Highly crosslinked waterborne polyurethane synthetic leather and preparation method thereof

Abstract

The application discloses a kind of high crosslinking water-based polyurethane synthetic leather and preparation method thereof, belong to environmental protection material technical field, the high crosslinking water-based polyurethane synthetic leather includes modified water-based polyurethane emulsion, bio-based plasticizer, modified nanocellulose, composite crosslinking agent, defoaming agent, leveling agent;The preparation method includes the preparation of bottom coating modified water-based polyurethane emulsion, the preparation of surface coating high crosslinking water-based polyurethane emulsion and the preparation such as synthetic leather step.The application utilizes hyperbranched prepolymer, sulfonic acid group modified water-based polyester emulsion, realizes the characteristics of high tenacity of bottom layer adhesion, high crosslinking and hydrolysis resistance of surface layer;The application utilizes silane modified nanocellulose doping and composite crosslinking agent gradient crosslinking, realizes the high degree of combination between bottom layer, surface layer and bottom surface layer, so that the mechanical strength and moisture permeability resistance of finished synthetic leather are significantly improved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of environmental protection materials, and particularly relates to a high-crosslinking water-based polyurethane synthetic leather and a preparation method thereof. BACKGROUND

[0002] Synthetic leather is widely used in shoes, clothing, automotive interiors and other fields due to its excellent physical and chemical properties. Traditional polyurethane synthetic leather mainly relies on organic solvents, resulting in environmental pollution and health risks. With the increasing strictness of environmental protection regulations, water-based polyurethane synthetic leather has gradually become a research hotspot. However, the existing water-based polyurethane synthetic leather has the following key technical problems:

[0003] Firstly, traditional water-based polyurethane relies on a single crosslinking agent, which can only form limited covalent bonds with the carboxyl groups of the water-based polyurethane molecular chain, and the crosslinking density is less than or equal to 0.025 mmol / cm3, resulting in poor hydrolysis resistance of the synthetic leather, and the tensile strength retention rate is less than or equal to 70% after 72 hours of hot water immersion at 50°C, and low abrasion resistance. Secondly, high-mechanical-strength water-based synthetic leather needs to add a high content of organic solvents to adjust the viscosity, and the VOCs emission is greater than or equal to 50 mg / m3, which does not meet the requirements of the European Union REACH, China GB33372-2020 and other regulations on volatile organic compounds (VOCs) emissions. Thirdly, under high solid content, the viscosity of traditional water-based polyurethane is high, which is difficult to coat, and restricts the processing and application of water-based polyurethane. Therefore, it is urgent to develop a water-based polyurethane synthetic leather preparation technology with high crosslinking, hydrolysis resistance, high tensile strength, easy coating and low VOCs emission. SUMMARY

[0004] The purpose of the present application is to solve the problems existing in the prior art, and to provide a high-crosslinking water-based polyurethane synthetic leather and a preparation method thereof.

[0005] In order to achieve the above-mentioned purpose, the present application adopts the following technical solutions:

[0006] A high-crosslinking water-based polyurethane synthetic leather comprises a base cloth, a bottom layer modified water-based polyurethane film and a surface layer modified water-based polyurethane film, and the surface layer modified water-based polyurethane film comprises the following components by weight:

[0007] 100 parts of modified water-based polyurethane emulsion: an ultrabranched water-based polyurethane emulsion;

[0008] 2-5 parts of bio-based plasticizer: an epoxy soybean oil derivative;

[0009] 1-3 parts of modified nanocellulose: silane-modified nanocellulose;

[0010] 0.5-1.5 parts of composite crosslinking agent: a nitrogen propylene compound and a blocked isocyanate in a mass ratio of 1:1;

[0011] 0.1-0.3 parts of defoaming agent: polyether modified silicone;

[0012] 0.1-0.2 parts of leveling agent: polyether modified polydimethylsiloxane BYK-333;

[0013] The modified waterborne polyurethane film of the bottom layer comprises the following components by weight:

[0014] 100 parts of modified waterborne polyurethane emulsion: sulfonate modified waterborne polyurethane emulsion;

[0015] 2-5 parts of bio-based plasticizer: epoxy soybean oil derivative;

[0016] 0.5-1.5 parts of composite crosslinking agent: nitrogen propylene compound and blocked isocyanate in a mass ratio of 1:1;

[0017] 0.1-0.3 parts of defoaming agent: polyether modified silicone;

[0018] 0.1-0.2 parts of leveling agent: polyether modified polydimethylsiloxane BYK-333.

[0019] Preferably, the above substances are all commercially available products, wherein the solid content of the modified waterborne polyurethane emulsion is 40-50%, the diameter of the nanocellulose is 20 nm, and the type of the nitrogen propylene compound is XL-702.

[0020] The application also provides a preparation method of the aforementioned high crosslinking waterborne polyurethane synthetic leather, comprising the following steps:

[0021] S1, preparation of the modified waterborne polyurethane emulsion for surface coating

[0022] S101, adding diethylenetriamine and hexanediamine in a molar ratio of 7:4 into a reaction kettle, dissolving with methanol, passing nitrogen, stirring, and gradually adding methyl acrylate at a temperature of 10-25 DEG C, and then increasing the temperature to 30 DEG C, continuing to stir for 18 h, and reacting at a vacuum degree of-0.08 MPa and a temperature of 120-160 DEG C for 4 h to obtain an amino-terminated hyperbranched polyamide-amine prepolymer, which is washed with ethanol, dispersed in deionized water by ultrasonic, and then a dispersion liquid of the amino-terminated hyperbranched polyamide-amine prepolymer with a solid content of 30-40% is obtained;

[0023] In the added reagent, the amine reagent is excessive, and during the gradual drop of methyl acrylate at 10-25℃, the amino group in the amine reagent reacts with the carbon-carbon double bond in the methyl acrylate under alkaline conditions to generate a reaction intermediate, and low temperature inhibits the self-polymerization of the double bond; the temperature is increased to 120-160℃, the high temperature promotes the amidation reaction between the amino group in the reaction intermediate and the ester group in another intermediate, the generated methanol is removed under vacuum conditions, and the amidation reaction is promoted in the forward direction, so that a block hyperbranched polyamide-amine prepolymer is formed around diethylenetriamine;

[0024]

