Patent Description:
In a known low-refractive-index film, hollow or chainlike silicon oxide particles are used to form an empty space in the film and to contain air (refractive index <NUM>), thereby decreasing the refractive index to <NUM> or less. A technique of applying/drying a dispersion liquid of silicon oxide particles is widely used to form a low-refractive-index film having an empty space therein. Such a low-refractive-index film is suitably used as a layer constituting an antireflection film.

In a low-refractive-index film containing silicon oxide particles, an excessively high ratio of an empty space in the film results in low film strength, wear resistance, and scratch resistance. To solve this problem, in <CIT>, hollow silicon oxide particles and fine silicon oxide solid particles are mixed to form a film with increased strength.

However, a film described in <CIT> has a smaller empty space due to the fine silicon oxide particles and therefore has an increased refractive index. Thus, the increase in film strength is accompanied by the increase in refractive index and degradation of the performance of the low-refractive-index film. <CIT> is also cited as example of low-refractive-index fim of the prior art.

<CIT> discloses a coating composition and a plastic molded article made of the coating composition and describes a coated plastic molded article coated with a coating material composed of <NUM> parts by weight of organotrialcokysisilan represented by the general formula R<NUM>Si(O-R<NUM>)<NUM> (in the formula, R<NUM> denotes an alkyl group with a carbon number of <NUM> or <NUM>, and R<NUM> denotes an alkyl group with a carbon number of <NUM> to <NUM>), <NUM> to <NUM> parts by weight of organotrialcokysisilan represented by the general formula R<NUM>Si(OR<NUM>)<NUM> (in the formula, R<NUM> denotes an alkyl group with a carbon number of <NUM> to <NUM>, and R<NUM> denotes an organic group containing an epoxy group), <NUM> to <NUM> parts by weight of colloidal silica solution, and <NUM> to <NUM> parts by weight of dicarboxylate represented by the general formula (CF<NUM>)n(COOH)<NUM> (in the formula, n denotes an integer from <NUM> to <NUM>).

In view of such problems, the present disclosure provides a member having a porous layer containing particles and having a low refractive index and high film strength and a coating liquid for forming a porous layer containing particles.

The present disclosure in its first aspect provides a member as specified in claims <NUM> to <NUM>.

The present disclosure in its second aspect provides an optical apparatus as specified in claim <NUM>.

The present disclosure in its third aspect provides an imaging apparatus as specified in claim <NUM>.

The present disclosure in its third aspect provides a lens filter as specified in claim <NUM>.

The present disclosure in its fourth aspect provides a shield as specified in claim <NUM>.

<FIG> are schematic views of a member <NUM> according to an embodiment of the present disclosure. In <FIG>, the member <NUM> includes a substrate <NUM> and a porous layer <NUM> containing silicon oxide particles located on the substrate <NUM>.

If necessary, as illustrated in <FIG>, a functional layer <NUM>, such as an antifouling layer or a hydrophilic layer, may be provided on a surface of the porous layer <NUM> opposite the substrate <NUM>. The antifouling layer may be a fluoropolymer-containing layer, a fluorosilane monolayer, or a layer containing titanium oxide particles. The hydrophilic layer can be a hydrophilic polymer layer, particularly a layer containing a polymer with a zwitterionic hydrophilic group, such as a sulfobetaine group, a carboxybetaine group, or a phosphorylcholine group.

As illustrated in <FIG>, the member <NUM> may include an intermediate layer <NUM> between the substrate <NUM> and the porous layer <NUM> containing particles. The intermediate layer <NUM> can prevent diffusion of impurities from the substrate and improve antireflection performance. The intermediate layer <NUM> may be an inorganic compound layer, such as an oxide or nitride, or a polymer layer. The intermediate layer <NUM> may be a single layer formed of the above material or a laminate of a plurality of types of layers. To enhance antireflection performance, a high-refractive-index layer with a relatively high refractive index and a low-refractive-index layer with a relatively low refractive index can be alternately stacked. The high-refractive-index layer can have a refractive index of <NUM> or more and can be a layer containing any one selected from the group consisting of zirconium oxide, titanium oxide, tantalum oxide, niobium oxide, and hafnium oxide. The low-refractive-index layer can have a refractive index of less than <NUM> and can be a layer containing any one selected from the group consisting of silicon oxide and magnesium fluoride. Together with the intermediate layer <NUM>, the functional layer <NUM> may be provided on a surface of the porous layer <NUM>.

The member <NUM> according to the present disclosure can be used as a lens, a mirror, a filter, a functional film, or the like depending on the form of the substrate <NUM>. In particular, the member <NUM> according to the present disclosure is suitable for applications that require antireflection performance, for example, optical lenses and antireflection films. For example, the member <NUM> according to the present disclosure can be used to cover glasses of semiconductors and liquid crystal displays, for light-transmitting shield members, such as face shields and shield partitions, and for optical systems of various optical apparatuses. Among these, the member <NUM> according to the present disclosure is suitable for lenses constituting imaging optical systems of imaging apparatuses that require high antireflection performance. The member <NUM> according to the present disclosure can also be attached to another member via an adhesive layer.

Although some embodiments according to the present disclosure are more specifically described below, these embodiments can be appropriately modified without departing from the gist of the present disclosure, and the present disclosure is not limited to these embodiments.

The substrate <NUM> may be formed of glass, ceramic, resin, or metal. The substrate <NUM> may have any shape, such as a flat sheet, a curved substrate with a concave or convex surface, or a film. Depending on the intended use, a light-transmitting substrate may be used.

The composition of glass and ceramics is not particularly limited. Examples include zirconium oxide, titanium oxide, tantalum oxide, niobium oxide, hafnium oxide, lanthanum oxide, gadolinium oxide, silicon oxide, calcium oxide, barium oxide, sodium oxide, potassium oxide, boron oxide, and aluminum oxide. The substrate can be produced by a method such as grinding and polishing, molding, or float forming.

