallinity falls to the normal level; the hydrogen bonds of the side chain carboxamide group disperse again with the curling of the molecular chain, and the weak hydrogen bonds spontaneously dissociate, and the molecular chain returns to the disordered curling state; the low density and moderate bond energy of the small amount of covalent cross-linking bonds formed during the curing stage do not hinder the flexibility recovery of the chain segment, finally making the leather return to the initial super-soft state, realizing the long-term reversible balance of flexibility and mechanical properties.

[0039] The firm combination of the three-layer structure and the flexible support of the base cloth are derived from the dual action of chemical combination and physical anchoring. The intermediate layer of modified waterborne polyurethane controls the excess of aliphatic linear diisocyanate during preparation, ensuring that the terminal isocyanate groups remain at the end of the molecular chain. During the composite curing process, these terminal isocyanate groups condense with the terminal hydroxyl groups on the surface of the pure PCL-PEG-PCL film of the surface layer and the PCL-PEG-PCL film coated on the bottom layer, forming stable urethane covalent bonds to firmly connect the three layers into an integrated structure, avoiding the interlayer peeling problem caused by traditional physical bonding. At the same time, the intermediate layer is a full water-based emulsion, which has excellent wettability with the surface layer and the bottom layer, and all three contain PCL-PEG-PCL flexible segments, which have good material compatibility, further improving the interface bonding strength. After the base cloth 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 coated on the surface of the base cloth, it can penetrate into the micro-pores on the surface of the base cloth, and the mechanical anchoring effect can improve the film-base cloth interfacial bonding strength; while the alkali washing only acts on the surface of the base cloth without damaging the internal fiber structure, it retains the flexibility of the base cloth itself, so that the bottom layer has both support and does not affect the overall super-soft properties of the leather, achieving the synergy of support and softness.

[0040] The stable dispersion of the intermediate layer of modified waterborne polyurethane and the efficient removal of water during the curing process ensure the product performance and environmental protection. During preparation, the carboxyl groups introduced by 2,2-dimethylol propionic acid are neutralized by triethylamine to form hydrophilic carboxylate groups. Under high-speed stirring, the prepolymer is dispersed into nanoscale droplets to form a stable emulsion with deionized water, without the need for organic solvents to achieve the required viscosity for coating, ensuring workability and meeting the full-water environmental requirements. The two-step process of "pre-drying and curing" in the curing stage can completely remove water. Even if there is a very small amount of water that does not evaporate, it will react with the excess terminal isocyanate groups remaining in the intermediate layer to form urea bonds and release carbon dioxide, which is removed under the action of air blowing and pressure, without remaining in the film layer. The generated urea bonds can further enhance the strength of the film layer, achieving the dual effects of water removal and performance enhancement.

[0041] The leather can still maintain good performance in a low-temperature environment, which is due to the synergistic adaptation of the soft segment characteristics, hard segment structure and intermolecular force. The glass transition temperature of the PCL-PEG-PCL soft segment is extremely low, and the molecular chain can still maintain a high elastic state at low temperature and will not be brittle due to the decrease of temperature; the selected aliphatic linear diisocyanate (n = 4-8) has no cyclic structure, and the molecular chain has strong rotation ability, which still has a certain flexibility at low temperature, and does not affect the movement of the overall chain segment. At the same time, the N-H…O hydrogen bond formed by the side chain carboxamide group can still exist stably in a low-temperature environment, which can effectively stabilize the crystalline region that may be formed by the PCL segment, and avoid the brittle fracture of the crystalline region due to low temperature; the structure of the urethane covalent bond between the three layers is stable at low temperature, and will not be broken or brittle, which can continuously guarantee the interlayer bonding force and stress dispersion capacity. The fiber structure of the bottom base fabric still maintains flexibility at low temperature, and will not be hardened due to the decrease of temperature, which cooperates with the PCL-PEG-PCL film on the surface to make the leather still have good stress strength and elongation at break at low temperature, and adapt to the scene requirements of cold regions, automotive interiors, outdoor products and the like.

