Super-soft waterborne polyurethane synthetic leather and preparation method thereof

Abstract

The invention provides super-soft waterborne polyurethane synthetic leather and a preparation method thereof, belongs to the technical field of synthetic leather, and aims to solve the technical problem of poor mechanical properties of waterborne polyurethane synthetic leather. The preparation method of the super-soft waterborne polyurethane synthetic leather comprises the following steps: (1) adding ZIF-8 powder into a waterborne polyurethane solution to prepare a mixed solution; and (2) coating a substrate with the mixed solution, and drying to obtain the WPU / ZIF-8 thin film super-soft waterborne polyurethane synthetic leather, and (3) soaking the WPU / ZIF-8 thin film in an acid solution, and then washing and drying to obtain the WPU / ZIF-8 thin film. The WPU synthetic leather with a nano porous structure is prepared by synthesizing zeolite imidazate skeleton-8, adding the zeolite imidazate skeleton-8 as a pore-foaming agent into WPU and then carrying out a weak acid after-finishing process, so that high-efficiency compatibility of softness and mechanical properties is realized. The prepared WPU synthetic leather has a wide application prospect in production and manufacturing of synthetic leather products such as clothes and shoes.

Description

Technical Field

[0001] The invention belongs to the technical field of synthetic leather, and in particular relates to waterborne polyurethane synthetic leather. Background Art

[0002] Waterborne polyurethane (WPU) synthetic leather offers significant advantages over traditional solvent-based polyurethane (PU) and polyvinyl chloride (PVC) synthetic leather. Using water as its dispersion medium, it contains no organic solvents, eliminating volatile organic compound (VOC) emissions at the source. This makes it more environmentally friendly and safer, in line with the global trend toward green manufacturing. Furthermore, the high number of hydrophilic groups in the WPU molecular chain imparts excellent breathability and moisture permeability to the synthetic leather, making it more comfortable to wear than natural leather. Furthermore, the production process for WPU synthetic leather eliminates the need for toxic solvent recovery steps like N,N-dimethylformamide (DMF), reducing energy consumption and production costs. The finished product is easy to dye, resists yellowing, and is biodegradable, showing broad application prospects in automotive interiors, high-end footwear, fashion bags, and other fields. However, WPU synthetic leather struggles to simultaneously meet the demands for high softness and high mechanical properties. The primary reason for this difficulty in balancing flexibility and mechanical properties lies in the inherent contradictions between its molecular structure and processing technology. First, water-based systems must incorporate hydrophilic groups to achieve water dispersibility. However, these groups disrupt the regular arrangement of the hard segments, weakening hydrogen bonding and crystallinity, leading to a decrease in mechanical properties. Furthermore, the residual hydrophilic groups easily absorb moisture and soften the material, further reducing wear resistance and strength. Secondly, the ratio of hard segments (which provide strength) to soft segments (which provide flexibility) is difficult to optimize—too many hard segments will result in a brittle material, while too many soft segments will result in insufficient strength. Furthermore, the multilayer structure of synthetic leather (such as the base fabric and coating) must balance interfacial bonding and deformation coordination. While the foaming process improves softness, it also introduces pore defects, weakening load-bearing capacity. The slow evaporation of water during processing also easily forms micropores and a rough surface, affecting density and durability. These factors work together to make it difficult for waterborne polyurethane synthetic leather to simultaneously meet the requirements for high softness and high mechanical properties, requiring coordinated optimization through molecular design (such as block modification and self-crosslinking) and process innovation (such as gradient foaming and nanocomposites).

