A method of manufacturing an optical fiber includes applying a curable resin composition containing a photopolymerization initiator so that an outer circumference of a glass fiber including a core and a cladding is coated, and forming a coating layer by radiating an ultraviolet ray to the curable resin composition and curing the curable resin composition. Forming the coating layer includes providing two or more non-radiation periods and intermittently radiating the ultraviolet ray three or more times so that a total time of the non-radiation periods is 0.010 seconds or more.

To provide a glass composition for glass fiber having a low dielectric loss tangent, suppressing the occurrence of phase separation, having a reduced viscosity at high temperatures, and reducing the occurrence of striae. The glass composition for glass fiber includes 52.0 to 57.5% by mass of SiO.sub.2, 19.5 to 25.5% by mass of B.sub.2O.sub.3, 8.0 to 13.0% by mass of Al.sub.2O.sub.3, 0 to 2.0% by mass of MgO, 0 to 6.0% by mass of CaO, 0.5 to 6.5% by mass of SrO, and 0.1 to 3.0% by mass of TiO.sub.2, the ratio of Al.sub.2O.sub.3 to B.sub.2O.sub.3 is 0.35 to 0.54, and the content SI of SiO.sub.2, the content B of B.sub.2O.sub.3, the content M of MgO, the content C of CaO, the content SR of SrO, and the content T of TiO.sub.2 satisfy the following formula (1): 6.90≤100×(B/SI).sup.2×{SR/(C+SR)}.sup.2/3×{T/(M+T)}.sup.1/2≤12.30 (1).

The method for producing a resin composition for a secondary coating of an optical fiber comprises a step of reacting a polyol, a diisocyanate and a hydroxyl group-containing (meth)acrylate in the presence of a (meth)acrylate not having a hydroxyl group to obtain a mixture of urethane (meth)acrylate and the (meth)acrylate not having a hydroxyl group, and a step of adding a photopolymerization initiator to the mixture to obtain a resin composition, and a hydroxyl value of the (meth)acrylate not having a hydroxyl group is 12.0 mgKOH/g or less.

The method for producing a resin composition for a secondary coating of an optical fiber comprises a step of reacting a polyol, a diisocyanate and a hydroxyl group-containing (meth)acrylate in the presence of a (meth)acrylate not having a hydroxyl group to obtain a mixture of urethane (meth)acrylate and the (meth)acrylate not having a hydroxyl group, and a step of adding a photopolymerization initiator to the mixture to obtain a resin composition, and a hydroxyl value of the (meth)acrylate not having a hydroxyl group is 12.0 mgKOH/g or less.

[Problem] To provide a radiation curable resin composition that is suitable as a primary material of an optical fiber and has a fast curing rate by irradiation with radiation.

[Solution] A radiation polymerizable composition for forming a primary coating layer of an optical fiber, the radiation polymerizable composition comprising: (A) a urethane oligomer comprising a structure represented by formula (I) below:

##STR00001## wherein R is a vinyl group and * is a bond; (B) one or more compounds of: (i) maleic anhydride, (ii) a compound represented by formula (II):

##STR00002## wherein R.sup.1 is a single bond or an alkanediyl group comprising 1 to 6 carbon atoms, and R.sup.2 is a hydrogen atom, a hydroxy group, or a group represented by predetermined formula (II-1) or formula (II-2), or (iii) a compound represented by formula (III):

##STR00003## wherein R.sup.5 is an alkanediyl group comprising 1 to 6 carbon atoms; and (C) a radiation polymerization initiator.

[Problem] To provide a radiation curable resin composition that is suitable as a primary material of an optical fiber and has a fast curing rate by irradiation with radiation.

[Solution] A radiation polymerizable composition for forming a primary coating layer of an optical fiber, the radiation polymerizable composition comprising: (A) a urethane oligomer comprising a structure represented by formula (I) below:

##STR00001## wherein R is a vinyl group and * is a bond; (B) one or more compounds of: (i) maleic anhydride, (ii) a compound represented by formula (II):

##STR00002## wherein R.sup.1 is a single bond or an alkanediyl group comprising 1 to 6 carbon atoms, and R.sup.2 is a hydrogen atom, a hydroxy group, or a group represented by predetermined formula (II-1) or formula (II-2), or (iii) a compound represented by formula (III):

##STR00003## wherein R.sup.5 is an alkanediyl group comprising 1 to 6 carbon atoms; and (C) a radiation polymerization initiator.

A glass filler of the present invention includes: a glass substrate; and a coating film including a surface treatment agent, the coating film covering at least a portion of a surface of the glass substrate. The glass substrate is a glass fiber having a flat cross-sectional shape. For the glass substrate, when a length corresponding to a minor axis of the flat cross-section of the glass fiber is represented by L1, a length corresponding to a major axis of the flat cross-section of the glass fiber is represented by L2, and a length corresponding to a fiber length of the glass fiber is represented by L3, a ratio of an average of L2 to an average of L1, (average of L2)/(average of L1), is 1.5 or more and 8 or less, the average of L1 is 1.0 μm or more and 10 μm or less, an average of L3 is 3 μm or more and less than 80 μm, and an average of L3 is larger than the average of L2. The surface treatment agent includes a silane coupling agent. A proportion of the coating film in the glass filler is 0.05 mass % or more and 2.5 mass % or less.

A resin composition comprises a base resin containing an urethane (meth)acrylate oligomer, a monomer having a phenoxy group, and a photopolymerization initiator, and hydrophobic inorganic oxide particles, wherein the viscosity is 300 mPa.Math.s or more and 4200 mPa.Math.s or less at 45° C. and the content of the monomer having a phenoxy group is 1% by mass or more and 30% by mass or less based on the total amount of the base resin.

A resin composition comprises a base resin containing an urethane (meth)acrylate oligomer, a monomer having a phenoxy group, and a photopolymerization initiator, and hydrophobic inorganic oxide particles, wherein the viscosity is 300 mPa.Math.s or more and 4200 mPa.Math.s or less at 45° C. and the content of the monomer having a phenoxy group is 1% by mass or more and 30% by mass or less based on the total amount of the base resin.

A reinforcing material is disclosed that includes coated glass flakes and coated glass strands. When the total amount of a glycidyl group-including resin and aminosilane contained in the coatings of the coated glass flakes corresponds to 100% by mass, the amount of the resin is 30% to 95% by mass. When the total amount of a glycidyl group-including resin, aminosilane, and a urethane resin contained in the coatings of the coated glass strands corresponds to 100% by mass, the amount of the glycidyl group-including resin is 10% to 90% by mass, the amount of the aminosilane is 0.1% to 40% by mass, and the amount of the urethane resin is 1% to 50% by mass. Both the coated glass flakes and the coated glass strands have an ignition loss of 0.1% to 2.0% by mass measured pursuant to JIS R3420 (2013).