[0042] The beneficial effects of the present application are as follows:

[0043] 1. The present application uses the PCL-PEG-PCL block flexible diol designed by molecular design, and the low glass transition temperature characteristics of the intermediate segment PEG and the two end PCL segments, and the weak hydrogen bond formed by the side chain carboxamide group, so that the molecular chain of the leather can be freely curled and slid in a normal state, and has an extremely soft touch. When stressed, the PCL segment is oriented and crystallized along the stress direction, the hydrogen bond further stabilizes the crystalline region, and the stress dispersion effect of the urethane covalent bond between the three layers is combined, so that the strength is significantly improved. After the external force disappears, the crystalline region melts, the hydrogen bond dissociates, and the molecular chain returns to the disordered curled state, balancing the "ultra-soft touch" and "high strength under stress", and solving the industry pain point of "softness and weakness, rigidity and rigidity" of traditional technology.

[0044] 2. The present application does not need to add a curing agent in the curing stage, relies on the slight self-crosslinking of the residual isocyanate group and hydroxyl group, and a small amount of water, and combines the reversibility of the side chain hydrogen bond to form a low crosslinking density structure. The structure not only avoids the increase of rigidity caused by excessive crosslinking, but also stabilizes the film layer structure, and the chain segment can be restored to flexibility through thermal motion and hydrogen bond reconstruction during long-term use, and is not easy to appear brittle, deformation or interlayer peeling, which significantly prolongs the service life of the product.

[0045] 3. The present application solves the problem of interlayer peeling caused by traditional physical lamination or adhesive bonding by forming stable chemical covalent bonds through the condensation reaction of intermediate layer terminal isocyanate groups with surface layer and bottom layer terminal hydroxyl groups, firmly connecting the three layers into an integrated structure. After the bottom layer base cloth is treated with a low-concentration sodium hydroxide solution, a micro-rough structure is formed on the surface, which not only removes impurities but also does not damage the internal fibers, so that the PCL-PEG-PCL flexible film and the base cloth are enhanced in binding force through mechanical anchoring. The support of the base cloth and the flexibility of each layer work together to ensure the stability of the overall structure of the leather and do not affect the super-soft touch and bending flexibility.

[0046] 4. The present application selects acyclic aliphatic linear diisocyanate, which forms hard segments with good internal rotation ability, and cooperates with PCL-PEG-PCL soft segments with extremely low glass transition temperature, so that the leather still maintains high elasticity in low temperature environment. The hydrogen bond of the side chain carboxamide group stably exists at low temperature, which can effectively avoid the brittle fracture of the PCL crystalline region, and the covalent bond structure between the three layers will not be broken due to low temperature. This feature allows the leather to maintain good stress strength and elongation at break in cold regions, such as automotive interiors and outdoor products, greatly expanding the application range. BRIEF DESCRIPTION OF DRAWINGS

[0047] Figure 1 Functional caprolactone prepared in Preparation Example 2 of the present application 1 H NMR spectrum. DETAILED DESCRIPTION

[0048] In order to make the purpose, technical scheme and advantages of the present application clearer and more apparent, the present application will be further described in detail below with specific examples.

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

[0050] S1. 100g 4-oxocyclohexanecarboxamide is added to 800g dichloromethane, then 122.22g m-chloroperbenzoic acid is added, the temperature is raised to 35℃, and the reaction is carried out for 18h, then the temperature is cooled to room temperature, filtration is carried out, the filtrate is subjected to rotary evaporation to remove the solvent, the crude product is recrystallized in diethyl ether, and functional caprolactone is obtained;

[0051] S2. 100g PEG-400, 165.76g 6-caprolactone and 34.24g functional caprolactone are added to a reaction container, after water removal, 15g stannous octoate is added, then nitrogen is replaced for three times, the temperature is raised to 120℃, and the reaction is carried out for 8h, then the temperature is cooled to room temperature, the obtained product is added to 2.5kg dichloromethane, then 4kg diethyl ether at 0℃ is added under stirring, the obtained precipitate is subjected to suction filtration and dried, and the product with an average molecular weight of 3000 is collected, and PCL-PEG-PCL flexible dihydric alcohol is obtained.

