Patent Description:
An optical fiber has generally a coating resin layer for protecting a glass fiber that is an optical transmission medium. The optical fiber is required to reduce the stress between the glass fiber and the coating resin layer to suppress the generation of defects such as voids, especially when used at low temperatures.

The coating resin layer is composed of, for example, a primary resin layer and a secondary resin layer. In <CIT>, it is investigated to prevent peeling between the glass fiber and the primary resin layer at low temperatures by reducing the difference in the effective linear expansion coefficient between the primary resin layer and the secondary resin layer. In <CIT>, it is investigated to form a cured layer that hardly generates residual stress in a cured film and easily absorbs external loads by using a liquid curable resin composition containing a urethane (meth)acrylate and a polymerizable monofunctional monomer. <CIT> further describes an optical fiber comprising a glass fiber with a core and a cladding with which the core is covered, and a coating resin layer that includes titanium oxide.

An optical fiber according to the present invention is defined by the appended claims.

<FIG> is a schematic cross-section diagram showing an example of the optical fiber according to the present invention.

It is conceivable that the difference in curing shrinkage rates of a resin composition that forms a primary resin layer and a resin composition that forms a secondary resin layer is large as one of the factors for the generation of voids in an optical fiber. An object of the present disclosure is to provide an optical fiber that reduces the cure shrinkage rate of the secondary resin layer and sufficiently suppresses the generation of voids.

The present disclosure can provide an optical fiber that reduces the cure shrinkage rate of a secondary resin layer and sufficiently suppresses the generation of voids.

First, the contents of the embodiment of the present disclosure will be described by listing them. An optical fiber according to the present invention comprises a glass fiber comprising a core and a cladding, a primary resin layer being in contact with a glass fiber and covering the glass fiber, and a secondary resin layer covering the primary resin layer, wherein the Young's modulus of the primary resin layer is <NUM> MPa or more and <NUM> MPa or less at <NUM> ± <NUM>, the secondary resin layer consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator, and hydrophobic inorganic oxide particles, and the content of the inorganic oxide particles is <NUM>% by mass or more and <NUM>% by mass or less based on the total amount of the resin composition, characterized in that the inorganic oxide particles are silicon dioxide.

Using a resin composition containing inorganic oxide particles in a specific range can reduce the curing shrinkage rate to form a secondary resin layer having excellent toughness. Combining the secondary resin layer with the primary resin layer having the Young's modulus in the above range can sufficiently suppress the generation of voids in the optical fiber.

Due to excellent dispersion properties in the resin composition and easy formation of a hard coating film, the above inorganic oxide particles are silicon dioxide.

From the viewpoint of further reducing the curing shrinkage rate, the average primary particle size of the inorganic oxide particles may be <NUM> or less.

From the viewpoint of imparting appropriate strength to the coating resin layer, the Young's modulus of the secondary resin layer may be <NUM> MPa or more and <NUM> MPa or less at <NUM> ± <NUM>.

Specific examples of a resin composition and an optical fiber according to embodiments of the present disclosure will be described referring to the drawing as necessary. The present invention is not limited to these illustrations but is indicated by the claims and intended to include meanings equivalent to the claims and all modifications within the claims. In the following description, the same reference numerals are given to the same elements in the description of the drawing, and redundant explanations are omitted.

<FIG> is a schematic cross-section diagram showing an example of the optical fiber according to the present embodiment. The optical fiber <NUM> comprises the glass fiber <NUM> including the core <NUM> and the cladding <NUM>, and the coating resin layer <NUM> including the primary resin layer <NUM> provided on the outer periphery of the glass fiber <NUM> and the secondary resin layer <NUM>.

The cladding <NUM> surrounds the core <NUM>. The core <NUM> and the cladding <NUM> mainly include glass such as silica glass, germanium-added silica can be used, for example, in the core <NUM>, and pure silica or fluorine-added silica can be used in the cladding <NUM>.