[0003] The pore structure of WPU synthetic leather has an important influence on its mechanical properties and softness. The porogens commonly used in the preparation process mainly include physical foaming agents (such as mechanical stirring to introduce air), chemical foaming agents (sodium bicarbonate, azodicarbonamide, etc.), water-soluble salts (NaCl, sucrose) and volatile organic compounds (low-boiling point alcohols). However, these porogens have significant problems in practical applications: First, the uniformity of the pores is difficult to control, and it is easy to form uneven pore size distribution or closed-cell structures, affecting the consistency of the product's mechanical properties; second, chemical foaming agents may leave toxic decomposition products, and if water-soluble salts are not completely removed, they will absorb moisture and deteriorate the material properties; in addition, some porogens have poor compatibility with aqueous systems, which may lead to decreased emulsion stability or coating defects. More importantly, although high porosity improves the softness and breathability of synthetic leather, it will significantly reduce its tensile strength, tear strength and wear resistance, resulting in a performance contradiction. In terms of environmental protection, traditional chemical foaming agents may release harmful substances such as ammonia, which does not meet the requirements of green manufacturing. Currently, the industry is exploring new technologies such as supercritical CO2 foaming and bio-based porogens (such as starch granules) to balance pore structure, performance and environmental friendliness.

[0004] In 2011, Wang Quanjie et al. (Wang Quanjie, Zhang Yuzhou, Yang Yang, et al. Effects of different porogens on the permeability of polyurethane microporous membranes. Leather Science and Engineering, 2011, 21(03):23-28) studied the effects of urea, polyvinyl pyrrolidone and wood powder as porogens on the moisture permeability, mechanical properties, moisture absorption properties, porosity and microporous structure of polyurethane microporous membranes. The results showed that the moisture permeability of polyurethane microporous membranes with the addition of porogens can reach a maximum of 1876.81 g / (m 2·24h), a 315% improvement over untreated polyurethane. After comprehensive consideration of various influencing factors, urea and wood flour were found to be more effective as porogens. In 2013, Fang Chengyuan et al. (Fang Chengyuan, Li Yanjing, Jiang Beier, Li Ting, Chen Dajun, Study on the Moisture Permeability of Cyclodextrin / Polyurethane Blend Membranes, Shanghai Coatings, 2013, 51) proposed using cyclodextrin as a moisture permeability modifier and benzene as a porogen to prepare cyclodextrin / polyurethane blend membranes using a dry process. They investigated the effect of porogen dosage on the moisture permeability, thermal properties, and tensile properties of the polyurethane blend membranes. The results showed that cyclodextrin effectively improved the moisture permeability of the blend membranes. With increasing benzene content, the moisture permeability of the film increased linearly, the thermal properties remained essentially unchanged, and the tensile strength decreased slightly. In 2017, He Beibei et al. (He Beibei, Jie Yijun. Preparation and properties of polyurethane microporous membranes [J]. Chemical Industry Progress, 2017, 36(07): 2562-2567) prepared polyurethane microporous membranes by adding inorganic porogen NH4HCO3 and ZnO modified acrylonitrile-butadiene-styrene copolymer foaming agent. Both porogens successfully prepared microporous membranes with open pore structures, and their moisture permeability and porosity were greatly improved. According to experimental measurements, the moisture permeability of the polyurethane microporous membrane treated with NH4HCO3 can reach 929g / (m 2 ·24h); The moisture permeability of the polyurethane microporous membrane treated with modified azodicarbonamide (AC) foaming agent can reach 1064g / (m 2·24h), the moisture permeability of the untreated polyurethane microporous membrane was increased by 49.8% and 71.6%, respectively. NH4HCO3 and modified AC foaming agents have excellent pore-forming properties. In 2024, Shi et al. (Shi Z, Sheng Y, Wu J, Cui J, Lin W, Ngai T, Porous waterborne polyurethane films templated from pickering foams for fabrication of synthetic leather. Langmuirm, (2024), 40 (9): 4751-4761.) proposed the use of hydrophobic fumed silica particles (SPs) as stabilizers to form a highly stable foam emulsion through mechanical foaming, which was then blended with WPU; the bubbles in the foam acted as a template to form a continuous porous structure after WPU solidification. The porosity and pore size can be regulated by particle concentration and foaming process, resulting in a WPU-SP composite membrane with a richer porous structure. In 2017, Chen Yubo et al. (Chen Yubo, Li Hui, Zuo Danying, "Effects of the Porogen PVP on the Structure and Morphological Properties of CSP / PU Porous Membranes," Polyurethane Industry, 2017, 32, 19-21) prepared CSP / PU porous membranes using chitosan (CSP) powder, medical-grade thermoplastic polyurethane (PU), and the porogen polyvinylpyrrolidone (PVP). The authors discussed the effects of PVP and CSP dosage on the structure and properties of the CSP / PU porous membranes. The results showed that the porogen PVP significantly increased the porosity and pore size of the membranes; that PVP was associated with membrane structure, pore size, moisture permeability, and degradation stability; that CSP / PU porous membranes could absorb more water than their own weight; and that the addition of CSP and PVP improved the membrane's degradation stability. Patent publication number CN118725398A discloses a multifunctional polyurethane microporous membrane and its preparation method, comprising the following steps: uniformly stirring polyurethane and a porogen in a solvent to obtain a precursor solution; the porogen comprises triethyl phosphate and ammonium polyphosphate; coating the precursor solution on a release material and drying to obtain a polyurethane film; and displacing the polyurethane film in water to obtain a multifunctional polyurethane microporous membrane. Both triethyl phosphate and ammonium polyphosphate are readily soluble in water, but triethyl phosphate is a liquid while ammonium polyphosphate is a solid. Their dissolution and pore-forming rates upon contact with aqueous solution differ, and the resulting pores differ significantly in size. The resulting microporous structure exhibits a multiscale effect, forming a "large pore within small pore" structure. Patent publication number CN118955992A discloses a porous polyurethane composite material, its preparation method, and its application.The method comprises the following steps: mixing polyurethane, an organic solvent, a curing agent and a porogen I to obtain slurry I; mixing polyurethane, an organic solvent, a curing agent and a porogen II to obtain slurry II; then leveling the slurry I and the slurry II to form films respectively; obtaining polyurethane film I and polyurethane film II; contacting the two surfaces of the polyurethane film I and the polyurethane film II to dry and adhere them to obtain a double-layer film; and removing the porogen to obtain a porous double-layer film.