[0025] S102, the nanocellulose aqueous dispersion is selectively oxidized into carboxylated nanocellulose, which is added into an ethanol aqueous solution with a volume fraction of 20%, the pH is adjusted to 7.5-8.0 by triethylamine, and ultrasonic dispersion is performed to obtain a carboxylated cellulose aqueous solution for standby; an epoxy silane coupling agent KH-560 powder with a mass fraction of 5-6% is dissolved in the ethanol aqueous solution, the pH is adjusted to 4.0-4.5 by 0.1 mol / L HCl, and stirring is performed for 20 min; the solution is slowly dropped into the carboxylated cellulose aqueous solution, the temperature is increased to 50℃, and reaction is performed for 1-2 h; centrifugation is performed, the precipitate is washed with deionized water, dialysis is performed, and freeze-drying is performed to obtain a silane-modified nanocellulose powder;

[0026] The nanocellulose is treated by the silane coupling agent KH-560, under alkaline conditions, the carboxyl group forms a negative ion, and the epoxy structure in the silane coupling agent is not damaged; the silane coupling agent is easy to hydrolyze, the trimethoxysilane group (-Si(OCH3)3) is hydrolyzed under acidic conditions to generate a silicon hydroxyl group (-Si(OH)3), and the silicon hydroxyl group can condense with the hydroxyl group in the cellulose skeleton to generate a silicon-oxygen-carbon bond (-Si-O-C-); the nanocellulose modified by the silane contains a large number of hydroxyl groups and carboxyl groups, which provides more active sites for subsequent crosslinking with a carboxyl waterborne polyurethane emulsion, ensures the high crosslinking characteristics of the polyurethane emulsion, and simultaneously, after the addition of the silane-modified nanocellulose and drying and curing, the presence of the nanocellulose skeleton significantly improves the tensile strength of the finished synthetic leather, and the introduction of the hydrophobic group improves the moisture permeability of the finished synthetic leather;

[0027]

[0028] S103, the carboxyl waterborne polyurethane emulsion is placed in a reaction kettle, stirred and dispersed, triethylamine is added to adjust the pH to 7.5-8.0, 3-5% of the total system of the amino-terminated hyperbranched polyamide-amine prepolymer dispersion and 1-3% of the total system of the silane-modified nanocellulose powder are gradually added, ultrasonic stirring is performed for 30 min, 0.3% of dicyclohexyl carbodiimide is added, the temperature is increased to 40-50°C, and reaction is performed for 1.5 h, 0.1 mol / L hydrochloric acid is added to neutralize, filtration is performed, and a face-coating modified waterborne polyurethane emulsion is obtained;

[0029] The carboxyl waterborne polyurethane emulsion, the silane-modified nanocellulose, and the hyperbranched prepolymer are mixed in a corresponding proportion, under the condition of basicity adjusted by triethylamine and the catalyst dicyclohexyl carbodiimide, the carboxyl groups of the carboxyl waterborne polyurethane emulsion and the silane-modified nanocellulose are dissociated, at the same time, the terminal amino groups of the hyperbranched polyamide-amine prepolymer are activated, the amide reaction is preferentially performed under the condition of 40-50°C, preliminary pre-crosslinking is performed, reaction is performed for 1.5 h, the system is adjusted to neutral by hydrochloric acid to terminate the reaction, and under the neutral condition, the ring-opening reaction of the epoxy groups with the carboxyl groups and the amino groups is inhibited;

[0030]

[0031] S2, preparation of a base-coating modified waterborne polyurethane emulsion

[0032] The pH of the ethylenediamine ethanesulfonic acid sodium A95 solution is adjusted to 7.5-8.0 by triethylamine, the carboxyl waterborne polyurethane emulsion is placed in a reaction kettle, stirred and dispersed, the temperature is increased to 60°C, 2-6% of the total system of the ethylenediamine ethanesulfonic acid sodium A95 solution is gradually added dropwise, reaction is performed for 2 h, dehydration is performed under reduced pressure, a sulfonic acid-modified waterborne polyurethane emulsion with a solid content of 45-55% is obtained, an epoxy soybean oil derivative, a polyether-modified silicone, and a leveling agent are sequentially added, stirring is performed for 10 min, a composite crosslinking agent is added, stirring is performed for 3-5 min, and a base-coating modified waterborne polyurethane emulsion is obtained;

[0033] Under the alkaline condition, the -COOH of the carboxyl waterborne polyurethane emulsion is dissociated to generate -COO - , the amino group in the ethylenediamine ethanesulfonic acid sodium is activated to generate -COO - NH3 + an ionic bond, and the carboxyl groups in the waterborne polyester emulsion can also undergo an amide reaction with the amino groups in the ethylenediamine ethanesulfonic acid sodium to generate a covalent bond, the hydrophilic sulfonic acid group is introduced to the molecular main chain, the solid content of the base-coating modified waterborne polyurethane emulsion is improved, the film is soft and has high viscosity;

[0034] The epoxy soybean oil derivative as a toughening agent penetrates between the cross-linking networks to relieve the brittleness of the cross-linking networks; the silicone segment in the polyether-modified silicone reduces the surface energy of the emulsion to form a lubricating film on the surface of the synthetic leather, thereby improving the ultraviolet resistance, high and low temperature resistance, and moisture resistance of the synthetic leather; and the leveling agent helps to form a uniform coating during application.

[0035] S3, Preparation of a top layer high cross-linking waterborne polyurethane solution

[0036] The top coating modified waterborne polyurethane emulsion obtained in S103 is ultrasonically dispersed for 10 minutes, and then the epoxy soybean oil derivative, the polyether-modified silicone, and the leveling agent are sequentially added and stirred for 10 minutes, and then the composite cross-linking agent is added and stirred for 3-5 minutes to obtain a top layer high cross-linking waterborne polyurethane emulsion.

[0037] S4, Preparation of a synthetic leather

[0038] The bottom coating modified waterborne polyurethane emulsion in S3 is uniformly applied to the surface of the pretreated base cloth by the dipping roller coating method, the wet film thickness is 0.1 mm, and the film is pre-dried at 80°C for 30 seconds, then the top layer high cross-linking waterborne polyurethane solution in S3 is applied, dried at 80°C for 2 minutes, and dried at 120°C for 1 minute, and then embossed by a continuous embossing machine, and finally wound to obtain a finished cross-linking waterborne polyurethane synthetic leather.

[0039] After the corresponding emulsion of the bottom coating, cross-linking reactions begin to occur under heating conditions. The aziridine compound contains a three-membered ring structure, which can undergo ring-opening reactions with the carboxyl groups in the emulsion and the modified cellulose at 80°C to form urethane bonds (-NH-COO-) or ester bonds, thereby forming a cross-linking network structure.

