Preparation process of secondary foaming type water-based synthetic leather

Abstract

The application discloses a preparation process of a secondary foaming type water-based synthetic leather and relates to the technical field of high polymer foaming materials.The preparation process comprises the following steps: 1) epoxy modification of thermal expansion microspheres;2) side chain amino modification of polyurethane;3) preparation of foaming slurry;4) preparation of synthetic leather;firstly, the thermal expansion microspheres are subjected to epoxidation, the water-based polyurethane is grafted with side chain amino groups protected by tert-butyloxycarbonyl, and the crosslinking degree is improved by using triphenylmethane triisocyanate, and then the thermal expansion microspheres are mixed with the epoxy modified thermal expansion microspheres;after the first foaming by mechanical means, the tert-butyloxycarbonyl is dissociated to generate CO2 gas under heat in the step 4), active amino groups are released, crosslinked with the epoxy of the thermal expansion microspheres, and the thermal expansion microspheres are expanded at the same time, so that chemical foaming, namely the second foaming, is realized.The mechanical and chemical secondary foaming process is used in cooperation, the pain points of the traditional water-based leather, such as hard hand feeling, easy collapse of foaming cells and single performance, are solved, and the comprehensive performance is obviously improved compared with the prior art.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of high polymer materials, and particularly relates to a preparation process of a secondary foaming type water-based synthetic leather. BACKGROUND

[0002] As an important material widely used in shoe materials, luggage, furniture and automotive interiors, etc., the performance and environmental protection of synthetic leather are increasingly concerned. Traditional synthetic leather mostly uses solvent-based polyurethane systems, using organic solvents such as dimethylformamide (DMF) and methyl ethyl ketone (MEK). Not only does it have high volatile organic compound (VOCs) emissions, environmental pollution and safety hazards in the production process, but also poses a potential threat to human health. With the increasingly stringent environmental regulations and the increasing demand for green products from consumers, the development of high-performance water-based synthetic leather has become an important development direction for the industry.

[0003] Water-based polyurethane uses water as the dispersion medium, has the advantages of low VOCs, non-toxicity and non-flammability, but its mechanical properties, water resistance and structural stability are usually inferior to those of solvent-based systems, especially in foaming synthetic leather. Foaming process is one of the key technologies to achieve lightweight, high elasticity and soft feel of synthetic leather. Currently, physical foaming (such as mechanical foaming) and chemical foaming (such as thermal expansion microspheres) are commonly used foaming methods. Among them, thermal expansion microspheres are favored due to their easy-to-control foaming process and uniform pore structure.

[0004] However, the foaming process using thermal expansion microspheres in the prior art still has significant defects:

[0005] Weak interfacial bonding and insufficient mechanical properties; thermal expansion microspheres, as physical fillers, lack chemical bonding between the polyurethane matrix and have poor interfacial compatibility, which easily become stress concentration points, leading to interfacial peeling of the material under stress, seriously affecting the mechanical strength of the finished product. This problem is particularly prominent under high foaming ratio (low density) conditions, often leading to the inability to balance low density and high strength.

[0006] Poor pore structure stability; traditional thermal expansion microspheres rely on physical processes for foaming, and the pore wall strength formed after foaming is low, and gas easily escapes, leading to collapse and deformation during subsequent processing or use, resulting in decreased resilience and unstable size. Although some technologies attempt to introduce chemical cross-linking to enhance the structure, the foaming temperature and cross-linking reaction temperature often do not match, leading to insufficient foaming or cross-linking lag, and the simultaneous construction of the expanded structure and cross-linked network cannot be achieved.

[0007] Therefore, there is an urgent need in the industry for a new foaming synthetic leather preparation technology that can achieve low density, high resilience and high mechanical properties in an environmentally friendly water-based system. SUMMARY

[0008] The application aims at solving the defects in the prior art and provides a preparation process of a secondary foaming type water-based synthetic leather.

