White leather special high shielding and yellowing resistant compound finishing agent and preparation method thereof

Abstract

This invention relates to the field of leather finishing agents, and discloses a high-opacity, yellowing-resistant compound finishing agent for white leather and its preparation method. The compound finishing agent comprises a film-forming resin, a masking function, a yellowing resistance component, a hand feel adjustment component, and an emulsifying and dispersing composition. The masking function composition contains structured pre-composite microspheres and free nano-titanium dioxide. The structured pre-composite microspheres are acrylate polymer microspheres with a shell polymer network embedded with nano-titanium dioxide particles. The yellowing resistance composition contains a UV absorber, a hindered amine light stabilizer, and an antioxidant. This invention constructs a highly efficient and synergistic masking function composition by designing and preparing structured pre-composite microspheres with a shell embedded with nano-TiO2 and optimizing their compounding with free nano-TiO2. Simultaneously, it integrates a triple yellowing resistance defense system to establish a long-lasting yellowing resistance mechanism. The two core compositions work synergistically, resulting in significant improvements in masking efficiency and long-term yellowing resistance performance of the finishing agent.

Description

Technical Field

[0001] This invention relates to the field of leather finishing agents, and in particular to a high-opacity, yellowing-resistant compound finishing agent for white leather and its preparation method. Background Technology

[0002] White and light-colored leather products are highly favored in footwear, bags, clothing, and home décor due to their pure, stylish, and elegant appearance. However, two major technical bottlenecks have long existed in their production and use: first, insufficient coverage, making it difficult to effectively conceal defects, color differences, and processing marks on the leather raw material; second, a tendency to yellow, where the surface of white leather gradually turns yellow after exposure to ultraviolet light, high temperatures, or long-term storage, seriously affecting the product's appearance and lifespan.

[0003] Currently, there are various technical solutions for white leather finishing agents on the market, but each has its own limitations. For example, an existing patent (CN112876958A) discloses a high-performance water-based leather finishing agent, which uses a water-based self-matting polyurethane resin, film-forming polyurethane resin, and aliphatic polyurethane matting microspheres with specific particle sizes. It primarily aims to improve the physical and mechanical properties of the coating, such as scratch resistance, abrasion resistance, and chemical resistance, especially meeting the high standards required for automotive leather. Although its heat resistance to yellowing has been tested, its technical solution does not include a dedicated stabilizer system for photo-oxidative aging; its yellowing resistance mainly depends on the characteristics of the selected resin itself. The core function of the matting microspheres (particle size 5-30μm) in this document is to matte and improve physical properties, but it does not reveal any indication that its combination with high-refractive-index pigments (such as titanium dioxide) can produce a synergistic effect to enhance hiding power.

[0004] Patent publication number CN107892854A directly addresses the need for leather to resist yellowing, disclosing a composite resin containing hollow polymer spheres and a specially formulated yellowing-resistant acrylic emulsion (containing active zinc oxide). This solution provides some resistance to yellowing and masking properties, but its technical approach is relatively rudimentary: yellowing resistance is mainly achieved through resin modification (adding active zinc oxide), lacking a long-lasting, proactive, multi-layered chemical defense system; masking properties rely on the simple filling effect of fillers such as hollow polymer spheres, without addressing the need for precise microstructural design to maximize optical masking efficiency. Its formulation exhibits the characteristics of traditional composite resins, lacking sufficient functional integration and synergy.

[0005] In recent years, some studies have attempted to introduce nanomaterials (such as nano-ZnO and TiO2) or use UV absorbers to improve yellowing resistance or opacity, respectively. However, these technologies often suffer from trade-offs, being either too costly or having complex processes, making it difficult to achieve an ideal balance and synergistic improvement across multiple dimensions such as efficient opacity, long-lasting anti-yellowing, excellent feel, and cost control. In particular, how to achieve optimal opacity with the fewest coating layers through innovative component design and structural construction, while ensuring that white leather remains white for a long time in harsh environments, remains an unsolved technical challenge in this field.

[0006] Therefore, developing a special coating agent for white leather that can be scientifically compounded and synergistically combined to innovatively construct an efficient optical shielding system and a long-lasting anti-yellowing defense system, achieving perfect coverage with one or two sprays, maintaining whiteness and non-yellowing for a long time, and having a soft and natural feel while being environmentally friendly, has become a technological need that urgently needs to be addressed in this field. Existing technologies still need to be improved and developed. Summary of the Invention

[0007] In view of the shortcomings of the prior art, the purpose of this invention is to provide a high-opacity, yellowing-resistant compound coating agent for white leather and its preparation method, aiming to solve the problems of low covering efficiency and poor long-term yellowing resistance of existing white leather coating agents.

[0008] The technical solution of the present invention is as follows:

[0009] A high-opacity, yellowing-resistant compound coating agent for white leather, comprising, by weight percentage: 30%-50% film-forming resin composition; 10%-25% opacifying composition; 3%-8% yellowing-resistant composition; 5%-15% hand feel adjusting composition; 2%-5% emulsifying and dispersing composition; and the balance being deionized water. The opacifying composition comprises structured pre-composite microspheres and free nano-titanium dioxide, wherein the structured pre-composite microspheres are acrylate polymer microspheres with nano-titanium dioxide particles embedded in a shell polymer network. The yellowing-resistant composition comprises a UV absorber, a hindered amine light stabilizer, and an antioxidant.

[0010] The aforementioned high-opacity, yellowing-resistant compound coating agent for white leather contains structural pre-composite microspheres with an average particle size of 0.5-3 μm and free nano-titanium dioxide with an average particle size of 10-50 nm.

[0011] The aforementioned high-opacity, yellowing-resistant compound coating agent for white leather, wherein the dry matter mass ratio of the structured pre-composite microspheres and free nano-titanium dioxide in the opacifying functional composition is 1.5-4:1.

[0012] The aforementioned high-opacity, yellowing-resistant compound coating agent for white leather, wherein the film-forming resin composition is composed of waterborne polyurethane resin and waterborne acrylic resin in a mass ratio of 1-3:1.

[0013] The aforementioned high-opacity, yellowing-resistant compound coating agent for white leather, wherein the mass ratio of ultraviolet absorber, hindered amine light stabilizer, and antioxidant in the yellowing-resistant composition is (1.5-3):(1-2):1, wherein the ultraviolet absorber is selected from benzotriazole or triazine compounds; and the antioxidant is selected from hindered phenol or phosphite compounds.

[0014] The aforementioned high-opacity, yellowing-resistant compound coating agent for white leather includes a hand feel adjusting composition comprising an organosilicon elastomer and a wax emulsion; and an emulsifying and dispersing composition comprising a nonionic surfactant and a polycarboxylate dispersant.

[0015] A method for preparing a high-opacity, yellowing-resistant compound coating agent for white leather as described in this invention, comprising the following steps:

[0016] Pre-preparation of structured precomposite microsphere emulsions;

[0017] Add 40-60% of the total mass of deionized water in the formula to the reactor and heat to 45-55℃;

[0018] Add the emulsified dispersion composition while stirring, and stir at a stirring speed of 400-600 r / min until completely dissolved and homogeneous;

[0019] While maintaining the temperature, add free nano-titanium dioxide and structured pre-composite microsphere emulsion sequentially at a stirring speed of 1000-1500 r / min, and continue to disperse for 30-45 minutes until the system is uniform and fine.