In <FIG>, for example, the outside diameter (D2) of the glass fiber <NUM> is about <NUM>, and the diameter (D1) of the core <NUM> constituting the glass fiber <NUM> is about <NUM> to <NUM>.

The thickness of the coating resin layer <NUM> is typically about <NUM> to <NUM>. The thickness of each of the primary resin layer <NUM> and the secondary resin layer <NUM> may be about <NUM> to <NUM>, and for example, the thickness of the primary resin layer <NUM> may be <NUM> and the thickness of the secondary resin layer <NUM> may be <NUM>. The outside diameter of the optical fiber <NUM> may be about <NUM> to <NUM>.

The thickness of the coating resin layer <NUM> may be about <NUM> to <NUM>. In this case, the thickness of each of the primary resin layer <NUM> and the secondary resin layer <NUM> may be about <NUM> to <NUM>, and for example, the thickness of the primary resin layer <NUM> may be <NUM> and the thickness of the secondary resin layer <NUM> may be <NUM>. The outside diameter of the optical fiber <NUM> may be about <NUM> to <NUM>.

The secondary resin layer <NUM> can be formed by curing a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator and hydrophobic inorganic oxide particles.

(Meth)acrylate means an acrylate or a methacrylate corresponding to it. The same applies to (meth)acrylic acid and the like.

The inorganic oxide particles according to the present embodiment are spherical particles and have a surface subjected to hydrophobic treatment. The hydrophobic treatment according to the present embodiment is introduction of a hydrophobic group onto the surface of the inorganic oxide particles. The inorganic oxide particles having a hydrophobic group introduced have excellent dispersibility in the resin composition. The hydrophobic group may be a reactive group such as a (meth)acryloyl group, or may be a non-reactive group such as a hydrocarbon group (for example, an alkyl group) or an aryl group (for example, a phenyl group). In the case of the inorganic oxide particles having a reactive group, the resin layer having high Young's modulus is easy to form.

The inorganic oxide particles according to the present embodiment are dispersed in a dispersion medium. Using the inorganic oxide particles dispersed in the dispersion medium allows for uniform dispersion of the inorganic oxide particles in the resin composition and then improvement of the storage stability of the resin composition. The dispersion medium is not particularly limited as long as curing of the resin composition is not obstructed. The dispersion medium may be reactive or non-reactive.

A monomer such as a (meth)acryloyl compound and an epoxy compound can be used as the reactive dispersion medium. Examples of the (meth)acryloyl compound include <NUM>,<NUM>-hexanediol di(meth)acrylate, EO-modified bisphenol A di(meth)acrylate, polyethylene glycol di(meth)acrylate, PO-modified bisphenol A di(meth)acrylate, polypropylene glycol di(meth)acrylate, and polytetramethylene glycol di(meth)acrylate. As the (meth)acryloyl compound, compounds exemplified by monomers described below may be used.

A ketone solvent such as methyl ethyl ketone (MEK), an alcohol solvent such as propylene glycol monomethyl ether (PGME), or an ester solvent such as propylene glycol monomethyl ether acetate (PGMEA) may be used as a non-reactive dispersion medium. In the case of the non-reactive dispersion medium, the resin composition may be prepared by mixing the base resin and the inorganic oxide particles dispersed in the dispersion medium and removing a part of the dispersion medium. When the dispersion medium including the inorganic oxide particles is observed with an optical microscope (about <NUM> times magnification) and no particles are observed, the inorganic oxide particles are dispersed as primary particles.

The inorganic oxide particles dispersed in the dispersion medium remain to be dispersed in the resin layer after curing of the resin layer. When a reactive dispersion medium is used, the inorganic oxide particles are mixed with the dispersion medium in the resin composition and are incorporated in the resin layer with the dispersion condition maintained. When a non-reactive dispersion medium is used, at least a part of the dispersion medium evaporates and disappears from the resin composition, but the inorganic oxide particles remain in the resin composition with the dispersion condition remained and are also present in the cured resin layer with the dispersion condition remained. Electron microscope observation shows that the inorganic oxide particles present in the resin layer are in the condition of dispersion of the primary particle.