The resin can be a thermoplastic resin or a thermosetting resin. Examples of the thermoplastic resin include poly(ethylene terephthalate) (PET), poly(ethylene naphthalate), polypropylene (PP), poly(methyl methacrylate) (PMMA, acrylic resin), cellulose triacetate, polycarbonate (PC), cycloolefin polymers, and poly(vinyl alcohol). Examples of the thermosetting resin include polyimides, epoxy resins, and urethane resins.

Examples of the metal include a metal composed of one metal element and alloys containing two or more elements.

<FIG> are schematic partial enlarged views of the porous layer <NUM> containing particles of a member according to the present disclosure. <FIG> illustrates hollow silicon oxide particles <NUM>, and <FIG> illustrates chainlike silicon oxide particles <NUM> (connected solid particles). The porous layer <NUM> has a plurality of empty spaces <NUM> between the silicon oxide particles <NUM> bound by an inorganic binder <NUM> and contains an acid <NUM> in the layer. As illustrated in <FIG>, the porous layer <NUM> includes the silicon oxide particles <NUM> almost uniformly stacked on the surface of the substrate <NUM>.

When used as an antireflection layer, the porous layer <NUM> preferably has a refractive index in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>. A refractive index of less than <NUM> results in the porous layer with insufficient strength due to a high ratio of empty spaces in the layer. A refractive index of more than <NUM> may result in an insufficient decrease in the refractive index difference between the air and the substrate <NUM> and insufficient antireflection effects.

The porous layer <NUM> can have a hydrophilic surface. More specifically, the contact angle of pure water at a room temperature of <NUM> and at a humidity in the range of <NUM>%RH to <NUM>%RH preferably ranges from <NUM> to <NUM> degrees, more preferably <NUM> to <NUM> degrees. A contact angle of pure water of less than <NUM> degrees tends to result in permeation of moisture in the porous layer <NUM> from the surface of the porous layer <NUM> and impaired environmental stability. A contact angle of pure water of more than <NUM> degrees tends to result in weak bonding between the silicon oxide particles <NUM> and result in the porous layer <NUM> with low wear resistance.

Each component is now described in detail below.

The acid <NUM> in the porous layer <NUM> according to the present disclosure has <NUM> to <NUM> acidic groups and satisfies one of the following formulae (<NUM>) to (<NUM>). Acids with five or more acidic groups are difficult to dissolve in solvents and therefore tend to separate in the coating liquid. Furthermore, acids with five or more acidic groups may cause steric hindrance in the porous layer, disturb the arrangement of the particles, and cause light scattering. <CHM>
<CHM>
<CHM>
<CHM>.

A<NUM> in the formula (<NUM>) denotes COOH. n denotes an integer in the range of <NUM> to <NUM>. An acid satisfying the formula (<NUM>) may be tetrafluorosuccinic acid, hexafluoroglutaric acid, octafluoroadipic acid, dodecafluorosuberic acid, or hexadecafluorosebacic acid.

At least one of A<NUM> and A<NUM> in the formula (<NUM>) denotes SO<NUM>H or PO<NUM>H<NUM>, and the other denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH. At least one of A<NUM> and A<NUM> denotes SO<NUM>H or PO<NUM>H<NUM>. R denotes a divalent organic group having <NUM> to <NUM> carbon atoms. An acid satisfying the formula (<NUM>) may be <NUM>,<NUM>'-biphenyldisulfonic acid, methylenediphosphonic acid, <NUM>-phosphonobenzoic acid, <NUM>-phosphonobutyric acid, <NUM>-amino-<NUM>-hydroxy-<NUM>-nitrobenzenesulfonic acid, <NUM>-amino-<NUM>-naphthol-<NUM>-sulfonic acid, <NUM>-amino-<NUM>-naphthol-<NUM>-sulfonic acid, <NUM>-amino-<NUM>-naphthol-<NUM>-sulfonic acid, <NUM>,<NUM>'-benzidinedisulfonic acid, or <NUM>,<NUM>'-diaminostilbene-<NUM>,<NUM>'-disulfonic acid.

Each of A<NUM>, A<NUM>, and A<NUM> in the formula (<NUM>) denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH. At least one of A<NUM>, A<NUM>, and A<NUM> denotes SO<NUM>H or PO<NUM>H<NUM>. R denotes a trivalent organic group having <NUM> to <NUM> carbon atoms. An acid satisfying the formula (<NUM>) may be nitrilotris, <NUM>-hydroxyethane-<NUM>,<NUM>-diphosphonic acid, alendronic acid, N,N-Bis glycine or <NUM>-sulfophthalic acid.

Each of A<NUM>, A<NUM>, A<NUM>, and A<NUM> in the formula (<NUM>) denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH. At least one of A<NUM>, A<NUM>, A<NUM>, and A<NUM> denotes SO<NUM>H or PO<NUM>H<NUM>. R denotes a tetravalent organic group having <NUM> to <NUM> carbon atoms. An acid satisfying the formula (<NUM>) may be N,N,N',N'-ethylenediaminetetrakis, <NUM>-phosphonobutane-<NUM>,<NUM>,<NUM>-tricarboxylic acid, or <NUM>-hydroxy-<NUM>-(<NUM>-hydroxy-<NUM>-sulfo-<NUM>-naphthylazo)-<NUM>-naphthoic acid.

The acidic group of an acid satisfying one of the formulae (<NUM>) to (<NUM>) can modify the surface of the silicon oxide particles <NUM>. Although an acid with only one acidic group can modify only one silicon oxide particle, an acid with two or more acidic groups as in the formulae (<NUM>) to (<NUM>) can modify a plurality of particles depending on the number of acidic groups. Consequently, the acid <NUM> can also bind particles together, increase the number of bonding sites between particles, and increase the strength of the porous layer <NUM>.

The acid <NUM> in the porous layer <NUM> can be identified as one of the formulae (<NUM>) to (<NUM>) by the elemental analysis of the porous layer <NUM> or by the separation and quantitative analysis of organic acids by ion-exclusion chromatography or the like.