[0020] Reduce the stirring speed to 500-700 r / min, slowly add the film-forming resin composition, and stir for 20-30 minutes to form a uniform resin emulsion composite system.

[0021] Reduce the stirring speed to 200-400 r / min, and add the yellowing resistance composition and the feel adjustment composition in sequence. Stir for 10-15 minutes after each component is added until the mixture is homogeneous.

[0022] Add the remaining deionized water to adjust the solid content, and use a pH adjuster to adjust the pH of the system to 7.5-8.5;

[0023] Finally, the product is filtered and discharged to obtain the white leather-specific high-opacity, yellowing-resistant compound coating agent.

[0024] The preparation method of the high-opacity, yellowing-resistant compound coating agent for white leather includes the step of pre-preparing a structured pre-composite microsphere emulsion, which comprises:

[0025] Hydrophobically treated nano-TiO2 was stirred and dispersed in the presence of a polycarboxylate dispersant to form a stable TiO2 slurry.

[0026] Prepare a seed monomer preemulsion containing methyl methacrylate, butyl acrylate, functional monomers and crosslinking agents;

[0027] Under nitrogen protection, seed monomer pre-emulsion and water-soluble initiator are added dropwise to TiO2 slurry and polymerization is initiated at 70-80℃ to generate primary composite latex particles with TiO2 aggregates as the core.

[0028] Styrene is added to the system in one step, and the mixture is first swollen at 60-70℃ for 1.5-2 hours. Then, the temperature is raised to 85-90℃ for secondary polymerization and crosslinking. During this process, phase separation occurs inside to form a hollow structure. At the same time, TiO2 particles are enriched and fixed in the polymer network of the microsphere shell under the drive of crosslinking kinetics. Finally, after cooling, filtration, and pH adjustment to 7.5-8.5, a structured pre-composite microsphere emulsion is obtained.

[0029] Beneficial effects: This invention creatively designs and prepares structural pre-composite microspheres with embedded nano-TiO2 shells, and optimizes their compounding with free nano-TiO2 to construct a unique and highly efficient synergistic shielding composition. Simultaneously, by systematically integrating a triple active defense system of UV absorber, hindered amine light stabilizer, and antioxidant, a long-lasting yellowing resistance mechanism is established. The synergistic effect of these two core compositions, combined with scientific resin compounding and preparation processes, enables the coating agent of this invention to achieve significant effects in terms of hiding power (reducing the number of sprayings by more than 50%) and long-lasting yellowing resistance. Attached Figure Description

[0030] Figure 1 The present invention provides a flowchart of a preparation method for a high-opacity, yellowing-resistant compound coating agent for white leather. Detailed Implementation

[0031] This invention provides a high-opacity, yellowing-resistant compound coating agent for white leather, its preparation method, and its application. To make the objectives, technical solutions, and effects of this invention clearer and more explicit, the invention is further described in detail below. It should be understood that the specific embodiments described herein are only for explaining the invention and are not intended to limit the invention.

[0032] This invention provides a high-opacity, yellowing-resistant compound coating agent specifically for white leather. The agent, by weight percentage, comprises: 30%-50% film-forming resin composition; 10%-25% opacifying composition; 3%-8% yellowing-resistant composition; 5%-15% hand feel adjusting composition; 2%-5% emulsifying and dispersing composition; and the balance being deionized water. The opacifying composition includes structured pre-composite microspheres and free nano-titanium dioxide. The structured pre-composite microspheres are acrylate polymer microspheres with nano-titanium dioxide particles embedded in a shell polymer network. The yellowing-resistant composition includes a UV absorber, a hindered amine light stabilizer, and an antioxidant.

[0033] Specifically, this invention breaks away from the traditional approach of simply stacking pigments for reflection or filling with fillers. It utilizes a uniquely structured pre-composite microsphere to pre-, uniformly, and firmly embed nano-titanium dioxide within the polymer network of the acrylic microsphere shell. The microsphere itself is a highly efficient optical functional unit: the TiO2 within its shell acts as a strong reflective point, and the polymer network constitutes a micrometer-scale light scatterer. When a large number of these microspheres coexist with free nano-TiO2 in the coating, a multi-level synergistic system of microsphere-level scattering / reflection and nanometer-scale reflection is formed. Incident light encounters an extremely complex optical path within the coating: it is first strongly scattered by the microsphere structure, and then repeatedly reflected by free TiO2 and the TiO2 within the microsphere shell as it propagates between the microspheres. This cyclical effect of scattering-reflection-rescattering greatly dissipates and blocks light energy at the top of the coating, making it extremely difficult for light to penetrate to the leather substrate. This synergistic effect grows exponentially. Therefore, the coating agent of the present invention only requires 1-2 sprays (about 8-16 g / m²) to achieve the covering effect of traditional products or even comparative products that require 3-5 sprays. The covering efficiency (covering area per unit mass of coating agent) is increased by more than 50%, which significantly improves production efficiency and saves raw material costs.

[0034] To address the extreme sensitivity of white leather to yellowing, this invention constructs a triple active defense system, rather than relying on the resin's inherent resistance or other single methods. The first line of defense uses an ultraviolet absorber (UVA) to preferentially absorb high-energy UV photons, converting them into harmless heat energy, acting as a light filter. Free radicals induced by UV light or heat that are not fully absorbed are efficiently captured and converted into stable products by the hindered amine light stabilizer (HALS) in the second line of defense, interrupting the free radical chain reaction. The third line of defense uses an antioxidant (AOX) to decompose already generated hydroperoxides, preventing the decomposition and generation of new free radicals. This combination produces a synergistic stabilizing effect, far superior to any single component or the simple sum of two components. This system comprehensively intervenes in the yellowing process from the initiation, propagation, and termination stages, ensuring that the resin molecular chains in the coating maintain structural stability under long-term light and heat stress, significantly improving the heat and UV yellowing resistance of the coated leather.

[0035] By rationally compounding film-forming resins (PU / PA complementarity), precisely adding feel-adjusting compositions, and using stable emulsifying and dispersing compositions, the coating agent of this invention provides revolutionary coverage and anti-yellowing properties while ensuring that the coating has good flexibility, flexural strength, dry and wet rubbing resistance, and an adjustable feel from smooth to soft. Moreover, the entire system uses water as a medium, has extremely low VOC content, and strictly avoids environmentally restricted substances such as bisphenol S, bisphenol F, and APEO, meeting the requirements of green chemistry and sustainable development.