[0009] In order to achieve the above-mentioned purpose, the application adopts the following technical scheme:

[0010] The application first provides a preparation process of a secondary foaming type water-based synthetic leather, which comprises the following steps:

[0011] S1, epoxy modified thermal expansion microspheres

[0012] In a reaction kettle, thermal expansion microspheres and deionized water are added, and stirring is started to form a suspension; NaOH solution is added dropwise to adjust the pH to be alkaline, and the temperature is raised to 70-80 DEG C, and the reaction is kept for 4-5 h;

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

[0014] After the reaction is completed, the temperature is cooled to room temperature, and the filter cake is washed with deionized water until it is neutral, and then the filter cake is dispersed in deionized water again, and the pH is adjusted to be acidic;

[0015] The 1,4-butanediol diglycidyl ether is mixed with the surfactant uniformly, and then slowly added into the reaction kettle, and after the addition is completed, the catalyst is added, and the stirring reaction is continuously carried out at 60-70 DEG C for 6-8 h;

[0016] Under the action of the catalyst, one epoxy group of the 1,4-butanediol diglycidyl ether undergoes ring-opening esterification reaction with the carboxyl group (-COOH) on the surface of the microspheres, so that the other epoxy group is connected 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 DEG C to obtain the epoxy modified thermal expansion microspheres;

[0019] S2, side chain amino modified polyurethane

[0020] In a reaction kettle, an amino-terminated polyurethane water dispersion is added, and under low-speed stirring, N alpha-Boc-lysine is added, and the pH of the system is adjusted to be weakly alkaline by using triethylamine, and 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride and N-hydroxysuccinimide are added, and the reaction is carried out at room temperature for 12-24 h, and after the reaction is completed, a polyurethane dispersion with Boc protection group in the side chain and amino group at the end, i.e., side chain amino modified polyurethane, is obtained;

[0021] The carboxyl group of lysine is connected with the terminal amino group on the polyurethane chain to form an amide bond, and the terminal of the product is still amino which can continue to react to make the polyurethane chain have side chain amino (Boc protection) ;

[0022]

[0023] S3, preparation of foaming slurry

[0024] The side chain amino modified polyurethane is slowly added into acetone, and triphenylmethane triisocyanate is slowly added dropwise under ice water bath cooling and strong stirring, and the temperature is strictly controlled to be less than 10℃, and the reaction is carried out for 2-3h;

[0025] After the reaction is completed, acetone is removed by distillation under reduced pressure, and a proper amount of deionized water is added to adjust the solid content and viscosity of the system, so that a high cross-linking modified polyurethane dispersion with Boc-protected amino side chain is obtained;

[0026] The high cross-linking modified polyurethane dispersion is transferred into a batching kettle, and epoxy modified thermal expansion microspheres and wetting leveling agent are added and dispersed at low speed, and the viscosity of the slurry is adjusted with a thickening agent as a foaming slurry for standby;

[0027] S4, preparation of synthetic leather

[0028] The foaming slurry is stirred at high speed by a high-speed dispersing machine, a large amount of air is rolled in, the slurry is foamed for the first time, the volume is expanded to 2.5-3.0 times of the original volume, and a stable foam slurry is formed;

[0029] The foam slurry is scraped on a release paper and sent into a multi-stage program temperature oven for heating, curing and secondary foaming, and the cured synthetic leather is pressed by an embossing roller at high temperature to obtain a secondary foaming type water-based synthetic leather.

[0030]

[0031] Preferably, in S1, the thermal expansion microspheres are acrylonitrile-methyl methacrylate copolymer thermal expansion microspheres, the foaming starting temperature is 110℃, the particle size is 20-40μm, and the content is an alkane foaming agent; the surfactant is a nonionic surfactant; and the catalyst is sulfamic acid;

[0032] The mass ratio of the thermal expansion 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 terminal amino polyurethane water dispersion is 40%;

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

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

[0036] After the lysine hydrochloride is dissolved by stirring with deionized water, NaOH is slowly added to adjust the pH to 10.5, and NaCl precipitate is separated by centrifugation to obtain lysine free base;

[0037] The lysine free base is transferred into a reaction kettle, tetrahydrofuran is added, and the temperature is lowered to 0℃; a tetrahydrofuran solution of Boc2O is added dropwise, and the reaction is carried out at 0-5℃ for 2h, and then the temperature is raised to 25℃ for continuous reaction for 4h;

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

[0039] Preferably, in the preparation process of the Nα-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 leveling agent is 80:3-5:12-15:0.3-0.5.