[0005] Current research at home and abroad reveals that porogen technologies for WPU synthetic leather generally suffer from significant drawbacks. While physical foaming is environmentally friendly, it produces poor pore uniformity, leading to significant fluctuations in mechanical properties (a 30%-50% decrease in strength). Chemical foaming agents (such as azodicarbonamide), while highly efficient, produce residual toxic decomposition products and require stringent temperature control accuracy (±2°C). While water-soluble salt leaching is cost-effective, it produces larger pores, and residual salts readily absorb moisture (increasing water absorption by 20%-30%). While the Pickering foam template method achieves uniform pore size, nanoparticle dispersion is difficult, and excessive addition can lead to material embrittlement. Volatile solvent porogens carry the risk of residual VOCs and fail to meet environmental standards. While bio-based porogens are sustainable, they significantly compromise mechanical properties (a 35%-45% decrease in strength). These drawbacks are primarily due to key issues such as uncontrollable pore structure, significant performance loss, difficulty meeting environmental standards, and high process costs. Therefore, preparing flexible WPU with excellent mechanical properties remains a challenge. Summary of the Invention

[0006] In response to the above technical problems, the present invention proposes an ultra-soft water-based polyurethane synthetic leather and a preparation method thereof. The prepared WPU synthetic leather has high porosity and excellent mechanical properties, and has broad application prospects in the production and manufacturing of synthetic leather products such as clothing, shoes, and luggage.

[0007] In order to achieve the above object, the technical solution of the present invention is achieved as follows:

[0008] A super-soft water-based polyurethane synthetic leather, the preparation method comprising the following steps:

[0009] (1) adding ZIF-8 powder to aqueous polyurethane solution to prepare a mixed solution;

[0010] (2) coating the mixed solution on a substrate and drying the mixed solution to obtain WPU / ZIF-8 film ultra-soft water-based polyurethane synthetic leather;

[0011] (3) The WPU / ZIF-8 film is immersed in an acidic solution, and then washed and dried.

[0012] The ZIF-8 powder is synthesized by a solvent method, and the steps are: dissolving zinc salt and 2-methylimidazole in a solvent to prepare a reaction solution, and then reacting to obtain the powder.

