Coated optical fiber

Abstract

In a coated optical fiber comprising a quartz glass fiber, a primary coating, and a secondary coating, the secondary coating is formed by curing a resin composition comprising (A) 20-90 wt % of polyurethane (meth)acrylate oligomer having a Mn of up to 20,000 and (B) 80-10 wt % of a mono-ethylenically unsaturated compound containing at least one N-vinyl compound, whose homopolymer has a Tg of at least 50° C., with electron beams accelerated at a low voltage of 50-125 kV. The coated optical fiber has a minimized transmission loss.

Description

[0001] This invention relates to a coated optical fiber having on a quartz glass fiber a primary coating and a secondary coating.

BACKGROUND OF THE INVENTION

[0002] Current optical communications fibers include a variety of types such as quartz glass, multi-component glass and plastic fibers. Among these, quartz glass fibers are vastly used in a wide variety of applications because of their light weight, heat resistance, noninductive nature, low loss and high transmission capacity.

[0003] Albeit the above advantages, quartz glass fibers for optical communications are very thin, brittle and prone to breakage by external factors, so that the transmission loss is increased under external stresses. Thus the quartz glass fibers are generally provided with a two-layer resin cover by previously applying a relatively soft primary or buffer coating material and then applying a secondary or top coating material around the primary coating layer.

[0004] Since the secondary coating layer serves to protect the soft primary coating layer from external forces and eventually improve the strength of optical fiber, the secondary coating material is required to have a high Young's modulus after curing, maintain the high Young's modulus at elevated temperature, and possess such properties as high elongation, high strength, low water pickup and hydrolytic resistance. The secondary coating material is also required to be fast curable and low viscous in order to comply with the increased drawing speed of optical fiber for increased productivity.

[0005] One exemplary secondary coating material is, as disclosed in JP-A 59-170155, a resin composition comprising a polymerizable monomer compound based on a urethane oligomer terminated with ethylenically unsaturated groups at both ends and a photopolymerization initiator. In the process of preparing a coated optical fiber using such a curable composition for secondary coating, a quartz glass fiber as spun is passed through a coating die where it is coated with a primary coating composition, after which the primary coating is cured upon exposure to radiation from an irradiation equipment having a UV lamp built therein. Similarly, a secondary coating resin as formulated above is applied to the primary coating and exposed to UV radiation from a suitable irradiation equipment for curing, completing the secondary coating.

[0006] The secondary coating materials are most often radiation, typically UV, curable compositions. These compositions and secondary coatings thereof formed by a curing system satisfy most of the desired properties of the cured coating, but encounter a certain limit in pursuit of the fast cure needed to comply with a high drawing speed.

[0007] More particularly, for improving the curing performance of the secondary coating composition through the curing system, it is essential to develop an effective photopolymerization initiator and a high intensity UV lamp, which is difficult. Among radiation curing techniques of the same category, the electron beam curing technique involving the polymerization of ethylenically unsaturated groups provides for fast curing because it does not need a photopolymerization initiator and permits the equipment to be designed over a wide dose range from low to high doses. However, electron beams accelerated at high voltage can penetrate through the coating material and reach the quartz glass core of the optical fiber (serving as an optical waveguide) to damage the core, resulting in an increased transmission loss.

SUMMARY OF THE INVENTION

[0008] An object of the invention is to provide a coated optical fiber coated with a secondary coating composition which is curable with electron beams to accomplish a fast cure rate and which in the cured state satisfies the above-mentioned requirements of secondary coating, so that the transmission loss of the fiber is minimized.

[0009] It has been found that when a conventional known UV-curable coating composition for optical fibers, specifically a resin composition comprising (A) a polyurethane (meth)acrylate oligomer and (B) an ethylenically unsaturated compound is applied to a primarily coated optical fiber and cured with electron beams accelerated at a low voltage of 50 to 125 kV, a coated optical fiber characterized by a reduced transmission loss is produced at a high productivity or at a speed corresponding to high speed drawing.