It is desirable that the acid content of the porous layer be preferably in the range of <NUM>% to <NUM>% by mass of the silicon oxide particles contained in the porous layer. An acid content of less than <NUM>% by mass tends to result in insufficient dispersion of particles, irregular arrangement of particles in the layer, and the layer with low strength. An acid content of more than <NUM>% by mass tends to result in an increase in the empty spaces in the layer due to hindrance by the acid and result in the layer with low strength.

The silicon oxide particles <NUM> may be spherical, cocoon-like, barrel-shaped, disk-shaped, rod-like, acicular, square or rectangular, or chainlike particles. When the porous layer <NUM> is used as an antireflection layer, the silicon oxide particles <NUM> can be hollow silicon oxide particles each having a vacancy within a shell as illustrated in <FIG> or chainlike silicon oxide particles, that is, connected solid particles as illustrated in <FIG>. It is desirable that the silicon oxide particles <NUM> be silicon oxide particles formed by a wet process. This is because silicon oxide particles formed by a wet process have a larger number of silanol groups (Si-OH) on the surface of the particles and therefore tend to interact more strongly with the acid <NUM> than silicon oxide particles formed by a dry process.

Hollow silicon oxide particles can decrease the refractive index of the porous layer <NUM> containing the particles due to air (refractive index <NUM>) in the vacancies.

Hollow silicon oxide particles can be produced by a known method, for example, described in <CIT> or <CIT>. When the silicon oxide particles <NUM> are hollow particles, a coating liquid is applied to a substrate and is dried to form layers of the hollow silicon oxide particles <NUM> stacked in the direction perpendicular to the surface of the substrate <NUM>, as illustrated in <FIG>.

The hollow silicon oxide particles preferably have an average particle size in the range of <NUM> to <NUM>, more preferably <NUM> to <NUM>. An average particle size of less than <NUM> makes it difficult to consistently produce the particles. An average particle size of more than <NUM> tends to result in a large void between particles and scattering by the silicon oxide particles. However, when the porous layer <NUM> is not used as an antireflection film, the average particle size is not necessarily <NUM> or less.

The average particle size of hollow silicon oxide particles is an average Feret diameter. The average Feret diameter can be determined by image processing of a transmission electron micrograph of hollow silicon oxide particles contained in the coating liquid. The image processing may be commercial image processing, such as image Pro PLUS (manufactured by Media Cybernetics, Inc. In a predetermined image region, the contrast can be appropriately adjusted if necessary, and the Feret diameter of each particle can be measured by particle measurement. The average value of a plurality of particles can be calculated.

The thickness of the shell of each hollow silicon oxide particle ranges from <NUM>% to <NUM>%, preferably <NUM>% to <NUM>%, of the average particle size. A shell thickness of less than <NUM>% results in particles with insufficient strength. A shell thickness of more than <NUM>% results in a small ratio of the empty space to the cubic content of the particle; therefore, the effects of hollow silicon oxide particles cannot be produced, that is, a layer with a refractive index of <NUM> or less cannot be formed.

Chainlike silicon oxide particles are secondary particles composed of a linearly or crookedly connected primary solid silicon oxide particles. The size of a chainlike particle can be expressed by a short diameter and a long diameter. The short diameter of a chainlike particle corresponds to the thickness of the chainlike particle, in other words, the average particle size of one primary particle. The short diameter can be calculated from the specific surface area of a chainlike particle extracted from a coating liquid determined by a nitrogen adsorption method. The chainlike silicon oxide particles preferably have an average short diameter in the range of <NUM> to <NUM>. A short diameter of less than <NUM> may result in an excessively large surface area of the silicon oxide particles <NUM> and a layer with low reliability due to absorption of moisture and chemical substances from the atmosphere. On the other hand, an average short diameter of more than <NUM> may result in unstable dispersion in a solvent and poor coatability.

The long diameter of a chainlike silicon oxide particle corresponds to the length of a secondary particle. The long diameter of particles in a coating liquid can be determined by a dynamic light scattering method. The long diameter of the chainlike silicon oxide particles preferably ranges from <NUM> to <NUM> times the short diameter. A long diameter less than <NUM> times the short diameter may result in a dense layer with an insufficiently decreased refractive index. A long diameter more than <NUM> times results in a coating liquid with high viscosity and poor coatability and leveling.

The short diameter and long diameter of chainlike silicon oxide particles in the layer can be calculated from a scanning electron micrograph. The short diameter and long diameter can be determined from an image taken with a scanning electron microscope.

Primary particles constituting chainlike silicon oxide particles may have individual clearly observed shapes or deformed shapes due to fusion of particles. The primary particles can have individual clearly observed shapes. Primary particles constituting chainlike silicon oxide particles may be spherical, cocoon-like, or barrel-shaped and can be cocoon-like or barrel-shaped. Primary particles constituting chainlike silicon oxide particles are particularly preferably particles with a short diameter in the range of <NUM> to <NUM> and a long diameter in the range of <NUM> to <NUM> times the short diameter.

The coating liquid may contain particles with shapes other than spherical, cocoon-like, barrel-shaped, disk-shaped, rod-like, acicular, and square or rectangular chainlike silicon oxide particles. However, an excessively high number of particles with shapes other than chainlike silicon oxide particles results in a high refractive index. It is desirable in terms of optical performance that such particles be added such that the refractive index does not exceed approximately <NUM>.

Regardless of their shapes, silicon oxide particles can have a surface that can be bound by a binder described later.

The inorganic binder <NUM> for binding silicon oxide particles can be a silicon oxide compound. The silicon oxide compound can be a cured product of a silicon oxide oligomer formed by hydrolysis and condensation of a silicate.

When the inorganic binder <NUM> is an inorganic material of the same quality as silicon oxide particles, the inorganic binder <NUM> can increase the bond strength between particles, and the porous layer is less likely to deteriorate in the use environment.

