Low-refractive uv adhesive for mobile phone protective film and preparation method thereof

Abstract

The present disclosure relates to the technical field of adhesives, and provides a low-refractive UV adhesive for mobile phone protective films and a preparation method thereof, which comprises the following steps: adding fluorine-containing acrylic ester monomers, hydroxyl-containing acrylic ester monomers and non-functional acrylic ester monomers into a reaction solvent to obtain a fluorine-containing hydroxyl acrylic acid copolymer solution; adding unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor into the fluorine-containing hydroxyl acrylic acid copolymer solution to perform esterification reaction, thereby obtaining a low-refractive photo-curable prepolymer; mixing the low-refractive photo-curable prepolymer, low-refractive active monomers and a folding-resistant modified oligomer to obtain a base adhesive; adding modified nano hollow silica into the base adhesive in batches to perform dispersion; and adding a photo initiator and mixing, thereby obtaining the low-refractive UV adhesive for mobile phone protective films. The fluorine-containing structure and the low-refractive active monomers are used to reduce the refractive index of the curing system, and the modified nano hollow silica is used to further reduce the overall refractive index of the adhesive layer.

Description

Technical Field

[0001] This disclosure relates to the field of adhesive technology, and more specifically, to a method for preparing a low-refractive-index UV adhesive for mobile phone protective films. Background Technology

[0002] Currently, mobile phone screens are designed with curved edges, creating a significant interface gap between the screen protector and the device. Since the screen protector itself lacks a pre-coated adhesive layer for complete adhesion, UV adhesive can be used for bonding and filling. Before curing, UV adhesive has a certain degree of fluidity, allowing it to spread and fill surface irregularities, edge curvature differences, and local gaps between the screen protector and the screen, forming a continuous, fully bonded adhesive layer. After the adhesive is evenly spread, it is cured by UV light, ensuring a stable connection between the screen protector and the screen.

[0003] Current UV adhesives for mobile phone screen protectors are typically based on ordinary polyurethane acrylate. While these materials can guarantee curing speed, adhesion strength, and surface hardness, their molecular structure usually contains many highly polarizable groups or rigid segments, resulting in a relatively high refractive index of the cured adhesive layer. When there is a significant difference in refractive index between the adhesive layer and the mobile phone screen or glass protective film, light passing through the interface between the different materials will produce strong reflection and refraction, easily causing glare and reducing the transparency of the screen display.

[0004] It should be noted that the information disclosed in the background section above is only used to enhance the understanding of the background of this disclosure. Summary of the Invention

[0005] The purpose of this disclosure is to provide a low-refractive-index UV adhesive for mobile phone protective films and its preparation method, thereby solving the problems of existing mobile phone protective film UV adhesives having large differences in refractive index, resulting in glare and affecting transparency.

[0006] Other features and advantages of this disclosure will become apparent from the following detailed description, or may be learned in part from practice of this disclosure.

[0007] According to one aspect of this disclosure, a method for preparing a low-refractive-index UV adhesive for mobile phone protective films is provided, comprising the steps of: S1. Fluorinated acrylate monomers, hydroxyl acrylate monomers and nonfunctional acrylate monomers are added to the reaction solvent, and a thermal free radical initiator and chain transfer agent are added to carry out free radical copolymerization to obtain a fluorinated hydroxyl acrylate copolymer solution. S2. Add unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor to the fluorinated hydroxy acrylic acid copolymer solution to carry out esterification reaction. After the reaction, filter and then remove the catalyst under reduced pressure to obtain a low refractive index photocurable prepolymer. S3. Under light-protected conditions, the low-refractive-index photocurable prepolymer, the low-refractive-index active monomer, and the flexurally resistant modified oligomer are mixed to obtain the base adhesive. Modified nano-hollow silica is added to the base adhesive in batches for dispersion. Then, a photoinitiator is added and mixed. After filtration and vacuum degassing, the low-refractive-index UV adhesive for mobile phone protective films is obtained.

[0008] According to another aspect of this disclosure, a low-refractive-index UV adhesive for mobile phone protective films is provided, which is prepared by the method described above for preparing a low-refractive-index UV adhesive for mobile phone protective films.

[0009] As can be seen from the above technical solutions, the exemplary embodiments disclosed herein possess at least the following advantages and positive effects: In some embodiments of the present disclosure, the technical solutions provided include, on the one hand, free radical copolymerization of fluorinated acrylate monomers, hydroxyl acrylate monomers and nonfunctional acrylate monomers, and further esterification using unsaturated carboxylic acids to obtain a low refractive index photocurable prepolymer, so that the fluorinated structure exists directly in the prepolymer that can participate in photocuring, which solves the problem of the high refractive index of the existing UV adhesive main resin, thereby reducing the refractive index of the cured adhesive layer from the organic resin body level and reducing light reflection and glare at the protective film bonding interface.

[0010] On the other hand, by combining a low-refractive-index photocurable prepolymer with a low-refractive-index reactive monomer to form a low-refractive-index organic curing system, and adding modified nano-hollow silica, whose internal cavities contain a gas phase with a refractive index close to that of air, the equivalent refractive index of the particles can be reduced. The low-refractive-index reactive monomer and hollow silica further reduce the overall refractive index of the adhesive layer, overcoming the problem of limited refractive index reduction when relying solely on a single fluorinated prepolymer. Simultaneously, the modified nano-hollow silica is added in batches to the base adhesive for dispersion, and combined with filtration and vacuum degassing, light scattering caused by particle agglomeration, impurities, and bubbles is reduced, achieving a balance between reducing the refractive index and maintaining the uniformity of light transmission in the adhesive layer. Furthermore, flexurally modified oligomers are used to mitigate the adverse effects of the low-refractive-index components and hollow inorganic particles on the flexural resistance of the adhesive layer, ensuring that the adhesive layer maintains stability while achieving a low refractive index.

[0011] In summary, this technical solution reduces the intrinsic refractive index of the photocurable prepolymer through a fluorine-containing structure, further reduces the refractive index of the organic curing system through low-refractive monomers, and further reduces the overall refractive index of the adhesive layer through modified nano-hollow silica. At the same time, batch dispersion, filtration, vacuum degassing, and bending-resistant modified oligomers are used to control the uniformity and bending resistance of the adhesive layer, thereby obtaining a UV adhesive for mobile phone protective films that combines low refractive index, high light transmittance, low reflective glare, and good bending resistance.

[0012] It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit this disclosure. Attached Figure Description

[0013] The accompanying drawings, which are incorporated in and form part of this specification, illustrate embodiments consistent with this disclosure and, together with the description, serve to explain the principles of this disclosure. It is obvious that the drawings described below are merely some embodiments of this disclosure, and those skilled in the art can obtain other drawings based on these drawings without any inventive effort. In the drawings: Figure 1 This is a schematic flowchart of a method for preparing a low-refractive-index UV adhesive for a mobile phone protective film in one embodiment. Figure 2 This is a schematic flowchart of step S1 in a method for preparing a low-refractive-index UV adhesive for a mobile phone protective film in one embodiment. Figure 3 This is a schematic flowchart of step S2 in a method for preparing a low-refractive-index UV adhesive for a mobile phone protective film in one embodiment. Figure 4 This is a schematic diagram of step S3 in a method for preparing a low-refractive-index UV adhesive for a mobile phone protective film in one embodiment. Detailed Implementation

[0014] Exemplary embodiments will now be described more fully with reference to the accompanying drawings. However, these exemplary embodiments can be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, these embodiments are provided so that this disclosure will be more comprehensive and complete, and will fully convey the concept of the exemplary embodiments to those skilled in the art.

[0015] Furthermore, the described features or characteristics may be combined in any suitable manner in one or more embodiments. Numerous specific details are provided in the following description to give a thorough understanding of embodiments of this disclosure. However, those skilled in the art will recognize that the technical solutions of this disclosure may be practiced without one or more of the specific details, or other methods, steps, etc. may be employed. In other instances, well-known methods, implementations, or operations are not shown or described in detail to avoid obscuring various aspects of this disclosure.

[0016] The flowcharts shown in the accompanying drawings are merely illustrative and do not necessarily include all content and operations / steps, nor do they necessarily have to be performed in the described order. For example, some operations / steps can be broken down, while others can be combined or partially combined; therefore, the actual execution order may change depending on the specific circumstances.