[0013] The zinc salt is zinc nitrate or zinc chloride, and the solvent is one or more of methanol, ethanol, water or dimethylformamide.

[0014] The molar ratio of the zinc salt to 2-methylimidazole is generally 1:2-10; the concentration of the zinc salt in the reaction solution is 0.01-0.2 mol / L; the reaction solution is reacted at room temperature for 6-24 hours.

[0015] In the step (1), the amount of ZIF-8 powder added is 0.2-0.6 wt % of the waterborne polyurethane.

[0016] The substrate is a release paper, which facilitates peeling of the WPU / ZIF-8 film from the substrate.

[0017] The acidic solution is glacial acetic acid or citric acid, which has a certain solubility for ZIF-8 and does not cause etching to WPU.

[0018] The pH of the acidic solution is 4-6, which can be regulated by adjusting the concentration of the acidic solution.

[0019] In the step (3), the WPU / ZIF-8 film is immersed in the acidic solution for 24-72 hours.

[0020] Beneficial effects of the present invention:

[0021] (1) With the help of precise control and degradation of ZIF-8 porogen, the synthetic leather is given high porosity while achieving a breakthrough in the high compatibility of softness and excellent mechanical properties - it not only maintains the soft and tough touch of the material, but also ensures the stability of key mechanical indicators such as tensile strength and wear resistance, solving the technical pain point of traditional synthetic leather "soft but insufficient strength, strong but hard texture".

[0022] (2) The weak acid post-treatment process is used to degrade ZIF-8, which does not require high temperature or strong corrosive conditions. It is not only gentle and environmentally friendly, but also the size and distribution of the pore structure can be precisely controlled by the amount of ZIF-8 added and the degradation process, providing a reliable guarantee for the stability of performance in large-scale production.

[0023] (3) The prepared WPU synthetic leather has comfortable feel, durability and environmental protection characteristics, which perfectly fits the fields such as clothing and footwear that have high demands for "light, soft and tough" materials. At the same time, it provides a new option for performance upgrades for synthetic leather products such as luggage and furniture, and has strong market application potential and industrialization value. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0025] Figure 1 This is the XRD pattern of ZIF-8.

[0026] Figure 2 SEM images of WPU / ZIF-8 films prepared in different embodiments, (ac: different ZIF-8 mass fractions: 0.2% (a Example 2), 0.4% (b Example 4), 0.6% (c Example 8); df: different immersion times: 24h (d Example 8), 48h (e Example 9), 72h (f Example 10); gi: different pH: 4 (g Example 6), 5 (h Example 8), 6 (i Example 7)).

[0027] Figure 3 Mechanical properties of WPU / ZIF-8 films with different ZIF-8 addition amounts in comparative example 3 and examples 2, 4 and 6, (a) tensile strength change curve; (b) elongation at break change curve.

[0028] Figure 4 Mechanical properties of WPU / ZIF-8 films at different immersion times in Comparative Example 1 and Examples 8, 9, and 10: (a) tensile strength change curve; (b) elongation at break change curve.

[0029] Figure 5 Mechanical properties of WPU films immersed in solutions with different pH values ​​in Examples 3, 4, and 5 and Comparative Example 2, (a) tensile strength change curve; (b) elongation at break change curve.

[0030] Figure 6 Porosity of WPU / ZIF-8 films prepared in different examples, (a) porosity of different ZIF-8 addition amounts in Examples 2, 4 and 8; (b) porosity of different immersion times in Examples 8, 9 and 10; (c) porosity of different pH values ​​in Examples 3, 4 and 5.

[0031] Figure 7 The water contact angles of WPU / ZIF-8 films under different experimental conditions: (a) water contact angles of different ZIF-8 addition amounts in Comparative Example 3 and Examples 1, 3, and 6; (b) water contact angles of different immersion times in Comparative Example 1 and Examples 8, 9, and 10; (c) water contact angles of different pH values ​​in Examples 3, 4, and 5 and Comparative Example 2.