[0010] The present invention provides a coated optical fiber comprising a quartz glass fiber, a primary coating thereon, and a secondary coating on the primary coating. The secondary coating is formed from a resin composition comprising (A) 20 to 90% by weight of a polyurethane (meth)acrylate oligomer having a number average molecular weight of up to 20,000 and (B) 80 to 10% by weight of a mono-ethylenically unsaturated compound containing at least one N-vinyl compound, whose homopolymer has a glass transition temperature (Tg) of at least 50° C., by curing it with electron beams accelerated at a voltage of 50 to 125 kV.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0011] The coated optical fiber of the invention includes a primary layer of primary coating material enclosing a quartz glass fiber, and a secondary layer of secondary coating material enclosing the primary layer.

[0012] The coating composition used to form the primary layer may be any well-known primary coating composition, which may be of any well-known curing type such as ultraviolet (UV) or electron beam (EB) curing type.

[0013] According to the invention, the secondary layer is formed around the primary layer from a resin composition comprising (A) a polyurethane (meth)acrylate oligomer having a number average molecular weight (Mn) of up to 20,000 and (B) a mono-ethylenically unsaturated compound containing at least one N-vinyl compound, whose homopolymer has a Tg of at least 50° C. This resin composition is described below in detail. (A) Polyurethane (meth)acrylate oligomer having a Mn of up to 20,000

[0014] The polyurethane (meth)acrylate oligomer serving as component (A) of the inventive resin composition can be prepared by urethane-forming reaction using (a) a polyol, (b) a polyisocyanate and (c) a (meth)acrylate compound having a hydroxyl group. The polyurethane (meth)acrylate oligomer should have a number average molecular weight (Mn) of up to 20,000, and preferably up to 10,000.

[0015] (a) Polyol

[0016] Suitable polyol components include polyether polyols, polyester polyols, polycarbonate polyols and alkyl diols.

[0017] Polyether polyol

[0018] Suitable polyether polyols include homopolymers or copolymers of alkylene oxides, typically C2-C5 alkylene oxides such as ethylene oxide, propylene oxide, butylene oxide, tetrahydrofuran and 3-methyltetrahydrofuran; homopolymers or copolymers of the foregoing alkylene oxides using aliphatic C12-C40 polyols such as 1,2-hydroxystearyl alcohol and hydrogenated dimer diols as the initiator; adducts of bisphenol A with alkylene oxides such as propylene oxide, butylene oxide and tetrahydrofuran; and adducts of hydrogenated bisphenol A with alkylene oxides such as propylene oxide, butylene oxide and tetrahydrofuran. These polyether polyols may be used alone or in admixture of any.

[0019] The preferred polyether polyols are homopolymers or copolymers of C2-C4 alkylene oxides, especially C3-C4 alkylene oxides such as propylene oxide and tetrahydrofuran, for example, polyoxypropylene glycol, polytetramethylene ether glycol and propylene oxide-tetrahydrofuran copolymers. In order to reduce the viscosity of resin or suppress the evolution of hydrogen gas upon curing so as to comply with a high speed of tape manufacture, it is preferred to combine the foregoing with polyether polyols having an oxypropylene structure or polypropylene glycol. The polyether polyols may have a Mn selected, for example, in the range of about 200 to about 10,000.

[0020] The polyether polyols are commercially available. For example, (1) polyethylene glycol is available under the trade name of PEG600, PEG1000 and PEG2000 from Sanyo Chemical Industries, Ltd.; (2) polyoxypropylene glycol is available under the trade name of Takelac P-21, Takelac P-22 and Takelac P-23 from Takeda Chemical Industries, Ltd.; (3) polytetramethylene ether glycol is available under the trade name of PTG650, PTG850, PTG1000, PTG2000, and PTG4000 from Hodogaya Chemical Co., Ltd.; (4) propylene oxide-ethylene oxide copolymers are available under the trade name of ED-28 from Mitsui Toatsu Chemicals, Inc. and Excenol 510 from Asahi Glass Co., Ltd.; (5) tetrahydrofuran-propylene oxide copolymers are available under the trade name of PPTG1000, PPTG2000 and PPTG4000 from Hodogaya Chemical Co., Ltd.; (6) tetrahydrofuran-ethylene oxide copolymers are available under the trade name of Unisafe DC-1100 and Unisafe DC-1800 from NOF Corp.; (7) adducts of bisphenol A with ethylene oxide are available under the trade name of Uniol DA-400 and Uniol DA-700 from NOF Corp.; and (8) adducts of bisphenol A with propylene oxide are available under the trade name of Uniol DB-400 from NOF Corp.