[0052] Preparation Example 2: A specific preparation method of PCL-PEG-PCL flexible diol, comprising the following steps:

[0053] S1. 100 g of 4-oxocyclohexane carboxamide was added to 1 kg of dichloromethane, and then 134.46 g of meta-chloroperoxybenzoic acid was added. The temperature was raised to 40°C, and the reaction was carried out for 22 h. After cooling to room temperature, filtration was carried out, and the filtrate was subjected to rotary evaporation to remove the solvent to obtain a crude product. The crude product was recrystallized in diethyl ether to obtain functionalized caprolactone;

[0054] S2. 100 g of PEG-400, 196.02 g of 6-caprolactone and 53.98 g of functionalized caprolactone were added to a reaction vessel. After water removal, 18 g of stannous octoate was added, and then nitrogen was replaced for three times. The temperature was raised to 130°C, and the reaction was carried out for 10 h. After cooling to room temperature, it was added to 3 kg of dichloromethane, and then 5 kg of diethyl ether at 0°C was added under stirring. The obtained precipitate was subjected to suction filtration, and then dried to collect the product with an average molecular weight of 3000 to obtain PCL-PEG-PCL flexible diol.

[0055] Preparation Example 3: A specific preparation method of PCL-PEG-PCL flexible diol, comprising the following steps:

[0056] S1. 100 g of 4-oxocyclohexane carboxamide was added to 1.2 kg of dichloromethane, and then 146.68 g of meta-chloroperoxybenzoic acid was added. The temperature was raised to 45°C, and the reaction was carried out for 24 h. After cooling to room temperature, filtration was carried out, and the filtrate was subjected to rotary evaporation to remove the solvent to obtain a crude product. The crude product was recrystallized in diethyl ether to obtain functionalized caprolactone;

[0057] S2. 100 g of PEG-400, 223.17 g of 6-caprolactone and 76.83 g of functionalized caprolactone were added to a reaction vessel. After water removal, 20 g of stannous octoate was added, and then nitrogen was replaced for three times. The temperature was raised to 140°C, and the reaction was carried out for 12 h. After cooling to room temperature, it was added to 3.5 kg of dichloromethane, and then 6 kg of diethyl ether at 0°C was added under stirring. The obtained precipitate was subjected to suction filtration, and then dried to collect the product with an average molecular weight of 3000 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, and in step (2), no functionalized caprolactone is added, and 196.02 g of 6-caprolactone and 53.98 g of functionalized caprolactone are replaced by 250 g of 6-caprolactone.

[0059] Example 1: A specific preparation method of super-soft waterborne polyurethane leather, comprising the following steps:

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

[0061] (2) The polyester knitted fabric was soaked in a 5 wt% sodium hydroxide solution for 10 min, washed with deionized water until the washing liquid was neutral, and dried for standby use. 1 kg of PCL-PEG-PCL flexible diol prepared in Preparation Example 1 was dried and dehydrated, and 1 g of antioxidant 1010 and 0.5 g of ultraviolet absorber UV-531 were heated to melt under stirring, and then coated on the surface of the base fabric with a wet film thickness of 0.3 mm. After pre-curing at 60°C for 10 min and cooling to room temperature, a PCL-PEG-PCL flexible film was formed to obtain a bottom layer.

[0062] (3) 500 g of PCL-PEG-PCL flexible diol prepared in Preparation Example 1 was dehydrated and added to a reaction vessel, heated to 65°C, and 300 g of hexamethylene diisocyanate was added. Under nitrogen protection, the mixture was stirred for 2.5 h, and then 50 g of 2,2-dimethylol propionic acid was added. The temperature was raised to 80°C, and the stirring was continued for 1 h to obtain a hydrophilic prepolymer.

