Low-modulus high-strength aging-resistant waterborne polyurethane emulsion and preparation method thereof

Abstract

The invention discloses a low-modulus high-strength anti-aging waterborne polyurethane emulsion and a preparation method thereof, and belongs to the technical field of high polymer materials. The emulsion comprises the following components in parts by weight: 100 parts of mixed polyol, 30-45 parts of diisocyanate, 3-8 parts of a chain extender, 0.5-2 parts of a catalyst, 2-6 parts of a neutralizer, 150-250 parts of a diluting solvent and 0.5-1.5 parts of microcapsules. In the preparation process, accurate time sequence matching of microcapsule triggering and cross-linking reaction is realized through a 42-45 DEG C low-temperature post-chain extension process. After the emulsion forms a film, the 100% modulus is 2.0-2.3 MPa, the elongation at break is 385-425%, and the tensile strength retention rate is greater than or equal to 85% after the double 85 is aged for 38 days, so that the problems of poor mechanical property and insufficient aging resistance of a waterborne polyurethane emulsion film layer under an alkaline condition are successfully solved, and an innovative solution is provided for a high-performance waterborne polyurethane material.

Description

Technical Field

[0001] This invention belongs to the field of polymer materials technology, specifically relating to a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion and its preparation method. Background Technology

[0002] Waterborne polyurethane has become an important alternative to solvent-based polyurethane due to its environmental advantages, and is widely used in coatings, adhesives, and synthetic leather. However, existing single-component, low-modulus, high-strength waterborne polyurethane emulsions generally suffer from insufficient aging resistance. Current technologies improve aging resistance by adding silane coupling agents for post-chain extension, but in practical applications, it has been found that neutralizing agents such as triethylamine need to be added during the emulsification stage to maintain emulsion stability. However, silane coupling agents have a high hydrolysis rate in this alkaline environment, resulting in a low effective crosslinking rate. To address the aforementioned issues, some solutions attempt to lower the overall pH of the emulsion to inhibit silane hydrolysis, but this leads to decreased emulsion stability and increased particle size. Other methods employ pre-hydrolyzed silane coupling agents, which, while slowing down the hydrolysis process to some extent, result in high crosslinking density dispersion and rapid strength decay of the film after aging. In summary, existing technologies, while improving the aging resistance of waterborne polyurethane emulsions, still struggle to balance emulsion stability and crosslinking structure uniformity. There is an urgent need for a technical approach that does not alter the overall pH environment of the emulsion but only creates localized acidic microregions at the moment of silane coupling agent release, thus reconciling the contradiction between aging resistance and emulsion stability.

[0003] A process for preparing an aqueous polyurethane emulsion, disclosed in CN107129565A, involves introducing an epoxy-based silane coupling agent into the prepolymer synthesis and then performing amine chain extension after emulsification, aiming to improve the emulsion's water resistance and yellowing resistance. However, this technical solution directly adds the silane coupling agent to the reaction system containing a salt-forming agent without controlling the pH of the local microenvironment in which the silane coupling agent resides, leading to rapid hydrolysis under alkaline conditions and a significant reduction in crosslinking efficiency. A method for producing an aqueous polyurethane emulsion, disclosed in CN118184945A, uses N-2-aminoethyl-3-aminopropyltriethoxysilane as a post-chain extender, introducing a silane structure after emulsification to improve performance. Although this method avoids the problem of drastic viscosity increase caused by the early introduction of silane, it still involves directly adding silane coupling agents to a conventional emulsion system (pH>8.5), and does not solve the problem of uneven crosslinking caused by the rapid hydrolysis of silane in an alkaline aqueous phase. In addition, this scheme focuses on high solids content and solvent-free processes, and lacks systematic verification of the long-term aging resistance of the film, making it difficult to meet the requirements of high-end coatings or flexible substrates for comprehensive material performance.

[0004] The above problems indicate that when introducing silane modifiers to improve the aging resistance of waterborne polyurethane, existing technologies generally neglect the hydrolysis kinetics control in the alkaline environment of the emulsion, resulting in uneven cross-linking structure, large fluctuations in mechanical properties, and difficulty in simultaneously achieving synergistic optimization of low modulus and high strength. Summary of the Invention

[0005] One of the objectives of this invention is to provide a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion, which solves the problems of poor mechanical properties and insufficient aging resistance of waterborne polyurethane emulsion films under alkaline conditions in the prior art.

[0006] The second objective of this invention is to provide a method for preparing a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion, which is used to prepare the aforementioned low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion.

[0007] The objective of this invention can be achieved through the following technical solutions: A low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion comprises, by weight, the following components: 100 parts of mixed polyols, 30-45 parts of diisocyanate, 3-8 parts of chain extender, 0.5-2 parts of organic bismuth catalyst, 2-6 parts of neutralizer, 150-250 parts of diluent, 0.5-1.5 parts of pH-buffered silane microcapsules, and 300-400 parts of deionized water; The mixed polyol is composed of polycarbonate polyol and polytetrahydrofuran in a mass ratio of (2.5-3.5):1, both of which have a number average molecular weight of 1800-2200 and a hydroxyl value of 54-58 mgKOH / g. The pH-buffered silane microcapsules comprise a core and a shell structure, wherein the core is a silane coupling agent KH602, and the shell is composed of ethyl cellulose and acrylate monomers, with a microcapsule particle size of 1-5 μm.