[0032] Figure 8 The softness of WPU under different experimental conditions: (a) the softness of Comparative Example 3 and Examples 1, 3 and 6 with different ZIF-8 addition amounts; (b) the softness of Comparative Example 1 and Examples 8, 9 and 10 with different immersion times; (c) the softness of Examples 3, 4 and 5 and Comparative Example 2 with different pH values. DETAILED DESCRIPTION

[0033] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the accompanying drawings. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without creative work are within the scope of protection of the present invention.

[0034] Example 1

[0035] A super-soft water-based polyurethane synthetic leather, the preparation method comprising the following steps:

[0036] (1) ZIF-8 was synthesized by solvent method. 2-Methylimidazole (12.5 mmol) and zinc nitrate hexahydrate (3.13 mmol) were weighed and dissolved in 25 mL of methanol solution respectively. The mixture was stirred at room temperature for 20 min to obtain a clear solution. The two solutions were then quickly mixed and stirred for another 3 min. The mixed solution was placed at room temperature for 24 h. The obtained product was separated by centrifugation and washed several times with methanol solution. Finally, it was placed in an oven at 60 ° C and dried for 24 h to obtain a white ZIF-8 solid. Figure 1 It can be seen that the characteristic diffraction peaks of the prepared ZIF-8 are 7.40°, 12.84°, and 18.19°, which correspond to the (011), (112), and (222) crystal planes in the cubic crystal structure of ZIF-8, respectively (PDF#00-62-1030), indicating that the prepared product is indeed ZIF-8.

[0037] (2) 0.01 g of the ZIF-8 powder obtained in step (1) was weighed and added to a solution containing 5 g of aqueous polyurethane, and stirred on a magnetic stirrer to ensure that the mixture was fully stirred.

[0038] (3) Cut three pieces of release paper of appropriate size with scissors. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the mixed solution evenly on the release paper and scrape the solution evenly with a scraper. Place the release paper with the solution coated in an oven at 100°C for 10 minutes and then take it out to obtain a WPU / ZIF-8 film.

[0039] (4) A certain amount of glacial acetic acid solution was mixed with deionized water, and pH test paper was used to measure the pH while stirring to obtain a weak acid solution with a pH of 4. The WPU / ZIF-8 film prepared in step (3) was immersed in the solution for 24 hours, and then washed and dried to obtain the WPU / ZIF-8 film.

[0040] Example 2-10

[0041] Example 2-10 WPU synthetic leather was prepared according to the above preparation steps. The specific process parameters are shown in Table 1:

[0042] Table 1

[0043]

[0044] Comparative Example 1

[0045] A waterborne polyurethane synthetic leather, the preparation method comprising the following steps:

[0046] (1) ZIF-8 was synthesized by the solvent method. 2-Methylimidazole (12.5 mmol) and zinc nitrate hexahydrate (3.13 mmol) were weighed and dissolved in 25 mL of methanol solution. The mixture was stirred at room temperature for 20 min to obtain a clear solution. The two solutions were then quickly mixed and stirred for another 3 min. The mixed solution was allowed to stand at room temperature for 24 h. The obtained product was separated by centrifugation, washed several times with methanol solution, and finally dried in an oven at 60°C for 24 h to obtain a white ZIF-8 solid.

[0047] (2) The ZIF-8 powder obtained in step (1) was weighed to a mass of 0.03 g, and was added to a solution containing 5 g of aqueous polyurethane, and stirred on a magnetic stirrer to fully stir the solution.

[0048] (3) Cut three pieces of release paper of appropriate size with scissors. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the mixed solution evenly on the release paper and scrape the solution evenly with a scraper. Place the release paper with the solution coated in an oven at 100°C for 10 minutes and then take it out to obtain a WPU / ZIF-8 film.

[0049] Comparative Example 2

[0050] A waterborne polyurethane synthetic leather, the preparation method comprising the following steps:

[0051] (1) ZIF-8 was synthesized by the solvent method. 2-Methylimidazole (12.5 mmol) and zinc nitrate hexahydrate (3.13 mmol) were weighed and dissolved in 25 mL of methanol solution. The mixture was stirred at room temperature for 20 min to obtain a clear solution. The two solutions were then quickly mixed and stirred for another 3 min. The mixed solution was allowed to stand at room temperature for 24 h. The obtained product was separated by centrifugation, washed several times with methanol solution, and finally dried in an oven at 60°C for 24 h to obtain a white ZIF-8 solid.

