Low refractive index optical adhesive based on hollow silica and method of making the same

Abstract

The application discloses a kind of low refractive index optical glue based on hollow silica, optical glue preparation. To solve the problem that optical glue cannot simultaneously realize ultra-low refractive index, high light transmittance, low haze and long-term stability. The optical glue comprises: surface modified hollow silica particles;Fluorine-containing acrylate prepolymer;Active diluent;And, photoinitiator;Wherein, the particle size D50 of the hollow silica particle is less than 50 nm, its shell material is silica, the inside is cavity structure, and its surface is grafted with the polymerizable functional group that can copolymerize with the fluorine-containing acrylate prepolymer or active diluent. The optical glue of the application realizes the low refractive index of 1.25-1.30, has high light transmittance and low haze, and is dispersed stable, aging-resistant.

Description

Technical Field

[0001] This invention relates to the field of optical materials technology, and more specifically, to a low-refractive-index optical adhesive based on hollow silica and its preparation method. Background Technology

[0002] Optical adhesives are special adhesives used to bond optical components (such as lenses, prisms, display panels, etc.). Their optical properties (especially refractive index) directly affect the imaging quality and light efficiency of the entire optical system. In fields such as OLED displays, AR / VR waveguides, and holographic optical components, there is an urgent need for low-refractive-index optical materials with a refractive index below 1.35 or even 1.30 in order to achieve total internal reflection, eliminate interface reflections, or match specific optical path designs.

[0003] Currently, there are two common approaches to reducing the refractive index of materials: one is to introduce fluorine and utilize the low polarizability of fluorine atoms to prepare fluorinated resins (refractive index approximately 1.39); the other is to add low-refractive-index particles to the resin matrix. However, the refractive index of pure fluorinated resins is already close to its theoretical lower limit, making further reduction difficult. Therefore, adding hollow silica microspheres has become a research hotspot.

[0004] However, in practical applications, directly using hollow silica as an optical adhesive faces three major technical bottlenecks: Poor compatibility and easy aggregation: The surface of hollow silica is rich in silanol groups, which are highly hydrophilic, while the acrylates or fluorinated resins commonly used in optical adhesives are mostly hydrophobic. The polarity of the two is mismatched, which makes the particles very easy to aggregate and settle, and unable to form a stable dispersion system.

[0005] Optical performance degradation: Aggregated particles or interfacial defects between particles and resin can cause strong light scattering, resulting in a sharp increase in the haze of the adhesive layer and a significant decrease in light transmittance, which cannot meet the requirements of high transparency for optical applications.

[0006] The contradiction between particle size and performance: To obtain a lower refractive index, it is usually necessary to increase the hollow particle size or shell porosity, but this leads to a larger dispersed particle size, which intensifies light scattering; conversely, reducing the particle size helps transmittance, but the effect of reducing refractive index is not significant. How to balance this contradiction is a difficult problem that existing technologies have not been able to solve well.

[0007] Therefore, developing a low-refractive-index optical adhesive that combines ultra-low refractive index, high light transmittance, low haze, and excellent stability remains a pressing technical problem to be solved in this field. Summary of the Invention

[0008] To address the technical challenge that existing low-refractive-index optical adhesives cannot simultaneously achieve ultra-low refractive index, high transmittance, low haze, and long-term stability, this invention provides a low-refractive-index optical adhesive based on hollow silica and its preparation method. This optical adhesive achieves a low refractive index of 1.25-1.30, while also possessing high transmittance and low haze, as well as stable dispersion and aging resistance.

[0009] In a first aspect, the present invention provides a low-refractive-index optical adhesive based on hollow silica, the optical adhesive comprising: Surface-modified hollow silica particles; Fluorinated acrylate prepolymers; Reactive diluent; and Photoinitiator; The hollow silica particles have a particle size D50 of less than 50 nm, their shell material is silica, their interior has a hollow structure, and their surface is grafted with polymerizable functional groups that can undergo copolymerization reactions with the fluorinated acrylate prepolymer or reactive diluent.

[0010] Furthermore, The optical adhesive also contains an organic solvent, which includes one or more of propylene glycol methyl ether (PGME), ethyl acetate, and butanone. The components of the optical adhesive, by weight, include: 10-60 parts of modified hollow silica particles; 20-70 parts of fluorinated acrylate prepolymer; 5-15 parts reactive diluent; 1-5 parts of photoinitiator; Solvent 40-60 parts.

[0011] Furthermore, the hollow silica particles have an equivalent refractive index of 1.1 to 1.2, and the polymerizable functional group is derived from a silane coupling agent containing unsaturated double bonds, wherein the silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, or vinyltrimethoxysilane.

[0012] Furthermore, the optical adhesive layer formed after curing has a transmittance of ≥92% at a wavelength of 550 nm, a haze of ≤1.0%, and a refractive index of 1.25 to 1.30.

[0013] Furthermore, The fluorinated acrylate prepolymer is a copolymer of hexafluorobutyl acrylate with a molecular weight of 5,000 to 20,000.

[0014] The reactive diluent is an acrylate monomer containing one or more unsaturated double bonds.

[0015] In a second aspect, this disclosure provides a method for preparing an optical adhesive as described in any of the preceding claims, comprising the following steps: Hollow silica particles with polymerizable functional groups grafted onto their surface are prepared, wherein the particle size D50 of the hollow silica particles is less than 50 nm. The obtained hollow silica particles with polymerizable functional groups grafted on their surface were dispersed in an organic solvent and ultrasonically treated to obtain a uniform dispersion. Fluorinated acrylate prepolymer, reactive diluent and photoinitiator are added to the dispersion, and the mixture is stirred and mixed under light-protected conditions to obtain a mixture. The resulting mixture was subjected to vacuum degassing and then allowed to stand for aging to obtain the low-refractive-index optical adhesive.

[0016] Furthermore, the preparation of hollow silica particles with polymerizable functional groups grafted onto their surface includes: Monodisperse polystyrene microsphere templates were synthesized using soap-free emulsion polymerization or seed emulsion polymerization, wherein the polystyrene microspheres had a particle size of 20-100 nm. The polystyrene microsphere template was dispersed in an alcohol solvent, and an alkaline catalyst and tetraethyl orthosilicate were added. A silica shell was then coated on the surface of the polystyrene microspheres using a sol-gel method to obtain polystyrene-silica core-shell microspheres. The obtained polystyrene-silica core-shell microspheres were dispersed in an organic solvent, and the polystyrene template was removed by dissolution to obtain hollow silica particles; The obtained hollow silica particles were dispersed in an alcohol solvent, and a silane coupling agent containing unsaturated double bonds was added. The pH value was adjusted to acidic, and a surface modification reaction was carried out under heating conditions. After the reaction was completed, the particles were separated, washed, and dried to obtain hollow silica particles with polymerizable functional groups grafted on their surface.

[0017] Furthermore, The raw materials used to synthesize polystyrene microsphere templates include: styrene monomer, initiator, and co-stabilizing monomer; the initiator is potassium persulfate or ammonium persulfate, and the amount used is 1%-3% of the mass of styrene monomer; the co-stabilizing monomer is acrylic acid or methacrylic acid, and the amount used is 0.5%-2% of the mass of styrene monomer. The process of synthesizing polystyrene microsphere templates includes: Water and initiator are added to the reaction flask under inert gas protection and in a constant temperature water bath at 65-80℃. Styrene and co-stabilized monomers are mixed and slowly added dropwise to the reaction system; After reacting for 5-15 hours, a milky white emulsion with a bluish sheen is obtained. The emulsion was centrifuged and washed to remove unreacted monomers and electrolytes, and then redispersed in anhydrous ethanol / isopropanol to obtain polystyrene microsphere templates.

[0018] Furthermore, the alcohol solvent is anhydrous ethanol or isopropanol; the alkaline catalyst is ammonia water to adjust the pH of the system to 9-12; the tetraethyl orthosilicate is added slowly dropwise over 1-3 hours; the coating reaction temperature is 30-50℃ and the reaction time is 12-24 hours.

[0019] Furthermore, the silane coupling agent containing unsaturated double bonds is γ-methacryloxypropyltrimethoxysilane (KH-570), γ-methacryloxypropyltriethoxysilane (KH-572), or vinyltrimethoxysilane (VTMO); the amount of the silane coupling agent is 3%-10% of the mass of the hollow silica particles; The process of adjusting the pH to acidic and carrying out the surface modification reaction under heating conditions includes: Adjust the pH to 3.5-5.5 and react under heating conditions of 50-70℃ for 4-8 hours.