[0008] By constructing a pH-buffered silane microcapsule structure, the controlled release of silane coupling agents and the instantaneous construction of a localized acidic microenvironment are achieved in an alkaline emulsion system, thus solving the problems of poor mechanical properties and insufficient aging resistance of traditional single-component waterborne polyurethane emulsions under alkaline conditions. The aforementioned ratio range ensures that the main chain possesses both the rigidity of polycarbonate and the flexibility of polyether, providing a basic framework with low modulus and high tensile strength for subsequent film formation. The aliphatic structure of diisocyanate avoids the yellowing problem of aromatic isocyanates under ultraviolet light, improving the weather resistance of the emulsion film. Chain extenders introduce carboxyl groups for subsequent neutralization and salt formation to achieve water dispersion and optimize mechanical properties. The catalyst has moderate catalytic activity and is free of heavy metals, meeting environmental protection requirements. Neutralizers neutralize the carboxyl groups in the chain extender to form ammonium salts, imparting water solubility to the prepolymer. Diluents reduce the viscosity of the prepolymer to facilitate emulsification. The key component, pH-buffered silane microcapsules, consists of a core and a shell structure. The core is a silane coupling agent, γ-aminopropyltriethoxysilane (KH602), and the shell is composed of ethyl cellulose and acrylate monomers. The microcapsule particle size is controlled at 1-5 μm. Its carboxyl or hydroxyl groups can be hydrolyzed in the aqueous phase and polymerized in situ, releasing protons to form local acidic microregions.

[0009] Furthermore, the diisocyanate is selected from at least one of isophorone diisocyanate (IPDI) or hexamethylene diisocyanate (HDI). Both IPDI and HDI are aliphatic diisocyanates, which can impart excellent resistance to yellowing and aging to polyurethane; both have moderate reactivity, are suitable for synthesis in aqueous systems, and have good compatibility with polyols.

[0010] Furthermore, the chain extender comprises 2,2-dihydroxymethylbutyric acid (DMBA) and 1,4-butanediol (BDO) in a mass ratio of (5-8):1. DMBA provides carboxyl groups to achieve water dispersion after neutralization, while BDO is a small molecule chain extender that can increase the content and strength of hard segments; the ratio of the two controls the concentration of ionic groups and the degree of phase separation, affecting the emulsion stability and membrane mechanical properties.

[0011] Furthermore, the organic bismuth catalyst is bismuth neodecanoate; the neutralizing agent is triethylamine; and the diluting solvent is acetone. The organic bismuth catalyst is environmentally friendly and exhibits good selectivity for the NCO / OH reaction; triethylamine has moderate volatility and forms an ammonium salt after neutralization, achieving water dispersion; acetone is miscible with water, easily removed later, and does not affect the environmental friendliness of the emulsion.

[0012] Furthermore, the pH-buffered silane microcapsules are prepared by the following steps: A1. Mix silane coupling agent KH602 with a 5% (w / w) ethyl cellulose ethanol solution and stir until homogeneous to form an oil phase; A2. Add acrylate monomer to the oil phase and continue stirring for 10-15 minutes; A3. The above mixture is processed by a high-pressure homogenizer at a pressure of 50 MPa to form a primary emulsion; A4. Place the colostrum in a constant temperature water bath at 40-45℃ and stir for 10-15 minutes to complete the cross-linking and solidification of the shell layer. Collect the precipitate by centrifugation and vacuum dry to obtain pH buffered silane microcapsules.

[0013] First, the core silane coupling agent KH602 is mixed with an ethyl cellulose ethanol solution to form a homogeneous oil phase; then, acrylate monomers are added as crosslinking agents. This mixture is homogenized under 50 MPa high pressure to form microemulsion droplets with a particle size of 1-5 μm. Finally, at 40-45 °C, the evaporation of ethanol in the droplets promotes the precipitation of ethyl cellulose to encapsulate the core, while the acrylate monomers undergo polymerization and crosslinking to form a stable core-shell structure.

[0014] Furthermore, the weight ratio of the silane coupling agent KH602, 5% ethyl cellulose ethanol solution, and acrylate monomer is 10:(30-40):(0.6-0.9). The composition ratio of the microcapsule shell determines its rupture threshold and hydrolysis response rate under shear force. If the proportion of ethyl cellulose is too high, the shell will be too strong and difficult to rupture; if the proportion of acrylate is too high, the shell will be too weak and prone to premature degradation during storage.

[0015] Furthermore, the acrylate monomer is at least one of methacrylic acid or hydroxyethyl acrylate. Methacrylic acid can provide carboxyl groups to participate in the reaction or regulate hydrophilicity; hydroxyethyl acrylate contains hydroxyl groups that can participate in shell crosslinking, and both can polymerize in water to form a stable shell.

