JP5375204B2 - Antireflection film manufacturing method, antireflection film and optical element - Google Patents

Description

  The present invention relates to a method for producing an antireflection film having a silica airgel film having nano-sized micropores and having a low refractive index and excellent scratch resistance, and an antireflection film and an optical element obtained by this method.

The antireflection film is formed by a physical vapor deposition method such as vacuum vapor deposition, sputtering, or ion plating, or a liquid phase method such as a sol-gel method. The antireflection film needs to have a refractive index smaller than that of the base material. For example, when glass or plastic (refractive index: about 1.5 to 1.9) is used as the base material, the ideal refractive index for the antireflection film is 1.2 to 1.25. is there. When an antireflection film is formed by physical vapor deposition, the refractive index is relatively high at 1.38 even with the smallest refractive index material (MgF ₂ ), and the reflectance in the visible light region (wavelength: 400 to 700 nm) Will be over 1%.

A silica airgel film obtained by the sol-gel method has a smaller refractive index than MgF ₂ obtained by physical vapor deposition. US Pat. No. 5,948,482 (Patent Document 1) discloses a method for producing an airgel that can be used as an antireflection film by aging a sol containing SiO ₂ into a gel and then modifying the surface with an organic group. Describes a method of producing a silica airgel membrane using The silica airgel film formed by this manufacturing method has a porosity of 99% or more and a low refractive index. However, this silica airgel film has a problem that the mechanical strength is extremely low and the scratch resistance is poor.

  "Journal of Sol-Gel Science and Technology", 2000, Vol. 18, pp. 219-224 (Non-Patent Document 1) is a nanoporous material with excellent scratch resistance. A method for producing a silica film is proposed. This nanoporous silica membrane is prepared by hydrolyzing and polycondensing tetraethoxysilane with ammonia in an ethanol / water mixed solvent at 80 ° C. for 2 to 20 hours to prepare an alkaline sol, to which tetraethoxysilane, water and hydrochloric acid are added. After aging at 60 ° C for 15 days, the obtained sol was applied on a substrate, dried at 80 ° C for 30 minutes, and then heat-treated in ammonia / water vapor mixed gas or in the atmosphere (at 400 ° C for 30 minutes). Form. However, since this method requires 15 days for aging, not only the production efficiency is low, but the resulting nanoporous silica film is particularly scratch resistant when used as an antireflection film for an optical element such as a camera lens. It is insufficient. Furthermore, such a porous film has a problem in that the refractive index varies with the passage of time after manufacture, and a desired antireflection performance cannot be obtained.

  Japanese Patent Application Laid-Open No. 9-1064 (Patent Document 2) discloses that a condensation composition obtained by mixing a basic condensation composition of tetraalkoxysilane and an acidic condensation composition is further mixed with a condensation composition of alkylalkoxysilane. It is stated that a film with less anti-reflection effect over time can be obtained by placing a quality film obtained by applying the condensation composition obtained on the base material in an environment with a relative humidity of 70% or more (humidity treatment). ing. However, the method described in JP-A-9-1064 is very low in productivity because the humidity treatment takes time. Japanese Patent Application Laid-Open No. 9-1064 describes that when the processing time is short, the effect of preventing fluctuations with time is insufficient, and further light irradiation is required. Therefore, it is necessary to add a new manufacturing process in order to improve productivity, and it is difficult to cope with the existing equipment.

US Patent 5,948,482 Japanese Patent Laid-Open No. 9-1064

“Journal of Sol-Gel Science and Technology”, 2000, Vol. 18, pp. 219-224

  Accordingly, an object of the present invention is to efficiently produce an antireflection film having a low refractive index silica airgel film having excellent scratch resistance and little change in refractive index, and an antireflection film formed by this method. And an optical element having the antireflection film.

  As a result of diligent research in view of the above object, the present inventors have found that the first acidic solution obtained by further adding an acidic solution to an alkaline sol prepared by hydrolysis and polycondensation of alkoxysilane under a basic catalyst. A mixed sol obtained by mixing a sol and a second acidic sol obtained by hydrolysis and condensation polymerization of alkoxysilane under an acidic catalyst is coated and dried on a substrate, and then stored in a high humidity condition. As a result, it was found that the mechanical strength was improved and the time variation of the refractive index was reduced, and the present invention was conceived.