[0040] After the corresponding emulsion of the top coating, the aziridine compound containing a three-membered ring and the three-membered ring of the silane reagent first undergo ring-opening reactions with the carboxyl groups in the emulsion and the modified cellulose at 80°C. At the same time, due to the presence of aziridine compounds and carboxyl groups in the bottom layer emulsion, ring-opening reactions also occur at the interface between the bottom layer and the top layer, greatly increasing the peel strength of the finished synthetic leather. As the temperature continues to rise to 120°C, a large number of highly active free -NCO groups are released from the isocyanate groups, which react with the hydroxyl groups to also form urethane bonds, thereby achieving deep cross-linking.

[0041] Preferably, in S102, 2,2,6,6-tetramethylpiperidine nitroxide is used as a selective catalyst, NaBr is used as a co-catalyst, and NaClO is used as an oxidant to selectively oxidize the primary alcohol carbon in the nanocellulose to a carboxyl group. On the one hand, this improves the compatibility of the nanocellulose in the carboxyl waterborne polyurethane emulsion, and on the other hand, it provides more active sites for subsequent modification with silane coupling reagents, resulting in a more dense structure. When added to the emulsion, the mechanical strength of the cured product is significantly increased.

[0042] Preferably, in S4, after the water-based polyurethane coating base layer with sulfonic acid groups is selected and introduced, and dried and cured, the surface layer is coated with a water-based polyurethane emulsion doped with modified cellulose, and gradient curing is performed to achieve high peel strength, high mechanical strength, and high hydrolysis resistance of the finished synthetic leather.

[0043] Compared with the prior art, the present application has the following advantages:

[0044] 1. Compared with the traditional water-based polyurethane which relies on a single crosslinking agent and has low crosslinking density, the present application realizes a breakthrough through the synergistic design of hyperbranched prepolymer and sulfonic acid groups: on the one hand, diethylenetriamine and hexanediamine are compounded to prepare an amino-terminated hyperbranched polyamide-amine prepolymer, the multiple amino groups of which can form multiple site amide bonds with carboxyl water-based polyurethane, exposing more crosslinking reaction sites, and at the same time, due to the core-shell structure of the hyperbranched polymer, it is not easy to entangle, the emulsion viscosity is low, and it is easy to coat; on the other hand, sulfonic acid groups are introduced into the primer, and through the synergistic effect of ionic bonds and covalent bonds at high solid content, the emulsion stability at high solid content is realized, the dry film adhesion and toughness are increased, high organic solvents are not needed for adjustment, the volatile organic compounds are few, and it meets the requirements of GB 33372-2020. At the same time, the primer and the topcoat are functionally differentiated, and gradient crosslinking is performed with aziridine / closed isocyanate composite crosslinking agent, finally the crosslinking density is significantly improved, the tensile strength retention rate after 72h hot water immersion at 50℃ is far higher than that of traditional synthetic leather, and the chemical connection between the primer and the base cloth and between the primer and the topcoat is significantly enhanced, the peel strength is increased by 30%-50%, and the problems of poor hydrolysis resistance and easy delamination between layers are completely solved.

[0045] 2. Traditional high mechanical strength synthetic leather needs to rely on high content of organic solvents or inorganic fillers, which can easily lead to excessive volatile organic compounds or brittle coating, and the present application realizes a breakthrough through silane-modified nanocellulose: first, the surface hydroxyl groups of nanocellulose are selectively oxidized to carboxyl groups by 2,2,6,6-tetramethylpiperidine nitroxide radical, providing sufficient reaction sites, then modified by KH-560 silane, and through the ring-opening of epoxy groups and the condensation of silicon hydroxyl groups, the compatibility with WPU is improved. The modified nanocellulose is embedded in the crosslinking network of the topcoat as a nanoskeleton, which not only improves the tensile strength and abrasion resistance through the stress dispersion effect of the fibers, but also avoids the coating defects caused by the agglomeration of traditional fillers; at the same time, the hydrophobic groups introduced by silane modification also improve the moisture permeability, solving the problem of difficult balance between high mechanical strength and low air permeability, and achieving a balance between environmental friendliness and mechanical performance. BRIEF DESCRIPTION OF DRAWINGS

[0046] Figure 1 Structure diagram of high crosslinking water-based polyurethane synthetic leather prepared by the present application. DETAILED DESCRIPTION

[0047] The technical solutions in the embodiments of the present application will be described clearly and completely below. Obviously, the described embodiments are only part of the embodiments of the present application, rather than all the embodiments. Embodiment

[0048] S1, Preparation of a top-coating modified water-based polyurethane emulsion

[0049] S101, Diethylenetriamine and hexanediamine were added into a reaction kettle according to a molar ratio of 7:4, dissolved with methanol, and stirred under nitrogen at a temperature of 15°C. Methyl acrylate was gradually added dropwise, and the temperature was increased to 30°C. The stirring reaction was continued for 18 h. The vacuum degree was -0.08 MPa, and the reaction was carried out at 140°C for 4 h to obtain an amino-terminated hyperbranched polyamide-amine prepolymer. Ethanol washing, ultrasonic dispersion with deionized water, and the like were performed to obtain an amino-terminated hyperbranched polyamide-amine prepolymer dispersion liquid with a solid content of 35%;

[0050] S102, The nanocellulose water dispersion liquid was selectively oxidized into carboxylated nanocellulose. The carboxylated nanocellulose was added into an ethanol aqueous solution with a volume fraction of 20%. Triethylamine was used to adjust the pH to 7.5-8.0. Ultrasonic dispersion was performed to obtain a carboxylated cellulose aqueous solution for standby use. An epoxy silane coupling agent KH-560 powder with a mass fraction of 5% was dissolved in the ethanol aqueous solution. 0.1 mol / L HCl was used to adjust the pH to 4.0-4.5. Stirring was performed for 20 min. The solution was slowly added dropwise into the carboxylated cellulose aqueous solution. The temperature was increased to 50°C. The reaction was carried out for 1-2 h. Centrifugation, deionized water washing of the precipitate, dialysis, and freeze-drying were performed to obtain a silane-modified nanocellulose powder.

[0051] S103, The carboxylated water-based polyurethane emulsion was placed in a reaction kettle. Stirring and dispersion were performed. Triethylamine was used to adjust the pH to 7.5-8.0. An amino-terminated hyperbranched polyamide-amine prepolymer dispersion liquid with a proportion of 4% of the total system and a silane-modified nanocellulose powder with a proportion of 1% of the total system were gradually added. Ultrasonic stirring was performed for 30 min. 0.3% of dicyclohexyl carbodiimide was added. The temperature was increased to 40-50°C. The reaction was carried out for 1.5 h. 0.1 mol / L HCl was used to adjust the solution to neutral. Filtration was performed to obtain a top-coating modified water-based polyurethane emulsion.

