Preparation process of secondary foaming type water-based synthetic leather

Abstract

The invention discloses a preparation process of secondary foaming type water-based synthetic leather, and relates to the technical field of polymer foaming materials, the preparation process comprises the following steps: 1) epoxy modified thermal expansion microspheres; 2) side chain amino modified polyurethane; 3) preparing foaming slurry; 4) preparing the synthetic leather; firstly, thermal expansion microspheres are epoxidized, waterborne polyurethane is grafted with side chain amino protected by t-butyloxycarboryl, the crosslinking degree is improved through triphenylmethane triisocyanate, and then the waterborne polyurethane is mixed with the epoxy modified thermal expansion microspheres; after the first time of foaming by a mechanical means, in the step 4), t-butyloxycarboryl is heated and dissociated, CO2 gas is generated, active amino is released, the t-butyloxycarboryl is crosslinked with epoxy of the thermal expansion microspheres, meanwhile, the thermal expansion microspheres are expanded, and chemical foaming, namely second time of foaming, is achieved. Through cooperation of mechanical and chemical secondary foaming processes, the problems that traditional water-based leather is hard in hand feeling, bubbles are prone to collapse and the performance is single are solved, and the comprehensive performance is remarkably improved compared with the prior art.

Description

Technical Field

[0001] The present invention relates to the technical field of polymer materials, in particular to a preparation process of secondary foaming water-based synthetic leather. Background Art

[0002] Synthetic leather, a key material used in a wide range of applications, including footwear, luggage, furniture, and automotive interiors, is attracting increasing attention for its performance and environmental friendliness. Traditional synthetic leather often utilizes solvent-based polyurethane systems, using organic solvents such as dimethylformamide (DMF) and methyl ethyl ketone (MEK). These processes not only result in high emissions of volatile organic compounds (VOCs), significant environmental pollution, and significant safety hazards, but also pose potential threats to human health. With increasingly stringent environmental regulations and growing consumer demand for green products, the development of high-performance water-based synthetic leather has become a key development direction for the industry.

[0003] Waterborne polyurethane uses water as a dispersion medium and offers advantages such as low VOCs, non-toxicity, and non-flammability. However, its mechanical properties, water resistance, and structural stability are generally inferior to those of solvent-based systems, especially in foamed synthetic leather. The foaming process is a key technology for achieving lightweight, high elasticity, and a soft feel in synthetic leather. Currently, physical foaming (such as mechanical foaming) and chemical foaming (such as thermally expandable microspheres) are commonly used. Thermally expandable microspheres are favored due to their easy-to-control foaming process and uniform cell structure.

[0004] However, the foaming process using heat-expandable microspheres in the prior art still has significant defects:

[0005] Weak interfacial bonding leads to insufficient mechanical properties. The thermally expandable microspheres, as physical fillers, lack chemical bonding with the polyurethane matrix, resulting in poor interfacial compatibility and a tendency to become stress concentration points. This can lead to interfacial delamination when subjected to stress, severely compromising the mechanical strength of the finished product. This problem is particularly pronounced at high expansion ratios (low densities), often resulting in products that cannot achieve both low density and high strength.

[0006] Poor cell structure stability. Traditional thermally expandable microspheres rely on a physical process for foaming. The resulting cell walls are weak, allowing gas to escape easily. This can lead to collapse and deformation during subsequent processing or use, resulting in reduced resilience and dimensional instability. While some technologies have attempted to introduce chemical cross-linking to enhance the structure, these often suffer from a mismatch between the foaming temperature and the cross-linking reaction temperature, leading to incomplete foaming or delayed cross-linking, making it impossible to simultaneously build the expanded structure and the cross-linked network.

[0007] Therefore, the industry urgently needs a new foamed synthetic leather preparation technology that can achieve low density, high rebound and high mechanical properties in an environmentally friendly water-based system. Summary of the Invention

[0008] The purpose of the present invention is to solve the shortcomings of the prior art and to propose a preparation process for secondary foaming water-based synthetic leather.

[0009] In order to achieve the above object, the present invention adopts the following technical solutions:

[0010] The present invention first proposes a preparation process of a secondary foaming water-based synthetic leather, comprising the following steps:

[0011] S1, Epoxy modified thermal expansion microspheres

[0012] Add heat-expandable microspheres and deionized water to the reactor and start stirring to form a suspension; add NaOH solution dropwise to adjust the pH to alkaline, raise the temperature to 70-80°C, and keep the reaction warm for 4-5 hours;

[0013] Under the alkaline condition of NaOH, the ester groups on the surface of the microspheres undergo hydrolysis reaction to generate sodium carboxylate salts, which introduce hydrophilic carboxylate ions on the surface of the microspheres, improve the dispersibility of the microspheres in water, and provide active sites for the next reaction;

[0014] After the reaction is completed, the mixture is cooled to room temperature, filtered, and washed with deionized water until neutral. The filter cake is redispersed in deionized water and the pH is adjusted to acidic.