[0036] In some embodiments, the average particle size of the structured precomposite microspheres is 0.5-3 μm, and the average particle size of the free nano-titanium dioxide is 10-50 nm. Specifically, the particle size of the structured precomposite microspheres (0.5-3 μm) is on the same order of magnitude as the visible light wavelength (0.4-0.76 μm), which is an ideal size range for light scattering, generating strong Mie scattering and effectively disrupting the optical path; the particle size of the free nano-TiO2 (10-50 nm) is much smaller than the visible light wavelength, mainly generating Rayleigh scattering and having a huge reflective surface area, making them highly efficient reflective / scattering particles; the matching design of the particle sizes of the two achieves high-efficiency optical interference across the entire wavelength range from nanometer to micrometer scale. This particle size combination gives the coating the widest scattering and reflection spectrum for visible light, achieving optimal masking ability. If the particle size of the structured precomposite microspheres is too large (>5μm), it may result in a rough coating surface, poor feel, and decreased scattering efficiency; if it is too small (<0.2μm), the scattering ability will be insufficient; if the free TiO2 particle size is too large (such as ordinary titanium dioxide >200nm), the reflective area per unit mass will be small, the covering efficiency will be low, and it will be easy to settle; if the particle size is too small (<5nm), the surface energy will be extremely high, it will be very easy to agglomerate, difficult to disperse, and the nano-effect will be lost.

[0037] In some embodiments, the dry matter mass ratio of the structured precomposite microspheres to free nano-titanium dioxide in the shielding functional composition is 1.5-4:1. Specifically, the dry matter mass ratio of the structured precomposite microspheres to free nano-titanium dioxide refers to the mass ratio of the two after removing the solvent. In this embodiment, the dry matter mass ratio of 1.5-4:1 ensures that the structured precomposite microspheres occupy a dominant skeletal position in the system, providing the main scattering framework and some reflection points. The free nano-TiO2 acts as a filler and synergist, precisely filling the gaps between the microspheres and forming a connection and enhancement of the reflection network with the TiO2 on the microsphere shell. This ratio is the optimal window for synergistic effect obtained through extensive experimental optimization. If the ratio is too low (too much free TiO2), it is prone to agglomeration, increasing costs and potentially damaging the structure formed by the microspheres; if the ratio is too high (too many microspheres), there are insufficient free reflection points, the optical path is not fully utilized, and the shielding force cannot reach its peak. At this ratio, the synergistic masking effect of the two components is maximized, and the covering area (mass masking efficiency coefficient) per unit mass of coating agent is the highest.

[0038] In some embodiments, the film-forming resin composition comprises waterborne polyurethane resin and waterborne acrylic resin in a mass ratio of 1-3:1. Specifically, waterborne polyurethane (PU) resin has excellent flexibility, abrasion resistance, low-temperature resistance, and high elasticity; waterborne acrylic (PA) resin has excellent adhesion, weather resistance, UV resistance, and cost advantages. Combining the two in a 1-3:1 mass ratio allows for the complementary use of their strengths, resulting in a continuous film with balanced mechanical properties, strong adhesion, and controllable cost. PU provides skeletal toughness and resilience, while PA provides adhesion and filling properties like muscle. The coating obtained by combining the two has good flexural strength, abrasion resistance, adhesion to leather, and a suitable modulus, avoiding problems such as excessive hardness, brittleness, or excessive softness and re-stickiness that may occur with a single resin.

[0039] In some embodiments, the mass ratio of UV absorber, hindered amine light stabilizer, and antioxidant in the anti-yellowing composition is (1.5-3):(1-2):1, wherein the UV absorber is selected from benzotriazole or triazine compounds; and the antioxidant is selected from hindered phenol or phosphite compounds. Specifically, the mass ratio of UV absorber (UVA), hindered amine light stabilizer (HALS), and antioxidant (AOX) (1.5-3):(1-2):1) is optimized based on their synergistic mechanism and consumption rate. UVA, as the first line of defense, requires a sufficient concentration to effectively absorb UV light; HALS can be recycled during the capture of free radicals, but needs to be matched with UVA to deal with any "slipped" free radicals; AOX, as a backup, is used in relatively small amounts but is essential. Benzotriazole / triazine derivatives exhibit strong UVA absorption in both UV-A and UV-B bands; high molecular weight HALS exhibit low migration and good long-lasting effects; hindered phenolic / phosphite-based AOX provides primary and secondary antioxidant effects, respectively. Under this specific ratio and composition, the triple defense system achieves the highest synergistic efficiency, maximally delaying the photo-oxidative degradation of the resin and minimizing changes in the yellowing index during accelerated aging tests. Imbalances in these ratios may lead to premature failure of any defense, resulting in a significant decrease in overall yellowing resistance. For example, insufficient HALS prevents effective blocking of free radical chain reactions; insufficient AOX leads to peroxide accumulation, accelerating subsequent yellowing. For example, the ultraviolet absorber may be selected from benzotriazoles (such as UV-326, UV-531, UV-P), triazines, etc.; the hindered amine light stabilizer may be selected from high molecular weight types (such as LS-622, LS-770, LS-944), etc.; the antioxidant may be selected from hindered phenols (such as antioxidants 1010, 1076) or phosphites (such as 168), but is not limited to these.

[0040] In some embodiments, the feel-modifying composition includes a silicone elastomer and a wax emulsion; the emulsifying and dispersing composition includes a nonionic surfactant and a polycarboxylate dispersant. Specifically, in the feel-modifying composition, the silicone elastomer (silicone) significantly reduces the surface tension of the coating, providing a smooth, soft feel and improving surface hydrophobicity, while the wax emulsion (PE, PTFE, etc.) imparts a special waxy feel, fineness, and abrasion resistance to the coating. The combination of the two allows for precise control of the feel. In the emulsifying and dispersing composition, the nonionic surfactant provides steric stabilization, and the polycarboxylate dispersant, through the combined action of electrostatic repulsion and steric hindrance, is particularly suitable for the long-term dispersion stability of nanoparticles (such as TiO2) and microspheres, preventing aggregation and sedimentation. For example, the silicone elastomer may be selected from amino-modified, epoxy-modified, or polyether-modified silicone oil microemulsions; the wax emulsion may be selected from polyethylene (PE) wax, polytetrafluoroethylene (PTFE) wax, polyamide wax, or oxidized polyethylene wax emulsion; the nonionic surfactant may be selected from isomeric alcohol ethers or fatty acid polyoxyethylene esters, but is not limited thereto.

[0041] In some embodiments, a method for preparing a high-opacity, yellowing-resistant compound coating agent specifically for white leather is also provided, such as... Figure 1 As shown, it includes the following steps:

[0042] S10. The hydrophobically treated nano-TiO2 is stirred and dispersed in the presence of a polycarboxylate dispersant to form a stable TiO2 slurry.

[0043] S20. Prepare a seed monomer preemulsion containing methyl methacrylate, butyl acrylate, functional monomers and crosslinking agents;

[0044] S30. Under nitrogen protection, the seed monomer pre-emulsion and water-soluble initiator are added dropwise to the TiO2 slurry and polymerization is initiated at 70-80℃ to generate primary composite latex particles with TiO2 aggregates as the core.

[0045] S40. Styrene is added to the system in one step. It is first swollen at 60-70℃ for 1.5-2 hours, and then heated to 85-90℃ for secondary polymerization and crosslinking. During this process, phase separation occurs inside to form a hollow structure. At the same time, TiO2 particles are enriched and fixed in the polymer network of the microsphere shell under the drive of crosslinking kinetics. Finally, after cooling, filtration and pH adjustment to 7.5-8.5, a structured pre-composite microsphere emulsion is obtained.