It is desirable that the binder content of the porous layer be in the range of <NUM>% to <NUM>% by mass, more desirably <NUM>% to <NUM>% by mass, of the silicon oxide particles contained in the porous layer. A binder content of less than <NUM>% by mass tends to result in the layer with low strength. A binder content of more than <NUM>% by mass may result in an increased refractive index and a low-refractive-index layer with insufficient optical performance.

<FIG> illustrates an imaging apparatus including a lens barrel (interchangeable lens) as an optical apparatus including a member according to the present disclosure. <FIG> illustrates a digital single-lens reflex camera to which a lens barrel (interchangeable lens) is attached.

The term "optical apparatus", as used herein, refers to an apparatus with an optical system, such as a binocular, a microscope, a semiconductor exposure apparatus, or an interchangeable lens.

The term "imaging apparatus", as used herein, refers to electronic equipment including an imaging device for receiving light passing through an optical device, for example, a camera system, such as a digital still camera or a digital camcorder, or a mobile phone. The imaging apparatus may also be a modular form mounted on electronic equipment, for example, a camera module.

Although a camera body <NUM> is coupled to a lens barrel <NUM>, which is an optical apparatus, in <FIG>, the lens barrel <NUM> is an interchangeable lens detachably mounted on the camera body <NUM>.

Light from an object passes through an optical system including lenses <NUM> and <NUM> arranged on the optical axis of an imaging optical system in a housing <NUM> of the lens barrel <NUM> and is received by an imaging device. A member according to the present disclosure can also be used as a lens constituting an optical system.

The lens <NUM> is movably supported by an inner tube <NUM> relative to an outer tube of the lens barrel <NUM> for focusing and zooming.

In the observation period before photographing, light from an object is reflected by a main mirror <NUM> in a housing <NUM> of the camera body, passes through a prism <NUM>, and then provides the photographer with an image to be photographed through a viewing lens <NUM>. The main mirror <NUM> is a half mirror, for example. Light transmitted through the main mirror is reflected by a sub-mirror <NUM> in the direction of an autofocusing (AF) unit <NUM>, and the reflected beam is used for focusing, for example. The main mirror <NUM> is attached to and supported by a main mirror holder <NUM> by adhesion or the like. For photographing, the main mirror <NUM> and the sub-mirror <NUM> are moved out of the optical path by a driving mechanism (not shown), a shutter <NUM> is opened, and an optical image to be photographed incident from the lens barrel <NUM> is focused on an imaging device <NUM>. Furthermore, the diaphragm <NUM> is configured to change the aperture area and thereby change the brightness and the depth of focus while photographing.

A member according to the present disclosure can be used as a lens constituting an optical system, suppress reflection and scattering in the optical system, and provide a good image.

Furthermore, due to the porous layer <NUM> with high mechanical strength, a member according to the present disclosure is suitable for a lens filter <NUM> to be installed on the outermost side of the optical system. Depending its type, the lens filter <NUM> has a function of protecting a lens or producing soft, color tone change, polarization, light reduction, and other effects on an image thus formed. <FIG> illustrates an example of the lens filter <NUM>. The lens filter <NUM> includes a member (filter member) <NUM> according to the present disclosure fitted into a frame <NUM> provided with a mounting portion <NUM>, such as a screw thread or a bayonet mount, for mounting the lens filter <NUM> on the housing <NUM> of the interchangeable lens. The filter member <NUM> has the porous layer <NUM> opposite the mounting portion <NUM> of the frame <NUM>. When the lens filter <NUM> is mounted on the housing <NUM>, the porous layer <NUM> is positioned on the light incident surface.

A member according to the present disclosure can be adopted as a light-transmitting member of a face shield. <FIG> is a schematic view of a face shield <NUM>. The face shield <NUM> includes a light-transmitting member <NUM> and a holder <NUM> for holding the light-transmitting member <NUM>. The holder <NUM> has a structure for fixing the light-transmitting member <NUM> to the user such that the light-transmitting member <NUM> covers at least part of user's face. The holder <NUM> includes a fixing portion <NUM> to which the light-transmitting member <NUM> is fixed, and a supporting portion <NUM> for fixing the fixing portion <NUM> to the user. The supporting portion <NUM> is connected to the fixing portion <NUM>. The fixing portion <NUM> is rod-like, and a peripheral portion of the light-transmitting member <NUM> is fixed to a side surface of the fixing portion <NUM> with a fixing component <NUM>, such as a pin or screw. The fixing component <NUM> can pass through a notch <NUM> and/or a hole <NUM> of the light-transmitting member <NUM> illustrated in <FIG>. The belt-like supporting portion <NUM> is attached to a wearer and supports the fixing portion <NUM>. The light-transmitting member <NUM> of the face shield <NUM> covers at least one or all of the eyes, nose, and mouth of the user, for example.

The outer surface (a surface opposite the face side) of the light-transmitting member <NUM> of the face shield <NUM> can be a front surface, and the inner surface (the surface on the face side) can be a back surface. A light source on the outer surface side can be a main factor of reflected beam, and the porous layer can face the outer surface side to produce antireflection effects. Furthermore, when the face shield <NUM> is used, scratches are more likely to occur on the outer surface than on the inner surface. Thus, considering the scratch resistance of the light-transmitting member <NUM>, the porous layer can be provided on the outer surface side. When the porous layer is provided on both sides, the porous layer on the outer surface side can be thicker than the porous layer on the inner surface side.

The face shield <NUM> may be provided with a ventilation fan <NUM> for ventilating the atmosphere adjacent to the light-transmitting member <NUM>. In this example, the ventilation fan <NUM> is provided inside the holder <NUM> (fixing portion <NUM>).

The use of the face shield <NUM> according to the present example not only protects the face of the wearer but also has the effects of improving the work efficiency of the wearer due to good visibility of the light-transmitting member <NUM> and allowing a person who faces the wearer to easily recognize the face and expression of the wearer.

Although the face shield has been described in <FIG>, a member according to the present disclosure is also suitable for a light-transmitting member of a shield partition.

Next, a coating liquid used to produce the porous layer <NUM> is described, and then a method for manufacturing the member <NUM> is described.