[0015] Mix 1,4-butanediol diglycidyl ether and surfactant evenly, slowly add to the reactor, add catalyst after addition, and continue stirring at 60-70°C for 6-8 hours;

[0016] Under the action of a catalyst, one of the epoxy groups of 1,4-butanediol diglycidyl ether undergoes a ring-opening esterification reaction with the carboxyl group (-COOH) on the surface of the microspheres, thereby connecting another epoxy group to the surface of the microspheres through a flexible chain segment:

[0017]

[0018] After the reaction is completed, the microspheres are collected by centrifugation, ultrasonically cleaned with ethanol, and vacuum dried at a low temperature of <50°C to obtain epoxy-modified thermal expansion microspheres;

[0019] S2, side chain amino modified polyurethane

[0020] Add amino-terminated polyurethane aqueous dispersion into a reaction kettle, add Nα-Boc-lysine under low-speed stirring, adjust the pH of the system to weak alkalinity with triethylamine, add 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide, and react at room temperature for 12-24 hours. After the reaction is completed, a polyurethane dispersion with a Boc protective group on the side chain and an amino group at the end is obtained, i.e., a side-chain amino-modified polyurethane;

[0021] The carboxyl group of lysine connects with the terminal amino group on the amino-terminated polyurethane chain to form an amide bond. At this time, the terminal of the product is still an amino group and can continue to react, so that the polyurethane chain has a side chain amino group (Boc protection);

[0022]

[0023] S3. Preparation of foaming slurry

[0024] Slowly add the side chain amino modified polyurethane into acetone, cool in an ice water bath and vigorously stir, then slowly add triphenylmethane triisocyanate dropwise, strictly control the temperature to <10°C, and react for 2-3 hours;

[0025] After the reaction is completed, acetone is removed by distillation under reduced pressure, an appropriate amount of deionized water is added, and the solid content and viscosity of the system are adjusted to obtain a highly cross-linked modified polyurethane dispersion with Boc-protected amino groups on the side chains;

[0026] The highly cross-linked modified polyurethane dispersion is transferred to the batching kettle, and epoxy modified thermal expansion microspheres and wetting and leveling agents are added. The mixture is stirred and dispersed at a low speed, and the viscosity of the slurry is adjusted with a thickener to prepare the foaming slurry for later use.

[0027] S4. Synthetic leather preparation

[0028] The foaming slurry is stirred at high speed by a high-speed disperser to draw in a large amount of air. The slurry foams for the first time and expands to 2.5-3.0 times its original volume to form a stable foam slurry.

[0029] The foam slurry is scraped onto release paper and sent into a multi-stage programmable temperature-controlled oven for heating, curing and secondary foaming. The cured synthetic leather passes through an embossing roller at high temperature to press the desired pattern to obtain secondary foamed water-based synthetic leather.

[0030]

[0031] Preferably, in S1, the heat-expandable microspheres are acrylonitrile-methyl methacrylate copolymer heat-expandable microspheres, the foaming starting temperature of which is 110° C., the particle size of which is 20-40 μm, the content of which is an alkane foaming agent; the surfactant is a nonionic surfactant; and the catalyst is aminosulfonic acid;

[0032] The mass ratio of heat-expandable microspheres, 1,4-butanediol diglycidyl ether, surfactant and catalyst is 5:3-4:0.1:0.05-0.1;

[0033] Preferably, in S2, the solid content of the amino-terminated polyurethane aqueous dispersion is 40%;

[0034] The mass ratio of amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:3.5-4.5:2.8:0.6;

[0035] The preparation process of Nα-Boc-lysine includes the following steps:

[0036] Lysine hydrochloride was dissolved in deionized water by stirring, and then NaOH was slowly added to adjust the pH to 10.5. The NaCl precipitate was separated by centrifugation to obtain lysine free base.

[0037] Transfer lysine free base into a reactor, add tetrahydrofuran, and cool to 0°C; add a tetrahydrofuran solution of Boc2O dropwise, react at 0-5°C for 2h, then heat to 25°C and continue to react for 4h;

[0038] Concentrated hydrochloric acid was added dropwise to pH = 3.0, and the precipitated white precipitate was washed with ice water to obtain Nα-Boc-lysine.

[0039] Preferably, in the preparation process of the Na-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride and Boc2O is 2:1:1.

[0040] Preferably, in S3, the mass ratio of the side chain amino modified polyurethane, triphenylmethane triisocyanate, epoxy modified thermal expansion microspheres and wetting and leveling agent is 80:3-5:12-15:0.3-0.5;

[0041] The wetting and leveling agent is a non-ionic wetting and leveling agent; the thickener is a polyurethane thickener;

[0042] Triphenylmethane triisocyanate was added as a solution of triphenylmethane triisocyanate in acetone;

[0043] The solid content of highly cross-linked modified polyurethane dispersion is 35%;

[0044] The viscosity of the foaming slurry is 25000-35000 mPa·s.