[0020] The optical adhesive product provided by this invention achieves a balance between ultra-low refractive index and high optical transparency by combining small-particle-size hollow silica particles with surface-grafted polymerizable functional groups with fluorinated acrylate resin. The hollow structure endows the particles with an extremely low equivalent refractive index, enabling the composite system to achieve a refractive index that is unattainable by traditional fluoropolymers. Meanwhile, the nanoscale particle size effectively suppresses Rayleigh scattering, ensuring high transmittance and extremely low haze in the visible light band. Simultaneously, the polymerizable functional groups on the particle surface chemically bond with the resin matrix during curing, forming a stable cross-linked network structure, completely solving the inherent defects of inorganic particles' tendency to aggregate and migrate in organic resins. The final product possesses excellent adhesion, flexibility, and aging resistance, meeting the stringent requirements for comprehensive optical adhesive performance in high-end fields such as OLED displays, AR / VR waveguides, and precision optical lenses.

[0021] This invention provides a method for preparing low-refractive-index optical adhesives, which constructs a complete, controllable, and easily scalable process route through the organic combination of template method, sol-gel method, and surface modification. First, a monodisperse polystyrene template is synthesized using soap-free emulsion polymerization, laying the foundation for subsequent uniform shell coating. Second, a silica shell is coated onto the template surface using the sol-gel method, with precise control of the TEOS dropping rate and reaction conditions to avoid secondary nucleation and ensure the integrity and thickness uniformity of the shell. Third, the template is gently removed using an organic solvent dissolution method, preserving the integrity of the shell structure and obtaining hollow silica with uniform particle size and complete cavity structure. Finally, surface grafting is performed using a silane coupling agent at specific pH and temperature to introduce polymerizable functional groups onto the particle surface, enabling it to chemically react with the resin matrix. The entire preparation process is mild, simple to operate, uses readily available raw materials, is environmentally friendly, and the key parameters of each step can be precisely controlled, exhibiting good batch stability and promising prospects for industrial scale-up. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0023] Figure 1 A schematic flowchart of a method for preparing a low-refractive-index optical adhesive provided for an embodiment of the present invention; Figure 2 A scanning electron microscope image of the modified hollow silica particles prepared in Example 1 of this invention; Figure 3 The figure shows the dynamic light scattering particle size distribution test results of the modified hollow silica particles prepared in Example 1 of this invention in propylene glycol methyl ether (PGME) dispersant; Figure 4 Fourier transform infrared spectrum of the modified hollow silica particles prepared in Example 1 of this invention; Figure 5 This is a physical image of the optical adhesive prepared in Example 1 of the present invention. Detailed Implementation

[0024] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0025] In this invention, the terms "first," "second," etc., are used for descriptive purposes only and should not be construed as indicating or implying their relative importance or implicitly specifying the number of technical features indicated. Therefore, a feature defined as "first" or "second" may explicitly or implicitly include at least one of that feature. In the description of this invention, "a plurality of" means at least two, such as two, three, etc., unless otherwise explicitly specified.

[0026] The present invention aims to overcome the technical defects of existing low-refractive-index optical adhesives, which are difficult to achieve ultra-low refractive index, high transmittance, low haze and long-term stability at the same time, and provides a low-refractive-index optical adhesive based on hollow silica and its preparation method.

[0027] Specifically, existing fluorinated resins have a lower limit on refractive index (approximately 1.35), which cannot meet the urgent need for high-end optical devices with refractive indices below 1.25 to 1.30. While adding hollow silica microspheres directly to the resin can reduce the refractive index, the hollow particles are rich in silanol groups and have strong hydrophilicity, resulting in poor compatibility with hydrophobic resins and easy aggregation. Furthermore, light scattering at the particle-resin matrix interface leads to a significant increase in the haze of the adhesive layer and a sharp decrease in light transmittance. In addition, there are problems such as poor storage stability and easy precipitation and stratification, which seriously restrict their application in practical optical devices.

[0028] To address the aforementioned technical bottlenecks, this disclosure aims to provide a low-refractive-index optical adhesive based on hollow silica, characterized by precisely adjustable refractive index to 1.25-1.30, high light transmittance and low haze, stable dispersion, and excellent overall performance. The optical adhesive comprises: Surface-modified hollow silica particles; Fluorinated acrylate prepolymers; Reactive diluent; and Photoinitiator; The hollow silica particles have a particle size D50 of less than 50 nm, their shell material is silica, their interior has a hollow structure, and their surface is grafted with polymerizable functional groups that can undergo copolymerization reactions with the fluorinated acrylate prepolymer or reactive diluent.

[0029] Meanwhile, this invention also aims to provide a preparation method that is process-controllable, easy to operate, and easy to scale up industrially. It synthesizes hollow silica with a specific structure through a template method and combines it with surface modification technology to fundamentally solve the interfacial compatibility problem between inorganic particles and organic resins, providing a feasible technical path for the large-scale production of high-performance optical adhesives.

[0030] The method includes the following steps: Hollow silica particles with polymerizable functional groups grafted onto their surface are prepared, wherein the particle size D50 of the hollow silica particles is less than 50 nm. The obtained hollow silica particles with polymerizable functional groups grafted on their surface were dispersed in an organic solvent and ultrasonically treated to obtain a uniform dispersion. Fluorinated acrylate prepolymer, reactive diluent and photoinitiator are added to the dispersion, and the mixture is stirred and mixed under light-protected conditions to obtain a mixture. The resulting mixture was subjected to vacuum degassing and then allowed to stand for aging to obtain the low-refractive-index optical adhesive.

[0031] This invention synthesizes hollow silica with a particle size D50 < 50 nm using a template method and modifies its surface with a silane coupling agent (such as KH-570) to form chemical bonds with a fluorinated resin. Based on the effective medium theory and Rayleigh scattering principle, the hollow structure reduces the equivalent refractive index of the particles to 1.1-1.2, the small particle size greatly suppresses light scattering, and the surface modification completely solves the problems of agglomeration and interfacial compatibility. The resulting optical adhesive has a refractive index that can be precisely controlled to 1.25-1.30, a transmittance ≥ 92%, a haze ≤ 1.0%, and exhibits no precipitation after 30 days of storage at 50℃. After aging at 85℃ / 85%RH for 1000 hours, its performance retention rate is > 95%. This solution successfully achieves a balance between ultra-low refractive index and high optical transparency, while also possessing excellent stability, meeting the urgent needs of cutting-edge fields such as OLED displays and AR / VR waveguides for high-performance optical adhesives.

[0032] This invention discloses a low-refractive-index optical adhesive based on hollow silica, the optical adhesive comprising: Surface-modified hollow silica particles; Fluorinated acrylate prepolymers; Reactive diluent; and Photoinitiator; The hollow silica particles have a particle size D50 of less than 50 nm, their shell material is silica, their interior has a hollow structure, and their surface is grafted with polymerizable functional groups that can undergo copolymerization reactions with the fluorinated acrylate prepolymer or reactive diluent.

[0033] The refractive index n of a material depends on its electron density and polarizability. For composite materials, the effective refractive index can be approximated by the Maxwell-Garnett effective medium theory. Hollow SiO2 has a shell of SiO2 (refractive index ≈ 1.46) and an interior of air (refractive index = 1.0), so the equivalent refractive index of the particles is much lower than that of solid SiO2. Theoretical calculations show that when the shell thickness and particle size ratio are appropriate, the equivalent refractive index of hollow particles can be as low as 1.1~1.2. When such ultra-low refractive index particles are dispersed in resin, the refractive index of the composite system can be reduced to 1.25~1.3, breaking the limit of fluoropolymers.

[0034] According to Rayleigh scattering theory, small particle size can avoid Rayleigh scattering. When the particle diameter is much smaller than the wavelength of light (400~700nm), the scattering intensity drops sharply. It is generally believed that scattering can be ignored when d < λ / 10 (i.e., < 50~60nm). This scheme controls D50 < 50nm to ensure that the transmittance in the visible light range is close to the theoretical limit. This is an optical prerequisite for achieving "high transmittance and low haze".

[0035] Surface modification addresses the issues of "compatibility" and "agglomeration." The problem with unmodified hollow SiO2 is that its surface is rich in silanol groups (Si-OH), making it highly hydrophilic; the resin (fluorinated acrylate) is hydrophobic. This polarity mismatch leads to particle agglomeration and precipitation, resulting in increased haze.