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

[0042] The triphenylmethane triisocyanate is added in the form of a triphenylmethane triisocyanate acetone solution.

[0043] The solid content of the high-crosslinking modified polyurethane dispersion is 35%.

[0044] The viscosity of the foaming slurry is 25,000-35,000 mPa·s.

[0045] Preferably, in S4, the thickness of the foaming slurry scraped on the release paper is 1.0-1.5mm.

[0046] The heating program is set as follows:

[0047] The temperature is kept at 80℃ for 4min; the polyurethane is melted into a film, the water part is evaporated, the slurry is gelled and shaped, and the fixation of the first foaming structure is completed.

[0048] 120℃, 5min; Nα-Boc group is removed quickly under the action of heat, generating free primary amino group (-NH2) of side chain, which immediately attacks the epoxy group on the surface of the epoxy modified thermal expansion microspheres to form a covalent bond, realizing chemical crosslinking; at the same time, the alkane inside the thermal expansion microspheres is gasified, the shell is softened and the second foaming occurs.

[0049] 150℃, 3min; ensure that the crosslinking reaction is complete, and the remaining water is completely evaporated.

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

[0051] Compared with the prior art, the beneficial effects of the present application are:

[0052] 1. Island-type synergistic reinforcing structure, significantly improving mechanical properties

[0053] In the traditional process, thermal expansion microspheres are used as functional fillers and are mainly physically blended with the polyurethane matrix, and the interfacial bonding force is weak. When stressed, the interface is prone to become a stress concentration point, leading to peeling of the microspheres and the matrix, and damage to the material. At the same time, in order to reduce the density of the product, it is necessary to increase the foaming ratio, but this usually sacrifices the mechanical strength of the material.

[0054] The present application changes the relationship between the epoxy modified microspheres and the amino modified polyurethane from physical blending to chemical bonding. The epoxy modified microspheres provide support and foaming power as "rigid cores"; the crosslinked polyurethane network serves as a "flexible shell" to wrap and anchor the microspheres, transfer and disperse stress, and form an island-type synergistic reinforcing structure: a large number of stable closed cells (island phase) formed by the expansion of microspheres are uniformly dispersed in the continuous and tough polyurethane crosslinked network (sea phase). The two are connected by strong C-N covalent bonds, and the microspheres are no longer weak interfacial fillers, but become reinforcing points in the crosslinked network. Therefore, even at a very high foaming ratio (low density), the material can still maintain excellent mechanical properties, breaking the shackles of traditional foaming materials "the lower the density, the worse the strength".

[0055] 2. Secondary foaming structure of super stable cells

[0056] Traditional process using thermal expansion microspheres relies on the physical expansion of microspheres under heat, and the stability of such cell structure is poor. When subjected to heat or pressure in subsequent processing or use, the gas may slowly escape or the cell may collapse, resulting in product deformation and reduced resilience. Some technologies introduce chemical crosslinking, but the triggering temperature and time of crosslinking reaction often do not match the microsphere foaming temperature. This may result in crosslinking first, inhibiting the full expansion of the microspheres; or foaming first and crosslinking later, but the expanded structure has been partially relaxed and failed before crosslinking.

[0057] The present application triggers foaming and crosslinking almost simultaneously in the same temperature range through molecular design. At 120℃, the microspheres begin to soften and expand, and the epoxy groups on their surface are immediately attacked by the newly generated free primary amino groups, resulting in ring-opening addition reaction.

[0058] During the expansion of the microspheres, the modified polyurethane simultaneously forms a strong, covalently bonded "anchoring network". This "expanding, crosslinking, and solidifying simultaneously" mechanism can lock the expanded state of the microspheres, resulting in extremely high cell wall strength that can effectively resist gas escape and external pressure, thus completely solving the fundamental problem of instability of traditional physical foaming structure. The resilience, durability, and shape retention of the final product are qualitatively improved.

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

[0060] High-performance synthetic leather, especially microfiber leather, uses a large amount of DMF, MEK, and other organic solvents as solvents for polyurethane resins, which poses environmental pollution and high occupational health and safety risks. Pure water-based polyurethane often does not perform as well as solvent-based polyurethane in terms of mechanical properties and hydrolysis resistance.