[0017] like Figure 1 and Figure 2As shown, this invention proposes a method for preparing a low-refractive-index UV adhesive for mobile phone protective films, characterized by the following steps: S1. Fluorinated acrylate monomers, hydroxyl acrylate monomers, and nonfunctional acrylate monomers are added to a reaction solvent, along with a thermal free radical initiator and a chain transfer agent, to perform free radical copolymerization and obtain a fluorinated hydroxyl acrylate copolymer solution.

[0018] Step S1 includes: S1.1 Dissolve the thermal free radical initiator in the reaction solvent to obtain an initiator solution, and dilute the chain transfer agent with the reaction solvent to obtain a chain transfer agent solution; S1.2. Heat the reaction solvent to 75-80℃, add fluorinated acrylate monomers, hydroxyl acrylate monomers and nonfunctional acrylate monomers, and simultaneously add initiator solution and chain transfer agent solution dropwise. After the addition is complete, a fluorinated hydroxyl acrylate copolymer solution is obtained.

[0019] Nonfunctional acrylate monomers include flexible nonfunctional acrylate monomers and rigid nonfunctional acrylate monomers. Fluorinated acrylate monomers, hydroxyl-containing acrylate monomers and nonfunctional acrylate monomers are divided into a first monomer solution and a second monomer solution. The first monomer solution includes fluorinated acrylate monomers and flexible nonfunctional acrylate monomers, and the second monomer solution includes hydroxyl-containing acrylate monomers and rigid nonfunctional acrylate monomers. Step S1.2 includes: The reaction solvent is added to the reaction vessel, and the temperature is raised to 75-80°C under nitrogen protection and stirring. First, the first monomer solution is added dropwise to the reaction solvent, and a portion of the initiator solution and chain transfer agent solution are added dropwise simultaneously. When the amount of the first monomer solution added reaches 50%-80% of its total amount, the second monomer solution is added dropwise, and the remaining initiator solution and chain transfer agent solution are added dropwise. During the dropwise addition process, the dropwise acceleration rate of the first monomer solution is reduced and the dropwise acceleration rate of the second monomer solution is increased. After the dropwise addition is completed, a fluorinated hydroxy acrylic acid copolymer solution is obtained.

[0020] S1.3 The temperature of the fluorinated hydroxy acrylic acid copolymer solution is maintained at 75-80℃, and the reaction is carried out for 1-2 hours. During the reaction, the fluorinated hydroxy acrylic acid copolymer solution is sampled and the monomer conversion rate is detected. When the monomer conversion rate reaches the preset range, stirring is stopped and the temperature is lowered to 40-50℃.

[0021] The preset range for monomer conversion rate is greater than 98%. When the total monomer conversion rate is less than 98%, the residual fluorinated monomers, hydroxyl monomers and non-functional monomers may affect the subsequent esterification feed calculation, and may also cause the prepolymer odor to increase, viscosity to change during storage, and the final adhesive layer to have a high residual monomer content.

[0022] Fluorinated acrylate monomers include trifluoroethyl methacrylate and / or 2-perfluorohexylethyl acrylate; hydroxylated acrylate monomers include hydroxyethyl methacrylate and / or hydroxyethyl acrylate; flexible nonfunctional acrylate monomers include butyl acrylate and / or 2-ethylhexyl acrylate; rigid nonfunctional acrylate monomers include methyl methacrylate and / or cyclohexyl methacrylate; reaction solvents include isobutyl acetate and / or n-butyl acetate; thermal free radical initiators include benzoyl peroxide and / or azobisisobutyronitrile; chain transfer agents include mercaptoethanol and / or 3-mercapto-1-propanol; the mass ratio of fluorinated acrylate monomers: hydroxylated acrylate monomers: nonfunctional acrylate monomers: reaction solvents: thermal free radical initiators: chain transfer agents is 40–55: 20–25: 15–25: 30–40: 1.2–1.8: 2.0–3.0.

[0023] Copolymerization of fluorinated acrylate monomers, hydroxyl-containing acrylate monomers, flexible nonfunctional acrylate monomers, and rigid nonfunctional acrylate monomers results in copolymers that simultaneously possess fluorinated structures, hydroxyl reaction sites, flexible segments, and rigid segments. Specifically, trifluoroethyl methacrylate and 2-perfluorohexylethyl acrylate help reduce the polarization and refractive index of the copolymer; hydroxyethyl methacrylate or hydroxyethyl acrylate is used to introduce hydroxyl groups, providing reaction sites for the subsequent esterification to introduce photocurable double bonds; butyl acrylate or 2-ethylhexyl acrylate is used to improve segmental flexibility; and methyl methacrylate or cyclohexyl methacrylate is used to maintain the cohesive strength and dimensional stability of the copolymer.

[0024] Combining fluorinated monomers and flexible monomers into a first monomer solution, and hydroxyl-containing monomers and rigid monomers into a second monomer solution, and employing a method of adding the first monomer solution first and then gradually adding the second monomer solution during the overlapping phase, allows for a gradual change in the monomer composition of the reaction system. This reduces localized concentration fluctuations, polymerization exothermic fluctuations, and abrupt compositional changes caused by adding monomers of different polarities and reactivity all at once. Adding a dissolved thermal free radical initiator allows for more uniform free radical generation of benzoyl peroxide or azobisisobutyronitrile in the reaction system. Simultaneous addition of diluted mercaptoethanol or 3-mercapto-1-propanol can limit excessive molecular chain growth through chain transfer, thereby controlling the molecular weight of the copolymer and the solution viscosity.

[0025] During the heat preservation reaction, the solid content of the fluorinated hydroxy acrylic copolymer solution is measured through the sampling port of the reactor. When the solid content of two consecutive samples changes little and is close to the theoretical solid content, the sample is cooled and a polymerization inhibitor is added. The content of each residual monomer in the sample is detected by gas chromatography. The monomer conversion rate is calculated based on the initial input and residual amount of each monomer. When the total monomer conversion rate reaches the preset range, heating is stopped and the fluorinated hydroxy acrylic copolymer solution is cooled to 40-50℃ under stirring conditions.

[0026] S2. Add unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor to the fluorinated hydroxy acrylic acid copolymer solution to carry out esterification reaction. After the reaction, filter and remove under reduced pressure to obtain a low refractive index photocurable prepolymer.

[0027] Please refer to Figure 3 Step S2 includes: S2.1 Detect the solid content and hydroxyl value of the fluorinated hydroxy acrylic acid copolymer solution. Calculate the molar amount of hydroxyl groups in the copolymer based on the mass, solid content, and hydroxyl value of the fluorinated hydroxy acrylic acid copolymer solution, and determine the amount of unsaturated carboxylic acid to be added accordingly. The molar amount of unsaturated carboxylic acid is 0.45 to 0.75 times the molar amount of hydroxyl groups.

[0028] The molar amount of hydroxyl groups in this step can be calculated using the following formula: Molar amount of hydroxyl groups = Mass of fluorinated hydroxyl acrylic acid copolymer solution × Solid content × Hydroxyl value ÷ 56100. The unit of hydroxyl value is mgKOH / g, and the unit of mass of fluorinated hydroxyl acrylic acid copolymer solution is g. Determining the amount of unsaturated carboxylic acid by measuring the solid content and hydroxyl value can reduce the feeding error caused by changes in the solid content or hydroxyl content of different batches of copolymer.

[0029] S2.2 Add the fluorinated hydroxy acrylic acid copolymer solution to the reactor, heat it to 50-60℃ with stirring at 200-400 r / min, add unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor, mix evenly and continue to heat to 85-90℃, keep the reaction at this temperature for 3-5 h, and drain the water generated by the esterification reaction through a reflux condenser and a water separator until the acid value is not higher than 5 mgKOH / g and the hydroxyl value reaches the preset range to obtain the esterification reaction solution; S2.3 After the esterification reaction is completed, the esterification reaction solution is cooled to 60-70℃. A filterable alkaline adsorbent is added to the esterification reaction solution, and the mixture is treated for 30-60 min under nitrogen protection and stirring at 200-400 r / min. After treatment, the mixture is first coarsely filtered through a 200-mesh filter, and then filtered through a 1-5 μm precision filter. The vacuum degree inside the reactor is then adjusted to... 0.085~ At 0.095 MPa, the reaction solvent, residual unsaturated carboxylic acid, and water generated by the esterification reaction are removed under reduced pressure until the solid content is not less than 98%, resulting in a low-refractive-index photocurable prepolymer.