Due to excellent dispersion properties in the resin composition and easy formation of hard coating film, the inorganic oxide particles are silicon dioxide (silica). From the view point of excellent inexpensiveness, easy surface treatment, permeability to ultraviolet ray, easy provision of a resin layer with appropriate hardness, and the like, hydrophobic silica particles are used as the inorganic oxide particles according to the present invention.

From the viewpoint of imparting appropriate toughness to the secondary resin layer, the average primary particle size of the inorganic oxide particles may be <NUM> or less, is preferably <NUM> or less, more preferably <NUM> or less, and further preferably <NUM> or less. From the viewpoint of reducing the curing shrinkage rate of the secondary resin layer, the average primary particle size of the inorganic oxide particles is preferably <NUM> or more, and more preferably <NUM> or more. The average primary particle diameter can be measured with image analysis of electron microscope pictures, a light scattering method or a BET method, for example. The dispersion medium in which the primary particle of the inorganic oxide is dispersed appears to be visually transparent when the diameter of the primary particle is small. When the diameter of the primary particle diameter is relatively large (<NUM> or more), the dispersion medium in which the primary particle is dispersed appears to be clouded, but the precipitate is not observed.

The content of the inorganic oxide particles is <NUM>% by mass or more and <NUM>% by mass or less, based on the total amount of the resin composition. A content of the inorganic oxide particles of <NUM>% by mass or more allows easy reduction in curing shrinkage. The content of the inorganic oxide particles of <NUM>% by mass or less allows easy formation of the secondary resin layer having excellent toughness.

A resin composition according to the present embodiment contains a urethane (meth)acrylate oligomer. As the urethane (meth)acrylate oligomer, an oligomer obtained by reacting a polyol compound, a polyisocyanate compound, and a hydroxyl group-containing (meth)acrylate compound can be used.

Examples of the polyol compound include polytetramethylene glycol, polypropylene glycol and bisphenol A-ethylene oxide addition diol. The number average molecular weight of the polyol compound may be <NUM> to <NUM>. Examples of the polyisocyanate compound includes <NUM>,<NUM>-tolylene diisocyanate, <NUM>,<NUM>-tolylene diisocyanate, isophorone diisocyanate, and dicyclohexylmethane <NUM>,<NUM>'-diisocyanate. Examples of the hydroxyl group-containing (meth)acrylate compound include <NUM>-hydroxyethyl (meth)acrylate, <NUM>-hydroxybutyl (meth)acrylate, <NUM>,<NUM>-hexanediol mono(meth)acrylate, pentaerythritol tri(meth)acrylate, <NUM>-hydroxypropyl (meth)acrylate, and tripropylene glycol mono(meth)acrylate.

As a catalyst for synthesizing a urethane (meth)acrylate oligomer, an organotin compound is generally used. Examples of the organotin compound include dibutyltin dilaurate, dibutyltin diacetate, dibutyltin maleate, dibutyltin bis(<NUM>-ethylhexyl mercaptoacetate), dibutyltin bis(isooctyl mercaptoacetate), and dibutyltin oxide. From the view point of easy availability or catalyst performance, it is preferable that dibutyltin dilaurate or dibutyltin diacetate be used as catalyst.

When the urethane (meth)acrylate oligomer is synthesized, lower alcohols having <NUM> or less carbon atoms may be used. Examples of the lower alcohols include methanol, ethanol, <NUM>-propanol, <NUM>-propanol, <NUM>-butanol, <NUM>-butanol, <NUM>-methyl-<NUM>-propanol, <NUM>-pentanol, <NUM>-pentanol, <NUM>-pentanol, <NUM>-methyl-<NUM>-butanol, <NUM>-methyl-<NUM>-butanol, <NUM>-methyl-<NUM>-butanol, <NUM>-methyl-<NUM>-butanol, and <NUM>,<NUM>-dimethyl-<NUM>-propanol.

