An optical grade epoxy resin adhesive with high refractive index and flexibility

Abstract

The present application relates to the technical field of optical adhesive, and discloses an optical-grade epoxy resin adhesive with high refractive index and flexibility. The present application aims to solve the technical problem that the existing high-refractive epoxy resin is brittle and easy to crack due to the rigid skeleton, and the traditional toughening modification leads to a significant decrease in the refractive index. The present application chemically toughens the bisphenol fluorene resin by using a sulfide chain extender, and physically enhances it by introducing double-surface modified nano-zirconium dioxide, and cooperates with an aromatic amine curing agent and a gradient curing process to solve the problems of the traditional high-refractive material, such as brittleness, poor heat resistance and optical performance decrease after toughening. The refractive index of the adhesive prepared by the present application is above 1.65, the elongation at break is higher than 7.0%, and the adhesive has high light transmittance and high heat resistance, and excellent aging resistance, realizes the perfect balance of optical performance and mechanical strength and toughness, and is suitable for precise packaging and bonding of high-end flexible optical devices.

Description

Technical Field

[0001] This invention relates to the field of optical adhesives, and more specifically, to an optical-grade epoxy resin adhesive that combines high refractive index and flexibility. Background Technology

[0002] With the rapid development of flexible display technologies (such as foldable screen phones and rollable screen TVs) and precision optical modules (such as AR / VR lenses and optical communication devices), extremely stringent performance requirements have been placed on optical encapsulation bonding materials. Among them, epoxy resin has become the most widely used matrix material in this field due to its excellent bonding strength, light transmittance, and molding processability. Especially in improving optical efficiency and eliminating interface light loss, optical epoxy adhesives with high refractive index properties are particularly important. They can effectively match high refractive index lenses or substrates, reduce interface reflection and total internal reflection losses, thereby significantly improving the transmission efficiency and imaging quality of the optical path.

[0003] However, existing high-refractive-index epoxy resin systems generally face the physical contradiction of being unable to simultaneously achieve high refractive index and high flexibility. Pursuing a high refractive index often requires the introduction of numerous rigid aromatic rings, fused rings, or halogen groups into the molecular structure. This results in extremely high crosslinking density and severe brittleness in the cured product, making it highly susceptible to cracking and debonding under flexible bending or high / low temperature impacts. While toughening modification with long-chain polyethers or rubber can improve brittleness, it causes a precipitous drop in refractive index due to the dilution effect, and significantly sacrifices heat resistance and aging resistance, failing to meet the requirements of reflow soldering processes and long-term service. Therefore, developing a modified epoxy resin adhesive that can maintain ultra-high refractive index and heat resistance while possessing excellent flexibility and aging resistance has become a key challenge urgently needing to be overcome in the industry. Summary of the Invention

[0004] In view of the aforementioned existing problems, the present invention is proposed.

[0005] Therefore, this invention provides an optical-grade epoxy resin adhesive that combines high refractive index and flexibility, solving the technical problem that existing high refractive index epoxy resins are brittle and prone to cracking due to their rigid skeleton, while traditional toughening modifications lead to a significant decrease in refractive index.

[0006] To solve the above-mentioned technical problems, the present invention provides the following technical solution: This invention provides a method for preparing an optical-grade epoxy resin adhesive that combines high refractive index and flexibility, comprising the following steps: S1. Disperse nano-zirconia in a solvent, add 5-20% of a composite silane coupling agent by mass of nano-zirconia, and perform a grafting reaction at 40-80℃ for 3-9 hours; after the reaction, centrifuge, wash and dry to obtain double-surface modified nano-zirconia. S2. Using bisphenol fluorene-type epoxy resin as a rigid skeleton, dissolve it in toluene and heat it to 70~110℃. Under the action of a catalyst, slowly add 15~25% of a sulfide-based flexible chain extender by mass of resin. React at a constant temperature for 2~8 hours to introduce flexible sulfide segments. After the reaction is completed, remove the solvent under reduced pressure to obtain the modified fluorene-based epoxy prepolymer. S3. Using the modified fluorene-based epoxy prepolymer as the matrix, add 4-12% of the double-surface modified nano-zirconia by mass of the prepolymer, disperse it evenly using high shear force, add 20-30% of the aromatic amine curing agent and 0.5-2% of the accelerator by mass of the prepolymer, stir and degas under vacuum conditions to obtain the adhesive solution. S4. Apply the adhesive liquid to the surface of the substrate and perform a step curing process within a temperature range of 80~200℃. After cooling, the final cured product is obtained.

[0007] In a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, the composite silane coupling agent is composed of phenyltrimethoxysilane and γ-glycidoxypropyltrimethoxysilane; the amount of phenyltrimethoxysilane added is 7.0% of the mass of the nano-zirconia, and the amount of γ-glycidoxypropyltrimethoxysilane added is 6.5% of the mass of the nano-zirconia.

[0008] As a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, the specific process of the grafting reaction is as follows: before the reaction, the system is first dispersed by ultrasonic treatment at a frequency of 40 kHz for 30 minutes; then the system is heated to 65°C and refluxed at a constant temperature for 6.0 hours.

[0009] As a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, the bisphenol fluorene-type epoxy resin is 9,9-bis[(2,3-epoxypropoxy)phenyl]fluorene; the thioether-based flexible chain extender is 2,2'-dimercaptodiethyl sulfide, with an addition amount of 20%; and the catalyst is triphenylphosphine, with an addition amount of 0.1%.

[0010] As a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, the specific operation of the isothermal reaction in step S2 is as follows: the system is heated to 90°C and kept at that temperature for 4.0 hours.

[0011] As a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, wherein: the amount of the double-surface modified nano-zirconia added in step S3 is 7.5%; the aromatic amine curing agent is 4,4'-diaminodiphenyl sulfone, and the amount added is 25%; the accelerator is 2-ethyl-4-methylimidazole, and the amount added is 1%.

[0012] As a preferred embodiment of the method for preparing optical-grade epoxy resin adhesive with both high refractive index and flexibility according to the present invention, the specific parameters of the high shear force dispersion are: the planetary mixer is set to 2000 revolutions / minute and 800 revolutions / minute, and the processing time is 10 minutes; the vacuum degree of the vacuum degassing is -0.098MPa.

[0013] As a preferred embodiment of the method for preparing the optical-grade epoxy resin adhesive with both high refractive index and flexibility described in this invention, the specific procedure of the stepped curing treatment is as follows: firstly, leveling and defoaming are performed by holding at 100°C for 1.0 hour, then the main reaction is fixed by holding at 150°C for 2.0 hours, and finally stress relief treatment is performed by holding at 180°C for 1.0 hour.

[0014] In a preferred embodiment of the method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility as described in this invention, the nano-zirconia particles in step S1 have a particle size of 10-20 nm; and the solvent used is anhydrous ethanol.

[0015] This invention provides an optical-grade epoxy resin adhesive curing product that combines high refractive index and flexibility, prepared by the above method, wherein: the adhesive curing product has an inorganic-organic hybrid network structure, wherein the rigid fluorene-based framework and inorganic nanoparticles provide high refractive index support, and the flexible sulfide segments provide deformation buffer.