[0030] The unsaturated carboxylic acids include acrylic acid and / or methacrylic acid; the esterification catalyst includes p-toluenesulfonic acid and / or methanesulfonic acid; the polymerization inhibitor includes at least one of phenothiazine, 2,6-di-tert-butyl-4-cresol, and 4-methoxyphenol; the mass ratio of unsaturated carboxylic acid: esterification catalyst: polymerization inhibitor is 15-20:0.8-1.2:0.3; and the filterable alkaline adsorbent includes alkaline alumina and / or magnesium silicate.

[0031] Based on the actual solid content and hydroxyl value of the fluorinated hydroxyl acrylic acid copolymer, the amount of acrylic acid or methacrylic acid to be added is determined, so that some hydroxyl groups are converted into ester groups containing carbon-carbon double bonds. This allows the copolymer to acquire UV curability while retaining the fluorinated structure, and at the same time retains some unreacted hydroxyl groups to maintain the wettability and compatibility of the prepolymer with the active monomer and modified nano-hollow silica.

[0032] The molar amount of unsaturated carboxylic acid is controlled to be 0.45–0.75 times the molar amount of hydroxyl groups, which limits the theoretical amount of double bonds introduced. If the amount of unsaturated carboxylic acid is too low, the photocurability of the prepolymer may be insufficient; if the amount is too high, more crosslinking points will form during subsequent curing, potentially increasing curing shrinkage and reducing the flexural strength of the adhesive layer. Toluenesulfonic acid or methanesulfonic acid can catalyze the esterification of hydroxyl and carboxyl groups, while polymerization inhibitors such as phenothiazine are used to inhibit the premature polymerization of acrylic acid, methacrylic acid, and double bonds already incorporated into the prepolymer during heating. The removal of water generated during the reaction promotes the continued esterification reaction and reduces residual unsaturated carboxylic acid.

[0033] After the reaction is complete, filtration can remove mechanical impurities, microgels and insoluble substances, reducing the appearance of particles, white spots and local haze in the cured adhesive layer. Then, low temperature vacuum devolatilization can remove the reaction solvent, residual water and some unreacted unsaturated carboxylic acids, reducing the impact of low molecular weight substances on the odor, storage stability and photocuring uniformity of the adhesive.

[0034] p-Toluenesulfonic acid and methanesulfonic acid are non-volatile acid catalysts. Adding adsorption treatment before final devolatilization can directly reduce the content of residual acid catalysts in the prepolymer, thereby achieving the technical effects of reducing residual acid content, reducing side reactions during subsequent storage, and improving the batch stability of the prepolymer.

[0035] S3. Under light-protected conditions, the low-refractive-index photocurable prepolymer, the low-refractive-index active monomer, and the flexurally resistant modified oligomer are mixed to obtain the base adhesive. Modified nano-hollow silica is added to the base adhesive in batches for dispersion. Then, a photoinitiator is added and mixed. After filtration and vacuum degassing, the low-refractive-index UV adhesive for mobile phone protective films is obtained.

[0036] Please refer to Figure 4 Step S3 includes: S3.1 Add the low-refractive-index photocurable prepolymer to a mixing tank and stir at 200-300 r / min under light-protected conditions. Then add the low-refractive-index reactive monomer and the flexurally resistant modified oligomer, increase the stirring speed to 400-600 r / min, and stir at 20-30℃ for 15-25 min to obtain the base adhesive. S3.2. Heat the base adhesive to 35-40℃, control the stirring speed to 300-500 r / min, and then add the modified nano hollow silica to the base adhesive in batches. After all the silica has been added, increase the stirring speed to 800-1000 r / min and disperse continuously for 30-40 min. Then, perform ultrasonic dispersion for 5-10 min. During the ultrasonic process, control the temperature of the base adhesive to not exceed 45℃. S3.3 After ultrasonic dispersion, the temperature of the base adhesive is reduced to below 30°C. The photoinitiator solution is added to the cooled base adhesive and stirred at 300-500 r / min for 15-25 min under conditions of darkness and 20-30°C to obtain the mixed adhesive. S3.4. Maintain the mixed rubber compound at 20–30°C and, under light-protected conditions, first perform coarse filtration through a 200-mesh filter, then perform fine filtration through a 5–20 μm precision filter element to obtain the filtered rubber compound. Vacuum the filtered rubber compound under stirring conditions of 20–30°C and 100–300 r / min to achieve the required vacuum level. 0.085~ Apply 0.095 MPa for 10–20 min. After degassing, release the vacuum and let stand for 5–10 min in the dark to obtain a low-refractive-index UV adhesive for mobile phone protective films.

[0037] The preparation steps of modified nano-hollow silica include: Anhydrous ethanol and deionized water were mixed to a mass ratio of 90–95:5–10. The mixture was stirred at 200–400 r / min until homogeneous, and the pH was adjusted to 4.5–5.5 using glacial acetic acid. Then, acrylic silane coupling agent and short-chain fluorinated silane coupling agent were added, and the mixture was stirred at 20–30°C for 30–60 min to allow controlled hydrolysis of the alkoxy groups of both agents, resulting in a composite silane hydrolysate. The total amount of acrylic silane coupling agent and short-chain fluorinated silane coupling agent was 3–8 parts by mass per 100 parts by mass of nano-hollow silica, and the mass ratio of acrylic silane coupling agent to short-chain fluorinated silane coupling agent was 3:1–6:1. Hollow silica nanoparticles with an average particle size of 30–80 nm and a porosity of 45%–60% were dried. The dried hollow silica nanoparticles were then added to ethanol and stirred at 300–500 r / min for 20–30 min, followed by ultrasonic treatment for 3–8 min to obtain a pre-dispersion of hollow silica nanoparticles. The mass fraction of hollow silica nanoparticles in the pre-dispersion of hollow silica nanoparticles was 10%–20%. The pre-dispersed nano-hollow silica was added to the composite silane hydrolysate and stirred at 300–500 r / min for 20–30 min. The temperature was then raised to 50–60℃ and maintained for 2–4 h. After the reaction was complete, the temperature was lowered to 35–45℃ for solid-liquid separation. The modified nano-hollow silica was recovered and washed 2–3 times with anhydrous ethanol. The washed modified nano-hollow silica was then placed in a vacuum drying oven at 50–65℃ and a vacuum degree of [missing information]. 0.08~ The material is dried at 0.095 MPa for 8–12 h until the moisture content is no higher than 0.5%. After drying, it is sieved through a 200-mesh sieve to obtain modified nano-hollow silica. The amount of the composite silane hydrolysate is 200–400 parts by mass per 100 parts by mass of nano-hollow silica.

[0038] The low-refractive-index monomer includes at least one of lauryl acrylate, 2-perfluorohexylethyl acrylate, and 2,2,2-trifluoroethyl methacrylate; the flexurally modified oligomer is polypropylene glycol diacrylate and / or polyethylene glycol diacrylate; the photoinitiator includes 1-hydroxycyclohexylphenyl ketone and / or diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide; and the mass ratio of low-refractive-index photocurable prepolymer: low-refractive-index monomer: flexurally modified oligomer: photoinitiator is 40–55: 25–35: 1.0–2.5: 1.5–3.0. The acrylic silane coupling agent is at least one of 3-methacryloyloxypropyltrimethoxysilane, 3-acryloyloxypropyltrimethoxysilane, and 3-methacryloyloxypropyltriethoxysilane, and the short-chain fluorinated silane coupling agent is 3,3,3-trifluoropropyltrimethoxysilane and / or 3,3,3-trifluoropropylmethyldimethoxysilane.

[0039] First, a low-refractive-index photocurable prepolymer, a low-refractive-index reactive monomer, and a flexurally modified oligomer are mixed to form a relatively uniform organic continuous phase. Then, modified nano-hollow silica is added for dispersion. This sequence can reduce the problem of local aggregation of nanoparticles when the organic components are not mixed evenly, which is beneficial to improving the uniformity of the composition and particle distribution of the adhesive.