As the monomer, a monofunctional monomer having one polymerizable group or a multifunctional monomer having two or more polymerizable groups can be used. A monomer may be used by mixing two or more monomers.

Examples of the monofunctional monomer include (meth)acrylate monomers such as methyl (meth)acrylate, ethyl (meth)acrylate, propyl (meth)acrylate, n-butyl (meth)acrylate, sec-butyl (meth)acrylate, tert-butyl (meth)acrylate, isobutyl (meth)acrylate, n-pentyl (meth)acrylate, isopentyl (meth)acrylate, hexyl (meth)acrylate, heptyl (meth)acrylate, isoamyl (meth)acrylate, <NUM>-ethylhexyl (meth)acrylate, n-octyl (meth)acrylate, isooctyl (meth)acrylate, isodecyl (meth)acrylate, lauryl (meth)acrylate, <NUM>-phenoxyethyl (meth)acrylate, <NUM>-phenoxybenzyl acrylate, phenoxydiethylene glycol (meth)acrylate, phenoxypolyethylene glycol (meth)acrylate, nonylphenoxypolyethylene glycol (meth)acrylate, <NUM>-tert-butylcyclohexanol (meth)acrylate, tetrahydrofurfuryl (meth)acrylate, benzyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyethyl (meth)acrylate, dicyclopentanyl (meth)acrylate, nonylphenol polyethylene glycol (meth)acrylate, and isobornyl (meth)acrylate; carboxyl group containing monomers such as (meth)acrylic acid, (meth)acrylic acid dimer, carboxyethyl (meth)acrylate, carboxypentyl (meth)acrylate, and ω-carboxy-polycaprolactone (meth)acrylate; heterocycle containing (meth)acrylates such as N-acryloyl morpholine, N-vinyl pyrrolidone, N-vinyl caprolactam, N-acryloylpiperidine, N-methacryloylpiperidine, N-acryloylpyrrolidine, <NUM>-(<NUM>-pyridine) propyl (meth)acrylate, and cyclic trimethylolpropane formal acrylate; maleimide monomers such as maleimide, N-cyclohexyl maleimide, and N-phenyl maleimide; amide monomers such as (meth)acrylamide, N, N-dimethyl (meth)acrylamide, N, N-diethyl (meth)acrylamide, N-hexyl (meth)acrylamide, N-methyl (meth)acrylamide, N-isopropyl (meth)acrylamide, N-butyl (meth)acrylamide, N-methylol (meth)acrylamide, and N-methylolpropane (meth)acrylamide; aminoalkyl (meth)acrylate monomers such as aminoethyl (meth)acrylate, aminopropyl (meth)acrylate, N, N-dimethylaminoethyl (meth)acrylate, and tert-butylaminoethyl (meth)acrylate; and succinimide monomers such as N-(meth)acryloyloxymethylene succinimide, N-(meth)acryloyl-<NUM>-oxyhexamethylene succinimide, and N- (meth)acryloyl-<NUM>-oxyoctamethylene succinimide.

From the viewpoint of adjusting the Young's modulus of the coating film formed from the resin composition, isobornyl (meth)acrylate or <NUM>-tert-butylcyclohexanol (meth)acrylate is preferable, and isobornyl (meth)acrylate is more preferable as a monofunctional monomer.