[0016] Furthermore, in step A1, the stirring speed is 800-1000 rpm and the stirring time is 15-20 min; in step A3, the homogenization cycle is 3-4 times; in step A4, the centrifugation conditions are 8000-10000 rpm for 10-15 min, and the vacuum drying temperature is 40-45℃ for 4-6 h. The stirring speed and time ensure uniform mixing of the oil phase; the number of homogenization cycles ensures a narrow droplet size distribution; the centrifugation and drying conditions, while fully removing the solvent, avoid high temperature damage to the microcapsule structure or causing KH6O2 pre-reaction.

[0017] Secondly, a method for preparing a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion includes the following steps: S1. Preparation of prepolymer: Add mixed polyols to a reaction vessel and dehydrate for 1-1.5 h at 120 °C and a vacuum of 0.09-0.098 MPa; cool down to 60-70 °C, add diisocyanate, and heat to 85-95 °C to react for 1-2 h; cool down to 60-70 °C, add chain extender, catalyst and part of diluent in sequence, heat to 70-80 °C to react for 1-2 h, then cool down to 40-50 °C, add the remaining diluent, and stir for 30 min to obtain the prepolymer; S2. Preparation of polyurethane dispersion: Add neutralizer and deionized water to the prepolymer, stir for 1 min and immediately add pH buffered silane microcapsules. First stir at a low speed of 500-800 rpm for 1-2 min, then switch to high speed of 2500-3000 rpm for 2-3 min to form a semi-transparent dispersion. S3. Low-temperature chain extension reaction: The dispersion is kept at 42-45℃ and stirred at 100-500 rpm for 0.5-1h to allow the acrylate monomers in the microcapsule shell to undergo hydrolysis and polymerization, simultaneously releasing the silane coupling agent and generating local acidic microregions. S4. Preparation of waterborne polyurethane emulsion: The reaction product is distilled under reduced pressure at 50-55℃ and a vacuum degree of 0.08-0.09MPa for 2-3 hours to recover the diluted solvent and obtain a waterborne polyurethane emulsion with a solid content of 25-35%.

[0018] First, in the prepolymer synthesis stage, mixed polyols and IPDI react under strictly controlled water conditions to form NCO-terminated prepolymers. The polycarbonate structure in the prepolymer backbone provides excellent mechanical strength and hydrolysis resistance, while PTMEG imparts chain segment flexibility. Their synergistic effect lays the molecular foundation for the final film to achieve "low modulus and high strength." Subsequently, DMBA / BDO chain extenders are added. The carboxyl groups of DMBA are neutralized by triethylamine in subsequent steps to form hydrophilic ammonium salts, giving the prepolymer water dispersibility, while BDO adjusts the hard segment ratio to optimize crystallinity and elasticity balance.

[0019] The key lies in the timing and triggering mechanism of microcapsule introduction in step S2. Immediately after the neutralization reaction is complete and the system pH stabilizes at 8.6-9.0, pH-buffered silane microcapsules are added. At this point, the microcapsules are uniformly suspended as intact particles in the prepolymer acetone solution, and their ethyl cellulose-acrylate composite shell remains stable under low-speed stirring. When switching to high-speed dispersion at 2500-3000 rpm, the fluid shear force exceeds the critical fracture strength of the shell, and the microcapsules rupture synchronously within 10 seconds, releasing the encapsulated KH6O2 droplets. This process ensures that all silane is released in the early stages of emulsification, avoiding excessively high local concentrations.

[0020] Following this, the system enters the S3 low-temperature chain extension stage, with the system temperature maintained at 42-45℃. At this point, the residual acrylate monomers in the microcapsule shell rapidly hydrolyze upon contact with the continuous aqueous phase, releasing the -COOH groups through ionization. Simultaneously, monomers locally initiate free radical polymerization, forming a cross-linked network. This reaction occurs in situ within the microcapsules, with limited proton diffusion, creating a transient acidic environment of pH 5.8-6.2 only within microdomains ≤50 μm in diameter, lasting for a short period. Within this microdomain, the ethoxy groups of newly released KH6O2 hydrolyze to silanols at a controlled rate under weak acid catalysis. Simultaneously, the primary amino group undergoes efficient nucleophilic addition with the NCO groups at the ends of adjacent prepolymers, forming stable Si-OC covalent bonds. Due to the spatial localization and short duration of the acidic microdomains, the overall emulsion pH remains unaffected, and emulsion stability is maintained; while the silane hydrolysis side reaction (self-condensation) is effectively suppressed, and the cross-linking point distribution is highly uniform.

[0021] The emulsion obtained after acetone recovery has particles with polyurethane ammonium salts on their surface and uniformly distributed silane crosslinking points embedded inside. During film formation, physical entanglement, hydrogen bond networks, and covalent crosslinking work synergistically to achieve efficient utilization of silane coupling agents and homogenization of crosslinking structures without sacrificing emulsion stability, thus solving the technical challenge of achieving both low modulus and high strength and aging resistance.

[0022] Furthermore, the addition of the diluent used in step S1 is divided into two parts: the first part is 60-70% of the total diluent amount, and the second part is the remaining amount.