A coating liquid contains the silicon oxide particles <NUM>, the acid <NUM>, and a component serving as the inorganic binder <NUM>, which constitute the porous layer <NUM>, and an organic solvent.

The silicon oxide particles <NUM> and the acid <NUM> are described above.

As illustrated in <FIG>, the porous layer <NUM> includes the silicon oxide particles <NUM> stacked on the surface of the substrate <NUM>. A layer composed of unevenly arranged silicon oxide particles <NUM> tends to have an uneven stress distribution and decreased strength. Thus, it is desirable that the silicon oxide particles <NUM> be aligned to form the porous layer <NUM> with high strength.

The arrangement of the silicon oxide particles <NUM> depends mainly on the dispersion state of the silicon oxide particles <NUM> in a coating liquid for forming the porous layer <NUM> containing the particles and the dispersion state of the silicon oxide particles <NUM> while a coating film is formed after the coating liquid is applied to the substrate.

The silicon oxide particles <NUM> uniformly dispersed in the coating liquid enables the silicon oxide particles <NUM> to be uniformly applied to the substrate <NUM> and tends to result in better arrangement of the silicon oxide particles <NUM> in the formed layer. When the silicon oxide particles <NUM> in the coating liquid are dispersed in an aggregated state under the influence of the dispersion medium or the component serving as the inorganic binder <NUM>, the particles in the aggregated state are applied to the substrate <NUM> and are poorly aligned.

Even when the silicon oxide particles <NUM> in the coating liquid have a good dispersion state, aggregation of the silicon oxide particles <NUM> during a drying process after the coating liquid is applied to the substrate disturbs the arrangement of the silicon oxide particles <NUM> in the layer.

Thus, to form a layer with high strength, it is desirable that the silicon oxide particles <NUM> be uniformly dispersed in the coating liquid and that the silicon oxide particles <NUM> do not aggregate in the drying process of the coating liquid applied to the substrate <NUM>.

The coating liquid used to produce the porous layer <NUM> according to the present disclosure contains at least one acid selected from the group of acids listed in the formulae (<NUM>) to (<NUM>), and the surface of the silicon oxide particles <NUM> in the coating liquid is modified with the acid <NUM>. The modification with the acid <NUM> causes the silicon oxide particles <NUM> to be charged and repel each other, suppresses aggregation of the particles, and uniformly disperses the particles. This state is maintained in the drying process of the coating liquid applied to the substrate. Thus, a coating liquid can be used to form a layer with high strength composed of regularly and densely arranged silicon oxide particles <NUM>.

The acid <NUM> preferably has a molecular weight in the range of <NUM> to <NUM>. A molecular weight of less than <NUM> tends to result in difficult application due to increased viscosity and the coating liquid with low temporal stability. A molecular weight of more than <NUM> results in the presence of an acid with a high molecular weight between the particles and tends to result in a large empty space in the layer and scattering.

The acid <NUM> content preferably ranges from <NUM>% to <NUM>% by mass, more preferably <NUM>% to <NUM>% by mass, of the silicon oxide particles. An acid <NUM> content of the coating liquid below <NUM>% by mass of the silicon oxide particles tends to make it difficult to prevent aggregation of the silicon oxide particles <NUM> after the coating liquid is applied to the substrate. Alternatively, due to the influence of the solvent and a component serving as a binder in the coating liquid, the silicon oxide particles <NUM> tend to have low dispersion stability, and the coating liquid tends to become more viscous or form a gel with time. The acid <NUM> constituting <NUM>% by mass of the silicon oxide particles disturbs the arrangement of the silicon oxide particles <NUM>, increases the number of empty spaces <NUM>, and decreases the strength.

The acid <NUM> preferably has an acid dissociation constant in the range of -<NUM> to <NUM> pKa, more preferably -<NUM> to <NUM> pKa. An acid dissociation constant of less than -<NUM> pKa tends to result in irregular arrangement of the silicon oxide particles <NUM> during the drying process after application to the substrate <NUM> and the layer with low strength. An acid dissociation constant of more than <NUM> pKa tends to result in poor dispersion of the silicon oxide particles <NUM> in the coating liquid, poor arrangement in the layer, and the layer with low strength.

The acids <NUM> represented by the formulae (<NUM>) to (<NUM>) are mostly safe while handling of raw materials and are mostly solid, and are easy to handle.

The component serving as the inorganic binder <NUM> can be a silicon oxide oligomer. Although silicon oxide particles originally have silanol (Si-OH) groups on the surface, the number of silanol groups on the surface can be increased by mixing with a silicon oxide oligomer in the coating liquid. Consequently, the silicon oxide particles <NUM> are more easily bound on the surface. When the coating liquid is applied and dried, the silicon oxide oligomer forms silicon oxide, fixes chainlike silicon oxide particles <NUM> in contact with each other, and can provide a layer with high scratch resistance.

In a coating liquid, the component serving as the inorganic binder <NUM> preferably constitutes <NUM>% to <NUM>% by mass, more preferably <NUM>% to <NUM>% by mass, of the silicon oxide particles <NUM>. When the component serving as the inorganic binder <NUM> constitutes less than <NUM>% by mass of the silicon oxide particles <NUM>, the silicon oxide particles <NUM> is insufficiently dispersed in the dispersion liquid, and the layer tends to have low strength. When the component serving as the inorganic binder <NUM> constitutes more than <NUM>% by mass of the silicon oxide particles <NUM>, the inorganic binder disturbs the arrangement of the particles, and the layer tends to more strongly scatter visible light and have an increased refractive index.