[0036] The modification mechanism of this invention, taking the action of KH-570 as an example: KH-570 is methacryloyloxysilane, and its molecular structure is: CH2=C(CH3)COO(CH2)3Si(OCH3)3; after hydrolysis, the methoxy group (-OCH3) condenses with the hydroxyl groups on the surface of SiO2 to form a covalent bond (Si-O-Si), which is firmly grafted; the methacryloyl group (C=C) copolymerizes with the acrylate double bond in the resin during curing to form chemical crosslinking points.

[0037] The effects of modification are as follows: the particle surface changes from hydrophilic to hydrophobic, significantly improving compatibility with the resin; the particles form chemical bonds with the resin, and after curing, the particles are fixed in the network, preventing migration and aggregation; the interface bonding is tight, reducing interfacial scattering and further reducing haze. The surface modification of this invention is the interfacial chemical basis for "long-term stability" and "low haze".

[0038] Furthermore, in the formulation of the optical adhesive, a refractive index gradient matching scheme was employed. Fluorine atoms have low polarizability, and the contribution of the CF bond refractive index is small. The resin itself already has a low refractive index (~1.39). Hexafluorobutyl acrylate was selected to further reduce the background refractive index. Through refractive index gradient design: resin matrix: 1.39; hollow particles: 1.1~1.2, the effective refractive index of the composite system is between the two, which can be precisely controlled to 1.25~1.3 by adjusting the particle content. Moreover, the difference between the refractive index of the particles and the resin is moderate, avoiding strong Mie scattering. Combined with small particle size, this achieves high overall optical uniformity and high transmittance in the optical adhesive.

[0039] This embodiment of the optical adhesive contains surface-modified hollow silica particles. The hollow structure endows the particles with an extremely low equivalent refractive index, enabling them to lower the refractive index of the composite system to a level unattainable by traditional fluoropolymers. Meanwhile, the nanoscale particle size effectively suppresses Rayleigh scattering, ensuring high transmittance and extremely low haze in the visible light band. Simultaneously, the polymerizable functional groups on the particle surface chemically bond with the resin matrix during curing, forming a stable cross-linked network structure, completely solving the inherent defects of inorganic particles' tendency to aggregate and migrate in organic resins.

[0040] Furthermore, The optical adhesive also contains an organic solvent, which includes one or more of propylene glycol methyl ether (PGME), ethyl acetate, and butanone. The components of the optical adhesive, by weight, include: 10-60 parts of modified hollow silica particles; 20-70 parts of fluorinated acrylate prepolymer; 5-15 parts reactive diluent; 1-5 parts of photoinitiator; Solvent 40-60 parts.

[0041] Furthermore, the hollow silica particles have an equivalent refractive index of 1.1 to 1.2, and the polymerizable functional group is derived from a silane coupling agent containing unsaturated double bonds, wherein the silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, or vinyltrimethoxysilane.

[0042] Silane coupling agents can achieve surface functionalization of hollow silica, introducing unsaturated double bonds that can be copolymerized with fluorinated acrylate prepolymers or reactive diluents.

[0043] Control of modification conditions

[0044] pH control: pH 3.5-5.5 is the optimal range for the hydrolysis of silane coupling agents. Too low or too high a pH will affect the grafting efficiency and stability. Reaction temperature: 50-70℃ can accelerate the hydrolysis-condensation reaction, but excessively high temperatures may cause the coupling agent to self-polymerize; Coupling agent dosage: 3-10% is the preferred range. Too low a dosage will result in insufficient grafting, while too high a dosage may lead to multilayer physical adsorption rather than chemical bonding.

[0045] Under the above conditions, three silane coupling agents containing unsaturated double bonds, KH-570, KH-572 and VTMO, can be successfully grafted onto the surface of hollow silica, giving the particles polymerizable functional groups and significantly improving their compatibility and dispersion stability with the resin.

[0046] Furthermore, the optical adhesive layer formed after curing has a transmittance of ≥92% at a wavelength of 550 nm, a haze of ≤1.0%, and a refractive index of 1.25 to 1.30.

[0047] Under optimized formulation and process conditions, the optical adhesive of the present invention achieves stable comprehensive performance of ≥92% light transmittance, ≤1.0% haze, and 1.25-1.30 after curing.

[0048] Furthermore, The fluorinated acrylate prepolymer is a copolymer of hexafluorobutyl acrylate with a molecular weight of 5,000 to 20,000.

[0049] The reactive diluent is an acrylate monomer containing one or more unsaturated double bonds.

[0050] A solution polymerization method was employed, using hexafluorobutyl acrylate as the main monomer, methyl methacrylate as the comonomer (to adjust the glass transition temperature), and azobisisobutyronitrile (AIBN) as the initiator. By controlling the amount of initiator and the reaction time, hexafluorobutyl acrylate copolymers with different molecular weights were synthesized. The molecular weight of the fluorinated acrylate prepolymer was controlled within the range of 5000-20000, achieving an optimal balance between film-forming properties, optical properties, and storage stability.

[0051] In practical applications, diluents with different functionalities can be combined and used as needed, for example: For high adhesion requirements: HEMA containing hydroxyl groups can be used; For high hardness requirements: TMPTA can be compounded with a monofunctional diluent to balance hardness and adhesion; For rapid curing requirements: Multifunctional diluents can be used.

[0052] The present invention also provides a method for preparing the optical adhesive as described in any of the preceding claims, comprising the following steps: Step S100: Prepare hollow silica particles with polymerizable functional groups grafted on their surface, wherein the particle size D50 of the hollow silica particles is less than 50 nm. Step S200: Disperse the obtained hollow silica particles with polymerizable functional groups grafted on their surface in an organic solvent and sonicate them to obtain a uniform dispersion. Step S300: Add fluorinated acrylate prepolymer, reactive diluent and photoinitiator to the dispersion, and stir and mix under light-protected conditions to obtain a mixture; Step S400: The obtained mixture is subjected to vacuum degassing treatment, and then allowed to stand for aging to obtain the low refractive index optical adhesive.

[0053] This embodiment uses a template method to synthesize hollow silica, graft polymerizable functional groups onto the surface, and compound it with fluorinated resin to finally obtain an optical adhesive product with excellent performance.

[0054] Specifically, the preparation of modified hollow silica particles includes: First, hollow silica particles were prepared using a template method. Monodisperse polystyrene microspheres were synthesized via emulsion polymerization as templates, with the particle size of the templates carefully selected and controlled to ensure the size requirements of the final product. The obtained polystyrene microsphere templates were dispersed in an alcohol solvent, and under alkaline conditions, using tetraethyl orthosilicate as the silicon source, a silica shell was coated onto the template surface using a sol-gel method, forming a polystyrene-silica core-shell structure. Subsequently, the core-shell particles were dispersed in an organic solvent, and the internal polystyrene template was removed by dissolution to obtain hollow silica particles. The resulting hollow particles possess a complete shell structure and internal cavities, and their particle size meets the requirements of the final product.

[0055] Then, the hollow silica particles were surface modified. The hollow silica particles were dispersed in an alcohol solvent, and a silane coupling agent containing unsaturated double bonds was added. The mixture was heated under acidic conditions to allow the silane coupling agent to be chemically grafted onto the particle surface. After the reaction, the particles were separated, washed, and dried to obtain modified hollow silica particles with polymerizable functional groups grafted onto their surface. These modified particles retain the low refractive index characteristics of the hollow structure while also possessing the ability to chemically bond with a resin matrix due to the introduction of polymerizable functional groups on their surface.

[0056] The preparation of optical adhesive includes: The modified hollow silica particles prepared above were dispersed in an organic solvent and then subjected to ultrasonic treatment to ensure thorough dispersion, forming a uniform and stable dispersion. Ultrasonic treatment helps to break up any potential soft agglomerates, ensuring that the particles are monodisperse in the system.

[0057] The fluorinated acrylate prepolymer, reactive diluent, and photoinitiator were added sequentially to the obtained dispersion, and the mixture was stirred and mixed under light-protected conditions. Light-protected operation prevents premature decomposition of the photoinitiator during preparation, ensuring its subsequent curing effect. Stirring and mixing ensures that all components are fully contacted and evenly distributed, forming a homogeneous mixture.

[0058] The mixture is subjected to vacuum degassing to remove air bubbles introduced during stirring, thus preventing optical defects in the cured adhesive layer. After degassing, the mixture is allowed to stand and mature for a period of time to allow the components to fully impregnate and balance, and the viscosity to stabilize, ultimately yielding a low-refractive-index optical adhesive.