[0021] Polyester Polyol

[0022] Suitable polyester polyols include adducts of diols compounds (e.g., ethylene glycol, propylene glycol, diethylene glycol, dipropylene glycol, 1,5-pentaglycol, 3-methyl-1,5-pentanediol, 1,6-hexanediol and neopentyl glycol) with &egr;-caprolactam or &bgr;-methyl-&dgr;-valerolactone; reaction products of the foregoing diol compounds with dibasic acids such as succinic acid, adipic acid, phthalic acid, hexahydrophthalic acid and tetrahydrophthalic acid; and reaction products of three components, the foregoing diol compounds, the foregoing dibasic acids and &egr;-caprolactam or &bgr;-methyl-&dgr;-valerolactone.

[0023] Polycarbonate Polyol

[0024] Suitable polycarbonate polyols include reaction products of diol compounds such as 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, 1,4-butanediol, 1,5-octanediol, 1,4-bis(hydroxymethyl)cyclohexane, 2-methylpropanediol, dipropylene glycol, dibutyrene glycol, and bisphenol A or adducts of these diol compounds with 2 to 6 mol of ethylene oxide, with short chain dialkyl carbonates such as dimethyl carbonate and diethyl carbonate.

[0025] Also useful are polyester diols which are addition reaction products of these polycarbonate polyols with ethylene oxide, propylene oxide, &egr;-caprolactam or ⊕-methyl-&dgr;-valerolactone.

[0026] The polycarbonate polyols are commercially available, for example, in the trade name of Desmophen 2020E from Sumitomo Bayer Co., Ltd. and DN-980, DN-982 and DN-983 from Nippon Polyurethane Co., Ltd.

[0027] Alkyl Diol

[0028] Suitable alkyl diols include 1,6-hexanediol, 3-methyl-1,5-pentanediol, neopentyl glycol, 1,4-butanediol, 1,5-octanediol, 1,4-dihydroxycyclohexane, 1,4-bis(hydroxymethyl)cyclohexane, 2-methylpropanediol, tricyclodecane dimethanol, 1,4-bis(hydroxymethyl)benzene and bisphenol A.

[0029] Of these polyols, polyether polyols and alkyl diols are preferred because a resin composition having durability and a good balance of physical properties is obtainable.

[0030] (b) Polyisocyanate

[0031] Suitable polyisocyanate components used herein include tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, hydrogenated 4,4′-diphenylmethane diisocyanate, xylylene diisocyanate, hydrogenated xylylene diisocyanate, hexamethylene diisocyanate, isophorone diisocyanate, 1,5-naphthalene diisocyanate, tolidine diisocyanate, p-phenylene diisocyanate, transcyclohexane-1,4-diisocyanate, lysine diisocyanate, tetramethylxylene diisocyanate, lysine ester triisocyanate, 1,6,11-undecane triisocyanate, 1,8-diisocyanato-4-isocyanatomethyloctane, 1,3,6-hexamethylene triisocyanate, bicycloheptane triisocyanate, and trimethylhexamethylene diisocyanate. Of these, those polyisocyanates having a cyclic structure are preferred because cured parts having a high Young's modulus are obtained.