[0046] S50. Add 40-60% of the total mass of deionized water in the formula to the reactor and heat to 45-55℃.

[0047] S60. Add the emulsified dispersion composition while stirring, and stir at a stirring speed of 400-600 r / min until completely dissolved and homogeneous;

[0048] S70, while maintaining a temperature of 45-55℃ and stirring at a speed of 1000-1500r / min, add free nano-titanium dioxide and structured pre-composite microsphere emulsion in sequence, and continue to disperse for 30-45 minutes until the system is uniform and fine;

[0049] S80. Reduce the stirring speed to 500-700 r / min, slowly add the film-forming resin composition, and stir for 20-30 minutes to form a uniform resin emulsion composite system.

[0050] S90. Reduce the stirring speed to 200-400 r / min, and add the yellowing resistance composition and the feel adjustment composition in sequence. Stir for 10-15 minutes after each component is added until the mixture is uniform.

[0051] S100, add the remaining deionized water to adjust the solid content, and use a pH adjuster to adjust the pH of the system to 7.5-8.5;

[0052] S110. Finally, the product is filtered and discharged to obtain the white leather-specific high-opacity, yellowing-resistant compound coating agent.

[0053] The preparation method provided by this invention is a sophisticated chemical engineering process with interconnected steps and a clear mechanism. First, through an ingenious design of seed polymerization-swelling-secondary polymerization, utilizing the differences in reaction kinetics and the principle of phase separation, a structural pre-composite microsphere with a unique structure of a hollow core and a TiO2 embedded shell is actively synthesized at the microscale, solving the core problem of efficient utilization and positioning of TiO2. Then, based on the principles of colloidal stability and shear force matching, through a step-by-step, rate-controlled compounding process, this core microsphere is orderly assembled with other components into a macroscopically uniform, stable composite coating agent system that maximizes the microscopic synergistic effect. Each step of this method has a clear scientific purpose and mechanistic support, and its overall complexity and sophistication far exceed that of simple mixing processes. It is this method that ensures that the coating agent of this invention can achieve perfect coverage and long-lasting anti-yellowing properties with very few sprays, which are unexpected and excellent properties. This constitutes the substantial feature and key innovation of this invention that distinguishes it from existing technologies.

[0054] Specifically, the preparation of the structured precomposite microsphere emulsion in steps S10-S40 is the technological cornerstone of the entire invention. It actively builds functional structures at the nano and micro scale through a three-step method (seed polymerization → swelling → secondary polymerization / crosslinking), rather than passive mixing.

[0055] In step S10, due to the large specific surface area and high surface energy of nano-TiO2, it is extremely easy to agglomerate into micron-sized particles due to van der Waals forces, thus losing the nano-effect. In this embodiment, high-speed shear force (mechanical energy) is used to physically break these soft agglomerates. The polycarboxylate dispersant is strongly adsorbed on the TiO2 surface through its anchoring groups (such as -COOH), and the hydrophilic chains extend into the aqueous phase to form an electric double layer and steric hindrance, preventing re-agglomeration and obtaining a kinetically stable dispersion. The hydrophobic treatment changes the TiO2 surface from hydrophilic to oleophilic (hydrophobic), which significantly enhances its affinity with the subsequently added oily acrylate monomers. The adsorption of the dispersant further adjusts the chemical microenvironment of the particle surface, making it more likely to become a "heterogeneous nucleation site" for subsequent polymerization reactions. This step is a prerequisite to ensure that TiO2 can participate in subsequent reactions uniformly at the nanoscale, rather than being simply wrapped as an inert filler.

[0056] In step S20, hydrophobic monomers such as methyl methacrylate and butyl acrylate are pre-emulsified into submicron droplets in water with the help of an emulsifier. This significantly increases the oil-water interface area, ensuring that the monomers are uniformly and slowly released into the aqueous phase and migrate to the surface of TiO2 particles during the droplet addition stage, avoiding homogeneous nucleation caused by excessively high local monomer concentrations. In this embodiment, the main monomer (such as MMA / BA) constitutes the main body of the polymer shell, adjusts the glass transition temperature (Tg) of the polymer, and affects the hardness of the microspheres. The functional monomer (such as acrylic acid or phosphate methacrylate) has special functional groups that can generate stronger chemical adsorption or coordination with the TiO2 surface (anchoring effect), which is the key to realizing the embedding of TiO2 in the shell rather than simple encapsulation. It acts like glue, enhancing the bonding force between the polymer chain and the TiO2 surface during polymerization. The crosslinking agent (such as ethylene glycol dimethacrylate and divinylbenzene) is introduced in the first step of polymerization to initially establish the network structure of the shell polymer. Appropriate crosslinking can improve the mechanical strength of the microspheres and provide the necessary network constraints for the structural reconstruction (phase separation) in the second step.

[0057] In step S30, oxygen is purged by introducing nitrogen gas because oxygen is an effective inhibitor of free radical polymerization, consuming initiator free radicals and leading to a slow or incomplete reaction. The temperature is controlled at 70-80℃: this temperature is suitable for the decomposition of water-soluble initiators (such as potassium persulfate) to generate free radicals, ensuring an appropriate initiation rate and stable polymerization. This step enables heterogeneous nucleation and core-shell structure formation. Specifically, free radicals generated in the aqueous phase initiate the polymerization of dissolved monomers, forming oligomeric free radicals. As the chain length increases... When hydrophobic, it tends to precipitate from the aqueous phase; due to the presence of a large number of hydrophobic TiO2 particles (or small aggregates) modified with dispersants in the system, they are better precipitation and nucleation sites than emulsifier micelles; oligomer radicals are captured on the TiO2 surface, and monomers continue to polymerize and grow on its surface, gradually encapsulating one or a cluster of TiO2 particles to form "primary composite latex particles" with TiO2 as the core and a preliminary cross-linked polymer shell on the outside; this step realizes the first chemical bonding and fixation of TiO2 and polymer.

[0058] In step S40, a sufficient amount of swellable monomer (styrene) with good swelling capacity for the primary polymer particles is provided at once, allowing it to fully penetrate into the particle interior, violently swelling the primary particles, significantly increasing their volume, and relaxing their internal polymer network, providing space and fluidity for large-scale structural recombination; the swelling stage temperature is set at 60-70℃, which is lower than the rapid polymerization temperature of styrene, but sufficient to promote the thermodynamic diffusion penetration of monomer into the particle interior, and sufficient time (1.5-2 hours) is given to ensure swelling equilibrium, so that each particle is like a "small sponge" saturated with monomer. If high temperature is applied directly, the monomer will polymerize rapidly on the particle surface and cannot penetrate into the interior.

[0059] Next, in the high-temperature secondary polymerization / crosslinking stage: the temperature is raised to the optimal temperature for styrene polymerization, 85-90℃. At this point, a series of intricate kinetic processes occur synergistically:

[0060] The shell layer cures rapidly and preferentially: the outer shell of the particle is in direct contact with the aqueous phase, which has good heat dissipation and a relatively high concentration of initiator fragments. Therefore, the polymerization and crosslinking reaction rate in the shell region is much faster than that in the particle interior, and the shell layer quickly forms a rigid polymer network shell with a high crosslinking density.