[0059] In the preparation stage of modified hollow silica, the template method ensures the controllability of particle size and the integrity of structure; surface modification endows the particles with reactivity with the resin matrix, fundamentally solving the problems of dispersion stability and interfacial compatibility of inorganic particles in organic resin.

[0060] In the optical adhesive preparation stage, ultrasonic dispersion, light-protected mixing, vacuum degassing, and static curing are performed in tandem to ensure the uniformity, stability, and optical purity of the final product. The order and operating conditions of each step can be adjusted appropriately according to the actual production equipment and process requirements, but the overall process remains unchanged.

[0061] The raw materials used in this method are all common in the field, the preparation process requires no special equipment, is simple to operate, and has good prospects for industrial application. By adjusting the type and amount of each component, the performance of the final optical adhesive can be controlled within a wide range to meet the needs of different application scenarios.

[0062] The optical adhesive prepared by the method described in this embodiment achieves an effective reduction in refractive index while maintaining excellent optical transparency due to the use of small-particle-size hollow silica particles with surface-grafted polymerizable functional groups. The hollow structure endows the particles with an ultra-low equivalent refractive index, the small particle size suppresses light scattering, and the surface modification ensures uniform dispersion and long-term stability of the particles in the resin matrix. The resulting optical adhesive exhibits a uniform microstructure after curing, with no obvious particle agglomeration or interface defects, demonstrating excellent overall performance.

[0063] This method, through the synergistic cooperation of each step, successfully solves the technical problems of easy aggregation and precipitation of inorganic particles in organic resins, as well as severe interfacial scattering, providing a reliable technical path for the preparation of high-performance, low-refractive-index optical adhesives.

[0064] Furthermore, the preparation of hollow silica particles with polymerizable functional groups grafted onto their surface includes: Monodisperse polystyrene microsphere templates were synthesized using soap-free emulsion polymerization or seed emulsion polymerization, wherein the polystyrene microspheres had a particle size of 20-100 nm. The polystyrene microsphere template was dispersed in an alcohol solvent, and an alkaline catalyst and tetraethyl orthosilicate were added. A silica shell was then coated on the surface of the polystyrene microspheres using a sol-gel method to obtain polystyrene-silica core-shell microspheres. The obtained polystyrene-silica core-shell microspheres were dispersed in an organic solvent, and the polystyrene template was removed by dissolution to obtain hollow silica particles; The obtained hollow silica particles were dispersed in an alcohol solvent, and a silane coupling agent containing unsaturated double bonds was added. The pH value was adjusted to acidic, and a surface modification reaction was carried out under heating conditions. After the reaction was completed, the particles were separated, washed, and dried to obtain hollow silica particles with polymerizable functional groups grafted on their surface.

[0065] In a preferred embodiment, the process of creating polystyrene microsphere templates is as follows: In a 500 mL four-necked flask equipped with a stirrer, condenser, and nitrogen inlet tube, add 100 mL of deionized water and place in a 70°C constant temperature water bath. Purge with nitrogen for 30 minutes to remove dissolved oxygen. Weigh a certain amount of potassium persulfate (KPS), dissolve it in a small amount of deionized water, and add it to the reaction flask (KPS dosage is 1%-3% of the styrene mass, preferably 1.5%, i.e., 0.15 g). Weigh 10 g of purified styrene and a certain amount of acrylic acid (AA), mix thoroughly (AA dosage is 0.5%-2% of the styrene mass, preferably 1.0%, i.e., 0.1 g), and slowly add it dropwise to the reaction system over 1.5 hours using a constant pressure dropping funnel. After the addition is complete, continue the reaction at 70°C under nitrogen protection for 5-15 hours (preferably 8 hours) to obtain a milky white emulsion with a bluish tint.

[0066] The resulting emulsion was centrifuged at 8000 rpm for 15 minutes, the supernatant was discarded, and the precipitate was redispersed with anhydrous ethanol. This process was repeated three times by centrifugation and washing. The washed PS microspheres were then redispersed in anhydrous ethanol to obtain a PS microsphere template dispersion with a solid content of approximately 5%. Laser particle size analysis showed that the D50 of the PS microspheres was 20-100 nm (65 nm when the preferred embodiment was selected for each step).

[0067] Using the above-mentioned raw materials and proportions, PS microsphere templates with uniform particle size and good monodispersity were successfully synthesized, laying the foundation for subsequent coating with a uniform shell. By controlling the amount of initiator and co-stabilizing monomer, the template particle size can be precisely controlled to ensure that the final hollow silica particle size D50 < 50 nm.

[0068] The specific operations for coating the silica shell include: Take 100 mL of the above PS ethanol dispersion (diluted to 1% solid content), add 200 mL of anhydrous ethanol and 4 mL of concentrated ammonia (28%, as an alkaline catalyst, to adjust the pH of the system to about 9-12, preferably 10-11), place it in a 500 mL beaker, and stir magnetically (500 rpm) for 30 minutes at room temperature.

[0069] Place 1.2 mL of tetraethyl orthosilicate (TEOS) in a syringe and slowly add it dropwise to the above system at a rate of 0.5 mL / min using a micro-injection pump over approximately 1-3 hours, preferably 2.5 hours. Maintain stirring during the addition process to prevent localized high concentrations that could lead to TEOS nucleation. After the addition is complete, continue the reaction at room temperature for 12-24 hours (reaction temperature 30-50℃, reaction time preferably 18 hours) to allow the silica shell to fully grow.

[0070] After the reaction was completed, the reaction solution was centrifuged at 4000 rpm for 10 minutes, and the precipitate was collected to obtain PS-SiO2 core-shell microsphere gel.

[0071] By controlling the TEOS dropping rate and reaction conditions, a uniform and continuous silica shell was successfully coated on the PS template surface, avoiding secondary nucleation and ensuring the integrity and thickness uniformity of the shell.

[0072] Removal of polystyrene template

[0073] The polystyrene template is removed by dissolving it. Specific steps include: The above PS-SiO2 core-shell microsphere gel was dispersed in 100 mL of an organic solvent, such as toluene, xylene, or tetrahydrofuran (preferably toluene). After ultrasonic-assisted dispersion for 10 minutes, the gel was transferred to a round-bottom flask and refluxed in a water bath at 40-60°C for 6-12 hours (preferably refluxed at 50°C for 8 hours) to fully dissolve the PS template.

[0074] The dissolved dispersion was centrifuged at 8000 rpm for 15 minutes, the supernatant was discarded, and the precipitate was repeatedly washed five times with anhydrous ethanol by centrifugation. After each washing, the precipitate was redispersed to completely remove residual toluene and dissolved PS. Hollow silica gel was finally obtained.

[0075] By using toluene to dissolve the PS template under heating conditions, the complete removal of the template was ensured while avoiding damage to the silica shell, resulting in hollow silica particles with intact structure and clear cavities.

[0076] Surface modification of hollow silica includes: Take the above-mentioned hollow silica gel, determine its solid content, and then take 5 grams of hollow silica (dry weight) and disperse it in 100 mL of anhydrous ethanol. Sonicate the mixture for 30 minutes (300 W) to ensure thorough particle dispersion. Add 0.25 g of γ-methacryloyloxypropyltrimethoxysilane (KH-570) to the dispersion and adjust the pH to 3.5-5.5 (preferably 4.5) with glacial acetic acid. Transfer the reaction system to a water bath at 50-70°C (preferably 60°C) and stir magnetically for 4-8 hours (preferably 6 hours).

[0077] After the reaction was completed, the reaction solution was centrifuged at 8000 rpm for 15 minutes, and the precipitate was collected. The precipitate was washed three times with anhydrous ethanol, and after each wash, it was redispersed and centrifuged. The washed product was placed in a vacuum drying oven at 50℃ and dried for 12 hours to obtain modified hollow silica particles with methacryloyloxy groups (polymerizable functional groups) grafted onto their surface.

[0078] Laser particle size analyzer measurements showed that the modified hollow silica particles obtained using optimized parameters had a D50 of 35 nm and a D90 of 52 nm, exhibiting uniform particle size distribution. Infrared spectroscopy analysis revealed that the modified particles had a particle size distribution at 1750 cm⁻¹. - ¹ and 1605 cm - The presence of characteristic absorption peaks for C=O and C=C at position ¹ confirms successful grafting of KH-570. Thermogravimetric analysis revealed a grafting rate of approximately 4.6%.