[0016] The beneficial effects of this invention are as follows: By employing a dual strategy of chemical block toughening and physical nano-reinforcement, the technical bottleneck of the mutual constraint between high refractive index and high flexibility in optical materials is successfully overcome. In terms of matrix design, a high molar refractive index sulfide chain extender is used to perform molecular-level modification on the rigid bisphenol fluorene skeleton. While introducing flexible segments to significantly improve the elongation at break of the material, the high electron density of sulfur compensates for optical losses, allowing the matrix resin to maintain an extremely high intrinsic refractive index while significantly toughening it. In conjunction with the use of a sulfone-containing aromatic amine curing agent, the material is further endowed with excellent heat resistance, rigidity, and resistance to oxidation and yellowing, ensuring that it can withstand the harsh tests of 260°C high-temperature reflow soldering and long-term photothermal aging.

[0017] In nanocomposite modification, this invention constructs a composite interface layer based on phenyl and epoxy-based bifunctional silanes, precisely solving the problems of refractive index mismatch and weak bonding between inorganic nano-zirconia and organic matrices. The introduction of phenyl groups effectively eliminates interfacial light scattering, achieving a high-transmittance optical path with low haze; while the chemical bonding of epoxy groups transforms rigid particles into reinforcing and toughening physical rivets, avoiding stress debonding caused by traditional physical doping. The resulting cured material has a refractive index of over 1.65, exhibiting excellent flexibility and weather resistance, perfectly meeting the bonding requirements of high-end flexible optical devices. Attached Figure Description

[0018] To more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings used in the following description of the embodiments will be briefly introduced. 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 these drawings without creative effort.

[0019] Figure 1 The figures show the results of the resin substrate optimization experiment. The left figure shows the optimization experiment of the flexible chain extender, and the right figure shows the optimization experiment of the curing agent dosage.

[0020] Figure 2 The figure shows the experimental results of the surface modifier formulation test.

[0021] Figure 3 The figure shows the results of the screening experiment for the dosage of nano-modifiers.

[0022] Figure 4 The figure shows the experimental results of process optimization of reaction temperature and time.

[0023] Figure 5 The figure shows the results of the optimization experiment of the flexible chain extension synthesis process. Detailed Implementation

[0024] To make the above-mentioned objects, features and advantages of the present invention more apparent and understandable, the specific embodiments of the present invention will be described in detail below with reference to the accompanying drawings.

[0025] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and those skilled in the art can make similar extensions without departing from the spirit of the invention. Therefore, the invention is not limited to the specific embodiments disclosed below.

[0026] Secondly, the term "one embodiment" or "example" as used herein refers to a specific feature, structure, or characteristic that may be included in at least one implementation of the invention. The appearance of an embodiment in different places in this specification does not necessarily refer to the same embodiment, nor is it a single or selective embodiment that mutually excludes other embodiments.

[0027] Example 1 Reference Figure 1 This is the first embodiment of the present invention. This embodiment aims to screen the most suitable core substrate material for the optical-grade adhesive of the present invention from a variety of representative epoxy resin systems through systematic comparative experiments. In the development of optical adhesives, the substrate resin, as a continuous phase carrier, directly determines the performance of the final product in key dimensions such as optical performance, thermal stability, and service life. Therefore, conducting comprehensive performance assessment and screening of resins with different molecular skeleton characteristics is the first step in constructing a high-performance formulation system.

[0028] To overcome the limitations of the refractive index (typically below 1.57) of traditional bisphenol A epoxy resins, the industry's conventional optimization approach mainly focuses on introducing atoms or groups with high molar refractive index into the molecular structure, such as halogen atoms (e.g., bromine, iodine), sulfur atoms, or fused-ring aromatic structures. However, in actual formulation exploration and preliminary experiments, we have found that the strategy of solely pursuing the extreme value of refractive index is often accompanied by the catastrophic collapse of other key properties. For example, while tetrabromobisphenol A epoxy resin with high bromine content can increase the refractive index to over 1.65, its carbon-bromine bonds are easily broken and generate free radicals in high-temperature reflow soldering tests (260°C) simulating assembly processes, causing the adhesive layer to yellow rapidly or even carbonize, completely losing its value as an optical material. Similarly, while cyclosulfide resin with high sulfur content has an impressive refractive index of 1.71, its sulfide bonds are easily oxidized and degraded in accelerated aging tests at 150°C, causing a significant drop in light transmittance in a short time. Furthermore, such materials typically have a strong, pungent odor and a very short shelf life, making them unsuitable for industrial production.

[0029] Based on the above exploration experience, we have established that while prioritizing ensuring that the base resin has excellent basic optical properties such as refractive index and light transmittance, we must also take into account its stability under actual working conditions.

[0030] 1.1 Screening experiment of base resin To eliminate the interference of other additive variables on the performance evaluation of the resin substrate, this experiment uniformly used methylhexahydrophthalic anhydride (MHHPA) as a universal curing agent, which is currently the most widely used standard in transparent optical adhesives; and selected 2-ethyl-4-methylimidazolium as a universal catalyst.

[0031] Experimental groups: Group A1 used cyclic sulfur resin as a substrate (liquid, solvent-free); Group A2 used hydrogenated bisphenol A epoxy resin as a substrate (liquid, solvent-free); Group A3 used high-phenyl silicone resin as a substrate (liquid, solvent-free); Group A4 used general-purpose bisphenol A epoxy resin as a substrate (liquid, solvent-free); Group A5 used 9,9-bis[(2,3-epoxypropoxy)phenyl]fluorene as a substrate and introduced an equal mass of toluene as a dissolving medium; Group A6 used tetrabromobisphenol A epoxy resin as a substrate (solid, dissolved in toluene); Group A7 used naphthalene-type epoxy resin as a substrate (semi-solid, with a small amount of toluene added).

[0032] Each group of resins was mixed with a general-purpose curing agent and catalyst at a stoichiometric ratio (epoxy equivalent: anhydride equivalent = 1:0.9) until homogeneous. For groups containing solvent (E, F, G), the solvent was first dissolved by stirring at 90°C and pre-reacted for 30 minutes. Then, toluene solvent was completely removed under vacuum using a rotary evaporator until no bubbles or fractions were generated. The solvent-removed adhesive solution was coated onto a PET release film and cured in an oven according to a program of 100°C / 1 hour + 140°C / 2 hours to obtain standard test samples.

[0033] The refractive index and transmittance were tested using an Abbe refractometer and a spectrophotometer, respectively, to evaluate the basic optical properties of the materials. Materials with poor heat resistance were eliminated by simulating reflow soldering under high temperature impact of 260℃ / 5min. Materials with high durability were screened by testing the change in yellowing index under long-term thermal aging at 150℃ / 168h.

[0034] The test results are as follows: Among them, cyclosulfide resin showed an absolute advantage in refractive index, but its extremely poor aging stability prevented it from meeting the requirements for long-term use; hydrogenated bisphenol A epoxy resin had excellent light transmittance and good weather resistance, but its low refractive index could not achieve optical brightening function; tetrabromobisphenol A epoxy resin, although it obtained a high refractive index through halogen modification, could not withstand the high-temperature reflow soldering process. 9,9-bis[(2,3-epoxypropoxy)phenyl]fluorene not only had good optical properties, but also performed well in thermal shock resistance and aging resistance tests. It was ultimately selected as the core base material for subsequent modification.