An organic solvent that can be used in the coating liquid may be any solvent that does not precipitate the silicon oxide particles <NUM> or drastically increase the viscosity of the coating liquid. Examples of such a solvent include the following solvents. Monohydric alcohols, such as methanol, ethanol, <NUM>-propanol, <NUM>-propanol, <NUM>-butanol, <NUM>-butanol, <NUM>-methylpropanol, <NUM>-pentanol, <NUM>-pentanol, cyclopentanol, <NUM>-methylbutanol, <NUM>-methylbutanol, <NUM>-hexanol, <NUM>-hexanol, <NUM>-hexanol, <NUM>-methyl-<NUM>-pentanol, <NUM>-methyl-<NUM>-pentanol, <NUM>-ethylbutanol, <NUM>,<NUM>-dimethyl-<NUM>-pentanol, <NUM>-ethylbutanol, <NUM>-heptanol, <NUM>-heptanol, <NUM>-octanol, and <NUM>-octanol. Dihydric or polyhydric alcohols, such as ethylene glycol and triethylene glycol. Ether alcohols, such as methoxyethanol, ethoxyethanol, propoxyethanol, isopropoxyethanol, butoxyethanol, <NUM>-methoxy-<NUM>-propanol, <NUM>-ethoxy-<NUM>-propanol, <NUM>-propoxy-<NUM>-propanol, and <NUM>-methoxy-<NUM>-butanol; and ethers, such as dimethoxyethane, diglyme (diethylene glycol dimethyl ether), tetrahydrofuran, dioxane, diisopropyl ether, dibutyl ether, and cyclopentyl methyl ether. Esters, such as ethyl formate, ethyl acetate, n-butyl acetate, methyl lactate, ethyl lactate, ethylene glycol monomethyl ether acetate, ethylene glycol monoethyl ether acetate, ethylene glycol monobutyl ether acetate, and propylene glycol monomethyl ether acetate. Aliphatic and alicyclic hydrocarbons, such as n-hexane, n-octane, cyclohexane, cyclopentane, and cyclooctane. Aromatic hydrocarbons, such as toluene, xylene, and ethylbenzene. Ketones, such as acetone, methyl ethyl ketone, methyl isobutyl ketone, cyclopentanone, and cyclohexanone. Chlorinated hydrocarbons, such as chloroform, methylene chloride, carbon tetrachloride, and tetrachloroethane. Aprotic polarized solvents, such as N-methylpyrrolidone, N,N-dimethylformamide, N,N-dimethylacetamide, and ethylene carbonate. Two or more of these solvents may be used in combination.

From the perspectives of the dispersibility of the silicon oxide particles <NUM> and the coating performance of the coating liquid, <NUM>% or more by mass of the solvent in the coating liquid can be a water-soluble solvent having <NUM> to <NUM> carbon atoms and a hydroxy group. In particular, at least one solvent selected from the group consisting of ethoxyethanol, propoxyethanol, isopropoxyethanol, butoxyethanol, <NUM>-methoxy-<NUM>-propanol, <NUM>-ethoxy-<NUM>-propanol, <NUM>-propoxy-<NUM>-propanol, ethyl lactate, and <NUM>-methoxy-<NUM>-butanol can be contained. The use of these solvents reduces radial coating marks while coating and residual liquid around the substrate and improves coatability.

A method for manufacturing the member <NUM> according to the present disclosure includes the steps of applying a coating liquid to the substrate <NUM> and drying and/or baking the substrate <NUM> to which the coating liquid has been applied.

The coating liquid may be applied to the substrate <NUM> by a spin coating method, a blade coating method, a roll coating method, a slit coating method, a printing method, a gravure coating method, or a dip coating method. A member with a three-dimensionally complicated shape, such as a concave surface, can be produced by the spin coating method, which facilitates coating with a uniform thickness.

The drying and/or baking step is the step of removing the organic solvent and bonding the silicon oxide particles <NUM> without disturbing the arrangement thereof to form the porous layer <NUM>. The drying and/or baking step is preferably performed in the temperature range of <NUM> to <NUM>, depending on the heat resistance temperature of the substrate <NUM>. The time of the drying and/or baking step may be such that the organic solvent in the layer can be removed without any influence on the substrate <NUM> and preferably ranges from <NUM> minutes to <NUM> hours, more preferably <NUM> minutes to <NUM> hours.

In Examples <NUM> to <NUM>, a coating liquid for forming the porous layer <NUM> was prepared by the following method, and a porous layer was formed on a substrate to prepare the member <NUM> with the porous layer <NUM>. The coating liquid and the porous layer <NUM> were examined as described below.

A coating liquid was dropped on a polished surface of a glass substrate (φ30 mm, <NUM> in thickness, synthetic quartz with one polished surface) and was spread with a spin coater such that the porous layer <NUM> had a thickness of approximately <NUM>. The appearance of the layer containing particles was visually inspected for a defect with an optical microscope and was rated in accordance with the following criteria.

The rating A was judged to be excellent coatability, and the rating B was judged to be good coatability.

The porous layer <NUM> was formed on a polished surface of a glass substrate (φ30 mm, <NUM> in thickness, synthetic quartz with one polished surface). A polyester wiper (Alpha Wiper TX1009 manufactured by Texwipe) was moved reciprocally <NUM> times at a load of <NUM>/cm<NUM> on the surface of the porous layer <NUM>. The appearance was evaluated with an optical microscope. The evaluation criteria were as follows:.

In the present disclosure, the rating A was judged to be very high strength, and the rating B was judged to be high strength without problems.

The porous layer <NUM> containing particles was formed on a polished surface of a glass substrate (φ30 mm, <NUM> in thickness, synthetic quartz with one polished surface). Light was incident on the porous layer <NUM> using a spectroscopic ellipsometer (VASE, manufactured by J. Woollam Japan Co. The reflected beam was measured in the wavelength range of <NUM> to <NUM> to calculate a refractive index. The refractive index at a wavelength of <NUM> was rated in accordance with the following criteria.

The rating A or B was judged that the porous layer was suitable for a low-refractive-index layer.