[0045] Preferably, in S4, the foam slurry is coated on the release paper to a thickness of 1.0-1.5 mm;

[0046] The heating program is set as:

[0047] Keep warm at 80℃ for 4 minutes; the polyurethane melts and forms a film, the water partially evaporates, the slurry gels and takes shape, and the first foaming structure is fixed.

[0048] The mixture was kept at 120°C for 5 minutes. The Nα-Boc group was rapidly removed under the action of heat, generating free primary amino groups (-NH2) on the side chain. These active -NH2 groups immediately attacked the epoxy groups on the surface of the epoxy-modified heat-expandable microspheres, causing a ring-opening addition reaction to form covalent bonds and achieve chemical cross-linking. At the same time, the alkanes inside the heat-expandable microspheres were vaporized, the shell softened, and a second foaming occurred.

[0049] Keep at 150℃ for 3 min to ensure the cross-linking reaction is complete and evaporate the remaining water thoroughly.

[0050] The secondary foaming water-based synthetic leather prepared by the above-mentioned preparation process has a total expansion ratio of the foaming slurry of 3.5-6 times; the final product density ranges from 0.18-0.35g / cm 3 .

[0051] Compared with the prior art, the present invention has the following beneficial effects:

[0052] 1. Island-type synergistic reinforcement structure significantly improves mechanical properties

[0053] In traditional processes, thermally expandable microspheres, as functional fillers, are primarily physically blended with the polyurethane matrix. This results in weak interfacial bonding, which can easily become a stress concentration point when subjected to stress, leading to separation of the microspheres from the matrix and material damage. Furthermore, to reduce product density, the expansion ratio must be increased, but this often sacrifices the mechanical strength of the material.

[0054] This invention transforms the relationship between epoxy-modified microspheres and amino-modified polyurethane from physical blending to chemical bonding. The epoxy-modified microspheres act as a "rigid core," providing support and foaming power; the cross-linked polyurethane network acts as a "flexible shell," encapsulating and anchoring the microspheres, transferring and dispersing stress. Together, they form a sea-island-type synergistic reinforcement structure: a continuous, strong polyurethane cross-linked network (sea phase) is uniformly dispersed within a large number of stable, closed cells (island phase) formed by the expansion of the microspheres. The two are connected by strong CN covalent bonds, and the microspheres no longer serve as weak interfacial fillers, but rather as reinforcement points within the cross-linked network. Therefore, even at very high expansion ratios (low densities), the material maintains excellent mechanical properties, breaking the shackles of traditional foaming materials, which often hold that "lower density means lower strength."

[0055] 2. Secondary foaming structure of ultra-stable cells

[0056] Traditional processes using heat-expandable microspheres rely on the microspheres' physical expansion upon heating. This poorly stable cell structure can lead to slow gas escape or cell collapse during subsequent processing or use when subjected to heat and pressure, resulting in product deformation and reduced resilience. Some technologies incorporate chemical crosslinking, but the triggering temperature and timing of the crosslinking reaction often don't match the microsphere foaming temperature. This can result in crosslinking occurring first, inhibiting full expansion of the microspheres; or foaming occurring first, followed by crosslinking, but the expanded structure partially relaxes and fails before crosslinking.

[0057] Through molecular design, the present invention enables foaming triggering and cross-linking triggering to occur almost simultaneously within the same temperature range. At 120°C, the microspheres begin to soften and expand, and the epoxy groups on their surfaces are immediately attacked by the newly generated free primary amino groups in the surrounding area, causing a ring-opening addition reaction.

[0058] During the microsphere expansion process, the modified polyurethane simultaneously forms a strong, covalently bonded "anchoring network" for the microspheres. This "expansion, cross-linking, and curing" mechanism locks the microspheres in their expanded state, creating extremely strong cell walls that effectively resist gas escape and external pressure, thus completely resolving the fundamental structural instability of traditional physical foaming. The resulting product exhibits significantly improved resilience, durability, and shape retention.

[0059] 3. Environmentally friendly high-performance water-based system

[0060] High-performance synthetic leather, especially microfiber leather, uses a large amount of organic solvents such as DMF and MEK as solvents for polyurethane resins, which poses problems of environmental pollution and high occupational health and safety risks. Pure water-based polyurethane is often inferior to solvent-based polyurethane in terms of mechanical properties and hydrolysis resistance.

[0061] The present invention successfully makes up for the shortcomings of waterborne polyurethane in performance through the above-mentioned chemical crosslinking and island-in-sea reinforcement mechanism, and prepares a synthetic leather product that is both environmentally friendly and high-performance.