[0035] 1.2 Screening and Optimization Experiments of Flexible Chain Extenders Although the bisphenol fluorene-based resin selected in 1.1 exhibits excellent refractive index and heat resistance, its unique rigid caldo ring backbone results in extremely high brittleness and modulus in the cured product. In preliminary film-forming bending tests, it breaks upon bending, completely failing to meet the basic requirements of flexible optical devices for bending resistance and impact resistance. However, traditional toughening modifications (such as introducing long-chain aliphatic or polyether segments) are often achieved by sacrificing optical properties (significantly reducing the refractive index), creating an irreconcilable contradiction between high refractive index and high flexibility.

[0036] 1.2.1 Screening Experiment of Flexible Chain Extenders The purpose of this phase of the experiment is to screen out a special flexible chain extender that can effectively introduce flexible segments into a rigid framework, significantly improve the elongation at break of the material, and at the same time retain the refractive index of the system to the greatest extent.

[0037] In addition to setting up a blank control group (pure bisphenol fluorene resin), six representative toughening materials were selected for comparative screening: B1 with 15 parts of 1,4-butanediol diglycidyl ether; B2 with 25 parts of polypropylene glycol diglycidyl ether; B3 with 12 parts of liquid CTBN rubber; B4 with 8 parts of terminal epoxy silicone oil; B5 with 20 parts of dimer acid diglycidyl ester; and B6 with 15 parts of 2,2'-dimercaptodiethyl sulfide (DMDS).

[0038] Using the bisphenol fluorene epoxy resin selected in section 1.1 as a constant substrate, the above six chain extenders were added, along with a universal curing agent (MHHPA) and catalyst, to prepare a cured film. In addition to continuing to monitor the refractive index and transmittance, according to ASTM D638 standard, the sample was made into a dumbbell shape, and tensile and bending tests were performed using a universal testing machine at a rate of 5 mm / min. The elongation percentage at fracture and the elastic modulus during bending were recorded to quantitatively characterize the microscopic toughness and stiffness of the material. According to ASTM D522 standard, a 50 μm thick adhesive film was coated onto a PET substrate and cured. Then, it was bent 180° against a cylindrical shaft with a diameter of 3 mm, and the presence of cracks or peeling of the adhesive layer was observed to simulate the tolerance of flexible devices under extreme curvature.

[0039] The test results are as follows: The selection of toughening agents presents a classic dilemma: significantly increasing elongation at break completely solves the brittleness problem, but at the cost of a precipitous drop in refractive index; low-polarity modified materials, while having minimal impact on refractive index, have extremely weak ability to unlock the rigid fluorene framework. 2,2'-dimercaptodiethyl sulfide has a relatively small impact on refractive index and provides the highest performance improvement per unit refractive index, thus making it the optimal flexible chain extender.

[0040] 1.2.2 Optimization Experiment of Chain Extender Dosage After determining 2,2'-dimercaptodiethyl sulfide as the optimal chain extender component, further refined gradient experiments were conducted to pinpoint its optimal addition amount. To accurately capture the nonlinear inflection point of material properties with varying modifier content, 11 high-density concentration gradients were established at 2-part intervals within an effective range of 5 to 25 parts. Cured samples were prepared according to the above formulations, and their refractive index, elongation at break, and flexural modulus were tested respectively.

[0041] Test results are as follows Figure 1 As shown in the left figure, in order to find the theoretically optimal solution that balances optics from discrete experimental data, nonlinear regression analysis was performed on the efficiency of toughness improvement and the rate of refractive index loss, respectively. By calculating the marginal benefit inflection point of the elongation at break growth curve, the optimal theoretical point for toughness gain was determined to be 21.84 parts; simultaneously, based on the slope change of the refractive index decrease curve, the critical tolerance point for optical loss was determined to be 18.36 parts. To achieve a balance between the two, the arithmetic mean of these two theoretical points (20.10 parts) was taken as the global optimal solution, and considering the convenience of industrial production, the optimal addition amount of DMDS was finally determined to be 20 parts.

[0042] 1.3 Screening Experiments for Synthetic Catalysts After establishing the optimal ratio of bisphenol fluorene substrate to thioether chain extender, the core challenge of the synthesis process shifted to the precise control of the thiol-epoxy addition reaction. This reaction falls under the category of click chemistry, which demands extremely high catalyst selectivity. If the catalyst is too basic, it can easily trigger the self-polymerization of epoxy groups, leading to uncontrollable crosslinking degree and a sharp increase in viscosity. If the catalyst activity is insufficient, the reaction will be incomplete, and the residual free thiol groups will emit a foul odor and corrode the circuit. In addition, as an optical material, the color of the catalyst itself and its oxidation products is also a crucial factor that must be considered. Therefore, this experiment aims to screen a synthesis catalyst that combines high selectivity, high activity, and low color.

[0043] Experimental groups: C1 used 2,4,6-tris(dimethylaminomethyl)phenol (DMP-30); C2 used 1,8-diazabicycloundec-7-ene (DBU); C3 used 2-ethyl-4-methylimidazolium (2E4MZ); C4 used tetrabutylammonium bromide (TBAB); C5 used boron trifluoride ether (BF3·Et2O); C6 used triphenylphosphine (TPP).

[0044] The molecular weight distribution of the product was analyzed by gel permeation chromatography (GPC), combined with the mercapto peak (2550 nm) in Fourier transform infrared spectroscopy (FTIR). The degree of disappearance of the chain extension rate is comprehensively judged. This is used to quantify the directional catalytic ability of the catalyst for the main reaction (thiol-epoxy addition). A higher chain extension rate indicates fewer side reactions (such as epoxy self-polymerization) and a molecular structure closer to the theoretical design. A colorimetric method is used, placing the synthesized resin solution in a standard colorimetric tube and visually comparing it with the platinum-cobalt standard color scale under a standard light source. This index directly reflects the influence of the catalyst itself and its thermal oxidation products on the resin transparency. The lower the color number (<50), the lighter the resin color, and the more it meets the stringent requirements of high-end optical applications for colorless and transparent resins. At a constant reaction temperature, the viscosity change is recorded every 30 minutes after the start of the reaction, and the ratio of the final viscosity to the initial viscosity is calculated. This index is used to assess whether the reaction is mild and controllable. An excessively large growth factor (>100) usually indicates uncontrollable cross-linking or explosive polymerization, which can easily lead to material overflow accidents in industrial production.

[0045] The test results are as follows: Among them, DBU has high catalytic activity and selectivity, and high chain extension rate, but its strong alkalinity makes the reaction process extremely difficult to control, and the instantaneous viscosity surge means extremely high industrial safety risks; boron trifluoride diethyl ether achieves the best color due to the absence of chromophores, but its catalytic mechanism tends to be cationic-induced epoxy ring-opening self-polymerization rather than the thiol addition we need, resulting in extremely low chain extension rate and direct gelation of the product.

[0046] 1.4 Screening and optimization experiments of curing agents As a key component with a high proportion in epoxy resin systems, the curing agent's molecular structure directly determines the rigidity density and electron cloud distribution of the final cross-linked network. Considering that the product needs to meet the extreme requirements of high refractive index in precision optical modules, as well as the harsh tests of 260°C high-temperature reflow soldering and long-term service environment in SMT processes, the goal is to provide the material with a rigid skeleton (high Tg) sufficient to withstand instantaneous high-temperature impact and chemical stability to resist long-term photothermal oxidation, while ensuring that the high refractive index is not diluted.