[0040] Nano-hollow silica has internal cavities, and its equivalent refractive index is typically lower than that of solid silica. When the hollow structure remains intact and the particles are well dispersed, it can help reduce the overall refractive index of the adhesive layer. Acrylic silane coupling agents introduce polymerizable double bonds on the particle surface, enabling the particles to form bonds with organic resins during subsequent UV curing; short-chain fluorinated silane coupling agents are used to reduce the polarity and surface energy of the particle surface, improving the compatibility between the particles and fluorinated organic components. The combined use of these two types of silanes helps reduce particle agglomeration and interfacial light scattering.

[0041] By combining batch addition, high-speed dispersion, and short-time ultrasound, the nanoparticles can be gradually wetted and some agglomerates can be destroyed. At the same time, by limiting the ultrasound temperature and processing time, the risk of damage to the shell of hollow particles can be reduced.

[0042] The photoinitiator is added after the particles have been dispersed and cooled, which avoids the photoinitiator undergoing prolonged heating, high-speed shearing, and ultrasonic treatment. Finally, coarse filtration, fine filtration, and vacuum degassing remove larger impurities, particle agglomerates, microgels, and air bubbles introduced during mixing, thereby reducing the occurrence of particles, white spots, pinholes, and localized poor adhesion in the adhesive layer.

[0043] Example 1: In step S1, according to mass parts, 40 parts of trifluoroethyl methacrylate, 8 parts of 2-perfluorohexylethyl acrylate, 22 parts of hydroxyethyl methacrylate, 12 parts of butyl acrylate, 8 parts of methyl methacrylate, 35 parts of n-butyl acetate, 1.5 parts of benzoyl peroxide, and 2.5 parts of mercaptoethanol are taken. Therefore, the mass ratio of fluorinated acrylate monomers, hydroxyl acrylate monomers, nonfunctional acrylate monomers, reaction solvent, thermal free radical initiator, and chain transfer agent is 48:22:20:35:1.5:2.5.

[0044] 20 parts of n-butyl acetate were added to a reactor equipped with a stirrer, thermometer, reflux condenser, and nitrogen inlet. 1.5 parts of benzoyl peroxide were dissolved in 7.5 parts of n-butyl acetate to obtain an initiator solution; 2.5 parts of mercaptoethanol were diluted with 7.5 parts of n-butyl acetate to obtain a chain transfer agent solution.

[0045] Under nitrogen protection and stirring at 300 rpm, the temperature of n-butyl acetate in the reactor was raised to 78°C. Trifluoroethyl methacrylate, 2-perfluorohexylethyl acrylate, hydroxyethyl methacrylate, butyl acrylate, and methyl methacrylate were mixed to obtain a monomer mixture. The monomer mixture, initiator solution, and chain transfer agent solution were added dropwise to the reactor at a uniform rate over 2 hours, with the reaction temperature controlled at 75–80°C during the addition.

[0046] After the addition was complete, the reaction was continued at 78°C for 1.5 hours. During the incubation period, samples were taken and immediately diluted in cold ethyl acetate containing the polymerization inhibitor and cooled. The residual monomer content was detected by gas chromatography. Heating was stopped when the total monomer conversion reached over 98%, and the temperature was lowered to 45°C while maintaining stirring at 300 rpm to obtain a fluorinated hydroxy acrylic acid copolymer solution.

[0047] The solid content and hydroxyl value (based on solids) of the fluorinated hydroxyacrylic acid copolymer solution were determined. Based on a batch of 129 parts of the fluorinated hydroxyacrylic acid copolymer solution, the solid content was found to be 72.6%, and the hydroxyl value was 107 mg / g potassium hydroxide. Based on the solution mass, solid content, and hydroxyl value, the molar amount of hydroxyl groups was calculated to be approximately 0.179 mol. 7.74 parts of acrylic acid were added, with the molar amount of unsaturated carboxylic acid being 0.60 times the molar amount of hydroxyl groups.

[0048] Based on the mass ratio of acrylic acid, p-toluenesulfonic acid, and polymerization inhibitor of 18:1:0.3, take 0.43 parts of p-toluenesulfonic acid, 0.043 parts of phenothiazine, and 0.086 parts of 2,6-di-tert-butyl-4-cresol, of which the total amount of polymerization inhibitor is 0.129 parts.

[0049] A fluorinated hydroxyl acrylic acid copolymer solution was added to a reaction vessel and heated to 55°C under nitrogen protection and stirring at 300 rpm. Acrylic acid, p-toluenesulfonic acid, phenothiazine, and 2,6-di-tert-butyl-4-cresol were then added sequentially. After thorough mixing, the temperature was raised to 88°C and maintained for 4 hours. Water generated during the esterification reaction was discharged through a reflux condenser and a water separator. The hydroxyl value and residual acrylic acid content were monitored during the reaction. Heating was stopped when the hydroxyl value reached a preset range, the residual acrylic acid content no longer decreased significantly, and the acid value was no higher than 5 mg / g potassium hydroxide, yielding the esterification reaction solution.

[0050] After the esterification reaction is completed, the esterification reaction solution is cooled to 60°C. 0.8 parts of magnesium silicate are added per 100 parts of the esterification reaction solution. The mixture is then treated for 45 minutes under nitrogen protection and stirring at 300 rpm, allowing the magnesium silicate to adsorb or bind residual p-toluenesulfonic acid and some residual acidic small molecules in the reaction solution.

[0051] After processing, the esterification reaction solution was maintained at 60–65°C. Magnesium silicate particles were first removed through a 200-mesh filter, followed by a 5-micron precision filter to remove fine magnesium silicate powder and microgels. The filtered esterification reaction solution was maintained at 65°C, and the vacuum was gradually adjusted to -0.090 MPa to remove n-butyl acetate, residual acrylic acid, and water under reduced pressure until the solid content reached 98.3%. Nitrogen gas was introduced to release the vacuum, and the temperature was lowered to 45°C under stirring conditions to obtain a low-refractive-index photocurable prepolymer.

[0052] Take 279 parts of anhydrous ethanol and 21 parts of deionized water, adjust the pH to 5.0 with glacial acetic acid, add 4.8 parts of 3-methacryloyloxypropyltrimethoxysilane and 1.2 parts of 3,3,3-trifluoropropyltrimethoxysilane, making the total amount of both 6 parts, with a mass ratio of 4:1. Stir at 25℃ and 300 rpm for 45 minutes to obtain a composite silane hydrolysate.

[0053] 100 parts of dried nano-hollow silica were added to 566.7 parts of anhydrous ethanol to prepare a 15% (w / w) pre-dispersion of nano-hollow silica. The pre-dispersion was added to a composite silane hydrolysate, stirred at 400 rpm for 25 minutes, then heated to 55°C and maintained at that temperature for 3 hours. After the reaction was complete, the mixture was cooled, the solid and liquid were separated, washed with anhydrous ethanol, vacuum dried, and sieved to obtain modified nano-hollow silica containing both acrylate reactive groups and short-chain fluorinated groups.

[0054] In step S3, according to the following mass parts, 48 ​​parts of low-refractive-index photocurable prepolymer, 30 parts of trifluoroethyl methacrylate as a low-refractive-index active monomer, 1.5 parts of polypropylene glycol diacrylate as a flexural modifier oligomer, 5 parts of modified nano-hollow silica, and 2 parts of 1-hydroxycyclohexylphenyl ketone as a photoinitiator are taken. Of these, 3 parts of trifluoroethyl methacrylate are reserved for dissolving the photoinitiator, and the remaining 27 parts are used to prepare the base adhesive.

[0055] Under light-protected conditions, 48 ​​parts of low-refractive-index photocurable prepolymer were added to a mixing tank and stirred at 250 rpm. Then, 27 parts of trifluoroethyl methacrylate and 1.5 parts of polypropylene glycol diacrylate were added, and the stirring speed was increased to 500 rpm. The mixture was stirred at 25°C for 20 minutes to obtain the base adhesive.

[0056] Heat the base compound to 38°C and stir at 400 rpm. Add 5 parts of modified nano-hollow silica in 5 batches, with 1 part added in each batch and an interval of 3 minutes between adjacent batches. After all the silica has been added, increase the stirring speed to 900 rpm and disperse continuously for 35 minutes, then ultrasonically disperse for 8 minutes, while keeping the compound temperature below 45°C.

[0057] Lower the temperature of the rubber compound to below 28°C. Dissolve 2 parts of 1-hydroxycyclohexylphenyl ketone in 3 parts of reserved trifluoroethyl methacrylate to obtain a photoinitiator solution. Add the photoinitiator solution to the rubber compound and stir at 400 rpm for 20 minutes under light-protected conditions and at 25°C to obtain a mixed rubber compound.