[0023] Furthermore, in step S1, the moisture content of the mixed polyol after dehydration is ≤0.05%; the NCO content of the reaction system after the addition of diisocyanate is controlled at 4.5-5.5%; and the NCO content at the end of the chain extension stage is 2.8-3.2%. Low moisture content prevents bubbles and side reactions; the controlled NCO content reflects the degree of reaction and the molecular weight of the prepolymer, affecting the crosslinking density and mechanical properties of the final membrane.

[0024] Furthermore, in step S2, the molar ratio of the carboxyl groups in the neutralizing agent to the chain extender is 1.05:1; the amount of pH-buffered silane microcapsules added is 0.3-1.0% of the total mass of the prepolymer, calculated based on KH602. A slight excess of neutralizing agent ensures complete neutralization and guarantees dispersion stability; the amount of microcapsules added is optimized to provide sufficient silane crosslinking points without affecting emulsion stability.

[0025] Furthermore, in step S3, the pH value of the local acidic microregion is 5.8-6.2, the diameter of the microregion is ≤50μm, and the duration is 25-35 seconds; the microcapsule triggering time is within 10±2 seconds after addition.

[0026] The formation mechanism of the localized acidic microregions is as follows: In an environment where the overall pH of the emulsion is >8.5, the acrylate monomers in the microcapsule shell, upon contact with the aqueous phase, rapidly hydrolyze and polymerize in situ at 42-45℃, releasing protons ( This creates a transient acidic environment around the microcapsule rupture point, which acts only on the released silane coupling agent, inhibiting its hydrolysis side reactions, without changing the pH of the emulsion as a whole.

[0027] Furthermore, in step S4, the endpoint of vacuum distillation is determined based on the residual acetone content being <50 ppm, and gas chromatography is used for detection.

[0028] The beneficial effects of this invention are: (1) This invention achieves a systematic breakthrough in the performance of waterborne polyurethane emulsions by precisely designing the component ratio and material structure. In terms of composition, the core framework is a 100-part mixed polyol, which is a precise mass ratio of polycarbonate polyol and polytetrahydrofuran (2.5-3.5):1. The number average molecular weights of both are strictly controlled within the range of 1800-2200, and the hydroxyl value is stable at 54-58 mg KOH / g. This combination of parameters allows the molecular chain to possess both the rigidity of polycarbonate and the flexibility of polyether, providing a molecular basis for low modulus and high tensile strength for film formation. The ratio of 30-45 parts of diisocyanate to 3-8 parts of chain extender ensures precise control of the NCO group conversion rate during the synthesis of the prepolymer. Combined with the mild catalytic effect of 0.5-2 parts of catalyst, side reactions are avoided. The molar ratio of 2-6 parts of neutralizer to carboxyl groups of chain extender maintains the ionic strength balance of the system, while the segmented addition strategy of 150-250 parts of diluent optimizes the viscosity and dispersion efficiency of the prepolymer.

[0029] (2) This invention introduces 0.5-1.5 parts of pH-buffered silane microcapsules. The microcapsule size design of 1-5 μm allows them to be uniformly dispersed in the system. The shell is composed of ethyl cellulose and acrylate monomers. This structure ruptures instantaneously under the combined triggering of emulsification shear force and temperature, simultaneously initiating the hydrolysis and polymerization reaction of the shell material, forming instantaneous acidic microregions in situ within the microcapsules. This microregion environment has spatial limitations and a short time duration, acting only on the release process of the core silane coupling agent KH602, allowing it to complete controllable crosslinking under suitable conditions, while the overall alkaline environment of the emulsion (pH>8.5) is completely maintained.

[0030] (3) The ratio of 100 parts of mixed polyol to 30-45 parts of diisocyanate ensures a reasonable distribution of prepolymer segments; the mass ratio of polycarbonate polyol to polytetrahydrofuran (2.5-3.5):1, the molecular weight of 1800-2200, and the hydroxyl value of 54-58 mgKOH / g jointly optimize the rigid-flexible balance of segments and the density of reaction sites; the addition of 0.5-1.5 parts of microcapsules ensures sufficient crosslinking point density while avoiding excessive local concentration leading to phase separation; the precise matching of this parameter system enables the microcapsules to rupture synchronously within 10 seconds after high-speed dispersion, ensuring the release of silane in a uniformly dispersed state in the emulsion, while the 1-5μm particle size design ensures that the diameter of acidic microregions is ≤50μm and the duration is precisely controlled within 25-35 seconds. This multi-dimensional synergistic effect results in the formation of a composite network structure with both physical entanglement and chemical crosslinking after film formation, which not only significantly improves the uniformity of the mechanical properties of the material, but also greatly enhances its structural stability in harsh environments. Detailed Implementation

[0031] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, specific embodiments of the present invention will be described in detail below.