[0052] (2) The ZIF-8 powder obtained in step (1) was weighed to a mass of 0.03 g, and was added to a solution containing 5 g of aqueous polyurethane, and stirred on a magnetic stirrer to fully stir the solution.

[0053] (3) Cut three pieces of release paper of appropriate size with scissors. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the mixed solution evenly on the release paper and scrape the solution evenly with a scraper. Place the release paper with the solution coated in an oven at 100°C for 10 minutes and then take it out to obtain a WPU / ZIF-8 film.

[0054] (4) Soaking the WPU / ZIF-8 film prepared in step (3) in deionized water at pH = 7 for 24 hours, and then drying it to obtain the WPU / ZIF-8 film.

[0055] Comparative Example 3

[0056] A waterborne polyurethane synthetic leather, the preparation method comprising the following steps:

[0057] (1) Add it to a solution containing 5g of aqueous polyurethane and stir it on a magnetic stirrer to fully stir it evenly.

[0058] (3) Use scissors to cut three pieces of release paper of appropriate size. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the waterborne polyurethane solution on the release paper and evenly apply the solution with a scraper. Place the release paper with the solution applied in an oven at 100°C for 10 minutes and then take it out to obtain a WPU film.

[0059] (4) A certain amount of glacial acetic acid solution was mixed with deionized water, and pH test paper was used to measure the pH while stirring to obtain a weak acid solution with a pH of 4. The WPU film was immersed in the solution for 24 hours, and then washed and dried to obtain the WPU film.

[0060] Test Case

[0061] Under the same conditions, Figure 2 (ac) are SEM cross-sectional images of WPU (ZIF-8) films with ZIF-8 mass fractions of 0.2%, 0.4%, and 0.6%, respectively. It can be seen that there is a connected pore structure inside, and with the increase of ZIF-8 mass fraction, the distribution range of its connected pores increases. Figure 2 (df) are SEM cross-sectional images of the WPU / ZIF-8 film after immersion for 24 h, 48 h, and 72 h, respectively. It can be seen that the internal pore size increases with increasing immersion time, reaching a maximum pore size of approximately 24 μm after immersion for 72 h. Figure 2 (gi) SEM cross-sectional images of the WPU / ZIF-8 film immersed in solution at pH 4, 5, and 6, respectively. It can be seen that at pH 4, the internal pore diameter is larger, approximately 22 μm. As the pH increases, the internal pore diameter gradually decreases. Furthermore, the interconnected pore structure formed by acid etching of the ZIF-8 film also ensures the required mechanical properties to a certain extent.

[0062] Figure 3 (a) is the curve of the tensile strength of WPU / ZIF-8 film changing with the mass fraction of ZIF-8. It can be seen that with the increase of ZIF-8 mass fraction, its tensile strength continues to decrease. When the ZIF-8 mass fraction is 0.2%, its tensile strength is lower than the original sample but higher than other addition amounts, reaching 3.7MPa. Figure 3 (b) It can be seen that when the ZIF-8 addition amount is 0.2%, its elongation at break is lower than that of other addition amounts, reaching 12.5%. When the ZIF-8 mass fraction is 0.6%, although its tensile strength is lower than that of other addition amounts, its elongation at break is the highest relative to other addition amounts, reaching 14.02%. The possible reason is that the porosity is too high or the pores are too large, resulting in stress concentration and a decrease in tensile strength. However, the presence of pores may make the molecular chains more slippery during stretching, which is manifested as an increase in elongation. When the ZIF-8 mass fraction is 0.4%, the comprehensive mechanical properties are better, with a tensile strength and elongation of 3.2 MPa and 13.5%, respectively.