[0063] (4) The hydrophilic prepolymer was cooled to 40°C, 30 g of triethylamine was added, and stirred for 15 min. Then 1 kg of deionized water was added under high-speed stirring at 2000 rpm, and the stirring was maintained for 15 min. After filtration through a 300-mesh filter cloth, the mixture was vacuum degassed for 10 min to obtain a water-based modified polyurethane.

[0064] (5) The bottom layer was laid flat, and the modified water-based polyurethane was coated on the surface with a wet film thickness of 0.2 mm to obtain an intermediate layer. The surface layer was then covered on the surface of the intermediate layer, and a pressure of 0.1 MPa was applied. The mixture was first dried at 60°C for 15 min, and then the temperature was raised to 85°C for curing for 30 min. After natural cooling to room temperature, the mixture was placed for 24 h to obtain a super-soft water-based polyurethane leather.

[0065] Example 2: A specific preparation method of a super-soft water-based polyurethane leather, comprising the following steps:

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

[0067] (2) The polyester knitted fabric is soaked in a 8wt% sodium hydroxide solution for 15 minutes, then washed with deionized water until the washing liquid is neutral, dried and reserved; 1 kg of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 2 is dried and dehydrated, and 2 g of antioxidant 1010 and 1.2 g of ultraviolet absorber UV-531 are heated to melt under stirring, then coated on the surface of the base fabric, with the wet film thickness controlled at 0.4 mm. After pre-curing at 65°C for 12 minutes and cooling to room temperature, a PCL-PEG-PCL flexible film is formed, obtaining the bottom layer;

[0068] (3) 600 g of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 2 is dehydrated and added to a reaction vessel, heated to 70°C, and 350 g of hexamethyl diisocyanate is added. After stirring for 3 hours under nitrogen protection, 60 g of 2,2-dimethylol propionic acid is added, and the temperature is raised to 85°C for continuous stirring for 1.2 hours, obtaining a hydrophilic prepolymer;

[0069] (4) The hydrophilic prepolymer is cooled to 43°C, 40 g of triethylamine is added, and stirred for 18 minutes. Then 1.2 kg of deionized water is added under high-speed stirring at 2200 rpm, and stirring is maintained for 20 minutes. After filtration through a 300-mesh filter cloth, vacuum degassing is performed for 15 minutes, obtaining a modified waterborne polyurethane;

[0070] (5) The bottom layer is laid flat, and the modified waterborne polyurethane is coated on its surface, with the wet film thickness controlled at 0.4 mm, obtaining the middle layer. Then the surface layer is covered on the surface of the middle layer, and a pressure of 0.2 MPa is applied. After air drying at 65°C for 18 minutes, the temperature is raised to 90°C for curing for 35 minutes, and then naturally cooled to room temperature. After standing for 24 hours, a super-soft waterborne polyurethane leather is obtained.

[0071] Example 3: A specific preparation method of a super-soft waterborne polyurethane leather, comprising the following steps:

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

[0073] (2) The polyester knitted fabric is soaked in a 10wt% sodium hydroxide solution for 20 minutes, then washed with deionized water until the washing liquid is neutral, dried, and used as a base fabric; 1 kg of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 3 is dried and dehydrated, and 3 g of antioxidant 1010 and 2 g of ultraviolet absorber UV-531 are added and heated to melt under stirring, then coated on the surface of the base fabric to control the wet film thickness to be 0.5 mm, and pre-cured at 70°C for 15 minutes and cooled to room temperature to form a PCL-PEG-PCL flexible film, thereby obtaining a bottom layer;