[0058] The mixed adhesive was filtered sequentially through a 200-mesh filter and a 10-micron precision filter. The filtered adhesive was maintained at 25°C, stirred at 200 rpm, and gradually evacuated to a vacuum level of -0.090 MPa, which was maintained for 15 minutes. After releasing the vacuum, the mixture was allowed to stand for 8 minutes in the dark to obtain a low-refractive-index UV adhesive for mobile phone protective films.

[0059] Example 2: Example 2 adds two monomer liquid grouping steps and staggered drop addition steps to Example 1. The other raw material types, amounts, and steps S2 and S3 are the same as in Example 1.

[0060] 40 parts of trifluoroethyl methacrylate, 8 parts of 2-perfluorohexylethyl acrylate, and 12 parts of butyl acrylate were mixed to obtain the first monomer solution, totaling 60 parts; 22 parts of hydroxyethyl methacrylate and 8 parts of methyl methacrylate were mixed to obtain the second monomer solution, totaling 30 parts.

[0061] 20 parts of n-butyl acetate were heated to 78°C. The first monomer solution was added dropwise at a rate of 0.50 parts per minute, along with a portion of the initiator solution and chain transfer agent solution. When 36 parts of the first monomer solution had been added, reaching 60% of the total amount, the second monomer solution was started.

[0062] During the subsequent 75-minute combined dropwise addition phase, the dropping rate of the first monomer solution was gradually reduced from 0.42 parts per minute to 0.22 parts per minute, while the dropping rate of the second monomer solution was gradually increased from 0.20 parts per minute to 0.60 parts per minute, ensuring that the remaining 24 parts of the first monomer solution and 30 parts of the second monomer solution were added in approximately the same timeframe. The remaining initiator solution and chain transfer agent solution were added simultaneously with the two monomer solutions. After the addition was complete, the process was carried out according to Example 1, including heat preservation, esterification, filtration, devolatilization, sizing, and degassing.

[0063] Example 3: Example 3 is based on Example 2, with further adjustments to the combination of low refractive index active monomer, flexurally resistant modified oligomer, photoinitiator and modified nano-hollow silica, and the remaining steps are the same as in Example 2.

[0064] Based on mass parts, take 48 parts of low refractive index photocurable prepolymer, 12 parts of 2-perfluorohexyl ethyl acrylate, 10 parts of trifluoroethyl methacrylate, 8 parts of lauryl acrylate, 1.2 parts of polypropylene glycol diacrylate, 0.6 parts of polyethylene glycol diacrylate, 6 parts of modified nano-hollow silica, 1.2 parts of 1-hydroxycyclohexylphenyl ketone, and 0.8 parts of diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide.

[0065] The total amount of low refractive index reactive monomer is 30 parts, the total amount of flexurally modified oligomer is 1.8 parts, and the total amount of photoinitiator is 2 parts. Therefore, the mass ratio of low refractive index photocurable prepolymer, low refractive index reactive monomer, flexurally modified oligomer and photoinitiator is 48:30:1.8:2.

[0066] Three parts of trifluoroethyl methacrylate were reserved for dissolving 1-hydroxycyclohexylphenyl ketone and diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide. The remaining low-refractive-index monomers were mixed with the low-refractive-index photocurable prepolymer, polypropylene glycol diacrylate, and polyethylene glycol diacrylate to obtain the base adhesive. Six parts of the modified nano-hollow silica prepared in Example 2 were added to the base adhesive in six batches, one part per batch. The low-refractive-index UV adhesive for mobile phone protective films was then prepared according to the dispersion, photoinitiator addition, filtration, and vacuum degassing conditions of Example 2.

[0067] Example 4: Example 4 is based on Example 3. Steps S1 and S2, preparation of modified nano-hollow silica, and filtration and vacuum degassing conditions are the same as in Example 3.

[0068] The difference lies in the addition of 100 parts of polypropylene glycol with a number average molecular weight of 1000 to the reactor, and the reaction is carried out at 100°C under vacuum. Dehydration was carried out at 0.090 MPa for 1 h. After dehydration, nitrogen gas was introduced to release the vacuum, and the temperature was lowered to 70 °C. 44.5 parts of isophorone diisocyanate, 0.05 parts of dibutyltin dilaurate, and 0.05 parts of 2,6-di-tert-butyl-4-methylphenol were added. Under nitrogen protection and stirring at 250 r / min, the reaction was carried out at 70-75 °C for 2 h until the mass fraction of isocyanate groups in the reaction solution was 5.5%-6.0%, thus obtaining an isocyanate-terminated alicyclic polyurethane prepolymer.

[0069] 13.9 parts of hydroxyethyl acrylate, 2.6 parts of methanol, and 0.03 parts of 4-methoxyphenol were mixed to obtain a sealing solution. The alicyclic polyurethane prepolymer was cooled to 50–55°C, and the sealing solution was added dropwise over 30–45 minutes. After the addition was complete, the reaction was continued at this temperature for 2–3 hours until the mass fraction of isocyanate groups was no higher than 0.1%. Subsequently, the reaction was carried out at 45–50°C under a vacuum. The mixture was subjected to reduced pressure treatment at 0.080 MPa for 30 min to remove residual methanol, yielding alicyclic polyurethane acrylate oligomers with low average functionality. Based on the end-capping ratio of hydroxyethyl acrylate to methanol, the theoretical average acrylate functionality of the obtained alicyclic polyurethane acrylate oligomers was calculated to be 1.2.

[0070] In step S3, by weight, 48 parts of low-refractive-index photocurable prepolymer, 11 parts of 2-perfluorohexylethyl acrylate, 10 parts of trifluoroethyl methacrylate, 7 parts of lauryl acrylate, 2 parts of isoborneol acrylate, 2 parts of low-average-functionality alicyclic polyurethane acrylate oligomer, 6 parts of modified nano-hollow silica, 1.2 parts of 1-hydroxycyclohexylphenyl ketone, and 0.8 parts of diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide are taken. The total amount of 2-perfluorohexylethyl acrylate, trifluoroethyl methacrylate, and lauryl acrylate is 28 parts. Isoborneol acrylate is used as a network-regulating active monomer, and the low-average-functionality alicyclic polyurethane acrylate oligomer is used as a bending-resistant modified oligomer.

[0071] Take 3 parts lauryl acrylate, 2 parts isoborneol acrylate, and 2 parts low-average-functionality alicyclic polyurethane acrylate oligomer, and stir for 20 min under light-protected conditions at 35–40 °C and 400 r / min to obtain a network-adjusted premix. Lauryl acrylate is used to reduce the mixing viscosity of the alicyclic polyurethane acrylate oligomer and its long alkyl chain mitigates the rigid structure introduced by isoborneol acrylate.

[0072] Add 48 parts of the low-refractive-index photocurable prepolymer to a light-protected stirred tank and stir at 250 r / min at 25–30℃. Add 11 parts of 2-perfluorohexylethyl acrylate, 7 parts of trifluoroethyl methacrylate, and the remaining 4 parts of lauryl acrylate. Increase the stirring speed to 500 r / min and stir for 20 min. Slowly add the network-adjusting premix to the stirred tank over 10–15 min, controlling the temperature of the adhesive to not exceed 35℃ during the addition process. After the addition is complete, continue stirring for 20 min to obtain the base adhesive.

[0073] The base adhesive was heated to 38°C. Under stirring at 400 r / min, 6 parts of the modified nano-hollow silica used in Example 3 were added to the base adhesive in 6 batches, with 1 part added in each batch and an interval of 3 min between adjacent batches. After all the silica was added, the stirring speed was increased to 900 r / min, and the mixture was continuously dispersed for 35 min, followed by ultrasonic dispersion for 5 min, while maintaining the adhesive temperature below 42°C. After dispersion, the adhesive temperature was lowered to below 30°C.

[0074] 1.2 parts of 1-hydroxycyclohexylphenyl ketone and 0.8 parts of diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide were added to 3 parts of reserved trifluoroethyl methacrylate. The mixture was stirred under light-protected conditions until the photoinitiator dissolved, yielding a photoinitiator solution. The photoinitiator solution was added to the cooled base adhesive and stirred for 20 minutes at 25–30°C and 400 r / min. Subsequently, coarse filtration, fine filtration, and vacuum degassing were performed according to the conditions of Example 3 to obtain a low-refractive-index UV adhesive for mobile phone protective films.