[0061] Internal phase separation and shrinkage: Inside the particles, newly formed polystyrene (PS) chains separate from the existing P(MMA-BA) polymer chains due to differences in thermodynamic compatibility. Simultaneously, internal monomers are consumed, leading to volume shrinkage.

[0062] The enrichment kinetics of TiO2 shells drive the formation of the "mosaic" structure. Under the inward pull generated by internal phase separation and contraction, the internal materials (polymer phase, unreacted monomers, TiO2 particles) are squeezed outward (towards the shell). Since the shell has been rapidly cross-linked and solidified, it forms a rigid porous network, which acts like a sieve or trapping net: small molecules and unreacted monomers can pass through; however, those nano-TiO2 particles that are tightly bound to polymer chains (especially those anchored by functional monomers) or are larger in size are effectively captured and locked in the pores or nodes of the rapidly solidified and densified shell network as they migrate outward.

[0063] Formation of hollow structures: As internal materials continuously migrate, accumulate and are fixed to the shell, the interior of the particles is gradually hollowed out, eventually forming a hollow or microporous structure.

[0064] Final Structure: After this step, the initial solid core-shell structure is reconstructed into: a low-density hollow / porous polymer core (primarily contributing to light scattering and weight reduction), and a robust polymer shell rich in high-density, highly cross-linked nano-TiO2 particles. The TiO2 particles are embedded in the shell network, resulting in stable positions and extremely high reflectivity.

[0065] Finally, the reaction is stopped by cooling; the trace amounts of gel or impurities that may be generated are removed by filtration to ensure the purity of the emulsion; the pH is adjusted to 7.5-8.5, and the electrostatic repulsion between emulsion particles is enhanced by the ionization of carboxyl groups and other groups to ensure the long-term storage stability of the microsphere emulsion.

[0066] Steps S50-S110 are mainly based on stepwise stabilization and shear force matching, which orderly introduces and harmoniously coexists components of different properties and sizes (nanoparticles, micron-sized composite microspheres, polymer resins, and small molecule additives) in the aqueous phase, avoiding agglomeration, demulsification, or performance degradation, thereby obtaining the white leather-specific high-opacity yellowing-resistant compound coating agent.

[0067] Specifically, in steps S50-S60, a temperature of 45-55°C can reduce the surface tension of water. Combined with a stirring speed of 400-600 r / min, this promotes the dissolution and diffusion of emulsifier and dispersant molecules, ensuring that the emulsifier and dispersant have formed a uniform molecular solution in the aqueous phase before the subsequent addition of solid particles. This allows them to be immediately adsorbed at the freshly formed particle-water interface, providing immediate protection and preventing direct contact and aggregation between particles.

[0068] In step S70, a high shear force of 1000-1500 r / min is provided, sufficient to break up any soft agglomerates that may form in the nano-TiO2 slurry during storage, and to ensure that the pre-composite microspheres (which are already a stable emulsion) are highly uniformly distributed in the system. The more easily agglomerated nano-TiO2 is added first, allowing it to disperse fully under high shear and with sufficient dispersant. Then, the structured microspheres are added; the addition of the microspheres does not interfere with the already dispersed nano-TiO2, and the two can be initially mixed under shear. Sufficient mass transfer and dispersion time are ensured, allowing the system to reach a uniform and fine state, forming a stable masking slurry. This is the physical basis for the high hiding power of the entire coating agent.

[0069] In step S80, the resin emulsion consists of submicron-sized polymer particles whose stability depends on the emulsifier layer on the surface. Excessive shear force (e.g., >1000 rpm) may damage this emulsifier film, leading to particle aggregation, demulsification, or shear thickening. Medium-speed stirring at 500-700 rpm provides sufficient mixing kinetic energy but avoids destructive shearing. Slowly adding the film-forming resin composition prevents excessively high local resin concentrations from causing a sharp increase in particle collision frequency and aggregation. This step ensures uniform mixing of the pre-dispersed shielding functional particles (nano-TiO2 and microspheres) with the resin particles, allowing the resin to begin encapsulating or filling around the functional particles, initially forming a system composed of a continuous phase (resin) and a dispersed phase (functional particles).

[0070] In step S90, the speed is reduced to 200-400 r / min, and the anti-yellowing composition and the feel-modifying composition are added sequentially. Since UV absorbers, light stabilizers, antioxidants, silicones, wax emulsions, etc., are mostly small molecules or surface-active substances, they are stable on their own, but severe shearing may cause them to generate unnecessary foam or destroy the structure of the already formed emulsion / dispersion system. The sequential addition ensures that each additive has enough time to diffuse and adsorb to its target position (e.g., the anti-yellowing agent diffuses into the polymer phase, and the feel-modifying agent tends to be on the coating surface) before adding the next one, avoiding competitive adsorption that leads to uneven distribution. The extremely low shear force (200-400 r / min) ensures that the constructed resin-functional particle composite structure is not destroyed, especially protecting the integrity of the structured pre-composite microspheres.

[0071] In steps S100-S110, water is first added to adjust the solid content, and then the pH of the entire compound system is adjusted to weak alkalinity. This allows all carboxyl-containing components (resin, dispersant, and some monomers) in the system to ionize, enhancing the electrostatic repulsion between particles. This, combined with steric hindrance, is the core factor in ensuring that the final product does not settle or separate during long-term storage. Filtration removes any trace amounts of impurities, undispersed clumps, or reaction byproducts that may be present, ensuring the product's appearance and application performance.

[0072] The present invention will be further described below with reference to the embodiments. However, the embodiments described below are only for illustrating the content of the present invention and are not intended to limit it. Therefore, any changes that are equivalent in meaning and scope to the claims of the present invention should be considered to be included within the scope of the claims.

[0073] Example 1

[0074] A high-opacity, yellowing-resistant compound coating agent specifically for white leather, with the following raw material composition by weight percentage:

[0075] Film-forming resin composition: 45% (30% waterborne polyurethane resin, 15% waterborne acrylic resin);

[0076] Shielding functional composition: 20% (12% dry matter of structured pre-composite microspheres, 8% dry matter of free nano-titanium dioxide);

[0077] Anti-yellowing composition: 6% (UV absorber UV-326 3%, hindered amine light stabilizer LS-622 2%, antioxidant 1010 1%).

[0078] Hand feel adjustment composition: 10% (6% silicone elastomer, 4% PE wax emulsion);

[0079] Emulsified dispersion composition: 4% (2% isomeric alcohol ether, 2% polycarboxylate dispersant);

[0080] The remainder is deionized water: 15%.

[0081] Its preparation method includes the following steps:

[0082] S10. The hydrophobically treated nano-TiO2 is stirred and dispersed in the presence of a polycarboxylate dispersant to form a stable TiO2 slurry.