[0047] 1.4.1 Screening experiment of curing agent Experimental groups: D1 with 6 parts ethylenediamine; D2 with 45 parts low molecular weight polyamide; D3 with 65 parts methylhexahydrophthalic anhydride (MHHPA) (with 1 part accelerator); D4 with 14 parts m-phenylenediamine (m-PDA); D5 with 85 parts methylnadic anhydride (NMA); D6 with 24 parts 4,4'-diaminodiphenylmethane (DDM); D7 with 30 parts 4,4'-diaminodiphenyl sulfone (DDS).

[0048] Using 100 parts of bisphenol fluorene epoxy resin as a base, 20 parts of dimercaptodiethyl sulfide (DMDS) and triphenylphosphine catalyst were compounded, and the above-mentioned curing agents were added respectively. After mixing and degassing, the standard curing procedure was performed according to the characteristics of each curing agent (stepwise temperature increase for amines and high temperature long-term curing for acid anhydrides) to obtain the final cured sample.

[0049] In addition to testing the refractive index, the glass transition temperature of the material needs to be tested by DSC; an aging test of 150℃ / 168h is also required, and the oxidation resistance life of the material is quantified by the yellowing index.

[0050] The test results are as follows: Among them, m-phenylenediamine significantly improved the refractive index, but the sample turned black after aging test; methylnadic anhydride has excellent heat resistance but reduced the refractive index; 4,4'-diaminodiphenyl sulfone, although it only slightly improved the refractive index, improved and stabilized the glass transition temperature and anti-aging degree, so it is the best curing agent.

[0051] 1.4.2 Optimization Experiment of Curing Agent Dosage After determining 4,4'-diaminodiphenyl sulfone (DDS) as the optimal curing agent component, further refined gradient experiments were conducted to pinpoint its optimal addition amount. Based on stoichiometry and the curing kinetics of aromatic amines, the upper limit for DDS addition in industrial applications is typically set at 40 parts (relative to 100 parts of resin) to avoid performance degradation caused by excessive free amine groups. Ten equally spaced concentration gradients were established within a wide range of 4 to 40 parts to capture the response of various performance parameters to changes in crosslinking density. Cured samples were prepared according to the above formulation, and their refractive index, glass transition temperature, and yellowing index were tested.

[0052] Test results are as follows Figure 1 As shown in the right figure, the glass transition temperature exhibits a typical parabolic curve pattern of first increasing and then decreasing with increasing curing agent dosage, peaking at 24.6 parts. The yellowing index is positively correlated with the residual amount of free amine groups. After the dosage exceeds 26.5 parts, the growth slope of the yellowing index becomes significantly larger, indicating a qualitative decline in the material's antioxidant capacity. The marginal benefit of refractive index narrows significantly after 22 parts, and further increases in dosage have negligible contribution to optics. Considering all factors, the optimal addition amount of 4,4'-diaminodiphenyl sulfone is determined to be 25 parts.

[0053] Example 2 refer to Figure 2 and Figure 3This is the second embodiment of the present invention. After optimizing the chemical structure of the base resin, flexible chain extender, and curing agent system, in order to further break through the upper limit of the refractive index of organic polymer materials, it is necessary to introduce a high-refractive-index inorganic nanophase for physical reinforcement. By introducing inorganic nanoparticles with ultra-high molar refractive index, and utilizing their uniform dispersion and interfacial effect in the resin matrix, an organic-inorganic hybrid optical gain network is constructed. This not only significantly improves the overall refractive index of the material, but also hinders crack propagation through the pinning effect of the nanoparticles, thereby giving the material an additional physical toughening effect.

[0054] 2.1 Screening Experiment of Nano Optical Particles To further enhance the refractive index of the material in a physical dimension, this experiment, based on the optimized organic matrix (bisphenol fluorene resin + 20 parts DMDS + 25 parts DDS) determined in Example 1, introduced different types of inorganic nanoparticles for composite reinforcement. To ensure the basic dispersibility of the nanoparticles in the organic phase, all particles were pre-treated with a general-purpose silane coupling agent (KH-570). Based on their specific surface area and oil absorption value, their optimal critical addition amount in the epoxy system (i.e., the maximum filler content while maintaining the fluidity of the adhesive and without significant macroscopic agglomeration) was determined in advance through rheological experiments.

[0055] Experimental group: E1 with 5 parts rutile ( E2 contains 8 parts zinc sulfide (ZnS); E3 contains 14 parts silicon dioxide (… E4 with 11 parts aluminum oxide ( E5 with 10 parts zirconium dioxide ( ).

[0056] The treated nanoparticles were dispersed in an optimized resin matrix, and cured samples were prepared after high-speed shearing and vacuum degassing. The gain amplitude and dispersion uniformity of the inorganic material relative to the overall optics were evaluated by refractive index and haze. The samples were placed in a full-spectrum xenon lamp aging chamber to simulate a high-intensity light environment (irradiance 0.55 W / m²) and continuously irradiated for 168 hours. The transmittance retention rate before and after irradiation was monitored, which objectively reflects the chemical stability of the material under photothermal energy excitation.

[0057] The test results are as follows: Titanium oxide can significantly increase the refractive index, but its photocatalytic activity, under strong light irradiation across the entire wavelength range, generates free radicals excited by high-energy photons that attack the organic matrix, accelerating material aging. While silica exhibits excellent dispersibility and stability, its low refractive index dilutes the optical advantages of the matrix. Zirconia, while increasing the refractive index, benefits from its wide bandgap and chemical inertness, exhibiting strong light stability, thus ultimately becoming the best optical modifier.

[0058] 2.2 Screening and Optimization of Surface Modifiers Inorganic nanoparticles and organic polymer matrices differ significantly in their physicochemical properties. The former possesses extremely high surface energy and hydrophilicity, while the latter is mostly composed of low-surface-energy, oleophilic structures. Without intervention, this oil-water immiscibility at the interface will directly lead to irreversible aggregation and sedimentation of nanoparticles in the adhesive solution, and the formation of numerous microscopic phase separation interfaces within the cured composite material, thus affecting material properties.

[0059] 2.2.1 Screening experiment of cross-linking modifier While simple physical dispersion can solve the problem of optical transparency, when subjected to external forces such as stretching or bending, the rigid inorganic core is very easy to detach and slip from the flexible organic network, resulting in a decrease in the mechanical strength of the material instead of an increase.

[0060] Based on the above objectives, 0.5 parts of six silane coupling agents containing different functional groups were selected to modify the surface of nano-zirconia and prepare a nanocomposite resin system. Experimental groups: F1 used γ-aminopropyltriethoxysilane (KH-550); F2 used γ-mercaptopropyltrimethoxysilane (KH-580); F3 used vinyltrimethoxysilane (A-171); F4 used octyltriethoxysilane (OTES); F5 used γ-propylisocyanatetriethoxysilane (A-Link35); and F6 used γ-glycidyloxypropyltrimethoxysilane (KH-560).