[0052] S2, Preparation of a base-coating modified water-based polyurethane emulsion

[0053] Triethylamine was used to adjust the pH of a sodium ethylenediamine ethanesulfonate A95 solution to 7.5-8.0. The carboxylated water-based polyurethane emulsion was placed in a reaction kettle. Stirring and dispersion were performed. The temperature was increased to 60°C. An ethylenediamine ethanesulfonate A95 solution with a proportion of 4% of the total system was gradually added dropwise. The reaction was carried out for 2 h. Dehydration under reduced pressure was performed to obtain a sulfonic acid modified water-based polyurethane emulsion with a solid content of 50%. An epoxy soybean oil derivative, a polyether modified organosilicon, and a leveling agent were sequentially added. Stirring was performed for 10 min. A composite crosslinking agent was added. Stirring reaction was carried out for 4 min to obtain a base-coating modified water-based polyurethane emulsion.

[0054] S3, Preparation of a top layer high crosslinking waterborne polyurethane solution

[0055] The top coating modified waterborne polyurethane emulsion obtained in S103 is ultrasonically dispersed for 10 min, and an epoxy soybean oil derivative, a polyether modified silicone, and a leveling agent are sequentially added and stirred for 10 min. A composite crosslinking agent is added and stirred for 4 min to obtain a top layer high crosslinking waterborne polyurethane emulsion.

[0056] S4, Preparation of synthetic leather

[0057] The bottom layer is coated by using an immersion roller coating method, and the bottom coating modified waterborne polyurethane emulsion in S3 is uniformly applied to the surface of the pretreated base cloth with a wet film thickness of 0.1 mm. The film is pre-dried at 80°C for 30 s, and the top layer high crosslinking waterborne polyurethane solution in S3 is applied. The film is dried at 80°C for 2 min and at 120°C for 1 min. The film is embossed by a continuous embossing machine, wound up, and the finished crosslinking waterborne polyurethane synthetic leather is obtained. Example

[0058] S1, Preparation of a top coating modified waterborne polyurethane emulsion

[0059] S101, Diethylenetriamine and hexanediamine are added to a reaction kettle in a molar ratio of 7:4, dissolved with methanol, stirred under nitrogen, and gradually added with methyl acrylate at a temperature of 15°C. The temperature is increased to 30°C, and the stirring reaction is continued for 18 h. The vacuum degree is -0.08 MPa, and the reaction is carried out at 140°C for 4 h to obtain an amino-terminated hyperbranched polyamide-amine prepolymer. The prepolymer is washed with ethanol and ultrasonically dispersed with deionized water to obtain a dispersion liquid of the amino-terminated hyperbranched polyamide-amine prepolymer with a solid content of 35%.

[0060] S102, The nanocellulose water dispersion is selectively oxidized to carboxylated nanocellulose. The carboxylated nanocellulose is added to an ethanol aqueous solution with a volume fraction of 20%, and the pH is adjusted to 7.5-8.0 with triethylamine. The mixture is ultrasonically dispersed to obtain a carboxylated cellulose aqueous solution for standby use. A silane coupling agent KH-560 with a powder mass of 5% is dissolved in the ethanol aqueous solution, and the pH is adjusted to 4.0-4.5 with 0.1 mol / L HCl. The mixture is stirred for 20 min, and then slowly added dropwise into the carboxylated cellulose aqueous solution. The temperature is increased to 50°C, and the reaction is carried out for 1-2 h. The mixture is centrifuged, the precipitate is washed with deionized water, dialyzed, and freeze-dried to obtain a silane-modified nanocellulose powder.

[0061] S103, the carboxyl waterborne polyurethane emulsion is placed in a reaction kettle, stirred and dispersed, triethylamine is added to adjust the pH to 7.5-8.0, 4% of the total system of the amino-terminated hyperbranched polyamide-amine prepolymer dispersion and 3% of the total system of the silane-modified nanocellulose powder are gradually added, ultrasonic stirring is performed for 30 min, 0.3% of dicyclohexyl carbodiimide is added, the temperature is raised to 40-50℃, and reaction is performed for 1.5 h, 0.1 mol / L hydrochloric acid is added to neutralize, filtration is performed, and a face-coating modified waterborne polyurethane emulsion is obtained;

[0062] S2, preparation of a base-coating modified waterborne polyurethane emulsion

[0063] The pH of a sodium ethylenediamine sulfonate A95 solution is adjusted to 7.5-8.0 with triethylamine, the carboxyl waterborne polyurethane emulsion is placed in a reaction kettle, stirred and dispersed, the temperature is raised to 60℃, 4% of the total system of the sodium ethylenediamine sulfonate A95 solution is gradually added dropwise, reaction is performed for 2 h, dehydration is performed under reduced pressure, a 50% sulfonic acid-modified waterborne polyurethane emulsion is obtained, an epoxy soybean oil derivative, a polyether-modified silicone, and a leveling agent are sequentially added, stirring is performed for 10 min, a composite crosslinking agent is added, stirring is performed for 4 min, and a base-coating modified waterborne polyurethane emulsion is obtained;

[0064] S3, preparation of a face-coating high-crosslinking waterborne polyurethane solution

[0065] The face-coating modified waterborne polyurethane emulsion obtained in S103 is ultrasonically dispersed for 10 min, an epoxy soybean oil derivative, a polyether-modified silicone, and a leveling agent are sequentially added, stirring is performed for 10 min, a composite crosslinking agent is added, stirring is performed for 4 min, and a face-coating high-crosslinking waterborne polyurethane emulsion is obtained;

[0066] S4, preparation of a synthetic leather

[0067] A base-coating modified waterborne polyurethane emulsion in S3 is uniformly applied to the surface of a pretreated base fabric in a manner of immersion roll coating, the wet film thickness is 0.1 mm, pre-drying is performed at 80℃ for 30 s, a face-coating high-crosslinking waterborne polyurethane solution in S3 is applied, drying is performed at 80℃ for 2 min, drying is performed at 120℃ for 1 min, embossing is performed on a continuous embossing machine, and winding is performed, and a finished crosslinking waterborne polyurethane synthetic leather is obtained. Example

[0068] S1, preparation of a face-coating modified waterborne polyurethane emulsion

[0069] S101, diethylenetriamine, hexanediamine are added into a reaction kettle according to a molar ratio of 7:4, methanol is added for dissolution, nitrogen is passed, stirring is performed, the temperature is 15℃, methyl acrylate is gradually added dropwise, the temperature is increased to 30℃, stirring is continuously performed for 18h, the vacuum degree is-0.08MPa, reaction is performed at 140℃ for 4h, an amino-terminated hyperbranched polyamidoamine prepolymer is obtained, ethanol washing is performed, deionized water ultrasonic dispersion is performed, and an amino-terminated hyperbranched polyamidoamine prepolymer dispersion liquid with a solid content of 35% is obtained;