[0083] S20. Prepare a seed monomer preemulsion containing methyl methacrylate, butyl acrylate, functional monomer phosphate methacrylate and crosslinking agents (ethylene glycol dimethacrylate and divinylbenzene);

[0084] S30. Under nitrogen protection, the seed monomer pre-emulsion and water-soluble initiator (potassium persulfate) are added dropwise to the TiO2 slurry and polymerization is initiated at 75°C to generate primary composite latex particles with TiO2 aggregates as the core.

[0085] S40. Styrene is added to the system at once, and it is first swollen at 65°C for 2 hours. Then, the temperature is raised to 85°C for secondary polymerization and crosslinking. During this process, phase separation occurs inside to form a hollow structure. At the same time, TiO2 particles are enriched and fixed in the polymer network of the microsphere shell under the drive of crosslinking kinetics. Finally, after cooling, filtration and pH adjustment to 7.5-8.5, a structured pre-composite microsphere emulsion is obtained.

[0086] S50. According to the formula of high-opacity, yellowing-resistant compound coating agent for white leather, add 50% of the total mass of deionized water in the formula to the reaction vessel and heat to 50℃.

[0087] S60. Add the emulsified dispersion composition while stirring, and stir at a stirring speed of 500 r / min until completely dissolved and homogeneous;

[0088] S70, while maintaining 50℃ and stirring at 1200r / min, add free nano-titanium dioxide and structured pre-composite microsphere emulsion in sequence, and continue to disperse for 40 minutes until the system is uniform and fine;

[0089] S80. Reduce the stirring speed to 600 r / min, slowly add the film-forming resin composition, and stir for 25 minutes to form a uniform resin emulsion composite system.

[0090] S90. Reduce the stirring speed to 300 r / min, and add the yellowing-resistant composition and the feel-modifying composition in sequence. Stir for 12 minutes after each component is added until the mixture is homogeneous.

[0091] S100, add the remaining deionized water to adjust the solid content, and use a pH adjuster to adjust the pH of the system to 8;

[0092] S110. Finally, the product is filtered and discharged to obtain the white leather-specific high-opacity, yellowing-resistant compound coating agent, denoted as E1.

[0093] Example 2

[0094] A high-opacity, yellowing-resistant compound coating agent specifically for white leather, with the following raw material composition by weight percentage:

[0095] Film-forming resin composition: 40% (20% waterborne polyurethane resin, 20% waterborne acrylic resin);

[0096] Shielding composition: 18% (12% dry matter of structured pre-composite microspheres, 6% dry matter of free nano-titanium dioxide);

[0097] Anti-yellowing composition: 5% (UV absorber UV-P 2.5%, hindered amine light stabilizer LS-770 1.5%, antioxidant 168 1%).

[0098] Hand feel modifier composition: 12% (8% silicone elastomer, 4% PTFE wax emulsion);

[0099] Emulsifying and dispersing composition: 3% (1.5% fatty acid polyoxyethylene ester, 1.5% polycarboxylate dispersant);

[0100] The remainder is deionized water: 22%.

[0101] The preparation method is the same as in Example 1, and the product obtained is denoted as E2.

[0102] Example 3

[0103] A high-opacity, yellowing-resistant compound coating agent specifically for white leather, with the following raw material composition by weight percentage:

[0104] Film-forming resin composition: 50% (37.5% waterborne polyurethane resin, 12.5% ​​waterborne acrylic resin);

[0105] Shielding composition: 15% (12% dry matter of structured pre-composite microspheres, 3% dry matter of free nano-titanium dioxide);

[0106] Anti-yellowing composition: 7% (triazine UV absorber 3.5%, hindered amine light stabilizer LS-944 2.5%, antioxidant 1076 1%).

[0107] Hand feel modifier composition: 8% (5% silicone elastomer, 3% polyamide wax emulsion);

[0108] Emulsifying and dispersing composition: 5% (2.5% nonionic surfactant, 2.5% polycarboxylate dispersant);

[0109] The remainder is deionized water: 15%.

[0110] The preparation method is the same as in Example 1, and the product obtained is denoted as E3.

[0111] Comparative Example 1 (Unstructured precomposite microspheres)

[0112] A high-opacity, yellowing-resistant compound coating agent for white leather differs from that in Example 1 in that the opacifying functional composition uses only an equal amount of free nano-titanium dioxide (20% dry matter) and does not add structured pre-composite microspheres. The remaining components and preparation process are the same as in Example 1, and the resulting product is designated as D1.

[0113] Comparative Example 2 (Ordinary hollow acrylic microspheres replacing structured precomposite microspheres)

[0114] A high-opacity, yellowing-resistant compound coating agent for white leather differs from that in Example 1 in that the opacifying functional composition uses ordinary hollow acrylic microspheres instead of structured pre-composite microspheres. The remaining components and preparation process are the same as in Example 1, and the resulting product is designated as D2.

[0115] Comparative Example 3 (using only hindered amine light stabilizer LS-622 as the yellowing-resistant compound)

[0116] A high-opacity, yellowing-resistant compound coating agent for white leather differs from that in Example 1 in that the yellowing-resistant composition uses only 6% of the hindered amine light stabilizer LS-622, without adding UV absorbers or antioxidants. The remaining components and preparation process are the same as in Example 1, and the resulting product is designated as D3.

[0117] Comparative Example 4 (using only UV absorber UV-326 as the anti-yellowing compound)

[0118] A high-opacity, yellowing-resistant compound coating agent for white leather differs from that in Example 1 in that the yellowing-resistant composition uses only 6% of the same amount of UV absorber UV-326, without adding hindered amine light stabilizers and antioxidants. The remaining components and preparation process are the same as in Example 1, and the resulting product is designated as D4.

[0119] Comparative Example 5 (Resin Ratio Imbalance)

[0120] A high-opacity, yellowing-resistant compound coating agent for white leather differs from that in Example 1 in that the mass ratio of waterborne polyurethane resin to waterborne acrylic resin in the film-forming resin composition is adjusted to 4:1. The remaining components and preparation process are the same as in Example 1, and the resulting product is designated as D5.

[0121] Comparative Example 6

[0122] A high-opacity, yellowing-resistant compound coating agent for white leather is prepared by mixing 40 parts of yellowing-resistant acrylic emulsion, 10 parts of paraffin emulsion, 5 parts of kaolin filler, 10 parts of commercial PMMA hollow microspheres (20μm), 1 part of thickener, and 1 part of defoamer, and adding water to adjust the solid content to be equivalent to that of Example 1, thus obtaining product D6; wherein, the yellowing-resistant acrylic emulsion is composed of ethyl acrylate, acrylic acid, sodium dodecyl sulfate, and active zinc oxide.