[0061] The modified nanofiller was dispersed in the resin matrix according to the tentative experimental amount (8 parts) in 2.1. Under constant temperature of 25℃, its zero-shear viscosity was tested using a rotational rheometer to detect the degree of improvement of the wetting energy of the particle surface by the modifier. The lower the viscosity, the better the dispersion and the easier the processing. The adhesive was poured into a standard polytetrafluoroethylene mold, and the cured film was cut into ASTM D638 Type IV standard dumbbell-shaped strips using a punching machine. The strips were tested using a universal testing machine at a tensile rate of 5 mm / min to examine whether the interface layer constructed by the modifier can effectively transfer stress and prevent brittle fracture caused by particle debonding.

[0062] The test results are as follows: Octyltriethoxysilane has low viscosity and is very easy to process, but its tensile strength and elongation are relatively low due to its lack of reactivity. γ-aminopropyltriethoxysilane has the best tensile strength, but its excessive reactivity causes the viscosity of the adhesive to spike in the early stages of mixing, severely shortening the operating window. γ-glycidoxypropyltrimethoxysilane has moderate viscosity and significantly improves tensile strength and elongation, thus making it the best crosslinking modifier.

[0063] 2.2.2 Screening Experiments for Optical Matching Modifiers After solving the mechanical bonding problem at the interface through crosslinking modifiers, another major challenge facing nanocomposites lies in the refractive index mismatch at the optical interface. Inorganic nanoparticles typically have extremely high refractive indices (>2.0). This huge refractive index gradient creates strong light scattering centers on the particle surface, leading to increased haze and decreased transmittance—the so-called Tyndall effect. To eliminate this optical defect, an optically matched modifier with a high molar refractive index structure (such as aromatic rings) must be introduced to construct an optical transition layer on the particle surface with a refractive index between the matrix and the filler, thereby smoothing light propagation and minimizing interfacial scattering losses.

[0064] To construct an ideal optical transition layer on the particle surface, KH-560 was added to ensure mechanical properties, and then optically modulating silanes with a content of less than 0.5 parts were compounded to investigate their effects on the interfacial optical path.

[0065] Experimental groups: G1 uses γ-chloropropyltrimethoxysilane; G2 uses β-phenylethyltrimethoxysilane; G3 uses styryltrimethoxysilane; G4 uses diphenyldimethoxysilane; G5 uses phenyltrimethoxysilane; G6 uses α-naphthyltrimethoxysilane.

[0066] The modified adhesive was observed after being left at room temperature for 72 hours to assess the risk of gelation or delamination. Cured samples were prepared for testing, with a focus on haze. The lower the haze value, the better the interfacial refractive index matching and the less scattering. Thermogravimetric analysis (TGA) was used to quantify the coverage density of organic matter on the particle surface.

[0067] The test results are as follows: The experimental results clearly demonstrate that high refractive index and high grafting rate are mutually exclusive. Diphenyldimethoxysilane and α-naphthyltrimethoxysilane offer good refractive indices due to their large aromatic structures, but excessive steric hindrance severely hinders the dense arrangement of silanes on the surface of nanoparticles, resulting in low grafting rates. This directly leads to particle aggregation and sedimentation, which manifests as persistently high haze in optics. γ-chloropropyltrimethoxysilane achieves a high grafting rate due to its small molecule advantage, but the refractive index contribution of chlorine atoms is limited, and there is a risk of hydrolysis producing acidic substances that corrode circuits, resulting in less than ideal optical haze. Styrenetrimethoxysilane exhibits excellent refractive index and haze, and the finished product has excellent optical transparency. However, the active double bonds in its molecule are prone to uncontrolled free radical polymerization during storage, causing the adhesive to gel and become unusable after 72 hours, which is a fatal defect for industrial products.

[0068] Phenylacetyltrimethoxysilane achieved high coverage with moderate steric hindrance and provided sufficient optical matching with its benzene ring structure, making it the best optical matching modifier.

[0069] 2.2.3 Optimization Experiment of Surface Modifier Ratio and Dosage After establishing the two-component composite modification strategy of "γ-glycidyl etheroxypropyltrimethoxysilane and phenyltrimethoxysilane", it is necessary to explore the competitive adsorption and synergistic effect mechanism of the two modifiers with different functions on the confined nano-surface. If the mechanical anchoring agent is used in insufficient amounts, the density of chemical bonding points at the interface will be insufficient, and the external stress cannot be effectively transferred, causing the material to brittlely debond under low deformation. If the amount is too large, the excess molecules will undergo self-condensation to form a multilayer structure due to the saturation of hydroxyl sites on the particle surface, which may become stress defects and interfere with optical transparency. If the amount of optical matching agent is too small, it means that the low refractive index gaps on the particle surface cannot be completely covered, resulting in strong light scattering and high haze. However, if the amount is too large, the loose organic layer formed by the accumulation of a large number of benzene rings will weaken the mechanical interlocking force of the interface like a lubricant, resulting in a decrease in modulus and a mismatch in the coefficient of thermal expansion.

[0070] Using the mass of nano-zirconia as a baseline (unit 1), the addition ratio (mass ratio) of the two was set separately. Referring to industrial dosage, the value range was set to 0.00 to 0.20, and 11 level groups with a step size gradient of 0.02 were set. Through pairwise orthogonal combinations, the dispersion viscosity in the liquid state (to evaluate anti-agglomeration ability), and the elongation at break in the cured state (to evaluate interfacial adhesion) and optical haze (to evaluate interfacial refractive index matching degree) were tested respectively.

[0071] Test results are as follows Figure 2 As shown, by fitting the response surface of the test results, the optimal point coordinates for dispersion viscosity were (0.058, 0.085), optimal point coordinates for elongation at break were (0.082, 0.045), and optimal point coordinates for optical haze were (0.052, 0.078). To balance these three factors, the average of the three vertex coordinates was used to obtain the coordinates of the comprehensive optimal point (0.064, 0.069). Considering the accuracy of material feeding and ease of operation in industrial production, the calculation results were finely adjusted and rounded. Finally, the optimal addition amount of the two-component modifier was determined to be: 6.5% KH-560 and 7.0% phenyltrimethoxysilane relative to the mass of nano-zirconia.

[0072] 2.3 Screening experiment on the dosage of nano-modifier After determining the optimal surface modification scheme for zirconium dioxide particles through preliminary steps, the modified nanoparticles possess ideal interfacial compatibility. However, the volume fraction of nanofillers in the matrix directly determines the macroscopic properties of the composite material. Too low a concentration results in insignificant refractive index gain, failing to reflect the advantages of the inorganic phase; too high a concentration leads to an average particle spacing below the critical value, easily inducing the loss of quantum size effects and macroscopic aggregation, resulting in an exponential increase in haze and mechanical embrittlement. This experiment aims to find the optimal balance concentration between optical gain and physical defects through precise gradient testing.

[0073] Using 100 parts of base resin (formulation containing 20 parts DMDS and 25 parts DDS) as the baseline, nano-zirconia particles optimized according to section 2.2.3 were used as variables. The addition amount was set to range from 1 part to 15 parts, with a step size of 1 part, for a total of 15 experimental groups. The refractive index (focusing on gain amplitude), elongation at break (focusing on mechanical peak), and optical haze (focusing on scattering threshold) were investigated in detail.

[0074] Test results are as follows Figure 3 As shown, the elongation at break exhibits a typical unimodal distribution with increasing dosage. The fitted curve shows that the theoretical peak of mechanical properties is located at 6.64 parts. The refractive index increases monotonically with increasing dosage, but the haze deteriorates exponentially after exceeding a certain threshold. By constructing a refractive index return-on-indices (ROI) function per unit haze cost, the optimal balance point for overall optical benefits is found to be at 8.17 parts. After averaging, the optimal point coordinates are found to be 7.405, and considering industrial requirements, 7.5 parts were ultimately selected.