[0070] S102, the nanocellulose water dispersion liquid is selectively oxidized into carboxylated nanocellulose, the carboxylated nanocellulose is added into an ethanol aqueous solution with a volume fraction of 20%, triethylamine is used to adjust the pH to 7.5-8.0, ultrasonic dispersion is performed, a carboxylated cellulose aqueous solution is obtained for standby, an epoxy silane coupling agent KH-560 powder with a mass fraction of 5% is dissolved in the ethanol aqueous solution, 0.1mol / L hydrochloric acid is used to adjust the pH to 4.0-4.5, stirring is performed for 20min, the solution is slowly added dropwise into the carboxylated cellulose aqueous solution, the temperature is increased to 50℃, reaction is performed for 1-2h, centrifugation is performed, the precipitate is washed with deionized water, dialysis is performed, and freeze-drying is performed, and a silane-modified nanocellulose powder is obtained;

[0071] S103, the carboxylated waterborne polyurethane emulsion is placed in a reaction kettle, stirring and dispersion are performed, triethylamine is used to adjust the pH to 7.5-8.0, the amino-terminated hyperbranched polyamidoamine prepolymer dispersion liquid with a proportion of 4% of the total system and the silane-modified nanocellulose powder with a proportion of 2% of the total system are gradually added, ultrasonic stirring is performed for 30min, 0.3% of dicyclohexyl carbodiimide is added, the temperature is increased to 40-50℃, reaction is performed for 1.5h, 0.1mol / L hydrochloric acid is used to adjust to neutral, and filtration is performed, and a topcoat modified waterborne polyurethane emulsion is obtained;

[0072] S2, preparation of a basecoat modified waterborne polyurethane emulsion

[0073] Triethylamine is used to adjust the pH of a sodium ethylenediamine ethanesulfonate A95 solution to 7.5-8.0, the carboxylated waterborne polyurethane emulsion is placed in a reaction kettle, stirring and dispersion are performed, the temperature is increased to 60℃, the sodium ethylenediamine ethanesulfonate A95 solution with a proportion of 2% of the total system is gradually added dropwise, reaction is performed for 2h, and dehydration is performed under reduced pressure, and a sulfonic acid modified waterborne polyurethane emulsion with a solid content of 50% is obtained, an epoxy soybean oil derivative, a polyether modified silicone and a leveling agent are sequentially added, stirring is performed for 10min, a composite crosslinking agent is added, stirring is performed for 4min, and a basecoat modified waterborne polyurethane emulsion is obtained;

[0074] S3, preparation of a topcoat high crosslinking waterborne polyurethane solution

[0075] The surface coating modified waterborne polyurethane emulsion obtained in S103 is ultrasonically dispersed for 10 min, and then an epoxy soybean oil derivative, a polyether modified silicone and a leveling agent are sequentially added and stirred for 10 min, a composite crosslinking agent is added and stirred for 4 min, and a surface coating high crosslinking waterborne polyurethane emulsion is obtained;

[0076] S4, preparation of synthetic leather

[0077] The bottom coating modified waterborne polyurethane emulsion in S3 is uniformly coated on the surface of the pretreated base cloth by the way of immersion roll coating, the wet film thickness is 0.1 mm, pre-drying is performed at 80℃ for 30 s, the surface layer high crosslinking waterborne polyurethane solution in S3 is coated, drying is performed at 80℃ for 2 min and at 120℃ for 1 min, embossing is performed by a continuous embossing machine, and the product crosslinking waterborne polyurethane synthetic leather is obtained by winding. Example

[0078] S1, preparation of surface coating modified waterborne polyurethane emulsion

[0079] S101, diethylenetriamine and hexanediamine are added into a reaction kettle according to a molar ratio of 7:4, methanol is added for dissolution, nitrogen is passed, stirring is performed, the temperature is 15℃, methyl acrylate is gradually added dropwise, the temperature is increased to 30℃, and stirring is continuously performed for 18 h, the vacuum degree is-0.08 MPa, and reaction is performed at 140℃ for 4 h, to obtain an amino-terminated hyperbranched polyamide-amine prepolymer, which is washed with ethanol and ultrasonically dispersed with deionized water to obtain an amino-terminated hyperbranched polyamide-amine prepolymer dispersion liquid with a solid content of 35%;

[0080] S102, the nanocellulose water dispersion liquid is selectively oxidized into carboxylated nanocellulose, which is added into an ethanol aqueous solution with a volume fraction of 20%, triethylamine is used to adjust the pH to 7.5-8.0, ultrasonic dispersion is performed, to obtain a carboxylated cellulose aqueous solution for standby use, an epoxy silane coupling agent KH-560 with a powder mass of 5% is dissolved in the ethanol aqueous solution, 0.1 mol / L HCl is used to adjust the pH to 4.0-4.5, stirring is performed for 20 min, the solution is slowly added dropwise into the carboxylated cellulose aqueous solution, the temperature is increased to 50℃, and reaction is performed for 1-2 h, centrifugation is performed, the precipitate is washed with deionized water, dialysis is performed, and freeze-drying is performed, to obtain a silane-modified nanocellulose powder;

[0081] S103, the carboxyl waterborne polyurethane emulsion is placed in a reaction kettle, stirring and dispersion are performed, triethylamine is used to adjust the pH to 7.5-8.0, the amino-terminated hyperbranched polyamide-amine prepolymer dispersion liquid with a proportion of 4% of the total system and the silane-modified nanocellulose powder with a proportion of 2% of the total system are gradually added, ultrasonic stirring is performed for 30 min, 0.3% of dicyclohexyl carbodiimide is added, the temperature is increased to 40-50℃, reaction is performed for 1.5 h, 0.1 mol / L hydrochloric acid is used to adjust to neutral, and filtration is performed, to obtain a surface coating modified waterborne polyurethane emulsion;

[0082] Preparation of S2, base coating modified water-based polyurethane emulsion

[0083] The pH of the sodium ethylenediamine sulfonate A95 solution was adjusted to 7.5-8.0 by triethylamine, the carboxyl water-based polyurethane emulsion was placed in a reaction kettle, dispersed by stirring, and heated to 60°C. 6% of the sodium ethylenediamine sulfonate A95 solution was gradually added to the system, and the reaction was carried out for 2h. After dehydration under reduced pressure, a sulfonic acid modified water-based polyurethane emulsion with a solid content of 50% was obtained. The epoxy soybean oil derivative, the polyether modified silicone, and the leveling agent were added in sequence, stirred for 10 min, and the composite crosslinking agent was added, stirred for 4 min, to obtain the base coating modified water-based polyurethane emulsion.