[0079] By optimizing the modification conditions, polymerizable functional groups were successfully introduced into the surface of hollow silica, which significantly improved the compatibility between the particles and the resin, laying the foundation for the subsequent preparation of stable and high-performance optical adhesives.

[0080] The particle size range of polystyrene microsphere templates is 20-100 nm. However, to achieve a final hollow silica particle size D50 of less than 50 nm, those skilled in the art should understand that it is necessary to select an appropriate particle size within this range (preferably 30-65 nm) and control the silica shell thickness (preferably 5-10 nm) and utilize the shell densification shrinkage effect during subsequent drying to ensure that the final product particle size meets the requirement of D50 < 50 nm. As shown in the preferred embodiment, using a PS template with a particle size of 55 nm, and controlling the shell thickness to approximately 8 nm, combined with drying shrinkage, hollow silica with a particle size of 35 nm was finally obtained. For PS templates with larger particle sizes (e.g., 80-100 nm), this can be achieved by preparing a thinner shell (e.g., 3-5 nm) and combining it with a higher shrinkage rate, but this may affect the shell integrity and particle strength, and is therefore not a preferred solution. Those skilled in the art can optimize suitable process parameters within the defined range through conventional experiments according to the target particle size.

[0081] Furthermore, The raw materials used to synthesize polystyrene microsphere templates include: styrene monomer, initiator, and co-stabilizing monomer; the initiator is potassium persulfate or ammonium persulfate, and the amount used is 1%-3% of the mass of styrene monomer; the co-stabilizing monomer is acrylic acid or methacrylic acid, and the amount used is 0.5%-2% of the mass of styrene monomer. The process of synthesizing polystyrene microsphere templates includes: Water and initiator are added to the reaction flask under inert gas protection and in a constant temperature water bath at 65-80℃. Styrene and co-stabilized monomers are mixed and slowly added dropwise to the reaction system; After reacting for 5-15 hours, a milky white emulsion with a bluish sheen is obtained. The emulsion was centrifuged and washed to remove unreacted monomers and electrolytes, and then redispersed in anhydrous ethanol / isopropanol to obtain polystyrene microsphere templates.

[0082] The initiator (potassium persulfate or ammonium persulfate) generates free radicals upon thermal decomposition, initiating the polymerization reaction of styrene monomer. The amount of initiator directly affects the number of nuclei, the polymerization rate, and the final particle size and distribution of microspheres. A dosage of 1%-3% of the styrene monomer mass achieves a moderate number of nuclei, ensures stable polymerization, and yields well-dispersed PS microspheres with a particle size of 20-100 nm, meeting the requirements for template application. Co-stabilizing monomers (acrylic acid or methacrylic acid) copolymerize onto the polystyrene chain during polymerization, introducing carboxyl (-COOH) functional groups. These carboxyl groups ionize under alkaline conditions, providing electrostatic repulsion, enhancing emulsion stability, and preventing particle agglomeration. A dosage of 0.5%-2% of the styrene monomer mass provides sufficient charge density on the particle surface, good emulsion stability, and a moderate carboxyl content, without affecting the subsequent silica shell coating and adhesion. The temperature of the constant-temperature water bath affects the decomposition rate of the initiator, the diffusion rate of the monomer, and the polymerization rate. Soap-free emulsion polymerization is typically carried out under heating conditions to provide sufficient activation energy. Within this temperature range, the initiator (KPS or APS) has a moderate half-life and a stable decomposition rate. Choosing 65-80℃, the polymerization reaction can be completed within 5-15 hours, resulting in high conversion rates and uniform microsphere size. Through a reaction time of 5-15 hours, monomer conversion can reach over 90%, the microsphere size reaches equilibrium, the particle size distribution is stable, and the emulsion exhibits a milky white appearance with a bluish sheen, indicating that the particle size is at the nanoscale and uniformly distributed.

[0083] The above parameter ranges are set based on the nucleation mechanism and particle size control theory of soap-free emulsion polymerization, aiming to obtain polystyrene microsphere templates with good monodispersity, controllable particle size, and emulsion stability. Those skilled in the art can reliably obtain template particles that meet subsequent coating requirements by making routine adjustments within the above ranges.

[0084] Furthermore, the alcohol solvent is anhydrous ethanol or isopropanol; the alkaline catalyst is ammonia water to adjust the pH of the system to 9-12; the tetraethyl orthosilicate is added slowly dropwise over 1-3 hours; the coating reaction temperature is 30-50℃ and the reaction time is 12-24 hours.

[0085] Alcohol solvents, used as reaction media, must dissolve tetraethyl orthosilicate (TEOS) and ammonia while maintaining good dispersion of the polystyrene template. The polarity and viscosity of the alcohol affect the hydrolysis and condensation rate of TEOS, as well as the dispersion stability of the particles in the system. Anhydrous ethanol: With moderate polarity, it has good solubility for both TEOS and ammonia, and the PS template is stably dispersed within it. It is the most commonly used solvent in the sol-gel method for coating silica, offering a moderate and easily controllable reaction rate. Isopropanol: With slightly lower polarity than ethanol, it can further reduce the hydrolysis rate of TEOS, which is beneficial for obtaining a more uniform and denser shell, suitable for scenarios requiring precise control of shell thickness. Ammonia acts as an alkaline catalyst, catalyzing the hydrolysis and condensation reactions of TEOS. Under alkaline conditions, the deprotonation degree of the silanol groups (Si-OH) increases, accelerating the condensation reaction rate. pH directly affects the hydrolysis rate, condensation mode, and shell density of TEOS. Adjusting the pH to 9-12 provides a suitable alkaline range for TEOS hydrolysis. Within this range, the condensation reaction primarily follows a core-shell growth pattern, with silanol groups preferentially condensing with existing Si-OH groups on the template surface. This promotes the formation of a continuous and dense shell layer, minimizing self-nucleation. The method of TEOS addition directly determines the concentration of silicon source in the reaction system. Slow addition maintains a low concentration of silica monomers, ensuring that the hydrolysis-condensation reaction occurs preferentially on the PS template surface rather than in solution, thus preventing self-nucleation. The timing of addition, the temperature of the coating reaction, and the reaction time are all carefully controlled to ensure a moderate TEOS hydrolysis-condensation rate, stable shell growth, and the acquisition of a uniformly thick and dense silica shell layer.

[0086] The above parameter range is set based on the nucleation and growth theory of the sol-gel method, aiming to achieve selective deposition of TEOS on the PS template surface, avoiding homogeneous self-nucleation, and obtaining core-shell particles with continuous shells, uniform thickness, and dense structure. Those skilled in the art can make routine adjustments within the above range to stably meet the requirements of subsequent template removal and hollow particle preparation.

[0087] Furthermore, the silane coupling agent containing unsaturated double bonds is γ-methacryloxypropyltrimethoxysilane (KH-570), γ-methacryloxypropyltriethoxysilane (KH-572), or vinyltrimethoxysilane (VTMO); the amount of the silane coupling agent is 3%-10% of the mass of the hollow silica particles; The process of adjusting the pH to acidic and carrying out the surface modification reaction under heating conditions includes: Adjust the pH to 3.5-5.5 and react under heating conditions of 50-70℃ for 4-8 hours.

[0088] Silane coupling agents act as "molecular bridges." The alkoxy group (-OCH3 or -OC2H5) at one end hydrolyzes and undergoes a condensation reaction with the silanol group (Si-OH) on the surface of hollow silica to form a strong Si-O-Si covalent bond. The polymerizable functional group (methacryloyloxy or vinyl) at the other end can copolymerize with fluorinated acrylate prepolymers or reactive diluents during curing to achieve chemical bonding between the particles and the resin matrix. KH-570, KH-572, and VTMO can be used. The amount of coupling agent used determines the grafting density on the particle surface. Insufficient dosage results in incomplete particle surface coverage, inadequate hydrophobicity, and limited improvement in resin compatibility. Excessive dosage may lead to multilayer physical adsorption rather than single-layer chemical bonding, wasting raw materials and potentially affecting particle surface properties. Within the 3%-10% range, complete monolayer grafting can be formed, transforming the particle surface from hydrophilic to hydrophobic, significantly improving compatibility with fluorinated resins. Simultaneously, the number of grafted functional groups is sufficient to form adequate chemical cross-links with the resin during curing. pH directly affects the hydrolysis and condensation reactions of silane coupling agents. Under acidic conditions, the alkoxy groups in silane coupling agents hydrolyze to generate silanols (Si-OH), which then condense with the silanols on the particle surface to form Si-O-Si bonds. Both excessively low and high pH values ​​affect the hydrolysis rate and condensation mode. The modification reaction pH should be controlled between 3.5 and 5.5. The modification reaction temperature is controlled at 50-70℃. Within this temperature range, the hydrolysis and condensation reaction rates of the silane coupling agent are moderate and can be completed within 4-8 hours. The grafting efficiency is high and it will not cause the coupling agent to self-polymerize or decompose.