Examples of the multifunctional monomer include ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, neopentyl glycol di(meth)acrylate, tripropylene glycol di(meth)acrylate, di(meth)acrylate of alkylene oxide adduct of bisphenol A, tetraethylene glycol di(meth)acrylate, hydroxypivalic acid neopentyl glycol di(meth)acrylate, <NUM>,<NUM>-butanediol di(meth)acrylate, <NUM>,<NUM>-hexanediol di(meth)acrylate, <NUM>,<NUM>-nonanediol di(meth)acrylate, <NUM>,<NUM>-dodecanediol di(meth)acrylate, <NUM>,<NUM>-tetradecanediol di(meth)acrylate, <NUM>,<NUM>-hexadecanediol di(meth)acrylate, <NUM>,<NUM>-eicosanediol di(meth)acrylate, isopentyl diol di(meth)acrylate, <NUM>-ethyl-<NUM>, <NUM>-octanediol di(meth)acrylate, EO adduct of bisphenol A di(meth)acrylate, trimethylol propane tri(meth)acrylate, trimethylol octane tri(meth)acrylate, trimethylol propane polyethoxy tri(meth)acrylate, trimethylol propane polypropoxy tri(meth)acrylate, trimethylol propane polyethoxy polypropoxy tri(meth)acrylate, tris[(meth)acryloyloxyethyl] isocyanurate, pentaerythritol tri(meth)acrylate, pentaerythritol polyethoxy tetra(meth)acrylate, pentaerythritol polypropoxy tetra(meth)acrylate, pentaerythritol tetra(meth)acrylate, ditrimethylol propane tetra(meth)acrylate, dipentaerythritol tetra(meth)acrylate, dipentaerythritol penta(meth)acrylate, dipentaerythritol hexa(meth)acrylate, and caprolactone-modified tris[(meth)acryloyloxyethyl] isocyanurate.

From the viewpoint of forming a coating film having a desired Young's modulus, tripropylene glycol di(meth)acrylate, <NUM>,<NUM>-hexanediol di(meth)acrylate, and trimethylolpropane tri(meth)acrylate may be used as a polyfunctional monomer. Among them, tripropylene glycol di(meth)acrylate is preferable as a polyfunctional monomer.

The above resin composition preferably contains <NUM>% by mass or more and <NUM>% by mass or less of a monomer based on the total amount of the base resin, more preferably contains <NUM>% by mass or more and <NUM>% by mass or less, further preferably contains <NUM>% by mass or more and <NUM>% by mass or less, and particularly preferably contains <NUM>% by mass or more and <NUM>% by mass or less. Containing the monomer in the above range allows easy preparation of the resin composition that is more excellent in the balance between application properties and coating film properties.

The photopolymerization initiator can be appropriately selected from known radical photopolymerization initiators and used. Examples of the photopolymerization initiator include <NUM>-hydroxycyclohexyl phenyl ketone, <NUM>,<NUM>-dimethoxy-<NUM>-phenylacetophenone, <NUM>-(<NUM>-isopropylphenyl)-<NUM>-hydroxy-<NUM>-methylpropan-<NUM>-one, <NUM>,<NUM>,<NUM>-trimethylpentylphosphine oxide, <NUM>,<NUM>,<NUM>-trimethylbenzoyldiphenylphosphine oxide, <NUM>-methyl-<NUM>-[<NUM>-(methylthio)phenyl]-<NUM>-morpholino-propan-<NUM>-one (Omnirad <NUM> manufactured by IGM Resins), <NUM>,<NUM>,<NUM>-trimethylbenzoyldiphenylphosphine oxide (Omnirad TPO manufactured by IGM Resins), and bis(<NUM>,<NUM>,<NUM>-trimethylbenzoyl) phenylphosphine oxide (Omnirad <NUM>, manufactured by IGM Resins).

Due to adjustment of the Young's modulus of the secondary resin layer, the resin composition may further contain an epoxy (meth)acrylate oligomer. As an epoxy (meth)acrylate oligomer, an oligomer obtained by reacting a compound having a (meth)acryloyl group with an epoxy resin having two or more glycidyl groups can be used.

The resin composition may further contain a silane coupling agent, a photoacid generator, a leveling agent, an antifoaming agent, an antioxidant, and the like.