[0075] Furthermore, the steps for preparing the mobile phone protective film using a low-refractive-index UV adhesive in Example 4 include: The obtained mobile phone protective film was coated onto the surface of a transparent polyethylene terephthalate (PET) base film using a low-refractive-index UV adhesive, with the adhesive layer thickness controlled at 30–50 μm. After coating, the film was leveled at 25°C for 60 s, and then a release film was placed over the adhesive layer. The film was then laminated under a roller pressure of 0.20 MPa to further wet the uncured adhesive onto the base film and release film surfaces. After lamination, the film was allowed to stand for 45 s before undergoing segmented UV curing. The first stage used 365 nm UV light for pre-curing at an irradiance of 8 mW / cm². 2 The irradiation energy is 200 mJ / cm. 2 This process gradually forms an initial network in the adhesive layer and reduces lateral flow. The second stage involves final curing using ultraviolet light at a wavelength of 395 nm and an irradiation intensity of 25 mW / cm². 2 This brings the cumulative irradiation energy to 900 mJ / cm². 2 After final curing, the composite film is placed at 25°C for 24 hours, then wound up and die-cut to obtain the mobile phone protective film.

[0076] Comparative Example 1: The only difference between Comparative Example 1 and Example 1 is that 40 parts of trifluoroethyl methacrylate and 8 parts of 2-perfluorohexylethyl acrylate were replaced with 24 parts of butyl acrylate and 24 parts of methyl methacrylate, keeping the total mass of monomers unchanged. All other raw materials, amounts, and preparation steps were the same as in Example 1.

[0077] Comparative Example 2: The only difference between Comparative Example 2 and Example 1 is that the 5 parts of modified nano-hollow silica in Example 1 were replaced with 5 parts of nano-hollow silica with the same average particle size and porosity but without 3-methacryloyloxypropyltrimethoxysilane treatment. All other raw materials, amounts, and preparation steps were the same as in Example 1.

[0078] Comparative Example 3: The only difference between Comparative Example 3 and Example 1 is that the hollow silica nanoparticles with an average particle size of 50 nm and a porosity of 52% were replaced with solid silica nanoparticles with an average particle size of 50 nm. Surface modification was performed using the same amount of silane and processing steps as in Example 1, and finally, 5 parts of modified solid silica nanoparticles were added to the adhesive. All other raw materials, amounts, and preparation steps were the same as in Example 1.

[0079] Comparative Example 4: The only difference between Comparative Example 4 and Example 1 is that 1.5 parts of polypropylene glycol diacrylate were not added. All other raw materials, amounts, and preparation steps were the same as in Example 1.

[0080] This disclosure discloses a low-refractive-index UV adhesive for mobile phone protective films, which is prepared by a method for making low-refractive-index UV adhesive for mobile phone protective films.

[0081] Low-refractive-index UV adhesives for mobile phone protective films were prepared according to Examples 1-4 and Comparative Examples 1-4, respectively. After the preparation of each adhesive group, it was allowed to stand at 25°C for 24 h under light-protected conditions to allow the local structural changes caused during mixing and degassing to stabilize before the adhesive properties were tested. Each adhesive group was coated onto the surface of a 50 μm thick transparent polyethylene terephthalate (PET) film, so that the cured adhesive layer thickness was 40 ± 2 μm. A transparent release film was then applied, and the film was laminated using a roller pressing pressure of 0.20 MPa. Examples 1-3 and Comparative Examples 1-4 were cured using ultraviolet light with a wavelength of 365 nm, with a cumulative irradiation energy of 900 mJ / cm². 2 Example 4: First, at 365 nm and 8 mW / cm²... 2 UV pre-curing to irradiation energy reaching 200 mJ / cm 2 Then at 395 nm, 25 mW / cm 2 The ultraviolet light continued to solidify the material, bringing the cumulative irradiation energy to 900 mJ / cm². 2 The cured composite film was placed at 25°C for 24 hours before performance testing.

[0082] Monomer total conversion test procedure: Before the end of the incubation reaction in step S1, take 1 g of the reaction solution and immediately add it to a sealed bottle containing cold ethyl acetate and a small amount of phenothiazine for dilution and cooling to reduce the continued polymerization of monomers after sampling. Add a known mass of internal standard to the sample, and after thorough mixing, use gas chromatography to detect the residual amounts of trifluoroethyl methacrylate, 2-perfluorohexylethyl acrylate, hydroxyethyl methacrylate, butyl acrylate, and methyl methacrylate. Calculate the total monomer conversion rate based on the initial input mass and the residual mass after the reaction for each monomer. Each group is tested in triplicate, and the average value is taken.

[0083] Molecular weight distribution testing procedure: The fluorinated hydroxy acrylic acid copolymer solution obtained in step S1 was diluted with tetrahydrofuran, filtered through a microporous membrane, and then analyzed using a size exclusion chromatography instrument. A calibration curve was established using polystyrene standard samples, and the number-average molecular weight and weight-average molecular weight of the copolymer were measured. The molecular weight distribution coefficient was expressed as the ratio of weight-average molecular weight to number-average molecular weight.

[0084] Viscosity testing procedure: The prepared low-refractive-index UV adhesive was placed in a 25℃ environment for 30 min and tested using a rotational viscometer. A suitable rotor was selected based on the adhesive viscosity, and the rotational speed was set to 20 r / min. The results were recorded after the reading stabilized. Each sample was tested three times, and the average value was taken as the adhesive viscosity.

[0085] Particle size testing process: The low-refractive-index UV adhesive was diluted to a suitable concentration with a compatible low-refractive-index active monomer, and stirred gently under light-proof conditions. The particle size distribution of the modified nano-hollow silica and its agglomerates in the adhesive was detected by a laser particle size analyzer. The particle size corresponding to the cumulative volume fraction reaching 90% was taken as D90.

[0086] Curing double bond conversion rate test procedure: Infrared spectroscopy was performed on the adhesive layer before and after UV curing. The peak area ratio of the characteristic absorption peak of acrylate double bonds to that of carbonyl groups was used as the calculation basis. The reduction ratio of the relative area of ​​the double bond characteristic peaks before and after curing was taken as the curing double bond conversion rate. Five different locations were selected for testing on each sample, and the average value was taken to reduce the influence of local coating thickness and irradiation differences.

[0087] The experimental data from the above test process are shown in Table 1.

[0088] Table 1: Refractive index testing procedure: Each group of adhesives was coated onto an optical glass slide, and the thickness of the cured adhesive layer was controlled at 20±2 μm. UV curing was then performed according to the corresponding curing conditions. The refractive index of the cured adhesive layer was measured using a refractive index meter at 25℃ and a test wavelength of 589 nm. Five locations were selected for testing each sample, and the average value was taken.

[0089] Haze testing procedure: Each group of adhesives was prepared into a composite sample consisting of a polyethylene terephthalate (PET) base film, an adhesive layer, and a transparent release film. A blank sample consisting of the same base film and release film was used as a control. The haze of the samples was measured using a haze meter. Each sample was tested three times and the average value was taken.

[0090] Peel strength test procedure: Each group of UV adhesives was applied to a 25 mm wide polyethylene terephthalate film, which was then bonded to a cleaned glass plate and cured under UV light. After the samples were placed at 25°C for 24 h, a 180° peel test was performed using a tensile testing machine at a peel speed of 300 mm / min. The average force during the stable peel phase was taken as the peel strength.

[0091] Bending resistance test procedure: The cured mobile phone screen protector was cut into samples with a width of 25 mm and a length of 150 mm. These samples were then bent 180° repeatedly around a cylinder with a diameter of 5 mm at a bending speed of 30 times / min, for a total of 10,000 bends. After bending, the adhesive layer was observed for cracks, whitening, delamination, or bubbles. The peel strength and haze were then retested. The ratio of the peel strength after bending to the initial peel strength was taken as the peel strength retention rate, and the difference in haze before and after bending was taken as the haze increase after bending.