[0084] S3, preparation of top layer high crosslinking water-based polyurethane solution

[0085] The top coating modified water-based polyurethane emulsion obtained in S103 was ultrasonically dispersed for 10 min, and the epoxy soybean oil derivative, the polyether modified silicone, and the leveling agent were added in sequence, stirred for 10 min, and the composite crosslinking agent was added, stirred for 4 min, to obtain the top layer high crosslinking water-based polyurethane emulsion.

[0086] S4, preparation of synthetic leather

[0087] The base coating modified water-based polyurethane emulsion in S3 was evenly applied on the surface of the pretreated base fabric by immersion roll coating, with a wet film thickness of 0.1 mm. Pre-drying was carried out at 80°C for 30s, and the top layer high crosslinking water-based polyurethane solution in S3 was applied. Drying was carried out at 80°C for 2 min and at 120°C for 1 min. Embossing was carried out on a continuous embossing machine, and the product was wound to obtain the finished crosslinking water-based polyurethane synthetic leather. Example

[0088] S1, preparation of top coating modified water-based polyurethane emulsion

[0089] S101, diethylenetriamine and hexanediamine were added to a reaction kettle in a molar ratio of 7:4, dissolved with methanol, stirred under nitrogen, and the temperature was maintained at 15°C. Methyl acrylate was gradually added, the temperature was raised to 30°C, and the stirring reaction was continued for 18h. The vacuum degree was -0.08MPa, and the reaction was carried out at 140°C for 4h to obtain an amino-terminated hyperbranched polyamide-amine prepolymer. Ethanol washing and ultrasonic dispersion with deionized water were carried out to obtain a dispersion liquid of amino-terminated hyperbranched polyamide-amine prepolymer with a solid content of 35%;

[0090] S102, the nanocellulose aqueous dispersion is selectively oxidized into carboxylated nanocellulose, which is added into an aqueous solution of 20% ethanol by volume, the pH is adjusted to 7.5-8.0 by triethylamine, and ultrasonic dispersion is performed to obtain a carboxylated cellulose aqueous solution for standby use; 5% of the powder mass of an epoxy silane coupling agent KH-560 is dissolved in the aqueous ethanol solution, the pH is adjusted to 4.0-4.5 by 0.1 mol / L HCl, and stirring is performed for 20 min; the solution is slowly dropped into the carboxylated cellulose aqueous solution, the temperature is raised to 50℃, and reaction is performed for 1-2 h; centrifugation is performed, the precipitate is washed with deionized water, dialysis is performed, and freeze-drying is performed to obtain silane-modified nanocellulose powder;

[0091] S103, the carboxylated waterborne polyurethane emulsion is placed in a reaction kettle, stirring and dispersion are performed, the pH is adjusted to 7.5-8.0 by triethylamine, 4% of the total system of an amino-terminated hyperbranched polyamide-amine prepolymer dispersion and 2% of the total system of silane-modified nanocellulose powder are gradually added, ultrasonic stirring is performed for 30 min, 0.3% of dicyclohexyl carbodiimide is added, the temperature is raised to 40-50℃, reaction is performed for 1.5 h, and the pH is adjusted to neutral by 0.1 mol / L hydrochloric acid; filtration is performed to obtain a face-coating modified waterborne polyurethane emulsion;

[0092] S2, preparation of a base-coating modified waterborne polyurethane emulsion

[0093] The pH of a sodium ethylenediamine sulfonate A95 solution is adjusted to 7.5-8.0 by triethylamine, the carboxylated waterborne polyurethane emulsion is placed in a reaction kettle, stirring and dispersion are performed, the temperature is raised to 60℃, 4% of the total system of the sodium ethylenediamine sulfonate A95 solution is gradually added, reaction is performed for 2 h, and dehydration is performed under reduced pressure to obtain a sulfonic acid-modified waterborne polyurethane emulsion with a solid content of 50%; an epoxy soybean oil derivative, a polyether-modified silicone, and a leveling agent are sequentially added, stirring is performed for 10 min, and a composite crosslinking agent is added, and stirring and reaction are performed for 4 min to obtain a base-coating modified waterborne polyurethane emulsion;

[0094] S3, preparation of a face-coating high-crosslinking waterborne polyurethane solution

[0095] The face-coating modified waterborne polyurethane emulsion obtained in S103 is ultrasonically dispersed for 10 min, an epoxy soybean oil derivative, a polyether-modified silicone, and a leveling agent are sequentially added, stirring is performed for 10 min, a composite crosslinking agent is added, and stirring and reaction are performed for 4 min to obtain a face-coating high-crosslinking waterborne polyurethane emulsion;

[0096] S4, preparation of a synthetic leather

[0097] The bottom layer is coated by using the method of immersion roller coating, the modified waterborne polyurethane emulsion in S3 is uniformly applied on the surface of the pretreated base cloth, the wet film thickness is 0.1 mm, pre-drying at 80°C for 30s, the top layer high crosslinking waterborne polyurethane solution in S3 is applied, drying at 80°C for 2min, drying at 120°C for 1min, embossing by continuous embossing machine, winding, and the finished crosslinked waterborne polyurethane synthetic leather is obtained.

[0098] Comparative Example 1: Compared with Example 5, the amount of modified cellulose powder added in S103 is 0.5%.

[0099] Comparative Example 2: Compared with Example 5, the amount of sodium ethylenediamine ethanesulfonate A95 solution added in S2 is 8%.

[0100] Comparative Example 3: Compared with Example 5, the sodium ethylenediamine ethanesulfonate A95 solution in S2 is replaced by sodium beta-alanine solution.

[0101] Comparative Example 4: Compared with Example 5, the step of adjusting pH 7.5-8.0 by triethylamine in S102 is removed.

[0102] Comparative Example 5: Compared with Example 5, unmodified nanocellulose is added in S103.

[0103] Performance test: according to the standards and test methods of ISO 3376:2020 "Leather-Determination of tensile strength and elongation", ISO 13935-1:2017 "Leather-Determination of peel strength and peel elongation", ISO 2419:2012 "Leather-Physical tests-Determination of resistance to hydrolysis", GB 33372-2020 "Adhesives Limit of Volatile Organic Compounds", the mechanical properties, hydrolysis resistance and VOC emission of the present application are detected.