[0123] The finishing agents prepared in Examples E1-E3 and Comparative Examples D1-D6 were applied to the same batch of white cowhide blanks using a uniform process, and performance tests were conducted. Specifically: Hiding power (comparative examples): measured using a hiding power tester according to GB / T 23981-2009 standard; higher values ​​indicate stronger hiding power. Number of coats required for complete coverage: sprayed layer by layer onto standard leather blanks until base color difference and defects were no longer visible to the naked eye, and the number of coats was recorded. UV yellowing resistance Δb*: after 168 hours of irradiation using a QUV accelerated aging test chamber (UVA-340 lamp), the change in b* value (Δb*) was measured using a colorimeter; a positive value indicates yellowing, and a smaller value indicates better yellowing resistance. Heat yellowing resistance Δb: the sample was placed in a 120℃ oven for 72 hours, and the change in b* value was measured after cooling. Hand feel score: averaged by 5 experienced leather engineers in a blind evaluation, with 10 points being the best (soft, smooth, natural). Abrasion resistance: according to QB / T... According to standard 2726-2005, using a Taber abrasion tester with a 500g load, the rotational speed at which the substrate exposed was recorded was recorded. Key performance results are compared in Table 1.

[0124] Table 1 Performance Test Results

[0125]

[0126] As shown in Table 1, the performance test results of all embodiments demonstrate excellent and balanced performance in key indicators such as hiding power (contrast ratio), number of sprays required for complete coverage, resistance to UV yellowing, resistance to heat yellowing, feel score, and abrasion resistance. The hiding power and contrast ratio of all embodiments are above 0.98, approaching the ideal value of 1.0 for complete coverage. The number of sprays required for complete coverage is only one (some require 1-2) sprays, significantly improving construction efficiency. The resistance to yellowing is between 1.8 and 2.0 for UV yellowing and between 2.1 and 2.3 for heat yellowing, indicating strong color stability under light and heat conditions. The feel score is 8.5-9.0, and the abrasion resistance is 7500-8000 revolutions, demonstrating good practicality and durability. In contrast, Comparative Examples 1-6 showed varying degrees of decline in all performance aspects, particularly in terms of coverage efficiency and resistance to yellowing. These comparative examples simulated scenarios such as omitting key components, replacing the core structure, simplifying the defense system, adjusting imbalanced proportions, and referring to existing technologies. Their performance degradation serves as a testament to the indispensability, structural uniqueness, and synergistic necessity of each component in the technical solution of this invention.

[0127] Specifically, the hiding power (comparative ratio) of Comparative Example 1 was only 0.88, significantly lower than that of the Example (≥0.98); complete coverage required 3–4 coats, drastically reducing efficiency; the UV yellowing resistance Δb was 4.5, and the heat yellowing resistance Δb was 5.0, indicating a significant increase in yellowing; the feel score was 7.0, and the abrasion resistance at 6000 revolutions, both lower than the Example. This is because Comparative Example 1 relied solely on the Rayleigh scattering and reflection of free nano-TiO2, lacking the synergy of micron-sized scatterers. Although the nanoparticles have high reflectivity, they are prone to aggregation due to their high surface energy, reducing the effective reflective area; furthermore, their particle size is much smaller than the visible light wavelength, resulting in a limited scattering angle and difficulty in forming multi-layered optical path interference; the TiO2 in Comparative Example 1 is in a free state, resulting in uneven distribution during film formation, easy sedimentation or aggregation, leading to poor optical uniformity of the coating; a large number of free nanoparticles may interfere with the continuity of resin film formation, affecting the coating's density and mechanical properties, thereby reducing abrasion resistance and feel. The above comparative results demonstrate that simply increasing nano-TiO2... 22 The dosage cannot be equivalent to the synergistic masking system of structured microspheres + free TiO2; microstructure design (such as embedding TiO2 in the shell of microspheres) is the key innovation to achieve efficient masking; the synergy of multi-scale scatterers (nano + micro) is the core path to improve masking efficiency.

[0128] In Comparative Example 2, ordinary hollow acrylic microspheres were used instead of structured pre-composite microspheres. The performance was as follows: hiding power 0.90, lower than the example but still better than Comparative Example 1; three coats were required for complete coverage; yellowing resistance Δb* was between 3.8 and 4.2, better than Comparative Example 1 but worse than the example; feel and abrasion resistance were slightly reduced. Analysis of the reasons: Although ordinary hollow microspheres have a certain light scattering ability (especially due to the refractive index difference caused by the hollow structure), their shells lack high-refractive-index TiO2 embeddings, resulting in a much lower reflection efficiency than structured microspheres; lack of optical synergy: the free TiO2 and hollow microspheres are only physically mixed, lacking the bridging and reflection network enhancement effect of the shell TiO2, leading to insufficient optical path optimization; poor structural stability: the shells of ordinary microspheres are not reinforced by TiO2 embeddings, potentially resulting in lower mechanical strength and durability, making them prone to breakage during shearing or film formation. The above comparison results show that a simple hollow structure is insufficient to achieve efficient coverage and must be combined with a high-reflectivity material (such as TiO2); the key to achieving efficient utilization and positioning of TiO2 is the inlay structure rather than simple encapsulation; the structured pre-composite microsphere is a functional structural unit, rather than an ordinary filler.

[0129] In Comparative Example 3, only hindered amine light stabilizer (HALS) was used as the yellowing-resistant composition. Its performance was as follows: hiding power and number of coats were comparable to the examples; however, the UV yellowing resistance Δb was as high as 5.2, and the heat yellowing resistance Δb was 4.8, indicating significant yellowing; the feel and abrasion resistance remained relatively good. Analysis of the reasons: Incomplete defense system: The main function of HALS is to capture free radicals and interrupt the chain reaction, but it cannot absorb ultraviolet light; in the absence of UV absorbers (UVA), a large amount of high-energy ultraviolet light directly attacks the resin molecular chain, leading to accelerated initial degradation; Lack of antioxidant synergy: Without antioxidants (AOX) to decompose hydrogen peroxide, the free radical reaction chain is prolonged, increasing the pressure on HALS and ultimately reducing the protective effect. The above comparison results show that yellowing resistance is a multi-stage and multi-mechanism synergistic protection process, and the triple defense is indispensable. UVA, HALS, and AOX correspond to the three stages of photodegradation initiation, propagation, and termination, respectively. The absence of any one of them will lead to the breakage of the defense chain. A single stabilizer cannot cope with the complex photo-thermal-oxygen aging environment. The triple defense system (UVA + HALS + AOX) has a significant synergistic enhancement effect.

[0130] Comparative Example 4 used only UV absorber (UVA) as the anti-yellowing composition, and its performance was as follows: good hiding power and number of coats; UV yellowing resistance Δb* was 3.5, better than Comparative Example 3 but still worse than the Example; heat yellowing resistance Δb* was as high as 6.0, the worst performance; good feel and abrasion resistance. Analysis of the reasons: No free radical scavenging mechanism: UVA can absorb ultraviolet light, but cannot deal with the free radicals already generated. Under heat aging or UV leakage, the free radical chain reaction will still occur; Insufficient heat aging protection: UVA has no protective effect against heat-induced oxidation reactions, leading to a significant decrease in heat yellowing resistance; Lack of long-term stability: UVA may become ineffective due to its own degradation or migration during long-term use, and without the supplementation of HALS and AOX, the system's durability is poor. The above comparative results indicate that: UV absorption is only the first line of defense against yellowing and cannot replace free radical stabilization and anti-oxidation; heat aging and photo-aging need to be treated differently, but can be defended together through a synergistic system; UVA alone cannot meet the high requirements of white leather for long-term color stability.