[0063] Figure 4 (a) shows the tensile strength of the WPU / ZIF-8 film as a function of immersion time in glacial acetic acid. As immersion time increases, the tensile strength first decreases, then increases, and then decreases again. At 48 hours, the tensile strength is lower than the original sample but higher than at other immersion times, reaching 3.5 MPa. Figure 4(b) shows the elongation at break of the WPU / ZIF-8 film as a function of immersion time in glacial acetic acid. It can be seen that the elongation at break remains essentially constant at different immersion times, approaching 14%. The best overall mechanical properties are achieved when the immersion time is 48 hours.

[0064] Figure 5 (a) shows the tensile strength of the WPU / ZIF-8 film as a function of the pH of the immersion solution. It can be seen that as the pH decreases, the tensile strength first decreases and then increases. At pH 4, the tensile strength is higher than in other acidic environments, reaching 3.3 MPa. Figure 5 (b) shows the elongation at break of the WPU / ZIF-8 film as a function of the pH of the immersion solution. It can be seen that the elongation at break decreases with decreasing pH. At pH 6, the elongation at break is 17.4%, exceeding that of other acidic environments. It also exhibits the best overall performance, with a tensile strength of 3.05 MPa.

[0065] Depend on Figure 6 (a) It can be seen that under different ZIF-8 mass fraction conditions, the porosity of WPU / ZIF-8 films is similar, but the porosity is relatively large, about 78%. Figure 3-6 (b) It can be seen that the porosity of the film decreases with the increase of immersion time. When the immersion time is 24h, the porosity is 78.1% at most. When the immersion time is 72h, the porosity drops to 45.6%. The possible reasons are the shrinkage of the polyurethane network and the blockage of the reactants. Figure 6 (c) It can be seen that when the pH of the immersion solution increases, the porosity first increases and then decreases, and the maximum porosity is 78.1% at pH = 5. When ZIF-8 is used as the porogen, its porosity is high and the porogenic effect is good.

[0066] Depend on Figure 7 (a) It can be seen that its water contact angle first increases and then decreases with the increase of mass fraction. When the mass fraction is 0.6%, its water contact angle is the smallest, which is 22°. Figure 7 (b) It can be seen that the water contact angle first decreases and then increases with the immersion time, and the minimum water contact angle is 22° when the immersion time is 24h. Figure 7 (c) It can be seen that when pH = 5, the water contact angle is the smallest at 22°. When the porosity is large, the film has better hydrophilicity.

[0067] Depend on Figure 8 (a) It can be seen that the softness of the WPU / ZIF-8 film increases with the increase of ZIF-8 addition. When the ZIF-8 mass fraction is 0.6%, its softness reaches a maximum of 4.5 mm. Figure 8(b) is the curve of the softness of the WPU (ZIF-8) film changing with the immersion time. As the immersion time increases, its softness first increases and then decreases. When the immersion time is 24 h, its softness reaches a maximum of 4.5 mm. Figure 8 (c) The softness of the WPU / ZIF-8 film increases first and then decreases with increasing pH, reaching a maximum softness of 4.5 mm at pH 5. This softness varies in line with porosity, with higher porosity indicating greater softness. This is due to the improved pore-forming effect of ZIF-8 as a porogen, which contributes to its higher softness.

[0068] Example 11

[0069] A super-soft water-based polyurethane synthetic leather, the preparation method comprising the following steps:

[0070] (1) ZIF-8 was synthesized by the solvent method. 2-Methylimidazole (6.25 mmol) and zinc nitrate hexahydrate (3.13 mmol) were weighed and dissolved in 50 mL of methanol solution. The mixture was stirred at room temperature for 20 min to obtain a clear solution. The two solutions were then quickly mixed and stirred for another 3 min. The mixed solution was allowed to stand at room temperature for 6 h. The obtained product was separated by centrifugation and washed several times with methanol solution. Finally, it was dried in an oven at 60°C for 24 h to obtain a white ZIF-8 solid.

[0071] (2) The ZIF-8 powder obtained in step (1) was weighed to a mass of 0.02 g, and was added to a solution containing 5 g of aqueous polyurethane, and stirred on a magnetic stirrer to fully stir the solution.