[0089] This disclosure employs a soap-free emulsion polymerization method to synthesize monodisperse polystyrene templates. By controlling the amounts of initiator and co-stabilizing monomers, template particles with uniform particle size and good monodispersity can be obtained. Based on this, a silica shell is coated onto the template surface using a sol-gel method, with precise control of the tetraethyl orthosilicate dropping rate and reaction conditions. This effectively avoids self-nucleation and ensures a uniform, continuous, and controllable shell thickness. After template removal, the resulting hollow silica particles have a complete structure and clear cavities. Through subsequent shell densification and shrinkage during drying, the final particle size can be stably controlled below 50 nanometers, providing a structural basis for high transmittance and low haze. Surface grafting modification of the hollow silica is performed using a silane coupling agent containing unsaturated double bonds. By precisely controlling the pH value, reaction temperature, and reaction time, the coupling agent forms a strong chemical bond with the silanol groups on the particle surface, introducing polymerizable functional groups onto the particle surface. This modification process maintains the integrity of the hollow structure while transforming the particles from hydrophilic to hydrophobic, significantly improving their compatibility with fluorinated acrylate resins and fundamentally solving the inherent defects of inorganic particles easily agglomerating and settling in organic resins. The grafted polymerizable functional groups can copolymerize with the resin matrix during subsequent curing, forming a chemical cross-linking network that firmly anchors the particles within the system, ensuring the long-term storage stability and reliability of the optical adhesive. Reasonable optimal ranges are provided for the key parameters of each step, allowing those skilled in the art to optimize and adjust them through routine experiments within these ranges to stably obtain the target product. Steps such as template synthesis, shell coating, template removal, and surface modification are all completed in conventional reaction equipment, requiring no special devices, making operation simple and easy to scale up for production. Operations such as ultrasonic dispersion, light-protected mixing, vacuum degassing, and static curing in the optical adhesive preparation process are all mature processes in the field, demonstrating good industrial feasibility. The optical adhesive prepared by the above method achieves a balance between ultra-low refractive index and high optical transparency by using small-particle-size hollow silica particles with surface-grafted polymerizable functional groups. The hollow structure endows the particles with an extremely low equivalent refractive index, enabling the composite system to have a refractive index as low as 1.25-1.30. The nanoscale particle size effectively suppresses Rayleigh scattering, and surface modification eliminates interface scattering, ensuring a transmittance ≥92% and haze ≤1.0%. The chemically bonded network structure provides excellent dispersion stability and aging resistance. The optical adhesive prepared by this method has broad application prospects in high-end fields such as OLED displays, AR / VR waveguides, and precision optical lenses.

[0090] This invention also provides a method for preparing a low-refractive-index optical adhesive based on hollow silica, comprising: Preparation of hollow silica 1. Synthesis of monodisperse polystyrene microsphere templates Formulation: Styrene (purified to remove polymerization inhibitor): 10 g; Deionized water: 100 mL; Initiator: Potassium persulfate (KPS): 0.1-0.2 g; Co-stabilizing monomer: Small amount of acrylic acid (AA, about 0.1 g) or methacrylic acid to provide surface charge and stabilize the emulsion.

[0091] Process: 1) Under nitrogen protection and in a constant temperature water bath at 70-75°C, add water and part of the initiator to the reaction flask.

[0092] 2) Styrene and acrylic acid are mixed and slowly added dropwise to the reaction system.

[0093] 3) React for 6-12 hours to obtain a milky white emulsion with a bluish tint.

[0094] 4) Centrifuge and wash to remove unreacted monomers and electrolytes, then redisperse in anhydrous ethanol / isopropanol for later use.

[0095] 2. Silica-coated shell

[0096] Formula: PS ethanol dispersion (1% solid content): 100 mL; anhydrous ethanol: 200 mL; concentrated ammonia (28%): 3-5 mL (catalyst, to create an alkaline environment, pH ~11); tetraethyl orthosilicate (TEOS): 0.5 - 2.0 mL.

[0097] Process and Control: 1) Mix the PS dispersion, ethanol, and ammonia, and stir with a strong magnetic stirrer (500 rpm) for 30 minutes at room temperature.

[0098] 2) Slowly add TEOS (using a microinfusion pump, to be completed within 1-2 hours).

[0099] 3) After the addition is complete, continue the reaction for 12-24 hours.

[0100] 4) The PS-SiO2 core-shell microsphere gel was obtained by centrifugation at 4000 r / min.

[0101] 3. Remove the PS template to obtain the hollow SI02.

[0102] Process: 1) Disperse the PS-SiO2 gel obtained by centrifugation in toluene, and use ultrasound assistance to soak or reflux for 12-24 hours to dissolve the PS core. Then centrifuge to obtain hollow SiO2 gel.

[0103] 2) Wash the hollow SIO2 gel by centrifugation five times with ethanol.

[0104] 3. Modification of hollow silica

[0105] To determine the solid content of the SIO2 gel, 5 grams of the active ingredient, hollow SIO2, was dispersed in 100 mL of anhydrous ethanol and ultrasonically dispersed for 30 minutes. 0.25 g of silane coupling agent KH-570 (equivalent to 5% of the silica mass) was added, and the pH was adjusted to 4-5 with glacial acetic acid. The mixture was stirred in a 60°C water bath for 6 hours. After the reaction, the mixture was centrifuged, and the precipitate was washed three times with anhydrous ethanol and vacuum dried at 50°C for 12 hours to obtain methacryloyloxy-modified hollow silica particles. Laser particle size analysis showed that the product particle size was D50 < 50 nm and D90 < 70 nm. 4. Preparation of low refractive index optical adhesive Component preparation: Modified hollow silica (prepared in Example 1): 25 parts by weight; Fluorinated acrylate prepolymer (hexafluorobutyl acrylate copolymer, molecular weight 10000): 60 parts by weight; Reactive diluent (hydroxyethyl methacrylate): 12 parts by weight; Photoinitiator (1173 / 184 / TPO-L): 3 parts by weight (one or two in combination); Solvent (PGME): 50 parts by weight; Preparation process: Under light-protected conditions, modified hollow silica was dispersed in PGME and ultrasonically treated for 20 minutes to ensure thorough dispersion. Then, fluorinated acrylate prepolymer, reactive diluent, and photoinitiator were added, and the mixture was stirred at 1500 rpm for 30 minutes in a high-speed disperser to ensure homogeneous mixing. The mixture was then placed in a vacuum degassing chamber and degassed at -0.08 MPa for 30 minutes, followed by standing and curing for 2 hours to obtain a low-refractive-index optical adhesive.

[0106] Example 1

[0107] This embodiment discloses a method for preparing a low-refractive-index optical adhesive based on hollow silica. The method involves preparing surface-modified hollow silica using optimal implementation parameters and then using the prepared surface-modified hollow silica to formulate a low-refractive-index optical adhesive, including the following steps.

[0108] 1. Preparation of hollow silica, including: Synthesis of PS template: 10 g purified styrene, 0.15 g potassium persulfate (KPS), 0.1 g acrylic acid (AA), and 100 mL deionized water were added to the reaction flask under nitrogen protection and in a 70°C constant temperature water bath. The styrene and acrylic acid were mixed thoroughly and slowly added dropwise to the reaction system, completing the addition within 2 hours. The reaction was continued for 8 hours to obtain a milky white emulsion with a bluish tint. The emulsion was centrifuged and washed three times at 8000 rpm to remove unreacted monomers and electrolytes, and then redispersed in anhydrous ethanol to obtain a PS microsphere template dispersion with a solid content of approximately 5%. Laser particle size analysis showed that the D50 of the PS microspheres was 65 nm.

[0109] Coating of PS-SiO2 core-shell microspheres: The PS ethanol dispersion with a solid content of 5% was diluted with anhydrous ethanol to a solid content of 1%. 100 mL of the PS ethanol dispersion obtained in Example A1 (1% solid content) was added to 200 mL of anhydrous ethanol and 4 mL of concentrated ammonia (28%), and the mixture was magnetically stirred at room temperature for 30 minutes. 1.2 mL of tetraethyl orthosilicate (TEOS) was slowly added dropwise at a rate of 0.5 mL / min using a micro-injection pump over approximately 2.5 hours. After the addition was complete, the reaction was continued at room temperature for 18 hours. After the reaction was complete, the mixture was centrifuged at 4000 rpm to obtain the PS-SiO2 core-shell microsphere gel.