A glass substrate (φ30 mm, <NUM> in thickness, synthetic quartz with polished surfaces on both sides) was placed in a substrate holder. An illuminometer (T-<NUM> manufactured by Konica Minolta Sensing) was installed in the substrate holder. While the illuminance was measured, the surface of the substrate was irradiated with white light such that the illuminance in the vertical direction was <NUM> lux. Next, a member having the porous layer <NUM> on the glass substrate was placed in a substrate holder such that white light was incident on the porous layer <NUM> side. The member was tilted at <NUM> degrees and was photographed with a camera (lens: EF50 mm F2. <NUM> Compact Macro manufactured by CANON KABUSHIKI KAISHA, camera: EOS-70D manufactured by CANON KABUSHIKI KAISHA) in the direction normal to the surface opposite the surface to be irradiated. The imaging conditions of the camera were ISO <NUM>, white balance: fair weather, diaphragm: <NUM>, and a shutter speed: <NUM> seconds. The average luminance of four positions in <NUM> pixels x <NUM> pixels in a captured image was calculated as a scattering value.

In the present disclosure, the porous layer <NUM> with a scattering value of <NUM> or less as calculated by this method was judged to have low scattering.

Pure water was dropped on the surface of the porous layer opposite the substrate, and the contact angle of the pure water was measured at a room temperature of <NUM> and at a humidity in the range of <NUM>%RH to <NUM>%RH. The contact angle was measured by taking an image <NUM> after the pure water was dropped.

While <NUM>-ethoxy-<NUM>-propanol was added to <NUM> of an isopropyl alcohol dispersion liquid of hollow silicon oxide particles (Thrulya <NUM> manufactured by JGC Catalysts and Chemicals Ltd. , average particle size: approximately <NUM>, shell thickness: approximately <NUM>, solid content: <NUM>% by mass), isopropyl alcohol was evaporated by heating. The isopropyl alcohol was evaporated to a solid content of <NUM>% by mass. Thus, <NUM> of a 1E2P solvent-substituted liquid of hollow silicon oxide particles (hereinafter referred to as a solvent-substituted liquid <NUM>) was prepared. An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (tetrafluorosuccinic acid manufactured by Tokyo Chemical Industry Co.

In a separate container, <NUM> of ethanol and an aqueous nitric acid (concentration: <NUM>%) were added to <NUM> of ethyl silicate. The mixture was stirred at room temperature for <NUM> hours to prepare a silica sol <NUM> (solid content: <NUM>% by mass). Gas chromatography showed that the raw material ethyl silicate was completely reacted.

The dispersion liquid <NUM> was diluted with ethyl lactate to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hollow silicon oxide particles.

The coating liquid <NUM> was dropped on a glass substrate and was spread with a spin coater such that the resulting porous layer had a thickness of approximately <NUM>, and was then baked in a thermostatic oven at <NUM> for <NUM> minutes to prepare a member <NUM>-<NUM> including a porous layer <NUM>-<NUM> containing particles. The contact angle of pure water on the surface of the porous layer <NUM>-<NUM> opposite the substrate was <NUM> degrees.

An acid was added to the solvent-substituted liquid <NUM> such that the ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (octafluoroadipic acid manufactured by Tokyo Chemical Industry Co.

The dispersion liquid <NUM> was diluted with ethyl lactate to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the mass ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hollow silicon oxide particles.

While <NUM>-methoxy-<NUM>-propanol (hereinafter abbreviated to PGME) was added to <NUM> of an aqueous dispersion liquid of hydrophilic silicon oxide particles (PL-<NUM> manufactured by Fuso Chemical Co. , average particle size: approximately <NUM>, long diameter/short diameter = <NUM>, solid content: <NUM>% by mass), water was evaporated by heating. The water was evaporated to a solid content of <NUM>% by mass. Thus, <NUM> of a PGME solvent-substituted liquid of hydrophilic silicon oxide particles (hereinafter referred to as a solvent-substituted liquid <NUM>) was prepared. An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hydrophilic silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with three acidic groups (nitrilotris manufactured by Tokyo Chemical Industry Co.

The dispersion liquid <NUM> was diluted with <NUM>-propoxy-<NUM>-propanol to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the ratio of hydrophilic silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hydrophilic silicon oxide particles.

The coating liquid <NUM> was dropped on a glass substrate and was spread with a spin coater such that the resulting porous layer had a thickness of approximately <NUM>, and was then baked in a thermostatic oven at <NUM> for <NUM> minutes to prepare a member <NUM>-<NUM> including a porous layer <NUM>-<NUM> containing chainlike silicon oxide particles. The contact angle of pure water on the surface of the porous layer <NUM>-<NUM> opposite the substrate was <NUM> degrees.

An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hydrophilic silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with four acidic groups (N,N,N',N'-ethylenediaminetetrakis manufactured by Tokyo Chemical Industry Co.

The dispersion liquid <NUM> was diluted with <NUM>-propoxy-<NUM>-propanol to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the mass ratio of hydrophilic silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hydrophilic silicon oxide particles.

An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hydrophilic silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (tetrafluorosuccinic acid manufactured by Tokyo Chemical Industry Co.

An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (tetrafluorosuccinic acid manufactured by Tokyo Chemical Industry Co.

An acid was added to the solvent-substituted liquid <NUM> such that the ratio of hydrophilic silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (tetrafluorosuccinic acid manufactured by Tokyo Chemical Industry Co.

An acid was added to the solvent-substituted liquid <NUM> such that the ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (dodecafluorosuberic acid manufactured by Tokyo Chemical Industry Co.

<NUM> of <NUM>-propoxy-<NUM>-propanol and <NUM> of methyl polysilicate (Methyl Silicate 53A manufactured by Colcoat Co. ) were slowly added to a separate container and were stirred at room temperature for <NUM> minutes to prepare a silica sol (hereinafter referred to as a silica sol <NUM>).

The dispersion liquid <NUM> was diluted with <NUM>-propoxy-<NUM>-propanol to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the mass ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hollow silicon oxide particles.

The dispersion liquid <NUM> was diluted with <NUM>-methoxy-<NUM>-butanol to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the mass ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hollow silicon oxide particles.

An acid was added to the solvent-substituted liquid <NUM> such that the ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (hexafluoroglutaric acid manufactured by Tokyo Chemical Industry Co.