[0061] The present application successfully addresses the performance shortcomings of water-based polyurethane through the above-mentioned chemical crosslinking and island-in-sea reinforcement mechanism, resulting in a synthetic leather product that combines environmental friendliness and high performance.

[0062] In summary, the present application achieves process controllability through Boc protecting group chemistry and precise synchronization of temperature-triggered deprotection and microsphere foaming. After expansion, the surface epoxy groups of the microspheres undergo in-situ crosslinking with the newly generated free primary amino groups, forming a covalently anchored island-in-sea reinforcement structure. This mechanism addresses the core problems of unstable physical foaming structure and weak interfacial bonding, and ultimately achieves excellent performance in an environmentally friendly water-based system, including low density, high resilience, and high mechanical strength. BRIEF DESCRIPTION OF DRAWINGS

[0063] Figure 1 NMR spectrum of Nα-Boc-lysine prepared by the present application;

[0064] Figure 2The infrared spectrum of the high crosslinking modified polyurethane dispersion prepared in the present application. DETAILED DESCRIPTION

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

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

[0067] Table 1. Raw drug information

[0068]

[0069] Example 1:

[0070] S1, epoxy modified thermal expansion microspheres

[0071] Thermal expansion microspheres and deionized water were added to a reaction kettle, stirring was started to form a suspension; NaOH solution was added dropwise to adjust the pH to alkaline, the temperature was raised to 75℃, and the reaction was kept for 4.5h;

[0072] After the reaction was completed, it was cooled to room temperature, filtered and washed with deionized water until neutral, and the filter cake was redispersed in deionized water, and the pH was adjusted to acidic;

[0073] 1,4-butanediol diglycidyl ether was mixed with a surfactant and slowly added to the reaction kettle, after the addition was completed, a catalyst was added, and the stirring reaction was continued at 65℃ for 7h;

[0074] After the reaction was completed, the microspheres were collected by centrifugation, ultrasonically cleaned with ethanol, and vacuum dried at 40℃ to obtain epoxy modified thermal expansion microspheres;

[0075] S2, side chain amino modified polyurethane

[0076] An amino-terminated polyurethane aqueous dispersion was added to a reaction kettle, under low speed stirring, Nα-Boc-lysine was added, the pH of the system was adjusted to weak alkaline with triethylamine, 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride and N-hydroxysuccinimide were added, and the reaction was carried out at room temperature for 12-24h, after the reaction was completed, a polyurethane dispersion with Boc protecting group in the side chain and amino group at the end was obtained, i.e., side chain amino modified polyurethane;

[0077] S3, preparation of foaming slurry

[0078] The side chain amino modified polyurethane was slowly added to acetone, under ice water bath cooling and strong stirring, triphenylmethane triisocyanate was slowly added dropwise, the temperature was strictly controlled to be <10℃, and the reaction was carried out for 2.5h;

[0079] After the reaction is completed, the 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 high crosslinking modified polyurethane dispersion with a side chain Boc protected amino group;

[0080] The high crosslinking modified polyurethane dispersion is transferred into a batching kettle, epoxy modified thermal expansion microspheres and wetting leveling agents are added, and low speed stirring and dispersion are performed, and a thickening agent is used to adjust the slurry viscosity as a foaming slurry for standby;

[0081] S4, synthetic leather preparation

[0082] The foaming slurry is stirred at high speed with a high-speed dispersing machine, a large amount of air is entrained, the slurry is foamed for the first time, the volume is expanded to 2.82 times the original volume, and a stable foam slurry is formed;

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

[0084] In the S1, the thermal expansion microspheres are acrylonitrile-methyl methacrylate copolymer thermal expansion microspheres; the surfactant is a non-ionic surfactant; and the catalyst is sulfamic acid;

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

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

[0087] The mass ratio of the amino-terminated polyurethane water dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide 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] After the lysine hydrochloride is dissolved by stirring with deionized water, NaOH is slowly added to adjust the pH to 10.5, and NaCl precipitate is centrifuged and separated to obtain lysine free base;

[0090] The lysine free base is transferred into a reaction kettle, tetrahydrofuran is added, and the temperature is lowered to 0℃; a tetrahydrofuran solution of Boc2O is added dropwise, and the reaction is carried out at 0-5℃ for 2h, and then the temperature is raised to 25℃ for continuous reaction for 4h;

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

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

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

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

[0095] The triphenylmethane triisocyanate is added in the form of triphenylmethane triisocyanate acetone solution;

[0096] The solid content of the high crosslinking modified polyurethane dispersion is 35%;

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

[0098] In the S4, the thickness of the foaming slurry scraped on the release paper is 1.0 mm;

[0099] The heating program is set as:

[0100] 80℃ for 4 min; 120℃ for 5 min; 150℃ for 3 min.