[0032] (c) Hydroxyl-Bearing (meth)Acrylate

[0033] Suitable (meth)acrylates having hydroxyl groups used herein include hydroxyalkyl (meth)acrylates, for example, hydroxy(C2-C10)alkyl (meth)acrylates such as 2-hydroxyethyl (meth)acrylate, 2-hydroxypropyl (meth)acrylate, 3-hydroxypropyl (meth)acrylate, 2-hydroxybutyl (meth)acrylate, 4-hydroxybutyl (meth)acrylate, pentanediol mono(meth)acrylate, hexanediol mono(meth)acrylate, and neopentyl glycol mono(meth)acrylate; 2-hydroxy-3-phenyloxypropyl (meth)acrylate, 2-hydroxyalkyl (meth)acryloyl phosphate, 4-hydroxycyclohexyl (meth)acrylate, cyclohexane-1,4-dimethanol mono(meth)acrylate, trimethylol propane di(meth)acrylate, and pentaerythritol tri(meth)acrylate. Also included are products of addition reaction of glycidyl or epoxy group-bearing compounds such as alkyl glycidyl ethers, allyl glycidyl ethers and glycidyl (meth)acrylates with (meth)acrylic acid. These hydroxyl-bearing (meth)acrylates may be used alone or in admixture of any. Preferred hydroxyl-bearing (meth)acrylates are hydroxy(C2-C4)alkyl (meth)acrylates, especially 2-hydroxyethyl (meth)acrylate and 2-hydroxypropyl (meth)acrylate.

[0034] It is noted that polyurethane (meth)acrylate oligomers can be prepared by reacting the aforementioned components. The proportion of the respective components constituting the polyurethane (meth)acrylate oligomer is often such that hydroxyl groups in the polyol component constitute about 0.1 to 0.8 mol, preferably about 0.2 to 0.7 mol, and especially about 0.2 to 0.5 mol, and the hydroxyl-bearing (meth)acrylate constitutes about 0.2 to 0.9 mol, preferably 0.3 to 0.8 mol, and especially about 0.5 to 0.8 mol, per mol of isocyanate groups in the polyisocyanate.

[0035] It is not critical how to react the aforementioned components to form a polyurethane (meth)acrylate oligomer. In one embodiment, all the components are mixed together and reacted. In another embodiment, the polyisocyanate is reacted with either one of the polyol component and hydroxyl-bearing (meth)acrylate, after which the reaction product is reacted with the remaining component.

[0036] The urethane-forming reaction may be effected in the presence of a catalyst which is selected from organometallic urethane-forming catalysts such as stanous octoate, dibutyltin diacetate, dibutyltin dilaurate, cobalt naphthenate, and lead naphthenate, and amine catalysts such as triethylamine, triethylene diamine and diazabicycloundecene as well as other well-known urethane-forming catalysts. (B) Mono-ethylenically unsaturated compound whose homopolymer has a Tg of at least 50° C.

[0037] The mono-ethylenically unsaturated compounds whose homopolymer has a Tg of at least 50° C., serving as component (B) in the inventive resin composition, include N-vinylacetamide compounds and compounds of the structure wherein (meth)acrylic acid is bonded with compounds having amino or hydroxyl groups through amidation or esterification reaction.

[0038] Examples of the N-vinylacetamide compounds include N-vinylpyrrolidone, N-vinylcaprolactam, N-vinylacetamide and N-vinylformamide. Examples of the compounds of the structure wherein (meth)acrylic acid is bonded with compounds having amino or hydroxyl groups through amidation or esterification reaction include cyclohexyl (meth)acrylate, isobornyl (meth)acrylate, benzyl (meth)acrylate, 4-butylcyclohexyl (meth)acrylate, dicyclopentadiene (meth)acrylate, 2-hydroxyethyl (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxy (meth)acrylate, tricyclodecanyl (meth)acrylate, tricyclodecanyloxy (meth)acrylate, morpholine (meth)acrylate, vinylpyridine, and vinylimidazole.

[0039] Of these compounds, any one of N-vinylpyrrolidone, N-vinylcaprolactam, N-vinylacetamide and N-vinylformamide is preferably contained as the N-vinyl compound when EB curing and the properties (e.g., elongation, Young's modulus and temperature dependence of Young's modulus) of cured film are taken into account.