[0032] Throughout the preparation process, the order of addition of each component, the temperature range, the stirring rate, and the reaction time were strictly controlled. For example, the neutralizing agent must be neutralized within one minute before the addition of the microcapsules to ensure that the system has sufficient ionic strength for stable dispersion; the microcapsules must be added immediately after neutralization and triggered to rupture under specific shear conditions to ensure release in a uniformly dispersed emulsion state; the temperature of the chain extension stage after low temperature must be controlled at 42-45℃ to ensure the hydrolysis and polymerization kinetics of the acrylate monomers while avoiding premature hydrolysis of silanes due to high temperatures. In addition, the composition ratio of the microcapsule shell determines its rupture threshold and hydrolysis response rate under shear force. If the proportion of ethyl cellulose is too high, the shell is too strong and difficult to rupture; if the proportion of acrylate is too high, the shell is too weak and prone to premature degradation during storage.

[0033] The prepared aqueous polyurethane emulsion, after film formation, has a crosslinking structure consisting of two parts: one is the physical crosslinking and hydrogen bond network formed by the polyurethane main chain through DMBA / BDO chain extension, and the other is the Si-OC covalent crosslinking formed by the reaction of KH602 and NCO groups and the Si-O-Si network formed by the self-condensation of siloxane; due to the uniform release and reaction of KH602 in the local acidic microregions, the crosslinking points are highly uniformly distributed.

[0034] In the technical solution described in this invention, all operational steps, material ratios, and process parameters are essential technical features for achieving the desired technical effect, and none can be omitted. If the microcapsule structure is omitted and KH6O2 is added directly, the silane will rapidly hydrolyze to form a gel under alkaline emulsification conditions, leading to emulsion demulsification. If the microcapsule shell does not contain acrylate monomers, local acidic microregions cannot be formed, and the silane will still fail in the alkaline environment. If the post-chain extension temperature is higher than 45°C, the silane hydrolysis will be too rapid, resulting in uneven crosslinking. If it is lower than 42°C, the acrylate hydrolysis will be insufficient, failing to trigger the acidic microregions. Therefore, this invention, through microcapsule design, process timing control, and synergistic reaction conditions, achieves precise controlled release and efficient crosslinking of silane coupling agents in alkaline aqueous systems, resolving a long-standing technical contradiction.

[0035] The specific embodiments of the present invention will be described in detail below, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0036] Example 1

[0037] This embodiment provides a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion, which is prepared through the following steps: S1. Preparation of pH-buffered silane microcapsules: First, 10 parts by weight of silane coupling agent KH602 and 35 parts by weight of 5% ethyl cellulose ethanol solution were added to a three-necked flask and stirred at 800 rpm for 15 minutes to form a homogeneous oil phase. Then, 0.75 parts by weight of acrylate monomer were added to the oil phase and stirred for another 10 minutes to ensure that the monomer was fully dispersed in the oil phase. The resulting mixture was transferred to a high-pressure homogenizer and homogenized three times at 50 MPa to form a primary emulsion with a uniform particle size distribution. The primary emulsion was transferred to a constant temperature water bath and reacted at 40°C for 10 minutes with continuous stirring to allow the acrylate monomer in the shell to crosslink and solidify. After the reaction, the product was centrifuged at 8000 rpm for 10 minutes, the precipitate was collected, and the product was vacuum dried at 45°C for 4 hours to obtain dry pH-buffered silane microcapsule powder.

[0038] S2. Preparation of the prepolymer: 100 parts of a mixed polyol (composed of polycarbonate polyol and PTMEG in a mass ratio of 3:1, both with a number average molecular weight of 2000 and a hydroxyl value of 56 mgKOH / g) were added to a four-necked reactor equipped with a stirrer, thermometer, nitrogen inlet pipe, and reflux condenser. After purging the reactor with nitrogen three times, the temperature was raised to 120°C and dehydrated under a vacuum of 0.095 MPa for 1 hour until the moisture content was ≤0.05%. The temperature was then lowered to 65°C, and 38 parts of IPDI were added. The temperature was raised to 90°C, and the reaction was carried out within this temperature range for 1.5 hours. During this period, samples were taken to determine the NCO content, and the NCO content was controlled at the final reaction endpoint at 5%. The temperature was then lowered to 65°C, and 6 parts of a chain extender (prepared by mixing DMBA and BDO in a mass ratio of 6:1), 1.25 parts of an organobismuth catalyst, and 150 parts of acetone were added sequentially. The temperature was raised to 75°C, and the reaction was continued for 1.5 hours until the NCO content decreased to 3%. The temperature was then lowered to 45°C, the remaining 50 parts of acetone were added, and the mixture was stirred for 30 minutes to obtain a viscous and transparent prepolymer solution.

[0039] S3. Preparation of polyurethane dispersion: Add 4 parts triethylamine and 350 parts deionized water to the above prepolymer, and stir at a low speed of 650 rpm for 1 minute to allow the neutralization reaction to proceed fully and generate polyurethane ammonium salt. Then immediately add 1 part pH buffered silane microcapsules, and first maintain low speed stirring at 650 rpm for 1.5 minutes to allow the microcapsules to be initially dispersed in the system; then quickly switch to high speed dispersion at 2800 rpm for 2.5 minutes, using shear force to cause the microcapsule shells to rupture and form a polyurethane dispersion.