[0062] In summary, the present invention achieves process control through Boc protecting group chemistry and precisely synchronizes temperature-triggered deprotection with microsphere foaming. This allows for in-situ crosslinking of surface epoxy groups with newly generated free primary amino groups after microsphere expansion, forming a covalently anchored island-in-the-sea reinforcement structure. This mechanism addresses the core challenges of structural instability and weak interfacial bonding associated with physical foaming, ultimately achieving the superior properties of low density, high resilience, and high mechanical strength in an environmentally friendly water-based system. BRIEF DESCRIPTION OF THE DRAWINGS

[0063] Figure 1 This is the H NMR spectrum of Nα-Boc-lysine prepared in the present invention;

[0064] Figure 2This is the infrared spectrum of the highly cross-linked modified polyurethane dispersion prepared in the present invention. DETAILED DESCRIPTION

[0065] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, rather than all the embodiments.

[0066] The purity of the drugs used in the experiment and their manufacturers are shown in Table 1:

[0067] Table 1. Raw material drug information

[0068]

[0069] Example 1:

[0070] S1, Epoxy modified thermal expansion microspheres

[0071] Heat-expandable microspheres and deionized water were added to the reactor, and stirring was started to form a suspension. NaOH solution was added dropwise to adjust the pH to alkaline, and the temperature was raised to 75°C and kept for 4.5 hours.

[0072] After the reaction is completed, the mixture is cooled to room temperature, filtered, and washed with deionized water until neutral. The filter cake is redispersed in deionized water and the pH is adjusted to acidic.

[0073] Mix 1,4-butanediol diglycidyl ether and surfactant evenly and slowly add them to the reactor. After the addition is complete, add the catalyst and continue stirring at 65°C for 7 hours.

[0074] After the reaction, the microspheres were collected by centrifugation, ultrasonically cleaned with ethanol, and dried in a vacuum at 40°C to obtain epoxy-modified thermally expandable microspheres.

[0075] S2, side chain amino modified polyurethane

[0076] Add amino-terminated polyurethane aqueous dispersion into a reaction kettle, add Nα-Boc-lysine under low-speed stirring, adjust the pH of the system to weak alkalinity with triethylamine, add 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide, and react at room temperature for 12-24 hours. After the reaction is completed, a polyurethane dispersion with a Boc protective group on the side chain and an amino group at the end is obtained, i.e., a side-chain amino-modified polyurethane;

[0077] S3. Preparation of foaming slurry

[0078] Slowly add the side chain amino modified polyurethane into acetone, cool in an ice water bath and vigorously stir, then slowly add triphenylmethane triisocyanate dropwise, strictly control the temperature to <10°C, and react for 2.5h;

[0079] After the reaction is completed, acetone is removed by distillation under reduced pressure, an appropriate amount of deionized water is added, and the solid content and viscosity of the system are adjusted to obtain a highly cross-linked modified polyurethane dispersion with Boc-protected amino groups on the side chains;

[0080] The highly cross-linked modified polyurethane dispersion is transferred to the batching kettle, and epoxy modified thermal expansion microspheres and wetting and leveling agents are added. The mixture is stirred and dispersed at a low speed, and the viscosity of the slurry is adjusted with a thickener to prepare the foaming slurry for later use.

[0081] S4. Synthetic leather preparation

[0082] The foaming slurry is stirred at high speed by a high-speed disperser to draw in a large amount of air. The slurry foams for the first time and its volume expands to 2.82 times of its original volume, forming a stable foam slurry.

[0083] The foam slurry is scraped onto release paper and sent into a multi-stage programmable temperature-controlled oven for heating, curing and secondary foaming. The cured synthetic leather passes through an embossing roller at high temperature to press the desired pattern to obtain secondary foamed water-based synthetic leather.

[0084] In S1, the heat-expandable microspheres are acrylonitrile-methyl methacrylate copolymer heat-expandable microspheres; the surfactant is a non-ionic surfactant; and the catalyst is aminosulfonic acid;

[0085] The mass ratio of thermal expansion microspheres, 1,4-butanediol diglycidyl ether, surfactant and catalyst is 5:3:0.1:0.1;

[0086] In S2, the solid content of the amino-terminated polyurethane aqueous dispersion is 40%;

[0087] The mass ratio of amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:4.5:2.8:0.6;

[0088] The preparation process of Nα-Boc-lysine includes the following steps:

[0089] Lysine hydrochloride was dissolved in deionized water by stirring, and then NaOH was slowly added to adjust the pH to 10.5. The NaCl precipitate was separated by centrifugation to obtain lysine free base.

[0090] Transfer lysine free base into a reactor, add tetrahydrofuran, and cool to 0°C; add a tetrahydrofuran solution of Boc2O dropwise, react at 0-5°C for 2h, then heat to 25°C and continue to react for 4h;

[0091] Concentrated hydrochloric acid was added dropwise to pH = 3.0, and the precipitated white precipitate was washed with ice water to obtain Nα-Boc-lysine.

[0092] During the preparation of the Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride and Boc2O is 2:1:1.