The silane coupling agent is not particularly limited as long as it does not disturb curing of the resin composition. Examples of the silane coupling agent include tetramethyl silicate, tetraethyl silicate, mercaptopropyl trimethoxysilane, vinyltrichlorosilane, vinyltriethoxysilane, vinyltris(β-methoxy-ethoxy)silane, β-(<NUM>,<NUM>-epoxycyclohexyl)-ethyltrimethoxysilane, dimethoxydimethylsilane, diethoxydimethylsilane, <NUM>-acryloxypropyltrimethoxysilane, γ-glycidoxypropyltrimethoxysilane, γ-glycidoxypropylmethyldiethoxysilane, γ-methacryloxypropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethoxysilane, N-(β-aminoethyl)-γ-aminopropyltrimethyldimethoxysilane, N-phenyl-γ-aminopropyltrimethoxysilane, γ-chloropropyltrimethoxysilane, γ-mercaptopropyltrimethoxysilane, γ-aminopropyltrimethoxysilane, bis-[<NUM>-(triethoxysilyl)propyl]tetrasulfide, bis-[<NUM>-(triethoxysilyl)propyl]disulfide, γ-trimethoxysilylpropyldimethylthiocarbamyl tetrasulfide, and γ-trimethoxysilylpropyl benzothiazyl tetrasulfide.

As the photoacid generator, an onium salt having an A+B- structure may be used. Examples of the photoacid generator include sulfonium salts such as UVACURE <NUM> (manufactured by Daicel-Cytec), CPI-100P, 110P (manufactured by San-Apro Ltd. ), <NUM> (manufactured by San-Apro Ltd. ) and iodonium salts such as Omnicat <NUM> (manufactured by IGM Regins), WPI-<NUM> (manufactured by FUJIFILM Wako Pure Chemical Corporation), Rp-<NUM> (manufactured by Rhodia Japan Ltd.

The Young's modulus of the secondary resin layer <NUM> is preferably <NUM> MPa or more at <NUM> ± <NUM>, more preferably <NUM> MPa or more and <NUM> MPa or less, and further preferably <NUM> MPa or more and <NUM> MPa or less. A Young's modulus of the secondary resin layer of <NUM> MPa or more is easy to improve the lateral pressure characteristics of the optical fiber, and the Young's modulus of <NUM> MPa or less can provide proper toughness to the secondary resin layer so that crack or the like in the secondary resin layer is hard to occur.

From the viewpoint of residual stress, the glass transition temperature (Tg) of the secondary resin layer may be <NUM> or more and <NUM> or less, is preferably <NUM> or more and <NUM> or less, more preferably <NUM> or more and <NUM> or less, and further preferably <NUM> or more and <NUM> or less.

The curing shrinkage rate in forming the secondary resin layer is preferably <NUM> to <NUM>%, more preferably <NUM> to <NUM>%, and further preferably <NUM> to <NUM>%. The difference between the curing shrinkage rate of the secondary resin layer and the curing shrinkage rate of the primary resin layer is preferably within <NUM>%. The method of measuring the cure shrinkage rate is not particularly limited and can be measured, for example, by the following method. A Teflon (registered trademark) ring (inner diameter of <NUM>) is placed on a glass plate, <NUM> of the resin composition is placed in the ring, and the resin composition is cured by irradiating it with <NUM> mJ/cm<NUM> of ultraviolet ray. The cure shrinkage rate is then determined from the heights of the resin surface before and after curing. Specifically, when the height of the resin surface before curing is T<NUM> and the height of the resin surface after curing is TD, the cure shrinkage rate is calculated by using the following formula.

In order to suppress the generation of voids in the optical fiber, the Young's modulus of the primary resin layer <NUM> is <NUM> MPa or more and <NUM> MPa or less at <NUM> ± <NUM>, and may be <NUM> MPa or more and <NUM> MPa or less, or <NUM> MPa or more and <NUM> MPa or less.

The primary resin layer <NUM> can be formed by curing a resin composition including a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator and a silane coupling agent. That is, the primary resin layer <NUM> can include a cured product of the resin composition containing a urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator, and a silane coupling agent. The content of urethane (meth)acrylate oligomer in the resin composition is preferably <NUM>% by mass or more and <NUM>% by mass or less, more preferably <NUM>% by mass or more and <NUM>% by mass or less, and more preferably <NUM>% by mass or more and <NUM>% by mass or less, based on the total amount of the resin composition.