[0075] Example 3 refer to Figure 4 This is the third embodiment of the present invention. Although the foregoing embodiments have established the optimal chemical ratios of each component under laboratory conditions, the transition from micro-beaker experiments to large-scale industrial production often faces numerous challenges due to scale-up effects. The mass and heat transfer efficiency, equipment shear capacity, and temperature and humidity fluctuations in the open environment of actual production differ significantly from the ideal controlled environment of the laboratory. These differences may lead to incomplete resin synthesis reactions, decreased nanoparticle dispersion efficiency, or excessive residual internal stress in the cured product, thereby causing performance fluctuations or even failures between product batches. This embodiment aims to systematically and adaptively optimize and fine-tune key preparation process parameters based on actual industrial production conditions to ensure that the final delivered optical-grade adhesive possesses high performance while also exhibiting excellent process robustness and mass production stability.

[0076] 3.1 Process optimization of modification reaction temperature and time Surface modification of nanoparticles is a crucial process determining the optical haze and mechanical strength of the final product. In industrial-scale production, the heating efficiency and solvent reflux conditions of the reaction vessel differ fundamentally from those of a laboratory flask. If the reaction temperature is too low or the time is insufficient, the hydrolysis and condensation kinetics of the silane coupling agent are limited, resulting in low grafting rates and incomplete surface coverage. Conversely, if the temperature is too high or the time is excessive, not only will energy consumption and solvent loss increase unnecessarily, but it will also easily induce the self-condensation of silane molecules in the liquid phase, forming ineffective oligomer impurities, and even causing irreversible thermal agglomeration of nanoparticles. Therefore, precise kinetic scanning and optimization of the temperature and time windows of the modification reaction are essential.

[0077] The temperature range was set from 40℃ to 80℃ with a step size of 5℃; the time range was set from 2 hours to 10 hours with a step size of 1 hour; a total of 81 independent process experimental groups were constructed by combining pairs.

[0078] Ten portions of nano-zirconia powder with a particle size of 10-20 nm were placed in a reaction vessel containing 100 portions of anhydrous ethanol and treated with a 40 kHz ultrasonic disperser for 30 minutes to construct a homogeneous suspension. Then, a mixture of phenyltrimethoxysilane and γ-glycidyl etheroxypropyltrimethoxysilane prepared according to the optimized ratio in section 2.2.3 was added. The reaction system was heated to the target temperature set for each experimental group and refluxed at a constant stirring speed of 500 rpm for the target time. After the reaction was terminated, the solid phase was separated using a high-speed centrifuge at 10,000 rpm, and washed three times with anhydrous ethanol to remove physically adsorbed silane monomers. Finally, the powder was dried in a vacuum oven at 80 °C for 12 hours, ground, and sieved to obtain the modified powder to be tested.

[0079] A small amount of dried powder was subjected to thermogravimetric analysis (TGA) to calculate the weight loss rate in the range of 30℃ to 800℃, in order to characterize the amount of chemical bonding of organic functional groups. Each group of powder was dispersed in a standard resin matrix at 7.5 parts (2.3 optimized value) and cured to form a film, and its haze value was tested.

[0080] Test results are as follows Figure 4 As shown, the grafting rate and haze did not exhibit a simple linear positive correlation, but rather a significant process window effect. With increasing temperature and time, the grafting rate initially increased rapidly and then slowed down, but a falsely high rate appeared in the high-temperature, long-duration zone. Although the TGA weight loss rate increased, this was mainly due to the deposition of silane self-polymers. Haze exhibited a typical valley characteristic: high haze in the low-temperature, short-duration zone due to incomplete coverage, and a rebound in haze in the high-temperature, long-duration zone due to light scattering caused by self-polymers. By performing an extreme value search on the haze response surface, the theoretical process point with optimal optical performance was determined to be around (63.5℃, 5.8h). Considering the temperature control accuracy and scheduling convenience of industrial operations, the final nano-modification process parameters were set at a reaction temperature of 65℃ and a reaction time of 6 hours.

[0081] 3.2 Optimization of Flexible Chain Extension Synthesis Process The synthesis of sulfide-modified fluorene epoxy resin involves a nucleophilic addition reaction between thiol groups and epoxy groups. This reaction is exothermic, and its kinetics are highly sensitive to temperature. Too low a temperature leads to a delayed reaction, and the residual free thiol groups not only produce a foul odor but also corrode metal circuits. Too high a temperature or too long a holding time easily induces homopolymerization side reactions and thermal oxidation of the epoxy groups, resulting in a darkening of the resin color (yellowing) or even gel failure.

[0082] The reaction temperature was set from 70℃ to 110℃ in 5℃ increments; the holding time was set from 1 hour to 8 hours in 1-hour increments, constructing 72 independent synthesis process groups. In a 500mL four-necked flask equipped with a mechanical stirrer, reflux condenser, and high-purity nitrogen protection interface, 100 parts of bisphenol fluorene diglycidyl ether and 100 parts of toluene were added. Stirring was started and the temperature was raised to the target temperature set for each experimental group. After the resin was completely dissolved and clarified, 20 parts of a pre-mixed mixture of 2,2'-dimercaptodiethyl sulfide and 0.1 parts of triphenylphosphine solution were added dropwise over 30 minutes using a constant-pressure dropping funnel. After the addition was complete, the reaction was maintained at the target temperature for the target time. After the reaction was completed, the mixture was rapidly cooled to room temperature to terminate the reaction, and then rotary evaporation was performed at 95℃ and -0.098MPa under reduced pressure for 1 hour to completely remove the toluene solvent, yielding the modified resin sample.

[0083] Iodometric titration was used to quantify the degree of reaction completion; the lower the residual amount, the more complete the chain extension reaction. After the reaction, the intrinsic viscosity was used to monitor whether cross-linking side reactions (abnormal viscosity spikes) occurred. The resin was dissolved into a 50% toluene solution, and the platinum-cobalt color number was determined by colorimetry. The lower the value, the less yellowing the resin was when heated, and the better the optical quality.

[0084] Regarding intrinsic viscosity, the reaction system exhibited a slow and controllable linear increase in the low-temperature region (<85℃), indicating that the chain extension reaction proceeded systematically. At 95℃ and with a reaction time exceeding 5 hours, the viscosity data showed an exponential increase, with some high-temperature groups even undergoing direct gelation. This indicates that under the intense thermal process, the previously suppressed homopolymerization side reaction of epoxy groups within the system was activated, leading to uncontrolled cross-linking of the molecular chain network. Therefore, to ensure the processing fluidity of the resin, the reaction temperature should not exceed 95℃.

[0085] like Figure 5 As shown, the residual thiol content and resin color exhibit a significant antagonistic relationship. While low temperature and short reaction time can maintain excellent low color number, the high residual thiol content results in a severely substandard reaction. Conversely, while high temperature and long reaction time can completely consume the thiol, the accompanying oxidative yellowing causes the color number to skyrocket, seriously impairing optical quality. Considering all factors, a constant temperature reaction at 90℃ for 4 hours was ultimately determined to be the optimal process parameter.