[0110] Template removal and acquisition of hollow SiO2: The obtained PS-SiO2 gel was dispersed in 100 mL of toluene. After ultrasonic-assisted dispersion, it was refluxed and stirred at 50 °C for 12 hours to dissolve the PS template. The gel was separated by centrifugation at 8000 rpm, and the precipitate was washed five times with anhydrous ethanol to obtain hollow silica gel.

[0111] 2. Surface modification of hollow SiO2, including: After determining the solid content of the obtained hollow silica gel, take the equivalent of 5 g dry weight of hollow SiO2 and disperse it in 100 mL anhydrous ethanol, then sonicate for 30 minutes. Add 0.25 g KH-570 (silane coupling agent, equivalent to 5% of the SiO2 mass), and adjust the pH to 4.5 with glacial acetic acid. Stir the reaction in a 60℃ water bath for 6 hours. After the reaction, centrifuge at 8000 rpm, wash the precipitate three times with anhydrous ethanol, and dry it under vacuum at 50℃ for 12 hours to obtain methacryloyloxy-modified hollow silica particles. The particle size of the product was determined by a laser particle size analyzer to be D50 of 45 nm and D90 of 68 nm.

[0112] 3. Formulation of low refractive index optical adhesive

[0113] Based on the modified hollow silica particles prepared above, a low-refractive-index optical adhesive was formulated.

[0114] Preparation of optical adhesive (modified SiO2 content 35 parts)

[0115] Prepare the ingredients according to the following weight proportions: 35 parts of the modified hollow silica particles obtained Fluorinated acrylate prepolymer (hexafluorobutyl acrylate copolymer, molecular weight 10000): 47 parts Reactive diluent (hydroxyethyl methacrylate, HEMA): 15 parts Photoinitiator (1173 and 184 mixed in a 1:1 ratio): 3 parts Solvent (propylene glycol methyl ether, PGME): 50 parts Preparation process: Under light-protected conditions, modified hollow silica particles were added to PGME and ultrasonically treated for 20 minutes (300W) to ensure full dispersion and obtain a uniform dispersion.

[0116] Add fluorinated acrylate prepolymer, HEMA and photoinitiator to the dispersion, and stir in a high-speed disperser at 1500 rpm for 30 minutes in the dark to ensure that the components are mixed evenly.

[0117] The mixture was placed in a vacuum degassing chamber and degassed at -0.08 MPa for 30 minutes, then allowed to stand and mature for 2 hours to obtain a low-refractive-index optical adhesive.

[0118] Figure 2 The image shows a scanning electron microscope (SEM) image (accelerating voltage 10.00 kV, magnification 100,000x, scale bar 100 nm) of the modified hollow silica particles prepared in Example 1 of this invention. As can be seen from the image, the particles exhibit a regular spherical morphology, a smooth surface, and a relatively uniform particle size distribution. The image shows the size measurements of six typical particles, with diameters of 53.59 nm, 53.59 nm, 40.19 nm, 41.31 nm, 53.59 nm, and 44.66 nm, respectively. The calculated average particle size is approximately 47.8 nm, and the median particle size is approximately 49.1 nm. All measured particles have diameters within the range of 40–55 nm, and no obvious aggregates or non-spherical particles were observed, indicating that the prepared hollow silica particles have good monodispersity and nanoscale size, meeting the requirement of this invention for a particle size D50 of less than 50 nm.

[0119] Figure 3This is a dynamic light scattering (DLS) particle size distribution (number distribution) curve of the modified hollow silica particles prepared in Example 1 of this invention in propylene glycol methyl ether (PGME) dispersant. The test temperature was 25.0 ℃, the refractive index of the dispersant was 1.403, and the refractive index of the sample was set to 1.60. As can be seen from the particle size distribution curve, the sample exhibits a single sharp peak with a peak particle size of approximately 37.8 nm, without secondary peaks or tailing, indicating that the particles are not agglomerated and free of large particle impurities. According to the instrumental analysis results, the number-average particle size D10 of this sample is 26.2 nm, and D50 is 35.6 nm, with D50 significantly less than 50 nm; the intensity-average particle size Z-Average is 64.47 nm, and the polydispersity index (PdI) is 0.143, indicating a narrow particle size distribution and good monodispersity. The DLS data further confirms that the hollow silica particles prepared in this invention have sub-50 nanometer size and excellent particle size uniformity, providing a structural basis for achieving optical adhesives with high light transmittance and low haze.

[0120] Figure 4 The Fourier transform infrared (FTIR) spectrum of the modified hollow silica particles prepared in Example 1 of this invention is shown. Several characteristic absorption peaks can be observed in the spectrum: at approximately 1075 cm⁻¹. - A strong and broad absorption band appears at ¹, which is attributed to the Si-O-Si asymmetric stretching vibration of the silica shell, confirming that the bulk of the particle is silica; at approximately 795 cm⁻¹... - ¹ and 506 cm - The absorption peaks at ¹ correspond to the symmetric stretching and bending vibrations of Si-O-Si, respectively, further confirming the existence of the silica network structure. Particularly important is the absorption peak at approximately 1750 cm⁻¹ in the spectrum. - A stretching vibration peak of the carbonyl group (C=O) appears at approximately 1605 cm⁻¹. - A stretching vibration peak of a carbon-carbon double bond (C=C) appeared at position ¹. Both characteristic peaks originated from the methacryloyloxy group in the surface-grafted γ-methacryloyloxypropyltrimethoxysilane (KH-570). Compared with unmodified hollow silica (containing only Si-O characteristic peaks), the appearance of C=O and C=C peaks in this spectrum directly proves at the chemical structure level that the silane coupling agent has been successfully grafted onto the surface of hollow silica particles via covalent bonds, introducing active unsaturated double bonds that can participate in subsequent photocuring reactions. This result strongly supports the conclusion of this invention that "polymerizable functional groups are grafted onto the surface," providing key structural evidence for improving the compatibility and interfacial bonding strength between particles and resin.

[0121] Figure 5 The image shown is a physical picture of the optical adhesive prepared in Example 1 of the present invention. It can be seen that the adhesive has good light transmittance and achieves a balance between ultra-low refractive index and high optical transparency during use.

[0122] Example 2: Preparation of optical adhesive (10 parts modified SiO2)

[0123] The process is basically the same as in Example 1, except that the amount of modified hollow silica particles is adjusted to 10 parts and the amount of fluorinated acrylate prepolymer is adjusted to 70 parts, while the other components and processes remain unchanged.

[0124] Example 3: Preparation of optical adhesive (60 parts modified SiO2)

[0125] The process is basically the same as in Example 1, except that the amount of modified hollow silica particles is adjusted to 60 parts and the amount of fluorinated acrylate prepolymer is adjusted to 20 parts, while the other components and processes remain unchanged.

[0126] Example 4: Preparation of optical adhesives (different solvents)

[0127] The process is basically the same as in Example 1, except that the solvent PGME is replaced with an equal amount of ethyl acetate, while the other components and processes remain unchanged.

[0128] Example 5: Preparation of optical adhesives (with different initiators)

[0129] The process is basically the same as in Example 1, except that the photoinitiator is replaced with an equal amount of TPO-L, while the other components and processes remain unchanged.

[0130] Comparative Example 1: No hollow silica

[0131] The process is basically the same as in Example 1, except that no hollow silica particles are added, the amount of fluorinated acrylate prepolymer is adjusted to 82 parts, and the other components and processes remain unchanged.

[0132] Comparative Example 2: Addition of unmodified hollow silica

[0133] The process is basically the same as in Example 1, except that the added hollow silica particles are the unmodified hollow silica particles obtained in Example 1 (without KH-570 modification), while the other components and processes remain unchanged.

[0134] Comparative Example 3: Adding solid silica

[0135] The process is basically the same as in Example 1, except that the modified hollow silica particles are replaced with an equal amount of solid silica nanoparticles (50 nm in diameter, unmodified), while the other components and processes remain unchanged.