As a urethane (meth)acrylate oligomer, an oligomer obtained by reacting a polyol compound, a polyisocyanate compound, a hydroxyl group-containing (meth)acrylate compound, and an alcohol may be used.

The Young's modulus of the primary resin layer <NUM> may be adjusted to a target range depending on the blending ratio of a hydroxyl group-containing (meth)acrylate compound and an alcohol used when synthesizing a urethane (meth)acrylate oligomer. The blending ratio of the alcohol is increased to decrease the oligomer having reactive (meth)acryloyl groups at both ends, easily decreasing the Young's modulus. In addition, the Young's modulus of the primary resin layer <NUM> may be adjusted to a target range depending on the molecular weight of the polyol compound used when synthesizing the urethane (meth)acrylate oligomer. The number average molecular weight of the polyol compound is preferably <NUM> to <NUM>, more preferably <NUM> to <NUM>, and further preferably <NUM> to <NUM>.

A urethane (meth)acrylate oligomer, a monomer, a photopolymerization initiator and a silane coupling agent may be appropriately selected from compounds exemplified in the above base resin. Note that the resin composition that forms the primary resin layer has a different composition from the base resin that forms the secondary resin layer.

Hereinafter, the results of evaluation test using Examples and Comparative Examples according to the present disclosure will be shown, and the present disclosure is described in more detail. The present invention is not limited to these examples.

As the oligomer, a urethane acrylate oligomer (UA) obtained by reacting a polypropylene glycol having a molecular weight of <NUM>, <NUM>,<NUM>-tolylene diisocyanate, and hydroxyethyl acrylate and an epoxy acrylate oligomer (EA) were prepared.

As the monomer, isobornyl acrylate (trade name "IBXA" of Osaka Organic Chemical Industry Co. ), tripropylene glycol diacrylate (trade name "TPGDA" of Daicel Ornex Co. ), and <NUM>-phenoxyethyl acrylate (trade name "Light Acrylate PO-A" of Kyoei Chemical Co. ) were prepared.

As the photopolymerization initiator, <NUM>-hydroxycyclohexyl phenyl ketone and <NUM>,<NUM>,<NUM>-trimethylbenzoyldiphenylphosphine oxide were prepared.

As the inorganic oxide particles, hydrophobic silica particles dispersed in MEK, having methacryloyl groups, and with an average primary particle size of <NUM> to <NUM> were prepared.

First, a base resin was prepared by mixing the above oligomer, monomer, and photopolymerization initiator. Next, the base resin and the silica particles were mixed, and then most of MEK as a dispersion medium was removed under reduced pressure to produce a resin composition for the secondary resin layer. The content of remaining MEK in the resin composition was <NUM>% by mass or less.

In Table <NUM>, Table <NUM> and Table <NUM>, the value of the monomer is the content based on the total amount of the base resin, the value of the oligomer is the content based on the total amount of the monomer, oligomer, and silica particles, and the value of silica particles is the content based on the total amount of the resin composition.

The following evaluation was performed by using the obtained resin composition for the secondary resin layer. The results are shown in Tables <NUM> to <NUM>.

A Teflon ring (inner diameter: <NUM>) was placed on a glass plate, <NUM> of the resin composition was placed in the ring, and the resin composition was cured by irradiating it with <NUM> mJ/cm<NUM> of ultraviolet ray. The cure shrinkage rate was determined from the heights of the resin surface before and after curing.

Urethane acrylate oligomers a1, a2, and a3 obtained by reacting polypropylene glycol with a molecular weight of <NUM>, isophorone diisocyanate, hydroxyethyl acrylate, and methanol were prepared. For the urethane acrylate oligomers a1, a2, and a3, the ratio of an oligomer having acryloyl groups at both ends and an oligomer having an acryloyl group at one end is adjusted by changing the blending ratio of hydroxyethyl acrylate and methanol.