<NUM> of an isopropyl alcohol dispersion liquid of hollow silicon oxide particles (Thrulya <NUM> manufactured by JGC Catalysts and Chemicals Ltd. , average particle size: approximately <NUM>, shell thickness: approximately <NUM>, solid content: <NUM>% by mass) was mixed with <NUM>-propoxy-<NUM>-propanol to prepare a dispersion liquid <NUM> with a solid content of <NUM>% by mass.

An acid was added to the dispersion liquid <NUM> such that the ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. The acid added was an acid with two acidic groups (dodecafluorosuberic acid manufactured by Tokyo Chemical Industry Co.

Furthermore, the silica sol <NUM> was added such that the mass ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hollow silicon oxide particles.

<NUM> of an isopropyl alcohol dispersion liquid of hollow silicon oxide particles (Thrulya <NUM> manufactured by JGC Catalysts and Chemicals Ltd. , average particle size: approximately <NUM>, shell thickness: approximately <NUM>, solid content: <NUM>% by mass) was mixed with ethyl lactate to prepare a dispersion liquid <NUM> with a solid content of <NUM>% by mass.

While <NUM>-propoxy-<NUM>-propanol was added to <NUM> of an isopropyl alcohol dispersion liquid of hollow silicon oxide particles (Thrulya <NUM> manufactured by JGC Catalysts and Chemicals Ltd. , average particle size: approximately <NUM>, shell thickness: approximately <NUM>, solid content: <NUM>% by mass), isopropyl alcohol was evaporated by heating. The isopropyl alcohol was evaporated to a solid content of <NUM>% by mass. Thus, <NUM> of a 1P2P solvent-substituted liquid of hollow silicon oxide particles (hereinafter referred to as a solvent-substituted liquid <NUM>) was prepared. An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was an acid with two acidic groups (tetrafluorosuccinic acid manufactured by Tokyo Chemical Industry Co.

An acid was added to the solvent-substituted liquid <NUM> such that the ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was <NUM>,<NUM>,<NUM>-trifluoropropionic acid (manufactured by Tokyo Chemical Industry Co. , number of acidic groups: <NUM>).

The dispersion liquid <NUM> was diluted with ethyl lactate to a solid content of <NUM>% by mass. The silica sol <NUM> was then added to the dispersion liquid <NUM> such that the mass ratio of hollow silicon oxide particles: silica sol component was <NUM>:<NUM>. The mixture was then mixed by stirring at room temperature for <NUM> hours to prepare a coating liquid <NUM> containing hydrophilic silicon oxide particles.

The coating liquid <NUM> was dropped on a glass substrate and was spread with a spin coater such that the resulting porous layer had a thickness of approximately <NUM>, and was then baked in a thermostatic oven at <NUM> for <NUM> minutes to prepare a member <NUM>-<NUM> including a porous layer <NUM>-<NUM> containing particles. The contact angle of pure water on the porous layer <NUM>-<NUM> was <NUM> degrees.

An acid was added to the solvent-substituted liquid <NUM> such that the mass ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared. The acid added was p-toluenesulfonic acid monohydrate (manufactured by Tokyo Chemical Industry Co. , number of acidic groups: <NUM>).

Phosphoric acid (manufactured by Tokyo Chemical Industry Co. , number of acidic groups: <NUM>) was added to the solvent-substituted liquid <NUM> such that the mass ratio of hollow silicon oxide particles:acid component was <NUM>:<NUM>. Thus, a dispersion liquid <NUM> was prepared.

Table <NUM> shows the evaluation results of the coating liquids <NUM> to <NUM> used in the examples and comparative examples and the members <NUM>-<NUM> to <NUM>-<NUM> produced using the coating liquids.

The results in Table <NUM> show that Examples <NUM> to <NUM> had high film strength while maintaining a low refractive index. Examples <NUM> to <NUM> also had a scattering value of <NUM> or less and had sufficient performance as optical functional films.

By contrast, Comparative Examples <NUM> to <NUM> had low film strength and a high scattering value of <NUM> or more. This probably shows that with an acid with only one acidic group it was difficult to maintain a highly dispersed state as the solvent evaporates during the formation of the coating film, thus resulting in irregular arrangement, linear scratches, and peeling. The irregular arrangement also probably resulted in a high scattering value.

As described above, the present disclosure can provide a member having a porous layer containing silicon oxide particles and having a low refractive index and high film strength.

Although optical members are mainly described above, a member according to the present disclosure can be used for other applications. Applications other than optical members may only require good coatability and high film strength and do not necessarily require a low refractive index or scattering value. For example, these applications include heat-insulating members and insulating members.

Claim 1:
A member (<NUM>) having a substrate and a porous layer (<NUM>) on the substrate (<NUM>), wherein
the porous layer (<NUM>) contains a plurality of silicon oxide particles (<NUM>) bound by an inorganic binder (<NUM>) and at least one acid (<NUM>) selected from the group consisting of the following formulae (<NUM>) to (<NUM>):
<CHM>
<CHM>
<CHM>
<CHM>
wherein
A<NUM> in the formula (<NUM>) denotes COOH, and n denotes an integer in the range of <NUM> to <NUM>,
one of A<NUM> and A<NUM> in the formula (<NUM>) denotes SO<NUM>H or PO<NUM>H<NUM>, and the other denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH, and R denotes a divalent organic group having <NUM> to <NUM> carbon atoms,
at least one of A<NUM>, A<NUM>, and A<NUM> in the formula (<NUM>) denotes SO<NUM>H or PO<NUM>H<NUM>, and the other denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH, and R denotes a trivalent organic group having <NUM> to <NUM> carbon atoms, and
at least one of A<NUM>, A<NUM>, A<NUM>, and A<NUM> in the formula (<NUM>) denotes SO<NUM>H or PO<NUM>H<NUM>, and the other denotes an acidic group selected from the group consisting of SO<NUM>H, PO<NUM>H<NUM>, COOH, and OH, and R denotes a tetravalent organic group having <NUM> to <NUM> carbon atoms.