[0074] (3) 700 g of the PCL-PEG-PCL flexible diol prepared according to Preparation Example 3 is dehydrated and added to a reaction vessel, heated to 75°C, and 400 g of hexamethylene diisocyanate is added, stirred under nitrogen protection for 3.5 hours, then 80 g of 2,2-dimethylol propionic acid is added, heated to 90°C, and stirred for 1.5 hours to obtain a hydrophilic prepolymer;

[0075] (4) The hydrophilic prepolymer is cooled to 45°C, 50 g of triethylamine is added, and stirred for 20 minutes, then 1.4 kg of deionized water is added under high-speed stirring at 2400 rpm, and stirring is maintained for 25 minutes, then filtered through a 300-mesh filter cloth, and vacuum degassed for 20 minutes to obtain a modified waterborne polyurethane;

[0076] (5) The bottom layer is laid flat, and the modified waterborne polyurethane is coated on the surface to control the wet film thickness to be 0.5 mm to obtain an intermediate layer, and then a top layer is covered on the surface of the intermediate layer, and a pressure of 0.3 MPa is applied, and then air-dried at 70°C for 20 minutes, and then heated to 95°C for curing for 40 minutes, and then naturally cooled to room temperature, and then placed for 24 hours to obtain a super-soft waterborne polyurethane leather.

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

[0078] .

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

[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 by 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 conditions and under stress: 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 stress 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, the dumbbell-shaped samples of Examples 1-5 and Comparative Examples 1-5 were tested by a universal material testing machine with real-time stress-strain collection function. The sample pretreatment was the same as before. The tensile speed was set to 500 mm / min. First, load at a rate of 1 MPa / min to 2 MPa and keep for 5 s to measure the tensile strength. Then continue to load to 5 MPa and keep for 10 s to measure the tensile strength. Finally, load to 8 MPa and keep for 10 s to measure the tensile strength. Calculate the increase of tensile strength at different stress stages compared with the tensile strength under normal conditions. According to the tensile strength increase under different stresses = (tensile strength under different stresses - normal tensile strength) / normal tensile strength x 100%, the experimental results are shown in Table 1.

[0092] 3. Constant stress retention fatigue strength test: Referring to GB / T 12000.2-2016 “Plastics - Polyurethane elastomers - Part 2: Requirements for moulding and extrusion materials”, the dumbbell-shaped samples of Examples 1-5 and Comparative Examples 1-5 were tested by constant stress retention. The sample was fixed on the universal material testing machine, loaded to 5 MPa constant stress and continuously retained for 100 h, and the real-time stress was recorded every 10 h. After 100 h, unload, test the residual tensile strength of the sample, calculate the strength retention rate (residual strength ÷ initial 5 MPa strength x 100%), the experimental results are shown in Table 1.

[0093] 4. Low temperature stress strength test: Referring to GB / T 1040.3-2006, the test environment was adjusted to -45℃±2℃, and the dumbbell-shaped samples of Examples 1-5 and Comparative Examples 1-5 were tested. The sample was first placed in a -45℃ low temperature box for 2 h, and after ensuring uniform temperature, a 5 MPa stress was applied on the universal material testing machine in the low temperature box (maintaining -45℃) and kept for 10 s. Then test the tensile strength and elongation at break after stress at a speed of 500 mm / min, the experimental results are shown in Table 1.

[0094] 5. Normal Shore hardness test: Referring to GB / T 2411-2008 “Plastics and ebonite Determination of indentation hardness (Shore hardness) by use of a hardness tester”, Shore A hardness tester was selected. The leather prepared from Examples 1-5 and Comparative Examples 1-5 was cut into 100 mm x 100 mm samples, ensuring that the surface was flat and wrinkle-free, and free of bubbles. After placing in an environment of 23℃±2℃, relative humidity 50%±5% for 24 h to eliminate environmental stress, the hardness tester needle was vertically pressed into the sample surface (the needle was at least 10 mm from the edge, and the distance between adjacent test points was at least 15 mm). Each sample was tested at 6 points, and the average value was obtained after removing the maximum and minimum values. The experimental results are shown in Table 1.