  That is, the method for producing an antireflection film of the present invention is a method for producing an antireflection film having at least one silica airgel film, and the silica airgel film hydrolyzes and polycondenses alkoxysilane under a basic catalyst. The first acidic sol having a median diameter of 100 nm or less obtained by further adding an acidic solution to the alkaline sol prepared above, and the median diameter of 10 obtained by hydrolysis and condensation polymerization of alkoxysilane under an acidic catalyst. A mixed sol made by mixing a second acidic sol of less than nm is coated and dried on a substrate, and then subjected to a humidity treatment that is stored for at least 1 hour in an environment with a temperature of 35 ° C or higher and a relative humidity of 70% or higher. It is characterized by doing.

  It is preferable to perform an alkali treatment after applying the silica airgel film. The alkali treatment is preferably performed by applying an alkali solution or leaving it in an ammonia atmosphere.

  The humidity treatment time is preferably 5 to 48 hours.

  The antireflection film of the present invention is produced by any one of the methods described above.

  The optical element of the present invention has the antireflection film.

  The refractive index of the substrate is preferably 1.35 to 2.00, and the refractive index of the silica airgel film is preferably 1.05 to 1.35. The silica airgel membrane preferably has a physical thickness of 15 to 500 nm.

  The silica airgel film formed by the method of the present invention has a low refractive index and excellent scratch resistance, and since the refractive index has little time fluctuation, the resulting antireflection film has a high antireflection effect and high storage stability. Have For this reason, this antireflection film is suitable for optical elements in a wide range of fields.

[1] Method for Producing Antireflection Film The antireflection film of the present invention has at least one silica airgel film. The silica airgel membrane is obtained by (1) adding an acidic solution to an alkaline sol prepared by hydrolysis and condensation polymerization of alkoxysilane under a basic catalyst to obtain a first acidic sol having a median diameter of 100 nm or less. A step, (2) a step of hydrolyzing and polycondensing alkoxysilane under an acidic catalyst to obtain a second acidic sol having a median diameter of 10 nm or less, and (3) a step of mixing the first and second acidic sols. (4) The obtained mixed sol is formed on a substrate by applying and drying, (5) an alkali treatment step, (6) a cleaning step, and (7) a humidity treatment step.

(1) Step of preparing the first acidic sol
(a) Alkoxysilane The alkoxysilane for the first acidic sol is preferably a tetraalkoxysilane monomer or oligomer (condensation product). When a tetrafunctional alkoxysilane is used, a sol of colloidal silica particles having a relatively large particle size can be obtained. Tetraalkoxysilane is represented by Si (OR) ₄ [R is an alkyl group having 1 to 5 carbon atoms (methyl, ethyl, propyl, butyl, etc.) or an acyl group having 1 to 4 carbon atoms (acetyl etc.)]. Those are preferred. Specific examples of tetraalkoxysilane include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, diethoxydimethoxysilane and the like. Of these, tetramethoxysilane and tetraethoxysilane are preferred. A small amount of trifunctional or lower functional alkoxysilane may be added to the tetraalkoxysilane as long as the effects of the present invention are not impaired.

(b) Hydrolysis and polycondensation in the presence of a basic catalyst Hydrolysis and polycondensation proceed by adding an organic solvent, a basic catalyst and water to the alkoxysilane. As the organic solvent, alcohols such as methanol, ethanol, n-propanol, i-propanol, and butanol are preferable, and methanol or ethanol is more preferable. As the basic catalyst, ammonia, amine, NaOH or KOH is preferable. Preferred amines are alcohol amines or alkyl amines (methylamine, dimethylamine, trimethylamine, n-butylamine, n-propylamine, etc.).

The amount ratio between the organic solvent and the alkoxysilane is preferably set so that the concentration of the alkoxysilane is 0.1 to 10% by mass (silica concentration) in terms of SiO ₂ . When the silica concentration is more than 10% by mass, the particle size of the silica particles in the obtained sol becomes too large. On the other hand, when the silica concentration is less than 0.1, the particle size of the silica particles in the obtained sol becomes too small. The molar ratio of organic solvent / alkoxysilane is preferably in the range of 5 × 10 ^{2 to} 5 × 10 ⁴ .

The basic catalyst / alkoxysilane molar ratio is preferably 1 × 10 ⁻⁴ to 1, more preferably 1 × 10 ^{−4 to} 0.8, and particularly preferably 3 × 10 ^{−4 to} 0.5. . If the molar ratio of basic catalyst / alkoxysilane is less than 1 × 10 ⁻⁴ , hydrolysis reaction of alkoxysilane does not occur sufficiently. On the other hand, even if the molar ratio exceeds 1 and the base is added, the catalytic effect is saturated.

  The water / alkoxysilane molar ratio is preferably 1-30. If the water / alkoxysilane molar ratio is more than 30, the hydrolysis reaction proceeds too quickly, making it difficult to control the reaction and making it difficult to obtain a uniform silica airgel film. On the other hand, when it is less than 1, hydrolysis of alkoxysilane does not occur sufficiently.