[0136] The performance of the optical adhesives in the examples and comparative examples was tested according to the following methods: Test items and test methods Refractive index: The optical adhesive was coated onto a quartz plate, cured under ultraviolet light to form a dry film, and measured using an Abbe refractometer at 25℃ and 550 nm wavelength. Transmittance: The transmittance at a wavelength of 550 nm was measured using a haze meter according to ASTM D1003 standard. Haze: The haze value was measured using a haze meter according to ASTM D1003 standard; Adhesion: Performed according to ASTM D3359 standard; Storage stability: After sealing the optical adhesive, place it in a 50℃ constant temperature oven and observe whether precipitation or stratification occurs after 30 days; Aging resistance: The cured film was placed in a constant temperature and humidity chamber at 85℃ / 85%RH for 1000 hours, and the light transmittance retention rate was measured.

[0137] The test results are summarized below:

[0138] To verify the rationality of the parameters of the present invention, based on Example 1, and with the remaining parameters selected according to the preferred implementation method, the effects of changing different key parameters on the optical adhesive were tested as shown in the following sub-examples.

[0139] Sub-example 1-1: Effect of different initiator dosages

[0140] The results show that PS templates with moderate particle size and good monodispersity can be obtained when the initiator dosage is controlled within the range of 1%-3%. If the dosage is exceeded, the particle size distribution becomes wider or the particle size exceeds the standard, which affects the final optical performance.

[0141] Sub-examples 1-2: Effect of different amounts of co-stabilizing monomers

[0142] The results show that the amount of co-stabilizing monomer in the range of 0.5%-2% can effectively stabilize the emulsion, while avoiding excessive use that could lead to shell defects.

[0143] Sub-examples 1-3: Effects of different TEOS dropping rates

[0144] The results show that controlling the TEOS dropping time within 1-3 hours can avoid self-nucleation and obtain core-shell particles with uniform shells.

[0145] Sub-examples 1-4: Effects of different types of silane coupling agents

[0146] The results showed that all three silane coupling agents containing unsaturated double bonds could be effectively grafted, imparting polymerizable functional groups to the particles and significantly improving the optical adhesive properties.

[0147] Sub-examples 1-5: Effect of different coupling agent dosages

[0148] The grafting rate of silane coupling agents is measured using thermogravimetric analysis (TGA). The calculated "grafting rate" is a relative quantitative indicator, not an absolute conversion rate.

[0149] The results showed that an amount of coupling agent in the range of 3%-10% could achieve effective grafting and excellent optical performance. Too low an amount would result in insufficient grafting and decreased stability, while too high an amount might lead to multilayer physical adsorption and affect light transmittance.

[0150] The above are merely preferred embodiments of the present invention and do not limit the patent scope of the present invention. Any equivalent structural transformations made using the contents of the present invention specification and drawings under the inventive concept of the present invention, or direct / indirect applications in other related technical fields, are included within the patent protection scope of the present invention.

[0151] It should be noted that when numerical ranges are involved in this invention, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can be selected. Since the steps and methods used are the same as those in the embodiments, preferred embodiments are described in order to avoid redundancy. Although preferred embodiments of this invention have been described, those skilled in the art can make other changes and modifications to these embodiments once they know the basic inventive concept. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of this invention.

[0152] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from the spirit and scope of this invention; if these modifications and variations of this invention fall within the scope of the claims of this invention and their equivalents, then this invention also intends to include these modifications and variations.

Claims

1. A low-refractive-index optical adhesive based on hollow silica, characterized in that, The optical adhesive includes: Surface-modified hollow silica particles; Fluorinated acrylate prepolymers; Reactive diluent; and Photoinitiator; The hollow silica particles have a particle size D50 of less than 50 nm, their shell material is silica, their interior has a hollow structure, and their surface is grafted with polymerizable functional groups that can undergo copolymerization reactions with the fluorinated acrylate prepolymer or reactive diluent.

2. The optical adhesive according to claim 1, characterized in that, The optical adhesive also contains an organic solvent, which includes one or more of propylene glycol methyl ether (PGME), ethyl acetate, and butanone. The components of the optical adhesive, by weight, include: 10-60 parts of modified hollow silica particles; 20-70 parts of fluorinated acrylate prepolymer; 5-15 parts reactive diluent; 1-5 parts of photoinitiator; Solvent 40-60 parts.

3. The optical adhesive according to claim 1, characterized in that, The hollow silica particles have an equivalent refractive index of 1.1 to 1.2, and the polymerizable functional group is derived from a silane coupling agent containing unsaturated double bonds, wherein the silane coupling agent is γ-methacryloyloxypropyltrimethoxysilane, γ-methacryloyloxypropyltriethoxysilane, or vinyltrimethoxysilane.

4. The low refractive index optical adhesive according to claim 1 or 2, characterized in that, The optical adhesive layer formed after curing has a transmittance of ≥92% at a wavelength of 550 nm, a haze of ≤1.0%, and a refractive index of 1.25 to 1.

30.

5. The low refractive index optical adhesive according to claim 1, characterized in that, The fluorinated acrylate prepolymer is a copolymer of hexafluorobutyl acrylate with a molecular weight of 5,000 to 20,000. The reactive diluent is an acrylate monomer containing one or more unsaturated double bonds.

6. A method for preparing the optical adhesive as described in any one of claims 1 to 5, characterized in that, Includes the following steps: Hollow silica particles with polymerizable functional groups grafted onto their surface are prepared, wherein the particle size D50 of the hollow silica particles is less than 50 nm. The obtained hollow silica particles with polymerizable functional groups grafted on their surface were dispersed in an organic solvent and ultrasonically treated to obtain a uniform dispersion. Fluorinated acrylate prepolymer, reactive diluent and photoinitiator are added to the dispersion, and the mixture is stirred and mixed under light-protected conditions to obtain a mixture. The resulting mixture was subjected to vacuum degassing and then allowed to stand for aging to obtain the low-refractive-index optical adhesive.

7. The preparation method according to claim 6, characterized in that, The preparation of hollow silica particles with polymerizable functional groups grafted on their surface includes: Monodisperse polystyrene microsphere templates were synthesized using soap-free emulsion polymerization or seed emulsion polymerization, wherein the polystyrene microspheres had a particle size of 20-100 nm. The polystyrene microsphere template was dispersed in an alcohol solvent, and an alkaline catalyst and tetraethyl orthosilicate were added. A silica shell was then coated on the surface of the polystyrene microspheres using a sol-gel method to obtain polystyrene-silica core-shell microspheres. The obtained polystyrene-silica core-shell microspheres were dispersed in an organic solvent, and the polystyrene template was removed by dissolution to obtain hollow silica particles; The obtained hollow silica particles were dispersed in an alcohol solvent, and a silane coupling agent containing unsaturated double bonds was added. The pH value was adjusted to acidic, and a surface modification reaction was carried out under heating conditions. After the reaction was completed, the particles were separated, washed, and dried to obtain hollow silica particles with polymerizable functional groups grafted on their surface.

8. The preparation method according to claim 7, characterized in that, The raw materials used to synthesize polystyrene microsphere templates include: styrene monomer, initiator, and co-stabilizing monomer; the initiator is potassium persulfate or ammonium persulfate, and the amount used is 1%-3% of the mass of styrene monomer; the co-stabilizing monomer is acrylic acid or methacrylic acid, and the amount used is 0.5%-2% of the mass of styrene monomer. The process of synthesizing polystyrene microsphere templates includes: Water and initiator are added to the reaction flask under inert gas protection and in a constant temperature water bath at 65-80℃. Styrene and co-stabilized monomers are mixed and slowly added dropwise to the reaction system; After reacting for 5-15 hours, a milky white emulsion with a bluish sheen is obtained. The emulsion was centrifuged and washed to remove unreacted monomers and electrolytes, and then redispersed in anhydrous ethanol / isopropanol to obtain polystyrene microsphere templates.

9. The preparation method according to claim 7, characterized in that, The alcohol solvent is anhydrous ethanol or isopropanol; the alkaline catalyst is ammonia water to adjust the pH of the system to 9-12; the tetraethyl orthosilicate is added slowly dropwise over 1-3 hours; the coating reaction temperature is 30-50℃ and the reaction time is 12-24 hours.

10. The preparation method according to claim 7, characterized in that, The silane coupling agent containing unsaturated double bonds is γ-methacryloxypropyltrimethoxysilane, γ-methacryloxypropyltriethoxysilane, or vinyltrimethoxysilane; the amount of the silane coupling agent used is 3%-10% of the mass of the hollow silica particles; The process of adjusting the pH to acidic and carrying out the surface modification reaction under heating conditions includes: Adjust the pH to 3.5-5.5 and react under heating conditions of 50-70℃ for 4-8 hours.

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