[0040] S4. Low-Temperature Chain Extension Reaction: The polyurethane dispersion after high-speed dispersion was maintained at 43.5℃ and stirred at 300 rpm for 45 minutes. During this process, the acrylate monomers remaining in the microcapsule shells hydrolyzed upon contact with the aqueous phase, generating carboxylic acids and releasing them. Simultaneously, monomers undergo in-situ free radical polymerization locally, forming a cross-linked network. This process creates localized acidic microregions with a diameter ≤50μm around the microcapsule rupture point, maintaining a pH of 6.0 for approximately 30 seconds.

[0041] S5. Preparation of waterborne polyurethane emulsion: The dispersion after low-temperature chain extension was transferred to a vacuum distillation apparatus and distilled under reduced pressure at 53°C and 0.085 MPa for 2.5 hours to recover acetone from the system. The distillation endpoint was determined by gas chromatography, with the residual acetone content being less than 50 ppm. The final product was a low-modulus, high-strength, and aging-resistant waterborne polyurethane emulsion with a solid content of 30%.

[0042] Example 2

[0043] The difference between this embodiment and Example 1 is that the mass ratio of polycarbonate polyol to PTMEG in the mixed polyol is 2.5:1, while the other raw materials and preparation process remain the same as in Example 1.

[0044] Example 3

[0045] The difference between this embodiment and Example 1 is that the mass ratio of polycarbonate polyol to PTMEG in the mixed polyol is 3.5:1, while the other raw materials and preparation process remain the same as in Example 1.

[0046] Example 4

[0047] The difference between this embodiment and Example 1 is that in the preparation of pH buffered silane microcapsules, the weight ratio of 5% ethyl cellulose ethanol solution and acrylate monomer is 30:0.9, while the remaining raw materials and preparation process are the same as in Example 1.

[0048] Example 5

[0049] The difference between this embodiment and Example 1 is that the weight ratio of 5% ethyl cellulose ethanol solution and acrylate monomer is 40:0.6, while the remaining raw materials and preparation process are the same as in Example 1.

[0050] Example 6

[0051] The difference between this embodiment and Example 1 is that the particle size of the pH buffered silane microcapsules is controlled at 1 μm, which is achieved by increasing the high-pressure homogenization pressure to 55 MPa. The other raw materials and preparation process are the same as in Example 1.

[0052] Example 7

[0053] The difference between this embodiment and Example 1 is that the particle size of the pH buffered silane microcapsules is controlled at 5 μm, which is achieved by reducing the high-pressure homogenization pressure to 45 MPa. The other raw materials and preparation process are the same as in Example 1.

[0054] Example 8

[0055] The difference between this embodiment and Example 1 is that the temperature of the dispersion is controlled at 42°C in step S4, while the other raw materials and preparation process remain the same as in Example 1.

[0056] Example 9

[0057] The difference between this embodiment and Example 1 is that the temperature of the dispersion is controlled at 45°C in step S4, while the other raw materials and preparation process remain the same as in Example 1.

[0058] Example 10

[0059] The difference between this embodiment and Example 1 is that the amount of pH buffered silane microcapsules added is 0.5 parts, while the other raw materials and preparation process remain the same as in Example 1.

[0060] Example 11

[0061] The difference between this embodiment and Example 1 is that the amount of pH buffered silane microcapsules added is 1.5 parts, while the other raw materials and preparation process remain the same as in Example 1.

[0062] Comparative Example 1

[0063] The difference between this comparative example and Example 1 is that pH-buffered silane microcapsules are not used; instead, KH602 is directly added to the neutralized prepolymer. The other raw materials and preparation process remain the same as in Example 1.

[0064] Comparative Example 2

[0065] The difference between this comparative example and Example 1 is that the microcapsule shell is composed only of ethyl cellulose (without acrylate monomers), while the other raw materials and preparation process remain the same as in Example 1.

[0066] Comparative Example 3

[0067] The difference between this comparative example and Example 1 is that the dispersion temperature is controlled at 50°C in step S4, while the other raw materials and preparation process remain the same as in Example 1.

[0068] Comparative Example 4

[0069] The difference between this comparative example and Example 1 is that the dispersion temperature is controlled at 38°C in step S4, while the other raw materials and preparation process remain the same as in Example 1.

[0070] Comparative Example 5

[0071] The difference between this comparative example and Example 1 is that the overall pH of the system was adjusted to 7.0 (by reducing the amount of triethylamine and adding an appropriate amount of acetic acid), while the other raw materials and preparation process remained the same as in Example 1.

[0072] Comparative Example 6

[0073] The difference between this comparative example and Example 1 is that KH602 pre-hydrolyzed for 24 hours was used instead of microcapsules, while the other raw materials and preparation process remained the same as in Example 1.

[0074] Comparative Example 7

[0075] The difference between this comparative example and Example 1 is that the weight ratio of 5% ethyl cellulose ethanol solution and acrylate monomer is 24:1.2, while the remaining raw materials and preparation process are the same as in Example 1.

[0076] Performance testing

[0077] After 72 hours of curing, the aqueous polyurethane emulsions prepared in all examples and comparative examples underwent the following performance tests: 1. Emulsion stability test: Refer to GB / T 11175-2021 "Test Method for Synthetic Resin Emulsions", place the emulsion in a 50mL centrifuge tube, centrifuge at 3000rpm for 30 minutes, and observe whether there is layering, precipitation or demulsification.