  The alkoxysilane solution containing the basic catalyst and water is preferably aged by standing at 10 to 90 ° C. for about 10 to 60 hours or slowly stirring. By aging, hydrolysis and condensation polymerization proceed, and silica sol is generated. The silica sol includes, in addition to a dispersion of colloidal silica particles, a dispersion in which colloidal silica particles are aggregated in a cluster.

(c) Hydrolysis and polycondensation in the presence of an acidic catalyst An acidic catalyst, and water and an organic solvent as necessary are added to the obtained alkaline sol, and hydrolysis and polycondensation are further advanced in an acidic state. Examples of the acidic catalyst include hydrochloric acid, nitric acid, sulfuric acid, phosphoric acid, acetic acid and the like. The same organic solvent as above can be used. In the first acidic sol, the molar ratio of acidic catalyst / basic catalyst is preferably 1.1 to 10, more preferably 1.5 to 5, and most preferably 2 to 4. If the molar ratio of acidic catalyst / basic catalyst is less than 1.1, polymerization by acidic catalyst does not proceed sufficiently. On the other hand, if it exceeds 10, the catalytic effect is saturated. The organic solvent / alkoxysilane molar ratio and the water / alkoxysilane molar ratio may be the same as described above. The sol containing the acidic catalyst is preferably aged by standing at 10 to 90 ° C. for about 15 minutes to 24 hours or slowly stirring. By aging, hydrolysis and condensation polymerization proceed to produce a first acidic sol.

  The median diameter of the silica particles in the first acidic sol is 100 nm or less, preferably 10 to 50 nm. The median diameter is measured by a dynamic light scattering method.

(2) Step of preparing the second acidic sol
(a) Alkoxysilane The alkoxysilane for the second acidic sol is Si (OR ¹ ) _x (R ² ) _4-x [x is an integer of 2 to 4. 2-4 functional groups represented by R ¹ is preferably an alkyl group having 1 to 5 carbon atoms (methyl, ethyl, propyl, butyl or the like) or an acyl group having 1 to 4 carbon atoms (acetyl or the like). R ² is preferably an organic group having 1 to 10 carbon atoms, for example, a hydrocarbon group such as methyl, ethyl, propyl, butyl, hexyl, cyclohexyl, octyl, decyl, phenyl, vinyl, allyl, and γ-chloropropyl, CF ₃ CH _2- , CF ₃ CH ₂ CH _2- , C ₂ F ₅ CH ₂ CH _2- , C ₃ F ₇ CH ₂ CH ₂ CH _2- , CF ₃ OCH ₂ CH ₂ CH _2- , C ₂ F ₅ OCH ₂ CH ₂ CH _2- , C ₃ F ₇ OCH ₂ CH ₂ CH _2- , (CF ₃ ) ₂ CHOCH ₂ CH ₂ CH _2- , C ₄ F ₉ CH ₂ OCH ₂ CH ₂ CH _2- , 3- (perfluoro (Cyclohexyloxy) propyl, H (CF ₂ ) ₄ CH ₂ OCH ₂ CH ₂ CH _2- , H (CF ₂ ) ₄ CH ₂ CH ₂ CH _2- , γ-glycidoxypropyl, γ-mercaptopropyl, 3,4 -Substituted hydrocarbon groups such as epoxycyclohexylethyl and γ-methacryloyloxypropyl.

  Specific examples of the bifunctional alkoxysilane include dimethyldialkoxysilane such as dimethyldimethoxysilane and dimethyldiethoxysilane. Specific examples of the trifunctional alkoxysilane include methyltrialkoxysilane such as methyltrimethoxysilane and methyltriethoxysilane, and phenyltrialkoxysilane such as phenyltriethoxysilane. Examples of the tetrafunctional alkoxysilane include tetramethoxysilane, tetraethoxysilane, tetrapropoxysilane, tetrabutoxysilane, and diethoxydimethoxysilane. The alkoxysilane is preferably trifunctional or more, more preferably methyltrialkoxysilane and tetraalkoxysilane.

(b) Hydrolysis and polycondensation in the presence of an acidic catalyst Hydrolysis and polycondensation proceed by adding an organic solvent, an acidic catalyst and water to an alkoxysilane monomer or oligomer (polycondensation product). The same organic solvent and acidic catalyst as described in the step of preparing the first acidic sol can be used. The acidic catalyst / alkoxysilane molar ratio is preferably 1 × 10 ⁻⁴ to 1, more preferably 1 × 10 ^{−4 to} 3 × 10 ^{−2, and} most preferably 3 × 10 ⁻⁴ to 1 × 10 ⁻² . The organic solvent / alkoxysilane molar ratio and the water / alkoxysilane molar ratio may be the same as described in the step of preparing the first acidic sol.