[0095] 6. Dynamic bending stiffness test: refer to QB / T 4463-2013 "Synthetic leather - Physical and mechanical tests - Determination of bending properties" for implementation, using a dynamic bending tester, cut 150mm x 25mm long strip samples of the leather prepared from Examples 1-5 and Comparative Examples 1-5, after 24h under the environment of 23℃±2℃, relative humidity 50%±5% to eliminate environmental stress, set the bending angle ±45°, bending rate 30 times / min, continuous bending 100 times, record the maximum bending force of the 1st and 100th times and calculate the bending force change rate, the experimental results are shown in Table 1.

[0096] Table 1 Performance test results

[0097]

[0098] Performance analysis:

[0099] From the experimental data in Table 1, it can be seen that the super-soft waterborne polyurethane leather prepared by Examples 1-5 of the application is significantly better than Comparative Examples 1-5 in terms of normal flexibility, stress strength improvement, long-term storage stability and low-temperature adaptability, and the comprehensive performance of Example 2 is the best.

[0100] Example 2 realizes the optimal balance of tensile strength and elongation at break in normal state, which is mainly due to the micro effect of molecular structure and three-layer synergy. In the block structure of PCL-PEG-PCL flexible diol, the middle segment PEG and the two end PCL segments are both low glass transition temperature soft segments, and the molecular chains are only combined by weak van der Waals force, which can freely coil and slide, providing a flexible basis for elongation at break; at the same time, the weak N-H…O hydrogen bond formed by the side chain carboxamide group moderately adjusts the segment arrangement, avoiding the insufficient tensile strength caused by excessive disorder of molecular chains. The middle layer forms an integrated structure with the surface layer and the bottom layer through urethane covalent bond, and the three layers cooperate to bear the stress when stressed, so that the stress is more evenly dispersed, so that the normal state has both suitable tensile strength and excellent elongation at break. By comparison, Comparative Example 1 lacks the side chain carboxamide hydrogen bond due to the absence of functional caprolactone, and the intermolecular force is weakened, so the tensile strength and elongation at break in normal state are both decreased; Comparative Example 2 uses pure PEG-400 instead of three-block diol, which lacks the rigid support of PCL segment, so although the elongation at break is acceptable, the tensile strength is significantly reduced, which cannot meet the practical demand; Comparative Examples 3-4 use long-chain aliphatic linear diisocyanate, which easily arranges the hard segment to form a crystalline region, limiting the sliding of the soft segment, resulting in a decrease in normal elongation at break; Comparative Example 5 uses short-chain isocyanate, and the intermolecular force of the hard segment is weak, so the normal tensile strength is obviously insufficient.

[0101] The embodiment 2 exhibits a significant strength improvement effect under stress state, which is due to the dual protection of molecular chain coordination and chemical bond. When external force acts, the PCL segments of the surface layer, the bottom layer and the middle layer are synchronously stretched and oriented along the direction of the stress. The linear structure of the PCL segments makes them have excellent crystallization tendency. During the orientation process, the molecular chains are regularly arranged to form crystalline regions, which serve as physical reinforcing nodes to improve the tensile strength. At the same time, the side chain carboxamide groups are close to each other with the orientation of the segments, and the intermolecular hydrogen bonds formed by N-H…O further fix the structure of the crystalline regions, avoiding the slippage of the crystalline regions under stress. The urethane covalent bond between the middle layer and the surface layer and the bottom layer can effectively disperse stress, avoiding local stress concentration, and the aliphatic straight chain hard segment can be oriented synchronously with the molecular chain, which neither interferes with the crystallization process of PCL nor helps to disperse stress, so that the strength increment after stress is significantly improved. The comparative example 1 lacks side chain hydrogen bonds, and the PCL segments are difficult to stabilize after being oriented and crystallized, and the crystalline regions are prone to slippage, so the strength increment after stress is lower than that of the embodiment 2. The comparative example 2 has no PCL segments, and cannot improve the strength by force-induced crystallization, so the strength increment after stress is significantly reduced. The long-chain isocyanate hard segments of the comparative examples 3-4 compete for orientation space with the PCL crystalline regions, which interferes with the efficiency of the force-induced crystallization of PCL, resulting in a significant decrease in the strength increment after stress. The short-chain isocyanate of the comparative example 5 cannot provide effective support for the soft segment, and the PCL crystalline region is prone to damage under stress, so the strength increment is less than that of the embodiment 2.