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

[0102] Example 2

[0103] The secondary foaming type water-based synthetic leather is prepared according to the preparation method of Example 1, but in the S1, the thermal expansion microspheres are acrylonitrile-methyl methacrylate copolymer thermal expansion microspheres; the surfactant is a non-ionic surfactant; and the catalyst is sulfamic acid;

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

[0105] The mass ratio of the terminal amino polyurethane water dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride and N-hydroxysuccinimide is 75:4:2.8:0.6;

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

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

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

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

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

[0111] The Nα-Boc-lysine after separation and purification is detected by nuclear magnetic hydrogen spectrum, and the results are shown in Figure 1 The high crosslinking modified polyurethane dispersion after separation and purification is detected by infrared spectrum, and the results are shown in Figure 2

[0112] Example 3

[0113] The secondary foaming type water-based synthetic leather is prepared according to the preparation method of Example 1, but in the S1, the heat-expandable microspheres are acrylonitrile-methyl methacrylate copolymer heat-expandable microspheres; the surfactant is a nonionic surfactant; and the catalyst is sulfamic acid;

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

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

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

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

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

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

[0120] Therefore, the following are also designed:

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

[0122] ​Comparative Example 2: The same as the formulation and experimental method of Example 2, but no Nα-Boc-lysine was used, and ordinary lysine was used instead.

[0123] Comparative Example 3: The same as the formulation and experimental method of Example 2, but in S2, the mass ratio of the terminal amino polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride and N-hydroxysuccinimide was 75:1:2.8:0.6.

[0124] Comparative Example 4: The same as the formulation and experimental method of Example 2, but in S2, the mass ratio of the terminal amino polyurethane aqueous dispersion, Nα-Boc-lysine, 1-(3-dimethylaminopropyl)-3-ethyl carbodiimide hydrochloride and N-hydroxysuccinimide was 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 wetting leveling agent was 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 wetting leveling agent was 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 wetting leveling agent was 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 wetting leveling agent was 80:4:25:0.4.

[0129] The mechanical properties, wear resistance, VOC emission, foaming ratio and density of the present application were detected according to the standards and detection methods of ISO 3376:2020 "Leather-Determination of tensile strength and elongation", ISO 17074:2019 "Leather-Determination of abrasion resistance", ISO 2419:2012 "Leather-Determination of resistance to hydrolysis in physical tests", GB / T 14522-2008 "Artificial climate aging test method for plastics, coatings and rubber materials for mechanical industry products", GB 33372-2020 "Limit of volatile organic compounds for adhesives";

[0130] The corresponding results, data were summarized and drawn into Table 2:

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

[0132]

[0133] Through data analysis, it can be known that:

[0134] Through the comparison of Example 2 and Comparative Example 1, it can be known that the secondary foaming structure of the present application is formed by the thermal expansion of the epoxy-modified thermal expansion microspheres. This structure is beneficial to the formation of a lightweight and soft-touch product, and will affect the elongation at break. The surface of the unmodified thermal expansion microspheres is inert, and there is lack of chemical bonding with the polyurethane matrix, only weak physical adsorption. When stressed, 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 constrain the expansion of the microspheres, resulting in insufficient foaming, uneven cell structure and poor stability, and therefore higher density.

[0135] Through the comparison of Example 2 and Comparative Example 2, it can be known that the Boc group protects the side chain amino group of lysine, ensuring that the modification reaction only occurs on the terminal amino group of the polyurethane. The obtained product is a polyurethane with a protected amino group in the side chain and an active amino group at the end;

[0136] During heat curing, the Boc protecting group is decomposed by heat, releasing the active side chain amino group. The newly generated side chain amino group quickly covalently bonds with the epoxy groups on the surface of the microspheres, and the adhesion of the microspheres and the final crosslinking curing are carried out synchronously, forming a uniform and firm overall network;

[0137] Comparative Example 2 uses ordinary lysine, and the side chain amino group of ordinary lysine is exposed and highly active. It will compete with the terminal amino group of the polyurethane for the reaction with the activator, generating uncontrollable and chaotic modified products.