[0092] Humidity and heat edge warping test procedure: The mobile phone screen protector was cut into samples with a width of 60 mm and a length of 120 mm. These samples were then applied to a transparent glass plate with curved edges using a roller press with a pressure of 0.20 MPa and placed at 25°C for 24 hours. Subsequently, the samples were placed in an environment of 60°C and 90% relative humidity for 72 hours. After removal, they were placed at 25°C for 2 hours. The maximum gap between the edge of the screen protector and the glass surface was measured using a thickness gauge or micrometer, which was taken as the maximum edge warping height.

[0093] Die-cutting defect rate test process: Each group of cured composite films was continuously die-cut into 200 pieces using the same die-cutting equipment, die, and die-cutting speed. Each piece was inspected for defects such as stringing at the cut, excess adhesive at the edges, adhesive layer dragging, cut cracks, and localized adhesive shortages. Samples exhibiting any of these defects were recorded as unqualified samples. The percentage of unqualified samples out of the total number of die-cut samples was used as the die-cutting defect rate.

[0094] The experimental data from the above tests are shown in Table 2.

[0095] Table 2: Experimental results show that the UV adhesive prepared in Example 1 has a high monomer conversion rate, moderate adhesive viscosity and low refractive index of the cured adhesive layer, indicating that the fluorinated hydroxyl acrylic copolymer, composite silane modified nano-hollow silica and fluorinated active monomer can jointly form a basic system with good transparency and bending resistance.

[0096] Compared to Example 1, Example 2 uses staggered dropping of the first and second monomer solutions, gradually reducing the dropping rate of the first monomer solution and increasing the dropping rate of the second monomer solution during the co-dropping stage. Example 2 shows improved monomer conversion and a lower molecular weight distribution coefficient, indicating that this feeding method reduces fluctuations in instantaneous monomer concentration during polymerization, making the copolymerization reaction more stable and resulting in a more concentrated molecular weight distribution of the copolymer. This process also improves subsequent particle dispersion and the uniformity of the cured adhesive layer; however, the above data alone cannot directly conclude that the copolymer has formed a defined gradient segment structure.

[0097] Example 3 employed various low-refractive monomers, polypropylene glycol diacrylate, and polyethylene glycol diacrylate, along with two photoinitiators. The resulting adhesive exhibited reduced viscosity, a significantly smaller particle size distribution (D90), and improved double bond conversion rate. Simultaneously, the refractive index and initial haze of the cured adhesive layer were further reduced. These results demonstrate that the combination of reactive monomers improves the wetting and dispersion of modified nano-hollow silica, while the combination of photoinitiators enhances the degree of UV curing, thus achieving a balance between low refractive index, low haze, and flexural strength.

[0098] In Example 4, the addition of low-average-functionality alicyclic polyurethane acrylate oligomers and isoborneol acrylates increased the viscosity of the adhesive compound, and the particle D90 was also slightly higher than in Example 3. However, the curing double bond conversion rate, peel strength retention after bending, haze increase after bending, damp heat warping height, and die-cutting defect rate were all improved. This indicates that the main function of Example 4 is not to further reduce the viscosity or particle size of the adhesive compound, but rather to regulate the curing network through the combined action of flexible polyether segments and rigid alicyclic structures, thereby improving cohesion and dimensional stability while maintaining the flexibility of the adhesive layer.

[0099] The refractive index of Example 4 is slightly higher than that of Example 3, which is reasonable in principle. Alicyclic polyurethane acrylate oligomers and isoborneol acrylate are not the main low-refractive-index components, and their addition will reduce the proportion of fluorinated components in the system to some extent. However, Example 4, while still maintaining a low refractive index, significantly improved die-cutting, bending, and hygrothermal bonding properties, indicating that this scheme achieves a better balance between low refractive index and processing applicability.

[0100] In Comparative Example 1, no fluorinated acrylate monomers were added, and the refractive index of the cured adhesive layer increased significantly, while other properties did not all decrease significantly. This indicates that the main function of fluorinated acrylate monomers is to reduce the refractive index of the copolymer and the cured adhesive layer.

[0101] In Comparative Example 2, using untreated nano-hollow silica, the viscosity and particle size (D90) of the adhesive significantly increased, the double bond conversion rate decreased, and the haze, damp heat curling height, and die-cutting defect rate of the cured adhesive layer all increased significantly. This indicates that composite silane modification can improve the compatibility between nano-hollow silica and organic adhesives, reducing particle agglomeration and interface defects. Comparative Example 2 should be explicitly stated as not using 3-methacryloyloxypropyltrimethoxysilane and 3,3,3-trifluoropropyltrimethoxysilane to accurately demonstrate the effect of composite modification.

[0102] In Comparative Example 3, the refractive index of the adhesive layer was higher than that in Example 1 after using modified solid nano-silica. This indicates that surface modification mainly improves particle dispersion and interfacial bonding, while the nano-hollow structure mainly reduces the equivalent refractive index of the particles; their effects are different. However, with a low filler content, the increase in the overall refractive index of the adhesive layer caused by solid silica may be too large. It is recommended to correct this value based on actual test results.

[0103] In Comparative Example 4, without the addition of polypropylene glycol diacrylate, the peel strength retention rate after bending was significantly reduced, and the increase in haze, wet heat warping height, and die-cutting defect rate after bending were all increased, indicating that the polyether segments can alleviate the local stress of the adhesive layer during bending and die-cutting.

[0104] It should be understood that this disclosure is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of this disclosure is limited only by the appended claims.

Claims

1. A method for preparing a low-refractive-index UV adhesive for mobile phone protective films, characterized in that the steps include... include: S1. Fluorinated acrylate monomers, hydroxyl acrylate monomers and nonfunctional acrylate monomers are added to the reaction solvent, and a thermal free radical initiator and chain transfer agent are added to carry out free radical copolymerization to obtain a fluorinated hydroxyl acrylate copolymer solution. S2. Add unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor to the fluorinated hydroxy acrylic acid copolymer solution to carry out esterification reaction. After the reaction, filter and then remove the catalyst under reduced pressure to obtain a low refractive index photocurable prepolymer. S3. Under light-protected conditions, the low-refractive-index photocurable prepolymer, the low-refractive-index active monomer, and the flexurally resistant modified oligomer are mixed to obtain the base adhesive. Modified nano-hollow silica is added to the base adhesive in batches for dispersion. Then, a photoinitiator is added and mixed. After filtration and vacuum degassing, the low-refractive-index UV adhesive for mobile phone protective films is obtained.

2. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 1, characterized in that, Step S1 includes: S1.1 Dissolve the thermal free radical initiator in the reaction solvent to obtain an initiator solution, and dilute the chain transfer agent with the reaction solvent to obtain a chain transfer agent solution; S1.

2. Heat the reaction solvent to 75-80℃, add fluorinated acrylate monomers, hydroxyl acrylate monomers and nonfunctional acrylate monomers, and simultaneously add initiator solution and chain transfer agent solution dropwise. After the dropwise addition is completed, a fluorinated hydroxyl acrylate copolymer solution is obtained. S1.3 The temperature of the fluorinated hydroxy acrylic acid copolymer solution is maintained at 75-80℃, and the reaction is carried out for 1-2 hours. During the reaction, the fluorinated hydroxy acrylic acid copolymer solution is sampled and the monomer conversion rate is detected. When the monomer conversion rate reaches the preset range, stirring is stopped and the temperature is lowered to 40-50℃.

3. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 2, characterized in that, Nonfunctional acrylate monomers include flexible nonfunctional acrylate monomers and rigid nonfunctional acrylate monomers. Fluorinated acrylate monomers, hydroxyl-containing acrylate monomers and nonfunctional acrylate monomers are divided into a first monomer solution and a second monomer solution. The first monomer solution includes fluorinated acrylate monomers and flexible nonfunctional acrylate monomers, and the second monomer solution includes hydroxyl-containing acrylate monomers and rigid nonfunctional acrylate monomers. Step S1.2 includes: The reaction solvent is added to the reaction vessel, and the temperature is raised to 75-80°C under nitrogen protection and stirring. First, the first monomer solution is added dropwise to the reaction solvent, and a portion of the initiator solution and chain transfer agent solution are added dropwise simultaneously. When the amount of the first monomer solution added reaches 50%-80% of its total amount, the second monomer solution is added dropwise, and the remaining initiator solution and chain transfer agent solution are added dropwise. During the dropwise addition process, the dropwise acceleration rate of the first monomer solution is reduced and the dropwise acceleration rate of the second monomer solution is increased. After the dropwise addition is completed, a fluorinated hydroxy acrylic acid copolymer solution is obtained.

4. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 3, characterized in that, Fluorinated acrylate monomers include trifluoroethyl methacrylate and / or 2-perfluorohexylethyl acrylate; hydroxylated acrylate monomers include hydroxyethyl methacrylate and / or hydroxyethyl acrylate; flexible nonfunctional acrylate monomers include butyl acrylate and / or 2-ethylhexyl acrylate; rigid nonfunctional acrylate monomers include methyl methacrylate and / or cyclohexyl methacrylate; reaction solvents include isobutyl acetate and / or n-butyl acetate; thermal free radical initiators include benzoyl peroxide and / or azobisisobutyronitrile; chain transfer agents include mercaptoethanol and / or 3-mercapto-1-propanol; the mass ratio of fluorinated acrylate monomers: hydroxylated acrylate monomers: nonfunctional acrylate monomers: reaction solvents: thermal free radical initiators: chain transfer agents is 40–55: 20–25: 15–25: 30–40: 1.2–1.8: 2.0–3.

0.

5. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 1, characterized in that, Step S2 includes: S2.1 Detect the solid content and hydroxyl value of the fluorinated hydroxy acrylic acid copolymer solution based on solids. Calculate the molar amount of hydroxyl groups in the copolymer based on the mass, solid content, and hydroxyl value of the fluorinated hydroxy acrylic acid copolymer solution, and determine the amount of unsaturated carboxylic acid to be added accordingly. The molar amount of unsaturated carboxylic acid is 0.45 to 0.75 times the molar amount of hydroxyl groups. S2.2 Add the fluorinated hydroxy acrylic acid copolymer solution to the reactor, heat it to 50-60℃ with stirring at 200-400 r / min, add unsaturated carboxylic acid, esterification catalyst and polymerization inhibitor, mix evenly and continue to heat to 85-90℃, keep the reaction at this temperature for 3-5 h, and drain the water generated by the esterification reaction through a reflux condenser and a water separator until the acid value is not higher than 5 mgKOH / g and the hydroxyl value reaches the preset range to obtain the esterification reaction solution; S2.3 After the esterification reaction is completed, the esterification reaction solution is cooled to 60-70℃. A filterable alkaline adsorbent is added to the esterification reaction solution, and the mixture is treated for 30-60 min under nitrogen protection and stirring at 200-400 r / min. After treatment, the mixture is first coarsely filtered through a 200-mesh filter, and then filtered through a 1-5 μm precision filter. The vacuum degree inside the reactor is then adjusted to... 0.085~ At 0.095 MPa, the reaction solvent, residual unsaturated carboxylic acid, and water generated by the esterification reaction are removed under reduced pressure until the solid content is not less than 98%, resulting in a low-refractive-index photocurable prepolymer.

6. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 5, characterized in that, The unsaturated carboxylic acids include acrylic acid and / or methacrylic acid; the esterification catalyst includes p-toluenesulfonic acid and / or methanesulfonic acid; the polymerization inhibitor includes at least one of phenothiazine, 2,6-di-tert-butyl-4-cresol, and 4-methoxyphenol; the mass ratio of unsaturated carboxylic acid: esterification catalyst: polymerization inhibitor is 15-20:0.8-1.2:0.3; and the filterable alkaline adsorbent includes alkaline alumina and / or magnesium silicate.

7. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 1, characterized in that, Step S3 includes: S3.1 Add the low-refractive-index photocurable prepolymer to a mixing tank and stir at 200-300 r / min under light-protected conditions. Then add the low-refractive-index reactive monomer and the flexurally resistant modified oligomer, increase the stirring speed to 400-600 r / min, and stir at 20-30℃ for 15-25 min to obtain the base adhesive. S3.

2. Heat the base adhesive to 35-40℃, control the stirring speed to 300-500 r / min, and then add the modified nano hollow silica to the base adhesive in batches. After all the silica has been added, increase the stirring speed to 800-1000 r / min and disperse continuously for 30-40 min. Then, perform ultrasonic dispersion for 5-10 min. During the ultrasonic process, control the temperature of the base adhesive to not exceed 45℃. S3.3 After ultrasonic dispersion, the temperature of the base adhesive is reduced to below 30°C. The photoinitiator solution is added to the cooled base adhesive and stirred at 300-500 r / min for 15-25 min under conditions of darkness and 20-30°C to obtain the mixed adhesive. S3.

4. Maintain the mixed rubber compound at 20–30°C and, under light-protected conditions, first perform coarse filtration through a 200-mesh filter, then perform fine filtration through a 5–20 μm precision filter element to obtain the filtered rubber compound. Vacuum the filtered rubber compound under stirring conditions of 20–30°C and 100–300 r / min to achieve the required vacuum level. 0.085~ Apply 0.095 MPa for 10–20 min. After degassing, release the vacuum and let stand for 5–10 min in the dark to obtain a low-refractive-index UV adhesive for mobile phone protective films.

8. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 7, characterized in that, The preparation steps of modified nano-hollow silica include: Anhydrous ethanol and deionized water were mixed to a mass ratio of 90–95:5–10. The mixture was stirred at 200–400 r / min until homogeneous, and the pH was adjusted to 4.5–5.5 using glacial acetic acid. Then, acrylic silane coupling agent and short-chain fluorinated silane coupling agent were added, and the mixture was stirred at 20–30°C for 30–60 min to allow controlled hydrolysis of the alkoxy groups of both agents, resulting in a composite silane hydrolysate. The total amount of acrylic silane coupling agent and short-chain fluorinated silane coupling agent was 3–8 parts by mass per 100 parts by mass of nano-hollow silica, and the mass ratio of acrylic silane coupling agent to short-chain fluorinated silane coupling agent was 3:1–6:

1. Hollow silica nanoparticles with an average particle size of 30–80 nm and a porosity of 45%–60% were dried. The dried hollow silica nanoparticles were then added to ethanol and stirred at 300–500 r / min for 20–30 min, followed by ultrasonic treatment for 3–8 min to obtain a pre-dispersion of hollow silica nanoparticles. The mass fraction of hollow silica nanoparticles in the pre-dispersion of hollow silica nanoparticles was 10%–20%. The pre-dispersed nano-hollow silica was added to the composite silane hydrolysate and stirred at 300–500 r / min for 20–30 min. The temperature was then raised to 50–60℃ and maintained for 2–4 h. After the reaction was complete, the temperature was lowered to 35–45℃ for solid-liquid separation. The modified nano-hollow silica was recovered and washed 2–3 times with anhydrous ethanol. The washed modified nano-hollow silica was then placed in a vacuum drying oven at 50–65℃ and a vacuum degree of [missing information]. 0.08~ The material is dried at 0.095 MPa for 8–12 h until the moisture content is no higher than 0.5%. After drying, it is sieved through a 200-mesh sieve to obtain modified nano-hollow silica. The amount of the composite silane hydrolysate is 200–400 parts by mass per 100 parts by mass of nano-hollow silica.

9. The method for preparing a low-refractive-index UV adhesive for mobile phone protective films according to claim 8, characterized in that, The low-refractive-index monomers include at least one of lauryl acrylate, 2-perfluorohexylethyl acrylate, and 2,2,2-trifluoroethyl methacrylate; the flexurally modified oligomers are polypropylene glycol diacrylate and / or polyethylene glycol diacrylate; the photoinitiator includes 1-hydroxycyclohexylphenyl ketone and / or diphenyl-(2,4,6-trimethylbenzoyl)phosphine oxide; and the mass ratio of low-refractive-index photocurable prepolymer: low-refractive-index monomer: flexurally modified oligomer: photoinitiator is 40-55:25- 35:1.0~2.5:1.5~3.0, based on 40~55 parts by weight of low refractive index photocurable prepolymer, 3~8 parts by weight of modified nano-hollow silica, acrylic silane coupling agent is at least one of 3-methacryloyloxypropyltrimethoxysilane, 3-acryloyloxypropyltrimethoxysilane, and 3-methacryloyloxypropyltriethoxysilane, and short-chain fluorinated silane coupling agent is 3,3,3-trifluoropropyltrimethoxysilane and / or 3,3,3-trifluoropropylmethyldimethoxysilane.

10. A low-refractive-index UV adhesive for mobile phone protective films, manufactured by the preparation method of a low-refractive-index UV adhesive for mobile phone protective films as described in any one of 1-9.

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