  The alkoxysilane solution containing an acidic catalyst and water is preferably aged by standing at 10 to 90 ° C. for about 30 minutes to 60 hours or by slowly stirring. By aging, hydrolysis and condensation polymerization proceed to produce a second acidic sol. When the aging time exceeds 60 hours, the median diameter of the silica particles in the sol becomes too large.

  The resulting colloidal silica particles in the second acidic sol have a smaller median diameter than the first acidic sol. The median diameter of the colloidal silica particles in the second acidic sol is 10 nm or less, preferably 1 to 5 nm. The median diameter ratio between the silica particles in the first acidic sol and the silica particles in the second acidic sol is preferably 5 to 50, and more preferably 5 to 35. When the median diameter ratio is less than 5 or more than 50, the scratch resistance of the silica airgel film is low.

(3) Step of preparing mixed sol It is preferable that the first acidic sol and the second acidic sol are mixed and stirred slowly at 1 to 30 ° C. for about 1 minute to 6 hours. You may heat a mixture at 80 degrees C or less as needed. The solid mass ratio of the first acidic sol and the second acidic sol is preferably 5 to 90, more preferably 5 to 80. When the solid content mass ratio is less than 5 or more than 90, the silica airgel film has low scratch resistance.

(4) Application and drying process
(a) Application Examples of methods for applying the mixed sol to the surface of the substrate include dip coating, spray coating, spin coating, and printing. When applying to a three-dimensional structure such as a lens, a dipping method is preferred. The pulling speed in the dipping method is preferably about 0.1 to 3.0 mm / sec.

  The organic solvent may be added as a dispersion medium in order to adjust the concentration and fluidity of the mixed sol and improve the coating suitability. The concentration of silica in the mixed sol at the time of application is preferably 0.1 to 20% by mass. If necessary, the mixed sol may be sonicated. Aggregation of colloidal particles can be prevented by ultrasonic treatment. The ultrasonic frequency is preferably 10 to 30 kHz, the output is preferably 300 to 900 W, and the treatment time is preferably 5 to 120 minutes.

(b) Drying The drying conditions for the coating film are appropriately selected according to the heat resistance of the substrate. In order to promote the polycondensation reaction, heat treatment may be performed at a temperature below the boiling point of water for 15 minutes to 24 hours, and then at a temperature of 100 to 200 ° C. for 15 minutes to 24 hours. The silica airgel film exhibits high scratch resistance by heat treatment.

(5) Alkali treatment step The scratch resistance is further improved by treating the silica airgel membrane with alkali. The alkali treatment is preferably performed by applying an alkali solution or leaving it in an ammonia atmosphere. The solvent of the alkaline solution can be appropriately selected according to the alkali, and water, alcohol and the like are preferable. The concentration of the alkaline solution is preferably 1 × 10 ^{−4 to} 20N, and more preferably 1 × 10 ^{−3 to} 15N.

  Examples of the alkali include inorganic alkalis such as sodium hydroxide, potassium hydroxide, and ammonia; inorganic alkali salts such as sodium carbonate, sodium bicarbonate, ammonium carbonate, and ammonium bicarbonate; monomethylamine, dimethylamine, trimethylamine, monoethylamine, diethylamine, Triethylamine, n-propylamine, di-n-propylamine, n-butylamine, di-n-butylamine, n-amylamine, n-hexylamine, laurylamine, ethylenediamine, hexamethylenediamine, aniline, methylaniline, ethylaniline, Cyclohexylamine, dicyclohexylamine, pyrrolidine, pyridine, imidazole, guanidine, tetramethylammonium hydroxide, tetraethylammonium hydroxide, Organic alkalis such as trabutylammonium hydroxide, monoethanolamine, diethanolamine, triethanolamine, choline; organic acid alkali salts such as ammonium formate, ammonium acetate, monomethylamine formate, dimethylamine acetate, aniline acetate, pyridine lactate, guanidinoacetic acid Etc. can be used.

When alkali treatment is performed by application of an alkaline solution, it is preferable to apply 10 to 200 mL per 1 cm ^{2 of} the silica airgel film. The application can be performed by the same method as that for applying the silica airgel film, and the spin coating method is preferred. The substrate rotation speed in the spin coating method is preferably about 1,000 to 15,000 rpm. The film after applying the alkaline solution is preferably stored at 1 to 40 ° C, more preferably at 10 to 30 ° C. The storage time is preferably 0.1 to 10 hours, more preferably 0.2 to 1 hour.