[0102] The embodiment 2 can still maintain a high strength retention rate after long-term constant stress preservation, which is due to the synergistic effect of the light self-crosslinking structure and the reversibility of the hydrogen bond. A small amount of -NCO groups, diol end hydroxyl groups and trace water are reacted to form low crosslinking density urethane bonds and urea bonds during the preparation of the prepolymer, which only stabilize the film structure without increasing the rigidity. At the same time, the side chain carboxamide hydrogen bond has reversibility, which can be slightly dissociated and reconstructed with stress fluctuation during long-term preservation, avoiding irreversible damage to the crystalline region due to continuous stress, thereby maintaining a high strength retention rate. The comparative example 1 has no side chain carboxamide hydrogen bond, and the PCL crystalline region is prone to slippage during long-term preservation, resulting in a low strength retention rate. The comparative example 2 uses pure PEG-400 instead of the triblock diol, and only relies on the weak intermolecular force of PEG to support, so the chain segment is prone to permanent deformation after long-term preservation, resulting in a significant decrease in the strength retention rate. The long-chain isocyanate hard segment of the comparative examples 3-4 has high rigidity, which is prone to cracking during long-term preservation, resulting in a decrease in the strength retention rate. The intermolecular force of the short-chain isocyanate hard segment of the comparative example 5 is weak, and the hard segment is prone to separate from the soft segment matrix during the preservation process, resulting in a significant strength attenuation and a lower retention rate than the embodiment 2.

[0103] The good stress strength and elongation at break of Example 2 in a low temperature environment is due to the synergistic adaptation of the soft segment characteristics, hard segment structure and chemical bonding. The glass transition temperature of the PCL-PEG-PCL soft segment is extremely low, far lower than the test temperature, and the molecular chain can still maintain a high elastic state at low temperature and will not be brittle due to the decrease in temperature; the selected aliphatic linear diisocyanate (n = 4-8) has no cyclic structure, and the molecular chain has strong rotation ability, and still has a certain flexibility at low temperature, which does not affect the overall chain segment movement; the N-H…O hydrogen bond formed by the side chain carboxamide group can still exist stably in a low temperature environment, which can effectively stabilize the crystalline region of the PCL segment that may be formed, and avoid the brittle fracture of the crystalline region due to low temperature; the urethane covalent bond between the three layers is stable in structure at low temperature, which can continuously guarantee the interlayer bonding force and stress dispersion ability. Comparative Example 1 has a decrease in stress strength and elongation at break at low temperature due to the lack of side chain hydrogen bond, and the PCL crystalline region is prone to slip and brittle fracture; Comparative Example 2 has a significant decrease in stress strength due to the lack of PCL segment, and the intermolecular force of pure PEG soft segment is further weakened at low temperature; the long-chain isocyanate hard segment of Comparative Examples 3-4 has a further increase in rigidity of the crystalline region at low temperature, which is prone to brittle fracture, resulting in a significant decrease in elongation at break; the short-chain isocyanate of Comparative Example 5 has a weaker intermolecular force at low temperature, which cannot effectively support the soft segment, and has poor low-temperature stress performance.