[0104] The corresponding results, data summary is drawn into Table 1:

[0105] Table 1. Performance test data of waterborne synthetic leather

[0106]

[0107] Data analysis:

[0108] The cellulose is introduced into the emulsion, that is, introduced into the cellulose skeleton, mainly to improve the mechanical properties of synthetic leather. The content of modified nanocellulose in Example 1 is lower than that in Example 5, lower than that in Example 2, and lower than that in Comparative Example 1. According to the synthetic leather performance data table in Table 1, the tensile strength and elongation at break of Example 2 are greater than those of Example 5, greater than those of Example 1, and greater than those of Comparative Example 1. This indicates that the mechanical properties of Example 2 are better than those of Example 5, better than those of Example 1, and better than those of Comparative Example 1. This can be explained as follows: the cellulose is uniformly distributed in the emulsion, acts as a structural skeleton, and when the synthetic leather is stretched longitudinally, the cellulose also bears the same tension. The cellulose itself has a compact structure and is not easy to break. After modification, the compatibility with the emulsion is good, and the gradient crosslinking of the composite crosslinking agent is combined to realize the construction of the three-dimensional network structure of the surface layer. Therefore, within a certain range, the mechanical properties increase with the increase of the doping amount of cellulose. As the doping amount increases, the mechanical properties change slightly. The mechanical properties of Example 2 and Example 5 differ slightly. If the content continues to rise, the excessive doping of cellulose increases the brittleness of the surface layer of synthetic leather, which is prone to breakage. Excessive doping can cause agglomeration of nanocellulose, uneven stress, and easy breakage. The data in Table 1 shows that the peeling strength and strength retention rate of Comparative Example 1 are greater than those of Example 2, greater than those of Example 5, and greater than those of Example 1. This indicates that the hydrolysis resistance of Comparative Example 1 is better than that of Example 2, better than that of Example 5, and better than that of Example 1. Because the waterborne polyurethane often contains hydrophilic groups such as carboxyl, amino, and hydroxyl groups, it is prone to hydrolysis, which limits the use of synthetic leather. After adding modified cellulose, the epoxy group and carboxyl group on the cellulose, as well as the hydroxyl group on the cellulose itself, significantly enhance the adhesion between the surface layer and the bottom layer under the action of the crosslinking agent, and are not prone to peeling. Generally, the peeling strength increases with the increase of the content of nanocellulose, and the peeling strength changes little before and after hydrolysis, and the hydrolysis resistance is stronger. The release amount of volatile organic substances is relatively small during the entire preparation process because there is little difference between the examples and the comparative examples.

[0109] In Comparative Example 5, ordinary nanocellulose is doped, and the mechanical properties and hydrolysis resistance are significantly lower than those of Example 5. Nanocellulose is not soluble in water and is prone to agglomeration in the emulsion, which leads to uneven distribution of cellulose in the emulsion. This causes uneven stress distribution in the synthetic leather when it is subjected to external force, which is prone to breakage and poor mechanical properties. In addition, only hydroxyl groups exist in ordinary cellulose, and there are no active groups for effective crosslinking with polyurethane molecules in the surface layer and polyurethane molecules in the bottom layer during subsequent thermal crosslinking. Therefore, the combination is weak, and the finished synthetic leather is prone to peeling. The small crosslinking degree allows water molecules to quickly enter between polyurethane molecules, resulting in poor moisture resistance. The hydrolysis of active functional groups in polyurethane greatly affects the use of synthetic leather.

[0110] The introduction of sulfonic acid group can regulate the hydrophilicity of the emulsion. According to the data in the table, the tensile strength, elongation at break and peel strength of Example 4 are greater than those of Example 3, indicating that as the content of sulfonic acid group increases, the mechanical properties and anti-peeling degree of synthetic leather increase, which can be explained as follows: -SO3Na is a strong hydrophilic group, which can form ionic crosslinking with waterborne polyurethane, thereby increasing the solid content of the emulsion, and after heat curing, the adhesion between the synthetic leather and the top layer increases, so within a suitable range, the mechanical properties and anti-peeling degree of synthetic leather are proportional to the content of sulfonic acid group. However, as the content of sulfonic acid group increases, the hydrophilic performance of sulfonic acid is greater than the increase in the solid content of the emulsion, which leads to easy water swelling of synthetic leather, which destroys the original crosslinking sites, resulting in a decrease in the mechanical properties and peel strength of synthetic leather. The data of Comparative Example 2 confirms this point. The strength retention rate of Example 3 is greater than that of Example 5, which is greater than that of Example 4, indicating that the water resistance of synthetic leather of Example 3 is better than that of Example 5, which is better than that of Example 4. This is because the peel strength retention rate depends on the water resistance of synthetic leather. The more the content of sulfonic acid group in Example 4, the more obvious the water absorption capacity, the poorer the hydrolysis resistance, and the smaller the strength retention rate. As the content of sulfonic acid increases to 8%, the strength retention rate of Comparative Example 2 decreases to 75%.

[0111] Comparative Example 3, compared with Example 5, replaces the ethylenediamine ethanesulfonic acid sodium A95 solution in S2 with a beta-alanine sodium solution, i.e., introducing more carboxyl groups into the polyurethane molecule to increase the solid content of the waterborne polyurethane. As can be seen from the table, the mechanical properties and peel strength of synthetic leather of Comparative Example 3 are poorer than those of Example 5, which is due to the fact that under the same amount of addition, the hydrophilic performance of sulfonic acid group is better than that of carboxyl group, thus resulting in a higher solid content of the emulsion of Example 5 than that of Comparative Example 3. After heat curing, the polyurethane film of Example 5 has greater adhesion, and the bonding force between the molecules of the bottom layer and the molecules of the top layer is stronger, making it difficult to break and peel, thus the mechanical properties and anti-peeling properties are excellent. However, due to the strong hydrophilicity of sulfonic acid, the water absorption capacity is stronger under the same content, and water molecules enter between the polyester molecules, which can cause hydrolysis of active groups, resulting in a decrease in the strength retention rate.

[0112] Compared with Example 5, the step of adjusting pH 7.5-8.0 by triethylamine in S102 is removed in Comparative Example 4. According to the data in the table, the mechanical properties, anti-peeling properties and hydrolysis resistance of the synthetic leather of Comparative Example 4 are poorer than those of Example 5, which is because the triethylamine in S102 is used to adjust the pH of the cellulose dispersion, and the silane coupling agent is hydrolyzed under acidic conditions to generate silicon hydroxyl groups which can condense with the hydroxyl groups in cellulose to generate silicon-oxygen-carbon bonds. However, since the KH-560 coupling agent contains an epoxy group, it is easily opened by carboxyl groups under acidic conditions, and the cross-linking between the surface layer modified waterborne polyurethane molecules and the bottom layer polyurethane molecules is weakened during subsequent thermal curing and cross-linking, so the synthetic leather is prone to breakage and peeling when subjected to external force, the cross-linking degree is low, water molecules can enter the intermolecular space through the pores in the film, and long-term hydrolysis of the synthetic leather occurs, so the performance of the synthetic leather of Comparative Example 4 is poorer than that of Example 5.