<NUM> parts by mass of a urethane acrylate oligomer a1, a2, or a3, <NUM> parts by mass of a nonylphenoxy polyethylene glycol acrylate, <NUM> parts by mass of N-vinylcaprolactam, <NUM> parts by mass of <NUM>,<NUM>-hexanediol diacrylate, <NUM> part by mass of <NUM>,<NUM>,<NUM>-trimethylbenzoyldiphenylphosphine oxide, and <NUM> part by mass of <NUM>-mercaptopropyltrimethoxysilane were mixed to produce each of a resin composition for the primary resin layer.

On the outer periphery of a <NUM> diameter glass fiber composed of a core and cladding, a primary resin layer with a thickness of <NUM> was formed by using a resin composition for the primary resin layer, and a secondary resin layer was formed on the outer periphery thereof by using a resin composition for the secondary resin layer to produce optical fibers in Examples and Comparative Examples. A linear speed was <NUM>/min.

The Young's modulus of the primary resin layer was measured by the Pullout Modulus (POM) method at <NUM>. Two parts of an optical fiber were fixed with two chuck devices, a coating resin layer (the primary resin layer and the secondary resin layer) between the two chuck devices was removed, and then one chuck device was fixed and another chuck device was slowly moved in the opposite direction of the fixed device. When the length of the portion sandwiched between the chuck devices to be moved in the optical fiber is L, the amount of movement of the chuck is Z, the outer diameter of the primary resin layer is Dp, the outer diameter of the glass fiber is Df, the Poisson's ratio of the primary resin layer is n, and the load in moving the chuck device is W, the Young's modulus of the primary resin layer was determined from the following formula.

The Young's modulus of the secondary resin layer was determined from <NUM>% secant value by using a pipe-shaped coating resin layer (length: <NUM> or more) obtained by taking out a glass fiber from an optical fiber to perform a tensile test (distance between marked lines: <NUM>) in an environment of <NUM> ± <NUM> and <NUM> ± <NUM>% RH.

For measurement of the glass transition temperature of the secondary resin layer, the dynamic viscoelasticity of a coating resin layer was measured by using a pipe-shaped coating resin layer obtained by taking out a glass fiber from an optical fiber and by using "RSA <NUM>" from TA Instruments, Inc. in the condition of tensile mode (distance between marked lines: <NUM>), a frequency of <NUM>, a heating rate of <NUM>/min, and a temperature range of <NUM> to <NUM>. The peak top temperature of measured tan δ was defined as the glass transition temperature (Tg) of the secondary resin layer.

An optical fiber of <NUM> was stored at <NUM> and <NUM>% humidity for <NUM> days and then left at -<NUM> for <NUM> hours, and the presence or absence of voids with a diameter of <NUM> or more was observed with a microscope. The case where the number of voids per <NUM> of the optical fiber was less than <NUM> was evaluated as "A", the case where the number of voids was <NUM> to <NUM> was evaluated as "B", and the case where the number of voids exceeded <NUM> was evaluated as "C". The results are shown in Tables <NUM> to <NUM>.

Claim 1:
An optical fiber (<NUM>) comprising a glass fiber (<NUM>) comprising a core (<NUM>) and a cladding (<NUM>); a primary resin layer (<NUM>) being in contact with the glass fiber (<NUM>) and covering the glass fiber (<NUM>); and a secondary resin layer (<NUM>) covering the primary resin layer (<NUM>),
wherein a Young's modulus of the primary resin layer (<NUM>) is <NUM> MPa or more and <NUM> MPa or less at <NUM> ± <NUM>, and
wherein the secondary resin layer (<NUM>) consists of a cured product of a resin composition comprising a base resin containing a urethane (meth)acrylate oligomer, a monomer, and a photopolymerization initiator; and hydrophobic inorganic oxide particles, wherein the monomer is selected from a (meth)acrylate monomer, a (meth)acrylic acid monomer, a maleimide monomer, a (meth)acrylamide monomer, a (meth)acryl succinimide monomer, or a combination thereof,
and a content of the inorganic oxide particles is <NUM>% by mass or more and <NUM>% by mass or less based on a total amount of the resin composition, characterized in that the inorganic oxide particles are silicon dioxide.