[0093] In S3, the mass ratio of side chain amino modified polyurethane, triphenylmethane triisocyanate, epoxy modified thermal expansion microspheres and wetting and leveling agent is 80:3:15:0.3;

[0094] The wetting and leveling agent is a non-ionic wetting and leveling agent; the thickener is a polyurethane thickener;

[0095] Triphenylmethane triisocyanate was added as a solution of triphenylmethane triisocyanate in acetone;

[0096] The solid content of highly cross-linked modified polyurethane dispersion is 35%;

[0097] The viscosity of the foaming slurry is 25000 mPa·s.

[0098] In S4, the foam slurry is coated on the release paper with a thickness of 1.0 mm;

[0099] The heating program is set as:

[0100] Keep warm at 80℃ for 4 minutes; keep warm at 120℃ for 5 minutes; keep warm at 150℃ for 3 minutes.

[0101] The total expansion ratio of the foaming slurry is 4.1 times; the density of the finished product is in the range of 0.26g / cm 3 .

[0102] Example 2

[0103] The secondary foaming water-based synthetic leather was prepared according to the preparation method of Example 1, except that in S1, the heat-expandable microspheres were acrylonitrile-methyl methacrylate copolymer heat-expandable microspheres; the surfactant was a nonionic surfactant; and the catalyst was aminosulfonic acid.

[0104] The mass ratio of thermal expansion microspheres, 1,4-butanediol diglycidyl ether, surfactant and catalyst is 5:3.5:0.1:0.075;

[0105] The mass ratio of amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:4:2.8:0.6;

[0106] In S3, the mass ratio of side chain amino modified polyurethane, triphenylmethane triisocyanate, epoxy modified thermal expansion microspheres and wetting and leveling agent is 80:4:13.5:0.4;

[0107] The viscosity of the foaming slurry is 30000 mPa·s.

[0108] In S4, the foam slurry is coated on the release paper to a thickness of 1.25 mm;

[0109] The total expansion ratio of the foaming slurry is 4.7 times; the density of the finished product is in the range of 0.23g / cm 3 .

[0110] The epoxy content on the microspheres was determined by titration with hydrochloric acid-acetone method, which was 1.1 mmol / g;

[0111] The Nα-Boc-lysine after separation and purification was detected by nuclear magnetic hydrogen spectrum. The results are as follows: Figure 1 As shown; the highly cross-linked modified polyurethane dispersion after separation and purification was tested by infrared spectroscopy, and the results were as follows Figure 2 shown.

[0112] Example 3

[0113] The secondary foaming water-based synthetic leather was prepared according to the preparation method of Example 1, except that in S1, the heat-expandable microspheres were acrylonitrile-methyl methacrylate copolymer heat-expandable microspheres; the surfactant was a nonionic surfactant; and the catalyst was aminosulfonic acid.

[0114] The mass ratio of thermal expansion microspheres, 1,4-butanediol diglycidyl ether, surfactant and catalyst is 5:4:0.1:0.05;

[0115] The mass ratio of amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:3.5:2.8:0.6;

[0116] In S3, the mass ratio of side chain amino modified polyurethane, triphenylmethane triisocyanate, epoxy modified thermal expansion microspheres and wetting and leveling agent is 80:5:12:0.5;

[0117] The viscosity of the foaming slurry is 35000 mPa·s.

[0118] In S4, the foam slurry is coated on the release paper with a thickness of 1.5 mm;

[0119] The total expansion ratio of the foaming slurry is 4.3 times; the density of the finished product is in the range of 0.24g / cm 3 .

[0120] Also designed accordingly:

[0121] Comparative Example 1: The formulation and experimental method are the same as those of Example 2, but epoxy-modified heat-expandable microspheres are not prepared and heat-expandable microspheres are used directly.

[0122] Comparative Example 2: The formulation and experimental method are the same as those of Example 2, but ordinary lysine is used instead of Nα-Boc-lysine.

[0123] Comparative Example 3: The formula and experimental method are the same as those of Example 2, but in S2, the mass ratio of the amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:1:2.8:0.6.

[0124] Comparative Example 4: The formula and experimental method are the same as those of Example 2, but in S2, the mass ratio of the amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:6:2.8:0.6.

[0125] Comparative Example 5: In S3, the mass ratio of the side chain amino-modified polyurethane, triphenylmethane triisocyanate, epoxy-modified thermal expansion microspheres and the wetting and leveling agent is 80:1:13.5:0.4.

[0126] Comparative Example 6: In S3, the mass ratio of the side chain amino-modified polyurethane, triphenylmethane triisocyanate, epoxy-modified thermal expansion microspheres and the wetting and leveling agent is 80:8:13.5:0.4.

[0127] Comparative Example 7: In S3, the mass ratio of the side chain amino-modified polyurethane, triphenylmethane triisocyanate, epoxy-modified thermal expansion microspheres and the wetting and leveling agent is 80:4:5:0.4.

[0128] Comparative Example 8: In S3, the mass ratio of the side chain amino-modified polyurethane, triphenylmethane triisocyanate, epoxy-modified thermal expansion microspheres and the wetting and leveling agent is 80:4:25:0.4.