When the alkali treatment is performed in an ammonia atmosphere, the treatment is preferably carried out in an ammonia gas partial pressure of 1 × 10 ⁻¹ to 1 × 10 ⁵ Pa. The treatment temperature is preferably 1 to 40 ° C, more preferably 10 to 30 ° C. The treatment time is preferably 1 to 170 hours, more preferably 5 to 80 hours.

  If necessary, the silica-treated airgel membrane is dried. Drying is preferably performed at a temperature of 100 to 200 ° C. for 15 minutes to 24 hours.

(6) Washing step The silica airgel membrane after the alkali treatment may be washed as necessary. The washing is preferably performed by a method of immersing in water and / or alcohol, a method of showering, or a combination thereof. You may ultrasonically treat, immersing. The washing temperature is preferably 1 to 40 ° C., and the time is preferably 0.2 to 15 minutes. The silica airgel membrane is preferably washed with 0.01 to 1,000 mL of water and / or alcohol per 1 cm ² . The silica airgel membrane after washing is preferably dried at a temperature of 100 to 200 ° C. for 15 minutes to 24 hours. As alcohol, methanol, ethanol, and isopropyl alcohol are preferable.

(7) Humidity treatment process The silica airgel membrane after application, alkali treatment, or washing is subjected to humidity treatment under high humidity conditions. It is thought that hydrolysis of unreacted alkoxysilane and polycondensation reaction of silanol groups proceed due to the humidity treatment, improving the mechanical strength of the silica airgel film and changing the refractive index over time after film formation. It is suppressed.

  Humidity treatment is performed by storing in an environment at a temperature of 35 ° C or higher and a relative humidity of 70% or higher for 1 hour or longer. If the humidity of the treatment is less than 70% RH, the above effect cannot be obtained sufficiently. The humidity is preferably 75% RH or more, more preferably 80% RH or more, and most preferably 90% RH or more. The treatment temperature is preferably 35 to 90 ° C, more preferably 40 to 80 ° C, and most preferably 50 to 80 ° C. When the treatment temperature is less than 35 ° C., the above effect cannot be obtained sufficiently, and even if it exceeds 90 ° C., the effect is saturated. Although the treatment time depends on the temperature condition and the humidity condition, the effect can be obtained by performing the treatment for 1 hour or more. Preferably it is 5-120 hours, More preferably, it is 5-48 hours. The effect is saturated when the treatment time exceeds 120 hours.

(9) Formation of a dense film A single layer of an antireflection film may be formed by applying only a silica airgel film on a substrate, but a multilayer is obtained by applying a silica airgel film on at least one previously formed dense film. An antireflection film may be formed. By forming the multilayer antireflection film, a high antireflection effect can be obtained in a wider wavelength range. The dense film is a layer made of an inorganic material such as a metal oxide (referred to as “inorganic layer”), a composite layer made of inorganic fine particles and a binder (referred to as “inorganic fine particle-binder composite layer” or simply “composite layer”), It is formed of a resin layer or the like. The refractive index of the dense film is set to a value smaller than the refractive index of the substrate and larger than the refractive index of the silica airgel film.

  Examples of inorganic materials that can be used for the inorganic layer include magnesium fluoride, calcium fluoride, aluminum fluoride, lithium fluoride, sodium fluoride, cerium fluoride, silicon oxide, aluminum oxide, zirconium oxide, cryolite, Examples include thiolite, titanium oxide, cerium oxide, silicon nitride, and mixtures thereof.

  Examples of inorganic fine particles that can be used in the composite layer include calcium fluoride, magnesium fluoride, aluminum fluoride, sodium fluoride, lithium fluoride, cerium fluoride, silicon oxide, aluminum oxide, zirconium oxide, cryolite, Examples thereof include at least one inorganic fine particle selected from the group consisting of thiolite, titanium oxide, indium oxide, tin oxide, antimony oxide, cerium oxide, hafnium oxide, and zinc oxide. The silicon oxide is preferably colloidal silica, and the colloidal silica may be surface-treated with a silane coupling agent or the like. The refractive index of the inorganic fine particle-binder composite layer depends on the composition of the inorganic fine particles and the binder composition as well as the content.