[0086] 3.3 Optimization of Curing Process Once the matrix resin is synthesized, while it is still in a molten or low-viscosity state, the filler compounding and final adhesive preparation should be carried out immediately. 7.5 parts of the double-modified nano-zirconia powder obtained in step 2.3 are added in batches to 120 parts of the sulfide-modified fluorene-based epoxy prepolymer obtained in step 3.2. Due to the high viscosity of the prepolymer, the mixture is placed in a planetary mixer for degassing at 2000 rpm and 800 rpm for 10 minutes. This utilizes high shear force to disperse the nanoparticles in the resin, while simultaneously utilizing the epoxy groups on the filler surface to form a physical entanglement with the prepolymer in a preparatory state. Subsequently, 25 parts of 4,4'-diaminodiphenyl sulfone as a latent curing agent and 1 part of 2-ethyl-4-methylimidazole as an accelerator are added to the mixture. The planetary mixer is restarted and stirred under vacuum for 15 minutes to completely remove micro-bubbles from the system, resulting in a uniform, bubble-free, pale yellow viscous adhesive.

[0087] For such high-refractive-index rigid systems, direct one-step high-temperature curing can easily lead to explosive polymerization and glue flow due to concentrated exothermic reaction, or the rapid cross-linking rate can trap air bubbles inside the adhesive layer, forming optical scattering points. More seriously, the huge internal stress generated by rapid shrinkage can cause the adhesive layer to crack or detach from the substrate after cooling. Therefore, it is necessary to design a three-stage gradient curing process: low-temperature leveling and bubble removal, medium-temperature main reaction shaping, and high-temperature post-treatment to relieve stress.

[0088] 3.3.1 Optimization of Leveling and Defoaming Processes This experiment aimed to find the optimal thermal window that minimizes the viscosity of the adhesive to facilitate the escape of microbubbles and wetting of the substrate, while avoiding premature curing and viscosity rebound. Based on the ignition temperature of the formulation measured by DSC thermal analysis, the holding time was uniformly set to the industry-standard 1 hour. With temperature as the variable, a 5°C increment gradient was set within the range of 80°C to 120°C, resulting in a total of 9 independent experimental groups.

[0089] The adhesive was poured into a standard mold and placed in an oven to perform the first stage of the process. Before removing it and quenching it, the surface of the sample was touched with a probe and its state was recorded. After the sample was removed and quenched to fully cure, the density of the final cured product was tested using the water displacement method. The theoretical true density is about 1.35 g / cm³. The closer the measured density is to this value, the fewer air bubbles remain inside and the more thorough the degassing is.

[0090] The test results are as follows: Within the 80-95℃ range, the resin melt viscosity remains high, and the microbubbles, affected by Stokes resistance, struggle to completely escape within one hour, resulting in a lower final density. When the temperature exceeds 105℃, the imidazole accelerator is activated, causing premature skinning and gelation on the surface of the adhesive, trapping the escaping bubbles within the system, thus reducing the density. The optimal product temperature is 100℃, hence it is chosen as the temperature for the first stage.

[0091] 3.3.2 Optimization of the main curing process The primary curing process is a crucial step in constructing the core framework of the cross-linked network. The temperature must be set higher than the activation energy threshold of the system to ensure the initiation of the curing reaction, while the rate of exothermic reaction must be controlled to prevent explosive polymerization caused by heat accumulation. Based on previous experiments, the holding time was uniformly fixed at 2 hours, and the temperature was used as a variable, with an incremental gradient of 5°C within the range of 130°C to 170°C.

[0092] After the first stage of processing, the sample is placed in an oven to perform the second stage procedure. A small amount of sample is taken for differential scanning calorimetry (DSC) analysis (heating rate 10℃ / min). The degree of curing is determined by calculating the ratio of the residual reaction exothermic enthalpy to the total enthalpy of complete resin curing. The surface of the sample is observed to see if it is flat and whether there are cracks, warping or scorching and yellowing.

[0093] The test results are as follows: The thermal energy provided in the 130-140℃ range was insufficient to drive the sterically hindered fluorene-based epoxy to react completely with DDS, resulting in a curing degree below 92% and incomplete release of material properties. While temperatures exceeding 160℃ resulted in high curing degree, the intense, concentrated exothermic reaction forcibly froze the polymer chains, preventing the release of internal stress generated by volume shrinkage and causing warping or even cracking of the samples. At 150℃, high curing degree was achieved while maintaining good appearance integrity, achieving the optimal balance between reaction degree and molding quality. Therefore, the second-stage process parameters were 150℃ for 2 hours.

[0094] 3.3.3 Post-curing and stress relief process optimization The curing process aims to impart short-term relaxation mobility to polymer chain segments through heat treatment above the glass transition temperature, thereby eliminating internal stress frozen during network formation and inducing a deep reaction of residual active groups to improve final heat resistance. Based on the 150℃ baseline determined in the second stage, the temperature range for this stage was set to 160℃ to 200℃ in 5℃ increments; the holding time was fixed at 1 hour; and a total of 9 independent experimental groups were constructed.

[0095] Its reflow soldering resistance was evaluated by DSC measurement; the proportion of cracked samples was recorded after 100 cycles between -40℃ and 125℃ to characterize the stress relief effect; the refractive index (nD25) was detected to determine whether high-temperature post-treatment would lead to optical performance degradation; and the elongation at break was used to determine whether high temperature would lead to over-crosslinking embrittlement.

[0096] The test results are as follows: The processing temperature of 160-165℃ was insufficient to provide enough chain segment relaxation energy, resulting in a low Tg of the finished product, and residual internal stress led to thermal shock cracking. As the temperature increased to 180℃, the material's Tg jumped to its peak and perfectly passed the thermal shock test. Crucially, after high-temperature densification treatment, the final refractive index of the finished product successfully exceeded 1.65, verifying the cumulative qualitative change effect of the multiple optimization strategies; simultaneously, the elongation at break (7.28%) remained at an excellent level. Above 185℃, excessive heat caused the sulfide flexible segments to break or become over-crosslinked, manifested as a sharp decrease in elongation at break and a drop in Tg. The third-stage process was ultimately determined to be a 180℃ holding time of 1 hour.

[0097] Example 4 This embodiment provides a process for preparing an optical-grade epoxy resin adhesive that combines high refractive index and flexibility. The specific steps are as follows: S1, Preparation of dual-surface modified nano-zirconia 100g of zirconia nanoparticles with a particle size of 10-20nm were dispersed in 1000g of anhydrous ethanol and ultrasonically dispersed at 40kHz for 30 minutes. Then, 6.5g of γ-glycidyl etheroxypropyltrimethoxysilane (KH-560) and 7.0g of phenyltrimethoxysilane were added, and the mixture was heated to 65℃ and refluxed at 500rpm for 6 hours. After the reaction, the mixture was centrifuged at 10000rpm, washed three times with ethanol, vacuum dried at 80℃ for 12 hours, and then ground and sieved to obtain a dual-modified nanofiller with phenyl and epoxy groups grafted onto its surface.