[0129] According to the standards and test methods of ISO 3376:2020 "Leather - Determination of tensile strength and elongation", ISO 17074:2019 "Leather - Determination of abrasion resistance", ISO 2419:2012 "Leather - Physical test - Determination of hydrolysis resistance", GB / T 14522-2008 "Artificial weathering test methods for plastics, coatings and rubber materials for mechanical industrial products", and GB 33372-2020 "Limits of volatile organic compounds in adhesives", the present invention is tested for mechanical properties, wear resistance, VOC emissions, foaming ratio, and density;

[0130] The corresponding results and data are summarized and plotted in Table 2:

[0131] Table 2. Performance test data of foamed water-based synthetic leather

[0132]

[0133] Data analysis shows that:

[0134] By comparing Example 2 with Comparative Example 1, it can be seen that the secondary foaming structure of the present invention is formed by the thermal expansion of epoxy-modified heat-expandable microspheres. This structure is conducive to the formation of lightweight, soft-touch products and affects their elongation at break. The surface of unmodified heat-expandable microspheres is inert, lacking chemical bonding with the polyurethane matrix and only having weak physical adsorption. When subjected to stress, the interface between the microspheres and the matrix is ​​prone to debonding, forming stress defect points, resulting in a sharp decrease in tensile strength and wear resistance. At the same time, the weak interfacial bonding force cannot effectively restrain the expansion of the microspheres, resulting in insufficient foaming, uneven cell structure, and poor stability, resulting in a higher density.

[0135] By comparing Example 2 with Comparative Example 2, it can be seen that the Boc group protects the side chain amino group of lysine, ensuring that the modification reaction occurs only at the terminal amino group of the polyurethane. The resulting product is a polyurethane with a well-defined structure, with protected amino groups on the side chains and a reactive amino group at the end.

[0136] During heat curing, the Boc protecting group decomposes under heat, releasing active side chain amino groups. The newly formed side chain amino groups quickly covalently bond with the epoxy groups on the surface of the microspheres. The bonding of the microspheres and the final cross-linking curing proceed simultaneously to form a uniform and strong overall network.

[0137] In contrast, in Comparative Example 2, ordinary lysine is used, and the side chain amino group of ordinary lysine is exposed and highly reactive. It will compete with the terminal amino group of the polyurethane for the reaction with the activator, generating an uncontrollable and chaotic modified product.

[0138] In step S3, when the isocyanate crosslinker is added at low temperature, all active amino groups in the system react with the crosslinker simultaneously. This results in a violent reaction with concentrated heat release, which can easily lead to localized over-crosslinking or even gelation, while other areas may be under-crosslinked. The entire reaction process is uncontrollable.

[0139] Because the crosslinking reaction has already occurred chaotically in the S3 stage, the system viscosity may increase abnormally, which is not conducive to the uniform dispersion of the epoxy microspheres. At the same time, the number of free amino groups available for reaction with the epoxy groups of the microspheres in the S4 step is also reduced and unevenly distributed, resulting in insufficient and uneven adhesion between the microspheres and the substrate, forming a crosslinked network structure with numerous defects. As a result, the mechanical properties, foaming efficiency, and environmental friendliness of the prepared product lag behind those of Example 2.

[0140] By comparing Example 2, Comparative Example 3, and Comparative Example 4, it can be seen that the side chain amino group of lysine is a potential cross-linking point introduced into the main chain. The amount of Nα-Boc-lysine determines the density of cross-linking points in the final polyurethane network. The amount of Nα-Boc-lysine used in Comparative Example 3 is too small. During the final curing, it can combine with the microspheres, and the number of covalent bonds generated is limited. Therefore, there is a lack of sufficient binding force between the molecular chains. Under the action of external force, the molecular chains are prone to relative slippage, causing the material to be easily broken or worn away. At the same time, due to the strong mobility of the molecular chains, the elongation at break is the highest. Due to the incomplete cross-linking network, many small molecular monomers and additives that do not participate in the reaction are loosely wrapped in it, making it easier to migrate and volatilize, resulting in a high VOC.

[0141] In Comparative Example 4, excessive amounts of Nα-Boc-lysine were used, introducing too many potential cross-linking points. During curing, an overly dense cross-linked network was formed. Although the tensile strength was high, the molecular chains were tightly locked, and the mobility was severely restricted. Excessive cross-linking also made the material hard and brittle, significantly reducing the elongation at break. When impacted, it may directly break rather than deform by stretching. At the same time, an overly rigid network would restrict the expansion of the microspheres, resulting in a decrease in the foaming multiple and an increase in density. Excessive lysine may also bring about steric hindrance, affecting the overall reaction efficiency. Excessive local cross-linking caused some small molecules to be encapsulated in the hardened network, which was slowly released later, resulting in a slight increase in VOC.