  Examples of the resin layer include a fluororesin layer, an epoxy resin layer, an acrylic resin layer, a silicone resin layer, and a urethane resin layer. The fluororesin includes crystalline fluororesins such as polytetrafluoroethylene resin, perfluoroethylene propylene copolymer, perfluoroalkoxy resin, polyvinylidene fluoride, ethylene-tetrafluoroethylene copolymer, and polychlorotrifluoroethylene. However, the amorphous fluororesin is preferable because it has excellent transparency. Specific examples of the non-crystalline fluororesin include a fluoroolefin copolymer, a polymer having a fluorine-containing aliphatic ring structure, and a fluorinated acrylate copolymer. An example of a fluoroolefin copolymer is a copolymer of 37 to 48 mass% tetrafluoroethylene, 15 to 35 mass% vinylidene fluoride, and 26 to 44 mass% hexafluoropropylene. Can be mentioned. Examples of the polymer having a fluorinated alicyclic structure include those obtained by polymerizing a monomer having a fluorinated alicyclic structure, and those obtained by cyclopolymerizing a fluorinated monomer having at least two polymerizable double bonds.

  The dense film may be a laminate of a plurality of layers. The plurality of dense films can be formed of any one of the inorganic layer, the composite layer, and the resin layer. When a plurality of dense films are formed by stacking inorganic layers, for example, magnesium fluoride, silicon oxide, aluminum oxide, zirconium oxide, titanium oxide, cerium oxide, silicon nitride, or the like can be used as a material.

  A layer made of only an inorganic material can be formed by a physical vapor deposition method such as a vacuum vapor deposition method, a sputtering method, or an ion plating method, or a chemical vapor deposition method such as thermal CVD, plasma CVD, or photo CVD. preferable. The inorganic fine particle-binder composite layer can be formed by a wet method such as a dip coating method, a spin coating method, a spray method, a roll coating method, or a screen printing method, but a dip coating method is preferred. The resin layer can be formed by a chemical vapor deposition method or a wet method. Among these methods, a method for producing an inorganic layer by vapor deposition and a method for producing an inorganic fine particle-binder composite layer and a fluororesin layer by dip coating are described in, for example, JP-A-2006-215542.

[2] Antireflection film The antireflection film is composed of only a silica airgel film or a multilayer film of a silica airgel film and one or more dense films. The physical film thickness of the silica airgel film is preferably 15 to 500 nm, more preferably 70 to 170 nm. The physical film thickness of the silica airgel film can be appropriately adjusted depending on the concentration of the mixed sol, the number of coatings, and the like.

  The silica airgel film is a porous film that has a skeleton having Si—O bonds, has uniform nano-sized fine pores, and has high transparency. The refractive index of the silica airgel film decreases as the porosity increases. The porosity of the silica airgel film is preferably 30 to 90%. A silica airgel film having a porosity of 30 to 90% has a refractive index of 1.05 to 1.35. For example, the refractive index of a silica airgel film having a porosity of 78% is about 1.1. When the porosity is more than 90%, the scratch resistance of the silica airgel film is lowered. If the porosity is less than 30%, the refractive index is too large.

  Since the silica airgel film obtained by the method of the present invention has a structure in which relatively small silica particles generated from the second acidic sol enter a gap between relatively large silica particles generated from the first acidic sol, While having a low refractive index, it has excellent scratch resistance and the time variation of the refractive index is reduced by the humidity treatment.

In the case where the antireflection film is composed of a two-layer film of a silica airgel film and a single dense film, it is preferable that the refractive index decreases in order from the substrate side to the dense film, the silica airgel film, and the incident medium. The optical film thicknesses d ₁ and d ₂ of the dense film and the silica airgel film are preferably in the range of λd / 5 to λd / 3 with respect to the design wavelength λd. The optical film thickness is the product of the refractive index of the film and the physical film thickness. The design wavelength λd used for determining the configuration of the thin film can be appropriately set according to the wavelength used for the optical element. For example, the visible wavelength [wavelength of 380 to 780 nm as defined by the CIE (International Commission on Illumination)] It is preferable that the center wavelength is substantially the same.

When the optical film thickness of the dense film and the silica airgel film is in the range of λd / 5 to λd / 3 with respect to the design wavelength λd, the optical film thickness of the antireflection film (optical film) The sum of the thicknesses d ₁ and d ₂ is in the range of 2λd / 5 to 2λd / 3, and the change of the refractive index with respect to the optical film thickness from the base material to the incident medium becomes a smooth stepped shape. When the optical film thickness of the antireflection film is in the range of 2λd / 5 to 2λd / 3, the optical path difference between the reflected light at the antireflection film surface and the reflected light at the boundary between the antireflection film and the substrate is the design wavelength. Since this is approximately half of λd, these rays cancel each other out due to interference. If the change in the refractive index with respect to the optical film thickness from the base material to the incident medium is made to be a smooth step, reflection of incident light that occurs at the boundary of each layer can be reduced over a wide wavelength range. Further, the reflected light generated at the interface of each layer cancels out by interfering with the light ray incident on each layer. Accordingly, the antireflection film exhibits an excellent antireflection effect for light rays in a wide wavelength range and a wide incident angle range. If the optical film thickness of the dense film and the silica airgel film is not in the range of λd / 5 to λd / 3, the refractive index change with respect to the optical film thickness from the substrate to the incident medium will not be smooth, so reflection at the interface of each film The rate will increase. The optical film thicknesses d ₁ and d ₂ are more preferably λd / 4.5 to λd / 3.5 with respect to the design wavelength λd.