[0104] The Shore hardness of Example 2 is in the super-soft interval, which is microscopically due to the synergistic regulation of molecular structure and interlayer action. The middle segment PEG and both end PCL segments of PCL-PEG-PCL flexible dihydric alcohol are low glass transition temperature soft segments, and the molecular chain always maintains a high elastic state under normal conditions, and only weak van der Waals force is used to combine the intermolecular chains without strong constraint of rigid segments; the N-H…O hydrogen bond formed by the side chain amide group is a weak interaction, which only slightly adjusts the chain segment arrangement to avoid excessive disorder, and does not increase the rigidity of the molecular chain; the selected aliphatic linear diisocyanate is a soft hard segment without cyclic structure, which can be uniformly dispersed in the soft segment matrix and does not form a rigid aggregation area, finally making the surface of the leather resist the ability to resist indentation weak, and showing low Shore hardness. Comparative Example 1 has a higher Shore hardness than Example 2 due to the lack of functional caprolactone and the lack of side chain carboxamide hydrogen bond, and the intermolecular chains are only combined by van der Waals force, and the chain segment arrangement has increased disorder, but the PCL segment itself still has a certain rigidity; Comparative Example 2 has a lower Shore hardness than Example 2, but the soft segment intermolecular force is extremely weak due to the replacement of the three-block dihydric alcohol with pure PEG-400 and the lack of rigid support of the PCL segment, and the membrane layer strength is insufficient, which has no practical value; the Shore hardness of Comparative Examples 3-4 is much higher than that of Example 2 due to the use of long-chain isocyanate, and the hard segment carbon chain is too long to form a hard segment crystalline region, and the crystalline region has high rigidity, which significantly enhances the surface resistance to indentation capacity; the Shore hardness of Comparative Example 5 is relatively high due to the weak intermolecular force of the short-chain isocyanate hard segment, which cannot effectively support the soft segment, and the intermolecular constraint is weaker than Example 2.

[0105] The dynamic bending force change rate of Example 2 is low, which reflects excellent repeated bending flexibility stability, which may be because of the dual advantages of base cloth-film interface synergy and molecular chain recovery. After the base cloth is immersed in a sodium hydroxide solution, a micro-rough structure is formed on the surface, and the PCL-PEG-PCL flexible film can penetrate into the micro-pores on the surface of the base cloth when coated, thereby improving the film-base cloth interface bonding strength through mechanical anchoring to avoid interface slipping during repeated bending; the linear structure of the PCL-PEG-PCL soft segment makes it have good chain segment recovery, and the side chain amide hydrogen bond can be reversibly dissociated and reconstituted during bending, and the chain segment is oriented along the stress direction during bending, and can quickly reset after unloading, without rigid aggregation or permanent deformation. Comparative Example 1 lacks side chain carboxamide hydrogen bonds, and the PCL chain segment lacks hydrogen bond stability after crystallization, and the crystalline region is prone to slip during repeated bending, resulting in an increase in the local rigidity of the film layer, and the weak intermolecular constraint makes the film-base cloth interface prone to micro-displacement, and the bending force change rate is higher than that of Example 2; Comparative Example 2 uses pure PEG-400 to replace the dihydric alcohol, and the film layer is low in strength, and the film layer is prone to slight extension along the surface of the base cloth during repeated bending, and the compatibility of PEG with the base cloth is poor, and the interface bonding strength is low, and the bending force change rate is relatively high; the long-chain isocyanate hard segment of Comparative Examples 3-4 has high crystalline region rigidity, and the crystalline region is prone to cracking to form local rigid sites during repeated bending, and the hard segment crystalline interferes with the film-base cloth cooperative deformation, and the bending force increases significantly with the number of times, and the change rate is much higher than that of Example 2; the short-chain isocyanate hard segment of Comparative Example 5 cannot effectively disperse the bending stress, and the soft segment chain is prone to permanent deformation, and the hard segment and the soft segment interface have weak bonding, and the bending force change rate is higher than that of Example 2.

[0106] The above merely illustrates the preferred embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art can make equivalent replacements or changes to the technical solutions and the inventive concept of the present application within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

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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