[0072] (3) Cut three pieces of release paper of appropriate size with scissors. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the mixed solution evenly on the release paper and scrape the solution evenly with a scraper. Place the release paper with the solution coated in an oven at 100°C for 10 minutes and then take it out to obtain a WPU / ZIF-8 film.

[0073] (4) A certain amount of glacial acetic acid solution was mixed with deionized water, and pH test paper was used to measure the pH while stirring to obtain a weak acid solution with a pH of 4. The WPU / ZIF-8 film prepared in step (3) was immersed in the solution for 48 hours, and then washed and dried to obtain the WPU / ZIF-8 film.

[0074] Example 12

[0075] A super-soft water-based polyurethane synthetic leather, the preparation method comprising the following steps:

[0076] (1) ZIF-8 was synthesized by the solvent method. 2-Methylimidazole (18.75 mmol) and zinc nitrate hexahydrate (3.13 mmol) were weighed and dissolved in 10 mL of methanol solution. The mixture was stirred at room temperature for 20 min to obtain a clear solution. The two solutions were then quickly mixed and stirred for another 3 min. The mixed solution was allowed to stand at room temperature for 12 h. The obtained product was separated by centrifugation and washed several times with methanol solution. Finally, it was dried in an oven at 60°C for 24 h to obtain a white ZIF-8 solid.

[0077] (2) The ZIF-8 powder obtained in step (1) was weighed to a mass of 0.03 g, and was added to a solution containing 5 g of aqueous polyurethane, and stirred on a magnetic stirrer to fully stir the solution.

[0078] (3) Cut three pieces of release paper of appropriate size with scissors. Place one piece of release paper on a scraper and spread it evenly on the glass plate. Then adjust the scraper thickness to 0.5 mm. Pour the mixed solution evenly on the release paper and scrape the solution evenly with a scraper. Place the release paper with the solution coated in an oven at 100°C for 10 minutes and then take it out to obtain a WPU / ZIF-8 film.

[0079] (4) A certain amount of citric acid was mixed with deionized water, and pH test paper was used to measure the pH value of the weak acid solution to obtain a pH value of 5. The WPU / ZIF-8 film prepared in step (3) was immersed in the solution for 48 hours, and then washed and dried to obtain the WPU / ZIF-8 film.

[0080] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A method for preparing super soft waterborne polyurethane synthetic leather, characterized in that: The following steps are involved: (1) adding ZIF-8 powder to aqueous polyurethane solution to prepare a mixed solution; (2) coating the mixed solution on a substrate and drying the mixed solution to obtain WPU / ZIF-8 film ultra-soft water-based polyurethane synthetic leather; (3) The WPU / ZIF-8 film is immersed in an acidic solution, and then washed and dried.

2. The method for preparing super soft waterborne polyurethane synthetic leather according to claim 1, wherein: The ZIF-8 powder is synthesized by a solvent method, and the steps are: dissolving zinc salt and 2-methylimidazole in a solvent to prepare a reaction solution, and then reacting to obtain the powder.

3. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 2, wherein: The zinc salt is zinc nitrate or zinc chloride, and the solvent is one or more of methanol, ethanol, water or dimethylformamide.

4. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 3, wherein: The molar ratio of the zinc salt to 2-methylimidazole is generally 1:2-10; the concentration of the zinc salt in the reaction solution is 0.01-0.2 mol / L; the reaction solution is reacted at room temperature for 6-24 hours.

5. The method for preparing the super soft waterborne polyurethane synthetic leather according to any one of claims 1 to 4, characterized in that: In the step (1), the amount of ZIF-8 powder added is 0.2-0.6 wt % of the waterborne polyurethane.

6. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 5, characterized in that: The substrate is release paper.

7. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 1, characterized in that: The acidic solution is glacial acetic acid or citric acid.

8. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 7, characterized in that: The pH of the acidic solution is 4-6.

9. The method for preparing the super soft waterborne polyurethane synthetic leather according to claim 1, characterized in that: In the step (3), the WPU / ZIF-8 film is immersed in the acidic solution for 24-72 hours.

10. Super soft waterborne polyurethane synthetic leather prepared according to the method according to any one of claims 1 to 9.

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