The refractive index differences R ₁ , R ₂ and R ₃ between the base material and the dense film, between the dense film and the silica airgel film, and between the silica airgel film and the incident medium are 0.02 to 0.4, respectively. preferable. As a result, the change in the refractive index with respect to the optical film thickness can be made smooth so that it can be approximated to a straight line, and the antireflection effect is further improved.

  When the antireflection film is composed of a multilayer film of a silica airgel film and a plurality of dense films, the plurality of dense films allow the reflected light generated at the interface of each layer and the light incident on each layer to cancel each other out by interference. In addition, it is preferable to design a plurality of films having different refractive indexes in appropriate combination. With such a design, the antireflection efficiency can be further increased.

[3] Optical element The optical element of the present invention has the antireflection film on the surface of an optical substrate. The material of the optical substrate may be any of glass, crystalline material, and plastic. Specific examples of optical substrate materials include optical glass such as BK7, LASF01, LASF016, LaFK55, LAK14, SF5, Pyrex (registered trademark) glass, quartz, blue plate glass, white plate glass, PMMA resin, PC resin, chain or Examples thereof include polyolefin resins mainly composed of cyclic olefins. The refractive index of these substrates is preferably in the range of 1.35 to 2.00. Examples of the shape of the optical substrate include a flat plate shape, a lens shape, a prism shape, a light guide shape, a film shape, and a diffraction element shape.

  The present invention will be described in more detail with reference to the following examples, but the present invention is not limited thereto.

Example 1
(1) Preparation of mixed sol After mixing 17.05 g of tetraethoxysilane and 69.13 g of methanol, 3.88 g of aqueous ammonia (3 N) was added and stirred at room temperature for 15 hours to prepare an alkaline sol. To 40.01 g of this alkaline sol, 2.50 g of methanol and 1.71 g of hydrochloric acid (12 N) were added and stirred at room temperature for 30 minutes to prepare 44.22 g of the first acidic sol (solid content: 4.94% by mass).

  After mixing 30 ml of tetraethoxysilane, 30 ml of ethanol and 2.4 ml of water at room temperature, 0.1 ml of hydrochloric acid (1 N) is added and stirred at 60 ° C. for 90 minutes to give a second acidic sol (solid content: 14.8% by mass) was prepared.

  0.22 g of the second acidic sol was added to the total amount of the first acidic sol (44.22 g) (solid content mass ratio of the first acidic sol / second acidic sol: 67.1), and the mixture was stirred at room temperature for 5 minutes. A mixed sol was prepared.

Measurement of median diameter The median diameters of the silica particles of the first acidic sol and the second acidic sol are 16.0 nm and 1.8 nm, respectively. The silica particles of the first acidic sol and the silica particles of the second acidic sol The median diameter ratio was 8.9. The median diameter was measured using a dynamic light scattering particle size distribution analyzer LB-550 (manufactured by Horiba, Ltd.).

  Table 1 summarizes the preparation conditions and median diameter of the first acidic sol, the second acidic sol, and the mixed sol.

注：(1) TEOSはテトラエトキシシランを表す。
(2) 第一の酸性ゾルと第二の酸性ゾルとの固形分質量比。
(3) 第一の酸性ゾル中のシリカ粒子と第二の酸性ゾル中のシリカ粒子とのメジアン径比。

Note: (1) TEOS represents tetraethoxysilane.
(2) Solid content mass ratio between the first acidic sol and the second acidic sol.
(3) Median diameter ratio between silica particles in the first acidic sol and silica particles in the second acidic sol.

(2) Formation of antireflection film
(a) Formation of a six-layer dense film
First layer in order from the substrate side by vacuum deposition using an apparatus having an electron beam deposition source on a LF5 glass flat plate (φ30 mm, refractive index nD = 1.585) as shown in Table 2 A dense film of a sixth layer was formed. A lens reflectometer (model number: USPM-RU, manufactured by Olympus Corporation) was used for measurement of physical film thickness and refractive index.