[0138] In the S3 step, when the isocyanate crosslinking agent is added at low temperature, all the active amino groups in the system will rush to react with the crosslinking agent. This will cause the reaction to be intense and the heat to be concentrated, which is extremely easy to cause local overcrosslinking or even gelation, while other areas may be insufficiently crosslinked. The entire reaction process is uncontrollable.

[0139] Since the crosslinking reaction has already occurred chaotically in the S3 stage, the viscosity of the system may abnormally increase, which is not conducive to the uniform dispersion of the epoxy microspheres. At the same time, the number of free amino groups available for the S4 step to react with the epoxy groups of the microspheres is also reduced and unevenly distributed, resulting in insufficient and uneven adhesion of the microspheres to the matrix, forming a crosslinked network structure with a large number of defects, thereby comprehensively falling behind Example 2 in terms of mechanical properties, foaming efficiency and environmental friendliness.

[0140] From Comparative Example 2, Comparative Example 3, Comparative Example 4, it can be seen that the side chain amino group of lysine introduces potential crosslinking points in the main chain, and the amount of Nα-Boc-lysine determines the density of crosslinking points in the final polyurethane network. The amount of Nα-Boc-lysine in Comparative Example 3 is too small, and during the final curing, it can be combined with the microspheres to generate a limited number of covalent bonds, so there is a lack of sufficient binding force between the molecular chains, and under the action of external force, the molecular chains are prone to relative slipping, resulting in the material being easily pulled apart or worn off. At the same time, because the molecular chain movement ability is strong, the elongation at break is the highest; due to the incomplete crosslinking network, many small molecular monomers, additives, etc. that do not participate in the reaction are loosely wrapped therein, and are more easily migrated and volatilized, so the VOC is high.

[0141] The amount of Nα-Boc-lysine in Comparative Example 4 is too large, introducing too many potential crosslinking points, and during curing, a too dense crosslinking network is formed. Although the tensile strength is high, the molecular chains are tightly locked and the movement ability is severely limited. The high crosslinking degree also makes the material hard and brittle, and the elongation at break is greatly reduced. When impacted, the material may directly break rather than stretch and deform. At the same time, the too rigid network will limit the expansion of the microspheres, resulting in a decrease in the foaming ratio and an increase in the density. Too much lysine may also cause steric hindrance, affecting the overall reaction efficiency. Local crosslinking too quickly causes some small molecules to be wrapped in the hardened network, which is slowly released later, and the VOC is slightly increased.

[0142] From Comparative Example 2, Comparative Example 5, Comparative Example 6, it can be seen that the amount of crosslinking agent triphenylmethane triisocyanate also revolves around the concept of crosslinking degree, which has a significant effect on the performance of the final product. That is, the three -NCO groups on the molecule of triphenylmethane triisocyanate can react with the -NH2 functional groups on the polyurethane chain to form a strong urea bond between the molecular chains, thereby constructing a three-dimensional network. The amount thereof directly determines the density of the main crosslinking network.

[0143] The amount of crosslinking agent in Comparative Example 5 is too small, and the crosslinking points are severely insufficient. The molecular chains are mainly combined by weak forces such as physical entanglement and hydrogen bonding, and cannot form an effective three-dimensional network. Also because there is a lack of sufficient chemical crosslinking points to resist external force, the material is soft and weak, and the molecular chains are prone to slipping. The material has low strength and poor wear resistance; the molecular chain movement is almost unobstructed, and can be highly stretched, so the elongation is high; a large amount of unreacted isocyanate monomers and other small molecules are loosely wrapped in the fragile network, and are easily volatilized, so the VOC is high; the weak network cannot effectively bind the microsphere expansion, resulting in easy rupture of the cell wall, insufficient foaming, low foaming efficiency, and high material density;

[0144] In Comparative Example 6, the crosslinking agent was in excessive amounts. The excess -NCO groups reacted with water, urea, and other substances in the system, generating a large amount of gas and providing additional chemical foaming for the system, resulting in a high foaming ratio. The excess -NCO groups also introduced a large number of rigid branches, leading to excessively high crosslinking density and extremely high network rigidity. Chain segment movement was completely suppressed. Although the tensile strength reached its peak, the elongation at break plummeted. This means that the material almost lost its elasticity and became very brittle, easily breaking upon impact. The high hardness provided resistance to abrasives, but due to its high brittleness, it may wear in the form of "chipping" rather than the "wear" of tough materials.