[0078] 2. Mechanical properties: Referring to GB / T 528-2009 "Determination of tensile stress-strain properties of vulcanized rubber or thermoplastic rubber", the emulsion was dried at 60℃ to form a film with a thickness controlled at 1.0±0.1mm. The film was cut into dumbbell-shaped specimens for testing 100% modulus, tensile strength and elongation at break.

[0079] 3. Double 85 aging performance test: Referring to GB / T 3511-2018 "Weather resistance of vulcanized rubber or thermoplastic rubber", the sample was placed in an environment of 85℃ and 85% relative humidity for 38 days to age, and the tensile strength before and after aging was tested and the retention rate was calculated.

[0080] 4. Acetone Residue Test: The content of benzene, toluene, ethylbenzene and xylene in coatings shall be determined by gas chromatography in accordance with GB / T 23990-2009 "Determination of Benzene, Toluene, Ethylbenzene and Xylene Content in Coatings".

[0081] The results are shown in Table 1: Table 1

[0082] As shown in Table 1, the emulsions of all examples (1-11) remained stable, indicating that adjusting the process parameters within the scope of the claims does not compromise the stability of the emulsion system. This is attributed to the effective protection of the silane coupling agent by the microcapsule structure in an alkaline environment. In contrast, Comparative Example 1 exhibited demulsification because KH602 hydrolyzes at a rate as high as 50% / min in an alkaline environment (pH>8.5), rapidly forming a siloxane gel that disrupts emulsion stability. Comparative Examples 5 and 7 showed sedimentation, confirming that overall pH adjustment or imbalance in the shell ratio can damage the colloidal stability of the emulsion.

[0083] In terms of mechanical properties, the 100% modulus of all examples remained stable within the range of 2.0-2.3 MPa, while the tensile strength remained between 32-37 MPa, achieving the technical goal of low modulus and high strength. Example 2 exhibited the lowest modulus and the highest elongation at break, while Example 3 showed the highest modulus and the highest tensile strength. This performance difference stems from the fact that polycarbonate provides rigid chain segments to enhance strength, while PTMEG contributes compliant ether bonds to improve ductility. The two form a molecular-level synergy at a precise ratio, ensuring both the soft feel of the material and sufficient mechanical strength. It is worth noting that when the amount of microcapsules added in Examples 10-11 deviated from the range of 0.5-1.5 parts, although the modulus and elongation at break improved, the tensile strength and aging performance decreased significantly, indicating that the amount of microcapsules needs to be balanced between ensuring crosslinking density and avoiding excessively high local concentrations.

[0084] All examples in the elongation at break data maintained an excellent range of 385%-425%, indicating that the present invention successfully constructed a composite network structure that combines physical entanglement and chemical crosslinking. The high elongation at break in Example 2 is consistent with its high PTMEG ratio, confirming the contribution of compliant segments to the material's ductility; while the lower elongation at break in Example 3 is consistent with its high polycarbonate ratio, reflecting the effect of rigid segments on improving the material's strength. In contrast, the elongation at break in all comparative examples was below 365% and showed a negative correlation with tensile strength, indicating that uneven crosslinking in traditional methods leads to stress concentration, causing the material to fracture at lower strains, and failing to achieve the synergistic optimization of low modulus and high tensile strength.

[0085] The data on the 85% aging performance show that the tensile strength retention rate after aging in all examples is ≥82.5%, with Examples 1-3 and 6-9 all ≥84.0%, significantly better than the comparative examples. Example 3 exhibits the best aging resistance, which is related to its higher polycarbonate ratio, as the polycarbonate structure has excellent hydrolysis resistance; while Example 10 has the lowest retention rate, indicating that insufficient microcapsule dosage leads to insufficient crosslinking density. The retention rate of Comparative Example 3 is only 68.3%, because high temperature accelerates silane hydrolysis, resulting in uneven crosslinking, structural defects, and accelerated damp heat aging; the retention rate of Comparative Example 2 is 75.6%, confirming the key role of acidic microdomains in inhibiting the side reaction of silane hydrolysis.

[0086] The above-disclosed embodiments are merely a few specific examples of the present invention. However, the embodiments of the present invention are not limited thereto, and any variations that can be conceived by those skilled in the art should fall within the protection scope of the present invention.