(b) Formation of silica airgel film A mixed sol was spin-coated on a 6-layer dense film formed by vacuum deposition, dried at 80 ° C for 30 minutes, and then heat-treated at 160 ° C for 30 minutes to obtain a physical film thickness of 97 A silica airgel film of nm was formed. Further, 400 μl of 0.001 N sodium hydroxide aqueous solution was spin-coated, allowed to stand at room temperature for 30 minutes, dried at 120 ° C. for 30 minutes, washed successively with water and isopropyl alcohol, and dried at 80 ° C. for 30 minutes. The substrate on which the silica airgel film was formed was subjected to humidity treatment at 60 ° C. and 90% RH for 72 hours to produce an antireflection film. The scratch resistance of the obtained antireflection film was evaluated by the following method. Further, the refractive index immediately after the formation of the silica aerosol film was measured. The refractive index of the silica aerosol film was determined by simulation after measuring the reflectance of the antireflection film. The results are shown in Table 4.

Scratch resistance evaluation method Silica airgel film on the substrate surface is 20 mm x 20 mm non-woven fabric (trade name “Spic Lens Wiper”, manufactured by Ozu Sangyo Co., Ltd. at a rate of 3600 mm / min while applying a pressure of 1 Kg / cm ² . ) Was rubbed 30 times and the surface scratches were visually confirmed and evaluated according to the following criteria.
<Criteria>
The silica aero membrane was not damaged at all ...
The silica aero film was scratched but did not peel off ...
Silica Aero membrane peeled off ×

Examples 2-7
An antireflection film was produced in the same manner as in Example 1 except that the humidity treatment conditions were changed as shown in Table 4. The scratch resistance of the obtained antireflection film and the refractive index of the silica aerosol film were measured in the same manner as in Example 1. The results are shown in Table 4.

Example 8
An antireflection film was produced in the same manner as in Example 1 except that a 6-layer dense film was formed using a LaSF03 glass plate (φ30 mm, refractive index nD = 1.810) with the structure shown in Table 3. The scratch resistance of the obtained antireflection film and the refractive index of the silica aerosol film were measured in the same manner as in Example 1. The results are shown in Table 4.

Comparative Examples 1 and 4
Antireflection films of Comparative Examples 1 and 4 were produced in the same manner as in Examples 1 and 8, respectively, except that the humidity treatment was not performed. The scratch resistance of the obtained antireflection film and the refractive index of the silica aerosol film were measured in the same manner as in Example 1. The results are shown in Table 4.

Comparative Examples 2 and 3
An antireflection film was produced in the same manner as in Example 1 except that the humidity treatment conditions were changed as shown in Table 4. The scratch resistance of the obtained antireflection film and the refractive index of the silica aerosol film were measured in the same manner as in Example 1. The results are shown in Table 4.

  Further, the antireflection films of Examples 1 and 8 and Comparative Examples 1 and 4 were stored at room temperature, and the refractive index of the silica airgel film after 1 month, 2 months and 3 months was measured in the same manner as in Example 1. . The results are shown in Table 5.

  As is apparent from Tables 4 and 5, the silica airgel films of Examples 1 to 8 subjected to the humidity treatment had a low refractive index and excellent scratch resistance. Further, as is clear from the results of Examples 1 and 8, the refractive index after storage for 3 months at room temperature does not vary from that immediately after film formation, and it is predicted that an antireflection film excellent in storage stability can be obtained. it can. In contrast, the silica airgel films of Comparative Examples 1 to 4 had a high refractive index and poor scratch resistance. Furthermore, it can be seen from the results of Comparative Examples 1 and 4 that the time variation of the refractive index is large.

Claims (4)

A method for producing an antireflection film having at least one silica airgel film, wherein the silica airgel film is obtained by further adding an acidic solution to an alkaline sol prepared by hydrolysis and condensation polymerization of alkoxysilane under a basic catalyst. The first acidic sol having a median diameter of 100 nm or less obtained by mixing with the second acidic sol having a median diameter of 10 nm or less obtained by hydrolysis and condensation polymerization of alkoxysilane under an acidic catalyst. A method for producing an antireflection film, wherein the mixed sol is applied and dried on a substrate and then subjected to a humidity treatment for 1 hour or longer in an environment having a temperature of 35 ° C. or higher and a relative humidity of 70% or higher. The method according to claim 1, wherein after the silica airgel film is applied, the alkali treatment is performed by applying an alkali solution or leaving it in an ammonia atmosphere. The method according to claim 2, wherein the alkali treatment is performed by applying an alkali solution. The method according to claim 1, wherein the humidity treatment time is 5 to 48 hours.