[0113] The above description is only the preferred specific implementation of the present application, but the protection scope of the present application is not limited thereto, any person skilled in the art can make equivalent replacements or changes within the technical range disclosed by the present application according to the technical solution and inventive concept of the present application, which should be covered within the protection scope of the present application.

Claims

1. A method for preparing highly cross-linked waterborne polyurethane synthetic leather, characterized in that, Includes the following steps: S1. Preparation of surface-coated modified waterborne polyurethane emulsion S101. Diethylenetriamine and hexamethylenediamine were added to a reaction vessel at a molar ratio of 7:

4. Methanol was added to dissolve them, nitrogen was introduced, and the mixture was stirred at a temperature of 10-25℃. Methyl acrylate was gradually added dropwise, and the temperature was raised to 30℃. The mixture was stirred and reacted for 18 hours at a vacuum of -0.08 MPa and 120-160℃ for 4 hours to obtain an amino-terminated hyperbranched polyamide-amine prepolymer. The prepolymer was washed with ethanol and ultrasonically dispersed in deionized water to obtain a dispersion of amino-terminated hyperbranched polyamide-amine prepolymer with a solid content of 30-40%. S102. Selectively oxidize the aqueous dispersion of nanocellulose to carboxylated nanocellulose, add it to a 20% (v / v) aqueous ethanol solution, adjust the pH to 7.5-8.0 with triethylamine, and disperse ultrasonically to obtain an aqueous solution of carboxylated cellulose for later use. Dissolve 5-6% (w / v) of the powdered epoxy silane coupling agent KH-560 in an aqueous ethanol solution, adjust the pH to 4.0-4.5 with 0.1 mol / L HCl, stir for 20 min, and slowly add it dropwise to the aqueous solution of carboxylated cellulose. Heat to 50℃ and react for 1-2 h. Centrifuge, wash the precipitate with deionized water, dialyze, and freeze dry to obtain silane-modified nanocellulose powder. S103. Place the carboxyl-based waterborne polyurethane emulsion in a reaction vessel, stir and disperse it, add triethylamine to adjust the pH to 7.5-8.0, gradually add 3-5% of the total system volume of terminal amino hyperbranched polyamide-amine prepolymer dispersion and 1-3% of the total system volume of silane-modified nanocellulose powder, ultrasonically stir for 30 min, add 0.3% of dicyclohexylcarbodiimide, heat to 40-50℃, react for 1.5 h, adjust to neutral with hydrochloric acid, filter, and obtain the surface-coated modified waterborne polyurethane emulsion; S2. Preparation of primer-modified waterborne polyurethane emulsion Adjust the pH of the sodium ethylenediamine ethanesulfonate A95 solution to 7.5-8.0 with triethylamine. Place the carboxyl-based waterborne polyurethane emulsion in a reaction vessel, stir and disperse, heat to 60℃, and gradually add 2-6% of the sodium ethylenediamine ethanesulfonate A95 solution. React for 2 hours, dehydrate under reduced pressure, and obtain a sulfonic acid-modified waterborne polyurethane emulsion with a solid content of 45-55%. Add epoxidized soybean oil derivative, polyether-modified organosilicon, and leveling agent BYK-333 in sequence, stir for 10 minutes, add aziridine compounds and blocked isocyanate composite crosslinking agents in a 1:1 mass ratio, and stir for 3-5 minutes to obtain a primer-modified waterborne polyurethane emulsion. S3, Additives to modified waterborne polyurethane emulsions for surface coating The surface-coated modified waterborne polyurethane emulsion obtained from S103 was ultrasonically dispersed for 10 min. Then, epoxidized soybean oil derivative, polyether-modified organosilicon and leveling agent BYK-333 were added sequentially and stirred for 10 min. A aziridine compound and a blocked isocyanate composite crosslinking agent with a mass ratio of 1:1 were added and stirred for 3-5 min to obtain a surface-coated highly crosslinked waterborne polyurethane emulsion. S4. Preparation of Synthetic Leather The base coat is applied by dip-coating roller coating. The modified waterborne polyurethane emulsion in S2 is evenly applied to the surface of the pretreated base fabric with a wet film thickness of 0.1 mm. It is pre-dried at 80°C for 30 seconds. The top coat of the highly cross-linked waterborne polyurethane emulsion in S3 is then applied. It is dried at 80°C for 2 minutes and then at 120°C for 1 minute. The fabric is then embossed using a continuous embossing machine and wound up to obtain the finished cross-linked waterborne polyurethane synthetic leather.

2. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, The carboxyl-based waterborne polyurethane emulsion contains 5-15% carboxyl groups, 40-50% solids, and 20nm diameter nanocellulose.

3. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, The molar ratio of methyl acrylate, diethylenetriamine, and hexamethylenediamine in S101 is 10:7:

4.

4. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, The selective oxidation method in S102 is as follows: 0.015% of 2,2,6,6-tetramethylpiperidine nitroxide radical and 0.1% NaBr are added to a 1% solid content aqueous solution of nanocellulose. The solution is stirred and dissolved at room temperature. The pH of the system is adjusted to 10.5-11.0 with 0.5 mol / L NaOH. NaClO solution is slowly added dropwise at 5-10% of the total system volume. The reaction is continued for 1-2 hours. The reaction is terminated by adding 5-10% anhydrous ethanol. The pH is adjusted to 2-3 with 0.1 mol / L HCl. The solution is centrifuged, the precipitate is washed with deionized water, dialyzed, and lyophilized to obtain carboxylated nanocellulose powder.

5. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, In S103, the silane-modified nanocellulose powder is added to the emulsion at an ultrasonic frequency of 40 kHz.

6. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, In S2, 15-20% of the moisture in the system is removed by decompression.

7. The method for preparing a highly cross-linked waterborne polyurethane synthetic leather according to claim 1, characterized in that, The pretreatment operation in S4 is as follows: immerse the base fabric in a 3-5% sodium hydroxide aqueous solution, stir at 50-55℃ for 30-40 minutes, rinse the base fabric repeatedly with deionized water until the pH of the rinsing solution is 7, dry at 80-85℃ for 15-20 minutes, and send it into a low-temperature plasma treatment device, controlling the treatment power to be 300-350W and the treatment time to be 30-40 seconds.

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