[0142] Comparing Examples 2, 5, and 6, we can see that the amount of triphenylmethane triisocyanate (TTI) used as a crosslinker also significantly impacts the properties of the final product, driven by the concept of crosslinking degree. Specifically, the three -NCO groups on TTI react with the -NH2 functional groups on the polyurethane chains to form strong urea bonds between the molecular chains, thereby building a three-dimensional network. The amount of TTI used directly determines the density of the primary crosslinked network.

[0143] In Comparative Example 5, the amount of crosslinking agent used was too small, resulting in a serious lack of crosslinking points. The molecular chains were mainly bound by weak forces such as physical entanglement and hydrogen bonds, and an effective three-dimensional network could not be formed. Furthermore, due to the lack of sufficient chemical crosslinking points to resist external forces, the material was soft and weak, the molecular chains easily slipped, and the material had low strength and poor wear resistance. The molecular chains moved almost unimpeded, allowing for high stretchability and high elongation. A large amount of unreacted isocyanate monomers and other small molecules were loosely wrapped in the fragile network, which was highly volatile and had high VOCs. The weak network could not effectively restrain the expansion of the microspheres, resulting in easy rupture of the cell walls, insufficient foaming, low foaming efficiency, and high material density.

[0144] The cross-linking agent in Comparative Example 6 is seriously excessive. The excessive -NCO groups react with water, urea, etc. in the system to produce a large amount of gas, which provides additional chemical foaming for the system and a high foaming multiple. The excessive -NCO groups also bring a large number of rigid side chains, resulting in an excessively high cross-linking density, extremely high network rigidity, and complete inhibition of segment movement. Although the tensile strength reaches a peak, the elongation at break plummets, which means that the material has almost lost its elasticity and is very brittle. It is very easy to break when impacted. The high hardness brings resistance to abrasives, but due to its high brittleness, it may wear in the form of "chipping" rather than "wear" of tough materials.

[0145] Comparison of Example 2, Comparative Example 7, and Comparative Example 8 demonstrates that the thermally expandable microspheres play a dual role as a foaming agent and organic filler in this composite material. Their dosage determines the final product's cell density and the integrity of the continuous matrix phase. When the microspheres are added in the appropriate amount, a "sea-island structure" forms after curing: a continuous, strong, cross-linked polyurethane network (sea phase) is uniformly dispersed with a large number of stable, closed cells (island phase) formed by the expanded microspheres. The polyurethane continuous phase provides excellent mechanical strength and toughness, while the uniform pores significantly reduce density, providing a soft feel and excellent thermal and sound insulation properties. The strong interfacial bonding ensures effective force transfer from the matrix to the microspheres, achieving a composite reinforcement effect.

[0146] In Comparative Example 7, the amount of microspheres used is too small, and the foaming points are insufficient, so a connected, high-proportion pore structure cannot be formed. Therefore, the density is the highest and the foaming multiple is the lowest. Due to the high content of matrix resin and the dense material, the tensile strength performance is good, but this is at the expense of the light and soft characteristics. It lacks the soft touch and compression resilience that synthetic leather should have. At the same time, the dense matrix has a higher internal temperature during curing, which encapsulates more non-volatile small molecules, and the VOC is relatively high.

[0147] In Comparative Example 8, excessive microsphere dosage severely diluted the polymer matrix, resulting in insufficient polyurethane to completely encapsulate and isolate each microsphere. This weakened the cell walls, making the microspheres more susceptible to agglomeration and forming stress concentration points. The large total interfacial area became a weak link in the material, preventing effective stress transmission and dispersion. This resulted in a decrease in both tensile strength and elongation at break.

[0148] In summary, Example 2 precisely controls the amounts of Na-Boc-lysine and crosslinker to construct a strong and resilient three-dimensional network structure. Furthermore, epoxy-modified microspheres are introduced at an optimal ratio, achieving a strong chemical bond between the matrix and the filler. This formulation allows the material to achieve a perfect balance of strength, elasticity, and low density during curing and foaming, resulting in an optimal combination of high expansion ratio, excellent mechanical properties, and low VOC emissions. The comparative examples failed to replicate this balanced effect due to uncontrolled crosslinking or interfacial bonding failures.

[0149] The above description is only a preferred specific embodiment of the present invention, but the scope of protection of the present invention is not limited thereto. Any technician familiar with the technical field, within the technical scope disclosed by the present invention, who makes equivalent replacements or changes based on the technical solution and inventive concept of the present invention, should be covered by the scope of protection of the present invention.