[0098] S2, Synthesis of sulfide-modified fluorene-based epoxy prepolymer In a reactor equipped with a mechanical stirrer and reflux condenser, 1000 g of 9,9-bis[(2,3-epoxypropoxy)phenyl]fluorene (bisphenol fluorene epoxy resin) and 1000 g of toluene were added, and the mixture was heated to 90 °C and stirred until dissolved. 200 g of 2,2'-dimercaptodiethyl sulfide (DMDS) and 1.0 g of triphenylphosphine were premixed and then added dropwise to the reactor at a uniform rate over 30 minutes. After the addition was complete, the reaction was maintained at 90 °C for 4 hours to allow the flexible sulfide segments to fully embed into the rigid framework. Finally, the toluene solvent was completely removed using a rotary evaporator under a vacuum of -0.098 MPa, yielding a pale yellow, transparent modified prepolymer.

[0099] S3, Compounding and Degassing of Adhesives Weigh 90g (7.5 parts) of the modified nanofiller prepared in S1 and add it in batches to the modified prepolymer prepared in S2 (containing a total of 1000g of resin matrix). Transfer the mixture to a planetary mixer. Set the revolution speed to 2000 rpm and the rotation speed to 800 rpm, and stir for 10 minutes to achieve high shear dispersion. Then add 250g of 4,4'-diaminodiphenyl sulfone (DDS) and 10g of 2-ethyl-4-methylimidazole, adjust the equipment to vacuum mode (-0.098MPa), and continue stirring for 15 minutes to obtain a uniform, bubble-free final solution.

[0100] S4, Gradient Curing Molding The adhesive was applied to the test substrate and placed in a precision oven for curing according to the following procedure: first, it was kept at 100℃ for 1 hour for leveling and bubble removal, then the temperature was raised to 150℃ and kept for 2 hours for cross-linking and shaping, and finally the temperature was raised to 180℃ and kept for 1 hour for high-temperature stress relief treatment. The finished product was obtained by naturally cooling to room temperature.

[0101] Test data of the cured sample: refractive index (nD25) is 1.6605; glass transition temperature (Tg) is 174.5℃; elongation at break is 7.45%; light transmittance (450nm) is 93.2% (no agglomeration, extremely low haze); yellowing index (ΔYI) is 2.5; no cracking or debonding after 100 cycles of thermal shock testing at -40℃ to 125℃.

[0102] In summary, by employing a dual strategy of chemical block toughening and physical nano-reinforcement, the technical bottleneck of the mutual constraint between high refractive index and high flexibility in optical materials has been successfully overcome. In terms of matrix design, a high molar refractive index sulfide chain extender is used to molecularly modify the rigid bisphenol fluorene skeleton. This introduces flexible segments, significantly improving the material's elongation at break, while the high electron density of sulfur compensates for optical losses, allowing the matrix resin to maintain an extremely high intrinsic refractive index while significantly toughening it. Furthermore, the use of a sulfone-containing aromatic amine curing agent further endows the material with excellent heat resistance, rigidity, and resistance to oxidation and yellowing, ensuring it can withstand the rigorous tests of 260℃ high-temperature reflow soldering and long-term photothermal aging.

[0103] In nanocomposite modification, this invention constructs a composite interface layer based on phenyl and epoxy-based bifunctional silanes, precisely solving the problems of refractive index mismatch and weak bonding between inorganic nano-zirconia and organic matrices. The introduction of phenyl groups effectively eliminates interfacial light scattering, achieving a high-transmittance optical path with low haze; while the chemical bonding of epoxy groups transforms rigid particles into reinforcing and toughening physical rivets, avoiding stress debonding caused by traditional physical doping. The resulting cured material has a refractive index of over 1.65, exhibiting excellent flexibility and weather resistance, perfectly meeting the bonding requirements of high-end flexible optical devices.

[0104] It should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A method for preparing an optical-grade epoxy resin adhesive that combines high refractive index and flexibility, characterized in that... Includes the following steps: S1. Disperse nano-zirconia in a solvent, add 5-20% of a composite silane coupling agent by mass of nano-zirconia, and perform a grafting reaction at 40-80℃ for 3-9 hours; after the reaction, centrifuge, wash and dry to obtain double-surface modified nano-zirconia. S2. Using bisphenol fluorene-type epoxy resin as a rigid skeleton, dissolve it in toluene and heat it to 70~110℃. Under the action of a catalyst, slowly add 15~25% of a sulfide-based flexible chain extender by mass of resin. React at a constant temperature for 2~8 hours to introduce flexible sulfide segments. After the reaction is completed, remove the solvent under reduced pressure to obtain the modified fluorene-based epoxy prepolymer. S3. Using the modified fluorene-based epoxy prepolymer as the matrix, add 4-12% of the double-surface modified nano-zirconia by mass of the prepolymer, disperse it evenly using high shear force, add 20-30% of the aromatic amine curing agent and 0.5-2% of the accelerator by mass of the prepolymer, stir and degas under vacuum conditions to obtain the adhesive solution. S4. Apply the adhesive liquid to the surface of the substrate and perform a step curing process within a temperature range of 80~200℃. After cooling, the final cured product is obtained.

2. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The composite silane coupling agent is composed of phenyltrimethoxysilane and γ-glycidoxypropyltrimethoxysilane; the amount of phenyltrimethoxysilane added is 7.0% of the mass of the nano-zirconia, and the amount of γ-glycidoxypropyltrimethoxysilane added is 6.5% of the mass of the nano-zirconia.

3. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The specific process of the grafting reaction is as follows: before the reaction, the system is first dispersed by ultrasonic treatment at a frequency of 40kHz for 30 minutes; then the system is heated to 65℃ and refluxed at a constant temperature for 6.0 hours.

4. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The bisphenol fluorene-type epoxy resin is 9,9-bis[(2,3-epoxypropoxy)phenyl]fluorene; the thioether-based flexible chain extender is 2,2'-dimercaptodiethyl sulfide, with an addition amount of 20%; and the catalyst is triphenylphosphine, with an addition amount of 0.1%.

5. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The specific operation of the isothermal reaction in step S2 is as follows: the system is heated to 90°C and kept at that temperature for 4.0 hours.

6. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, In step S3, the amount of the dual-surface modified nano-zirconia added is 7.5%; the aromatic amine curing agent is 4,4'-diaminodiphenyl sulfone, and the amount added is 25%; the accelerator is 2-ethyl-4-methylimidazole, and the amount added is 1%.

7. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The specific parameters for the high shear force dispersion are as follows: the planetary mixer is set to revolve at 2000 rpm and rotate at 800 rpm, with a processing time of 10 minutes; the vacuum degree for the vacuum degassing is -0.098 MPa.

8. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The specific procedure for the stepped curing process is as follows: first, the temperature is kept at 100℃ for 1.0 hour to perform leveling and defoaming; then, the temperature is kept at 150℃ for 2.0 hours to perform main reaction shaping; and finally, the temperature is kept at 180℃ for 1.0 hour to perform stress relief treatment.

9. The method for preparing an optical-grade epoxy resin adhesive with both high refractive index and flexibility according to claim 1, characterized in that, The nano-zirconia particles in step S1 have a particle size of 10-20 nm; the solvent used is anhydrous ethanol.

10. A cured optical-grade epoxy resin adhesive that combines high refractive index and flexibility, characterized in that, Prepared by the method according to any one of claims 1 to 9, the cured adhesive has an inorganic-organic hybrid network structure, wherein the rigid fluorene framework and inorganic nanoparticles provide high refractive index support, and the flexible sulfide segments provide deformation buffer.

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