[0145] Comparative Examples 2, 7, and 8 show that the thermally expanded microspheres play a dual role as both a foaming agent and an 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 microsphere dosage is appropriate, a "sea-island structure" is formed after curing: a continuous, tough polyurethane cross-linked network (sea phase) uniformly disperses a large number of stable closed-cell structures (islands phase) formed by the expansion of the microspheres. The continuous polyurethane phase provides excellent mechanical strength and toughness, while the uniform cell size significantly reduces density, providing a soft feel and good thermal and sound insulation properties. The strong interfacial bonding ensures that force is effectively transferred from the matrix to the microspheres, achieving a composite reinforcement effect.

[0146] Comparative Example 7 had too few microspheres, resulting in insufficient foaming points and an inability to form a connected, high-proportion cell structure. Therefore, it had the highest density and the lowest foaming ratio. Due to the high content of matrix resin, the material was dense, resulting in good tensile strength. However, this was at the cost of sacrificing its softness and lacking the soft touch and compression resilience that synthetic leather should have. At the same time, the dense matrix had a higher internal temperature during curing, which encapsulated more non-volatile small molecules, resulting in higher VOCs.

[0147] In Comparative Example 8, the excessive use of microspheres severely diluted the polymer matrix, resulting in insufficient polyurethane to completely encapsulate and isolate each microsphere. This led to thinner cell walls, making the microspheres more prone to aggregation and stress concentration points. The large total interfacial area became a weak point in the material, failing to effectively transfer and disperse stress. Consequently, both tensile strength and elongation at break decreased.

[0148] In summary, Example 2 constructed a strong and elastic three-dimensional network structure by precisely controlling the amounts of Nα-Boc-lysine and the crosslinking agent. Simultaneously, the optimal proportion of epoxy-modified microspheres was introduced, achieving a strong chemical bond between the matrix and the filler. This formulation enables the material to achieve a golden balance of strength, elasticity, and low density during curing and foaming, resulting in optimal overall performance with high foaming ratio, excellent mechanical properties, and low VOC emissions. The comparative examples, due to uncontrolled crosslinking or failed interfacial bonding, failed to reproduce this balanced effect.

[0149] The above merely describes preferred specific embodiments of the present application, but the protection scope of the present application is not limited thereto, and any person skilled in the art, according to the technical solution and inventive concept of the present application, makes equivalent replacement or change within the technical range disclosed by the present application, which should be covered within the protection scope of the present application.

Claims

1. A preparation process for a secondary foaming type of water-based synthetic leather, characterized in that, Includes the following steps: S1, Epoxy-modified thermally expandable microspheres Add acrylonitrile-methyl methacrylate copolymer thermally expanded microspheres and deionized water to the reaction vessel, start stirring to form a suspension; add NaOH solution dropwise to adjust the pH to alkaline, raise the temperature to 70-80℃, and keep the reaction at this temperature for 4-5 hours; After the reaction is complete, cool to room temperature, filter, wash with deionized water until neutral, redisperse the filter cake in deionized water, and adjust the pH to acidic. Mix 1,4-butanediol diglycidyl ether with a surfactant until homogeneous, and slowly add the mixture to a reaction vessel. After the addition is complete, add the catalyst and stir continuously at 60-70°C for 6-8 hours. After the reaction was completed, the microspheres were collected by centrifugation, ultrasonically cleaned with ethanol, and vacuum dried at a low temperature of <50℃ to obtain epoxy-modified thermally expandable microspheres. The mass ratio of acrylonitrile-methyl methacrylate copolymer thermally expandable microspheres, 1,4-butanediol diglycidyl ether, surfactant, and catalyst is 5:3-4:0.1:0.05-0.