Claims

1. A process for preparing a secondary foaming water-based synthetic leather, characterized in that: The following steps are involved: S1, Epoxy modified thermal expansion microspheres Add acrylonitrile-methyl methacrylate copolymer thermal expansion microspheres and deionized water into a reactor, start stirring to form a suspension; add NaOH solution dropwise to adjust the pH to alkaline, raise the temperature to 70-80°C, and keep the reaction warm for 4-5 hours; After the reaction is completed, the mixture is cooled to room temperature, filtered, and washed with deionized water until neutral. The filter cake is redispersed in deionized water and the pH is adjusted to acidic. Mix 1,4-butanediol diglycidyl ether and surfactant evenly, slowly add to the reactor, add catalyst after addition, and continue stirring at 60-70°C for 6-8 hours; After the reaction is completed, the microspheres are collected by centrifugation, ultrasonically cleaned with ethanol, and vacuum dried at a low temperature of <50°C to obtain epoxy-modified thermal expansion microspheres; S2, side chain amino modified polyurethane Add amino-terminated polyurethane aqueous dispersion into a reaction kettle, add Nα-Boc-lysine under low-speed stirring, adjust the pH of the system to weak alkalinity with triethylamine, add 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide, and react at room temperature for 12-24 hours. After the reaction is completed, a polyurethane dispersion with a Boc protective group on the side chain and an amino group at the end is obtained, i.e., a side-chain amino-modified polyurethane; S3. Preparation of foaming slurry Slowly add the side chain amino modified polyurethane into acetone, cool in an ice water bath and vigorously stir, then slowly add triphenylmethane triisocyanate dropwise, strictly control the temperature to <10°C, and react for 2-3 hours; After the reaction is completed, acetone is removed by distillation under reduced pressure, an appropriate amount of deionized water is added, and the solid content and viscosity of the system are adjusted to obtain a highly cross-linked modified polyurethane dispersion with Boc-protected amino groups on the side chains; The highly cross-linked modified polyurethane dispersion is transferred to the batching kettle, and epoxy modified thermal expansion microspheres and wetting and leveling agents are added. The mixture is stirred and dispersed at a low speed, and the viscosity of the slurry is adjusted with a thickener to prepare the foaming slurry for later use. S4. Synthetic leather preparation The foaming slurry is stirred at high speed by a high-speed disperser to draw in a large amount of air. The slurry foams for the first time and expands to 2.5-3.0 times its original volume to form a stable foam slurry. The foam slurry is scraped onto release paper and sent into a multi-stage programmable temperature-controlled oven for heating, curing and secondary foaming. The cured synthetic leather passes through an embossing roller at high temperature to press the desired pattern to obtain secondary foamed water-based synthetic leather.

2. The process for preparing a secondary foaming water-based synthetic leather according to claim 1, wherein: In S1, the foaming starting temperature of the heat-expandable microspheres is 110° C., the particle size is 20-40 μm, the content is an alkane foaming agent; the surfactant is a non-ionic surfactant; and the catalyst is aminosulfonic acid; The mass ratio of the heat-expandable microspheres, 1,4-butanediol diglycidyl ether, the surfactant and the catalyst is 5:3-4:0.1:0.05-0.

1.

3. The process for preparing a secondary foaming water-based synthetic leather according to claim 1, wherein: In S2, the solid content of the amino-terminated polyurethane aqueous dispersion is 40%; The mass ratio of amino-terminated polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide is 75:3.5-4.5:2.8:0.6; The preparation process of Nα-Boc-lysine includes the following steps: Lysine hydrochloride was dissolved in deionized water by stirring, and then NaOH was slowly added to adjust the pH to 10.

5. The NaCl precipitate was separated by centrifugation to obtain lysine free base. Transfer lysine free base into a reactor, add tetrahydrofuran, and cool to 0°C; add a tetrahydrofuran solution of Boc2O dropwise, react at 0-5°C for 2h, then heat to 25°C and continue to react for 4h; Concentrated hydrochloric acid was added dropwise to pH = 3.0, and the precipitated white precipitate was washed with ice water to obtain Nα-Boc-lysine.

4. The process for preparing a secondary foaming water-based synthetic leather according to claim 3, wherein: During the preparation of the Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride and Boc2O is 2:1:

1.

5. The process for preparing a secondary foaming water-based synthetic leather according to claim 1, wherein: In S3, the mass ratio of side chain amino modified polyurethane, triphenylmethane triisocyanate, epoxy modified thermal expansion microspheres and wetting and leveling agent is 80:3-5:12-15:0.3-0.5; The wetting and leveling agent is a non-ionic wetting and leveling agent; the thickener is a polyurethane thickener; Triphenylmethane triisocyanate was added as a solution of triphenylmethane triisocyanate in acetone; The solid content of highly cross-linked modified polyurethane dispersion is 35%; The viscosity of the foaming slurry is 25000-35000 mPa·s.

6. The process for preparing a secondary foaming water-based synthetic leather according to claim 1, characterized in that: In S4, the foam slurry is coated on the release paper with a thickness of 1.0-1.5 mm; The heating program is set as: Keep warm at 80℃ for 4 minutes; keep warm at 120℃ for 5 minutes; keep warm at 150℃ for 3 minutes.

7. The secondary foaming water-based synthetic leather prepared by the preparation process according to claim 1, characterized in that: The total expansion ratio of the foaming slurry is 3.5-6 times; the final product density range is 0.18-0.35g / cm 3 .

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