[0131] In Comparative Example 5, the resin ratio was imbalanced (PU:PA = 4:1), resulting in the following performance characteristics: acceptable hiding power (0.975), but requiring two coats; slightly decreased yellowing resistance; a hand feel score of only 6.5, indicating significant hardening; and a substantial decrease in abrasion resistance to 5000 revolutions. Analysis of the causes: Decreased resin compatibility and film-forming properties: An excessively high PU ratio increases the risk of resin phase separation, reducing coating continuity and adhesion; Hardened hand feel: Although PU is soft, excessive amounts have poor compatibility with PA, leading to the formation of localized hard segments in the coating, resulting in a hardened hand feel and decreased elasticity; Reduced abrasion resistance: Insufficient PA adhesion and filling properties result in weak bonding between the coating and the substrate, making it prone to detachment under friction. The above comparison results indicate that the PU to PA blend ratio must be strictly controlled between 1:1 and 3:1 to balance flexibility and adhesion; the resin combination affects not only mechanical properties but also the overall structure and function of the coating; the film-forming resin is the skeleton of the coating agent, and its ratio directly affects the overall performance of the coating.

[0132] Product D6, prepared in Comparative Example 6, exhibits the following performance characteristics:

[0133] The hiding power is only 0.85, requiring four coats; the yellowing resistance Δb* is between 4.0 and 4.5; the feel score is 6.0, and the abrasion resistance is 5500 revolutions, all lagging behind. Analysis of the reasons: The hiding mechanism is outdated: relying on simple filling with ordinary hollow microspheres and fillers, lacking nano / micro synergy and structural design; the yellowing resistance system is rudimentary: achieving yellowing resistance only through resin modification (adding active zinc oxide), lacking a multi-layered chemical defense system; poor component synergy: each component functions relatively independently, failing to form an integrated synergistic network of hiding-defense-film formation-feel as in this invention. The above comparison results indicate that existing technologies still have significant shortcomings in hiding efficiency and long-term yellowing resistance; simple component stacking cannot achieve performance breakthroughs, and structural innovation and system integration are necessary.

[0134] In summary, this invention, through the innovative design of a structured pre-composite microsphere and a triple anti-yellowing defense system, successfully constructs a coating agent specifically for white leather that combines high masking efficiency, long-lasting anti-yellowing effect, excellent feel, and good environmental adaptability. The performance degradation of each comparative example verifies the necessity and synergy of the key technical features, highlighting the breakthrough progress of this invention in component selection, structural design, and system integration. This invention not only solves the long-standing technical problems of insufficient masking and easy yellowing of white leather, but also provides reliable material assurance for the application of white leather products in high-end footwear, bags, and home furnishings through efficient application performance (fewer sprays) and long-lasting service performance (aging resistance), demonstrating significant economic benefits and market competitiveness.

[0135] It should be understood that the application of the present invention is not limited to the examples above. Those skilled in the art can make improvements or modifications based on the above description, and all such improvements and modifications should fall within the protection scope of the appended claims.

Claims

1. A high hiding and yellowing resistant special-purpose white leather coating agent, characterized in that, The raw material composition includes, by mass percentage: 30%-50% of a film-forming resin composition; 10%-25% of a hiding function composition; 3%-8% of a yellowing resistance composition; 5%-15% of a hand feeling adjusting composition; 2%-5% of an emulsifying and dispersing composition; and the balance of deionized water; the hiding function composition includes structured pre-composite microspheres and free nano titanium dioxide, the structured pre-composite microspheres are acrylic ester polymer microspheres in which nano titanium dioxide particles are embedded in a shell polymer network; the yellowing resistance composition includes an ultraviolet absorber, a hindered amine light stabilizer and an antioxidant; the average particle size of the structured pre-composite microspheres is 0.5-3 μm, and the average particle size of the free nano titanium dioxide is 10-50 nm; the film-forming resin composition is composed of a water-based polyurethane resin and a water-based acrylic resin at a mass ratio of 1-3:1; the steps for preparing the structured pre-composite microsphere emulsion include: stirring and dispersing hydrophobic treated nano TiO2 in the presence of polycarboxylate dispersant to form a stable TiO2 slurry; preparing a seed monomer pre-emulsion containing methyl methacrylate, butyl acrylate, functional monomers and crosslinking agent; under nitrogen protection, the seed monomer pre-emulsion and water-soluble initiator are added dropwise into the TiO2 slurry, polymerization is initiated at 70-80℃, and primary composite latex particles with TiO2 aggregates as the core are generated; styrene is added to the system at one time, swells at 60-70℃ for 1.5-2 hours, and then is heated to 85-90℃ for secondary polymerization and crosslinking, in this process, phase separation occurs inside to form a hollow structure, at the same time, TiO2 particles are enriched and fixed in the shell polymer network under the driving of crosslinking kinetics, and finally, the structured pre-composite microsphere emulsion is obtained through cooling, filtering and adjusting the pH to 7.5-8.

5.

2. The high hiding and yellowing resistant special white leather coating agent according to claim 1, characterized in that, In the hiding function composition, the dry mass ratio of the structured pre-composite microspheres to the free nano titanium dioxide is 1.5-4:

1. 3.The white leather special high-hiding and yellowing-resistant compound finishing agent of claim 1, characterized in that, In the yellowing resistance composition, the mass ratio of the ultraviolet absorber, the hindered amine light stabilizer and the antioxidant is (1.5-3):(1-2):1, wherein the ultraviolet absorber is selected from benzotriazole or triazine compounds; and the antioxidant is selected from hindered phenol or phosphite compounds. 4.The white leather special high-hiding, yellowing-resistant compound finishing agent of claim 1, characterized in that, The hand feeling adjusting composition includes silicone elastomer and wax emulsion; and the emulsifying and dispersing composition includes non-ionic surfactant and polycarboxylate dispersant.

5. A process for the preparation of a high hiding and yellowing resistant white leather special coating composition according to any one of claims 1 to 4, characterized in that, The method includes the steps of: preparing the structured pre-composite microsphere emulsion in advance; adding 40-60% of the total mass of deionized water in the formula to a reaction kettle and heating to 45-55℃; adding the emulsifying and dispersing composition under stirring, and stirring at a speed of 400-600 r / min until completely dissolved and uniform; maintaining the temperature, and adding the free nano titanium dioxide and the structured pre-composite microsphere emulsion in sequence under stirring at a speed of 1000-1500 r / min, and continuously dispersing for 30-45 minutes until the system is uniform and fine; reducing the stirring speed to 500-700 r / min, slowly adding the film-forming resin composition, and stirring for 20-30 minutes to form a uniform resin emulsion composite system; and The stirring speed is reduced to 200-400 r / min, the anti-yellowing composition and the hand feel adjusting composition are added in turn, and each component is stirred for 10-15 minutes after being added until it is uniformly mixed; The remaining deionized water is added to adjust the solid content, and the pH value of the system is adjusted to 7.5-8.5 by using a pH adjuster; Finally, the product is discharged after filtration, and the white leather special high-hiding anti-yellowing compound finishing agent is obtained.

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