1. S2, side-chain amino-modified polyurethane An amino-terminated polyurethane aqueous dispersion was added to a reaction vessel. Under low-speed stirring, Nα-Boc-lysine was added, and the pH of the system was adjusted to weakly alkaline with triethylamine. 1-(3-dimethylaminopropyl)-3-ethylcarbodiimide hydrochloride and N-hydroxysuccinimide were added, and the reaction was carried out at room temperature for 12-24 hours. After the reaction was completed, a polyurethane dispersion with Boc protecting groups on the side chain and amino groups at the end was obtained, namely, amino-modified polyurethane with side chains. The mass ratio of the 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 solid content of the amino-terminated polyurethane aqueous dispersion is 40%. S3, Preparation of foaming slurry Side-chain amino-modified polyurethane was slowly added to acetone. Triphenylmethane triisocyanate was slowly added dropwise under ice-water bath cooling and strong stirring, with the temperature strictly controlled below 10℃, and the reaction was carried out for 2-3 hours. After the reaction was completed, acetone was removed by vacuum distillation, an appropriate amount of deionized water was added, and the solid content and viscosity of the system were adjusted to obtain a highly crosslinked modified polyurethane dispersion with Boc protected amino groups on the side chain. The highly cross-linked modified polyurethane dispersion was transferred into a batching tank, and epoxy modified thermal expansion microspheres and wetting and leveling agents were added. The mixture was stirred and dispersed at low speed, and the viscosity of the slurry was adjusted with a thickener to prepare a foaming slurry for later use. 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. S4, Synthetic Leather Preparation The foaming slurry is stirred at high speed using a high-speed disperser, which incorporates a large amount of air. The slurry foams for the first time, expanding its volume to 2.5-3.0 times its original volume, forming a stable foam slurry. Foam paste 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 is then pressed with embossing rollers at high temperature to obtain secondary foamed water-based synthetic leather.

2. The preparation process of a secondary foaming type waterborne synthetic leather according to claim 1, characterized in that, In S1, the thermal expansion microspheres have a foaming initiation temperature of 110℃, a particle size of 20-40μm, and contain alkane-based foaming agents; the surfactant is a nonionic surfactant; and the catalyst is aminosulfonic acid.

3. The preparation process of a secondary foaming type water-based synthetic leather according to claim 1, characterized in that, The preparation process of Nα-Boc-lysine in S2 includes the following steps: After dissolving lysine hydrochloride in deionized water with stirring, NaOH was slowly added to adjust the pH to 10.

5. The NaCl precipitate was then separated by centrifugation to obtain free lysine base. The free base of lysine was transferred into the reaction vessel, tetrahydrofuran was added, and the temperature was lowered to 0℃; a tetrahydrofuran solution of Boc2O was added dropwise, and the reaction was carried out at 0-5℃ for 2 hours, and then the temperature was raised to 25℃ and the reaction was continued for 4 hours. Add concentrated hydrochloric acid until pH=3.0, and wash the precipitated white precipitate with ice water to obtain Nα-Boc-lysine.

4. The preparation process of a secondary foaming type waterborne synthetic leather according to claim 3, characterized in that, In the preparation of Nα-Boc-lysine, the molar ratio of NaOH, lysine hydrochloride, and Boc2O is 2:1:

1.

5. The preparation process of a secondary foaming type waterborne synthetic leather according to claim 1, characterized in that, In S3, the wetting and leveling agent is a nonionic wetting and leveling agent; the thickener is a polyurethane thickener. Triphenylmethane triisocyanate was added in the form of a triphenylmethane triisocyanate acetone solution; The solid content of the highly crosslinked modified polyurethane dispersion is 35%; The viscosity of the foaming slurry is 25,000-35,000 mPa·s.

6. The preparation process of a secondary foaming type waterborne synthetic leather according to claim 1, characterized in that, In step S4, the thickness of the foam paste applied to the release paper is 1.0-1.5 mm. The heating program is set as follows: Hold at 80℃ for 4 minutes; at 120℃ for 5 minutes; at 150℃ for 3 minutes.

7. The secondary foaming waterborne synthetic leather prepared using the preparation process described in claim 1, characterized in that, The total expansion ratio of the foamed slurry is 3.5-6 times; the final product density ranges from 0.18-0.35 g / cm³. 3 .

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