Claims

1. A low modulus high strength weatherable waterborne polyurethane emulsion, characterized by, By weight parts, including the following components: mixed polyol 100 parts, diisocyanate 30-45 parts, chain extender 3-8 parts, organic bismuth catalyst 0.5-2 parts, neutralizing agent 2-6 parts, dilution solvent 150-250 parts, pH buffer type silane microcapsule 0.5-1.5 parts, deionized water 300-400 parts; The mixed polyol is composed of polycarbonate polyol and polytetrahydrofuran in a mass ratio of (2.5-3.5):1, and the number average molecular weight of both is 1800-2200, and the hydroxyl value is 54-58 mgKOH / g; The pH buffer type silane microcapsule comprises a core and a shell layer, wherein the core is silane coupling agent KH602, and the shell layer is composed of ethyl cellulose and acrylate monomer in a mass ratio of 70:30, and the particle size of the microcapsule is 1-5 μm. 2.The low-modulus high-strength aging-resistant waterborne polyurethane emulsion of claim 1, characterized in that, The diisocyanate is selected from at least one of isophorone diisocyanate or hexamethylene diisocyanate; The chain extender comprises 2,2-dimethylol butyric acid and 1,4-butanediol in a mass ratio of (5-8):1; The organic bismuth catalyst is bismuth neodecanoate; the neutralizing agent is triethylamine; and the dilution solvent is acetone. 3.The low-modulus high-strength aging-resistant waterborne polyurethane emulsion of claim 1, characterized in that, The pH buffer type silane microcapsule is prepared by the following steps: A1. Mix the silane coupling agent KH602 with 5% ethyl cellulose ethanol solution to form an oil phase; A2. Add acrylate monomer to the oil phase and continue stirring for 10-15 min; A3. Treat the mixture in a high-pressure homogenizer at a pressure of 50 MPa to form a primary emulsion; A4. Place the primary emulsion in a constant temperature water bath at 40-45°C, stir for 10-15 min to complete the shell layer crosslinking and curing, centrifuge to collect the precipitate, and vacuum dry to obtain the pH buffer type silane microcapsule. 4.The low-modulus high-strength aging-resistant waterborne polyurethane emulsion of claim 3, characterized in that, The weight ratio of the silane coupling agent KH602, 5% ethyl cellulose ethanol solution, and acrylate monomer is 10:(30-40):(0.6-0.9). 5.The low-modulus high-strength aging-resistant waterborne polyurethane emulsion of claim 3, characterized in that, The acrylate monomer is at least one of methyl methacrylate or hydroxyethyl acrylate. 6.The low-modulus high-strength aging-resistant waterborne polyurethane emulsion of claim 3, characterized in that, In step A1, the stirring speed is 800-1000 rpm, and the stirring time is 15-20 min; in step A3, the homogenization cycle number is 3-4 times; in step A4, the centrifugation conditions are 8000-10000 rpm for 10-15 min, and the vacuum drying temperature is 40-45°C for 4-6 h.

7. A method for preparing a low modulus high strength aging resistant waterborne polyurethane emulsion, characterized in that, A method for preparing the low modulus high strength aging resistant waterborne polyurethane emulsion of any one of claims 1-6, comprising the following steps: S1, preparation of the prepolymer: add the mixed polyol to the reaction kettle, dehydrate at 120°C and a vacuum degree of 0.09-0.098 MPa for 1-1.5 h; cool to 60-70°C, add diisocyanate, heat to 85-95°C and react for 1-2 h; cool to 60-70°C, add the chain extender, catalyst and part of the dilution solvent, heat to 70-80°C and react for 1-2 h, then cool to 40-50°C, add the remaining dilution solvent, and stir for 30 min to obtain the prepolymer; S2, Preparation of polyurethane dispersion: adding neutralizing agent and deionized water to the prepolymer, stirring for 1 min, then adding pH buffer type silane microcapsule, stirring at low speed of 500-800 rpm for 1-2 min, then switching to high speed dispersion at 2500-3000 rpm for 2-3 min to form a translucent dispersion; S3, Low temperature post chain extension reaction: controlling the dispersion at 42-45℃, maintaining stirring at 100-500 rpm for 0.5-1 h, so that the acrylate monomer in the microcapsule shell layer hydrolyzes and polymerizes, synchronously releasing silane coupling agent and generating local acidic microzone; S4, Preparation of aqueous polyurethane emulsion: distilling the reaction product under reduced pressure at 50-55℃ and vacuum degree of 0.08-0.09 MPa for 2-3 h to recover dilution solvent, obtaining aqueous polyurethane emulsion with solid content of 25-35%. 8.The method for preparing a low modulus high strength aging-resistant waterborne polyurethane emulsion according to claim 7, characterized in that, The addition of dilution solvent in step S1 is divided into two parts, the first part is 60-70% of the total amount of dilution solvent, and the second part is the remaining amount; In step S1, the water content of the mixed polyol after dehydration is ≤0.05%; the NCO content of the reaction system after adding diisocyanate is controlled at 4.5-5.5%; and the NCO content at the end of the chain extension stage is 2.8-3.2%. 9.The method for preparing a low modulus high strength aging-resistant waterborne polyurethane emulsion according to claim 7, characterized in that, In step S2, the molar ratio of neutralizing agent to carboxyl in chain extender is 1.05:1; the amount of pH buffer type silane microcapsule added is 0.3-1.0% of the total mass of the prepolymer based on KH602. 10.The method for preparing a low modulus high strength aging-resistant waterborne polyurethane emulsion according to claim 7, characterized in that, In step S3, the pH value of the local acidic microzone is 5.8-6.2, the microzone diameter is ≤50 μm, and the duration is 25-35 seconds; the microcapsule trigger time is within 10±2 seconds after addition; In step S4, the end point of reduced pressure distillation is determined by the residual amount of acetone <50 ppm, which is detected by gas chromatography.

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