[0040] The proportion of the polyurethane (meth)acrylate oligomer (A) and the mono-ethylenically unsaturated compound (B) blended is selected depending on the type of the respective components (A) and (B), the desired viscosity of resin composition, and physical properties of cured products. Often the polyurethane (meth)acrylate oligomer (A) constitutes 20 to 90% by weight, preferably 40 to 80% by weight, and the mono-ethylenically unsaturated compound (B) constitutes 80 to 10% by weight, preferably 60 to 20% by weight, based on the total weight of components (A) and (B). Preferably the N-vinyl compound is further contained as component (B) in an amount of 3 to 20% by weight, more preferably 5 to 15% by weight.

[0041] There may also be blended a compound having a plurality of ethylenically unsaturated groups in a molecule. Of these compounds, exemplary difunctional compounds include di(meth)acrylate of 2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate, ethylene gyclol di(meth)acrylate, tetraethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, 1,4-butanediol di(meth)acrylate, 1,6-hexanediol di(meth)acrylate, glycol di(meth)acrylate, neopentyl glycerin di(meth)acrylate, hydroxypivalic acid neopentylglycol di(meth)acrylate, di(meth)acrylate of bisphenol A-ethylene oxide adduct, di(meth)acrylate of bisphenol A-propylene oxide adduct, di(meth)acrylate of 2,2′-di(hydroxyethoxyphenyl)propane, di(meth)acrylate of tricyclodecane dimethylol, dicyclopentadiene di(meth)-acrylate, pentane di(meth)acrylate, and di(meth)acrylic acid adduct of 2,27-di(glycidyloxyphenyl)propane.

[0042] Exemplary polyfunctional compounds include trimethylolpropane tri(meth)acrylate, trimethylolpropane trioxyethyl(meth)acrylate, pentaerythritol tri(meth)-acrylate, pentaerythritol tetra(meth)acrylate, dipenta-erythritol hexa(meth)acrylate, tetramethylolmethane tri(meth)acrylate, tetramethylolmethane tetra(meth)acrylate, tris(acryloxy)isocyanurate, tris(2-hydroxy)isocyanurate, tri(meth)acrylate of tris(hydroxypropyl)isocyanurate, tri(meth)acrylate of isocyanurate, triallyl trimellitic acid, and triallyl isocyanurate.

[0043] The amount of such a compound having ethylenically unsaturated groups blended is not critical as long as the secondary coating composition satisfies the desired properties.

[0044] If desired, a polymerization initiator may be added. Any well-known polymerization initiator is useful. Exemplary initiators include 1-hydroxy-cyclohexyl-phenyl-ketone, 2,2-dimethoxy-2-phenylacetophenone, phenyl-acetophenone diethyl ketal, alkoxyacetophenones, benzyl methyl ketal, benzophenone and benzophenone derivatives such as 3,3-dimethyl-4-methoxybenzophenone, 4,4-dimethoxybenzophenone and 4,4-diaminobenzophenone, alkyl benzoylbenzoates, bis(4-dialkylaminophenyl) ketones, benzyl and benzyl derivatives such as benzyl methyl ketal, benzoyl and benzoin derivatives such as benzoin butyl methyl ketal, benzoin isopropyl ether, 2-hydroxy-2-methylpropiophenone, thioxanthone derivatives such as 2,4-diethylthioxanthone and 2,4-dichlorothioxanthone, fluorene, 2-methyl-1-[4-(methylthio)phenyl]-2-morpholinopropane-1,2-benzyl-2-dimethylamino-1-(morpholinophenyl)-butanone-1, phosphine oxide derivatives such as 2,4,6-trimethylbenzoyl diphenyl phosphine oxide and bis(2,6-dimethoxybenzoyl)-2,4,4-trimethylpentyl phosphine oxide, organic peroxides such as benzoyl peroxide, t-butylperoxide and cumene hydroperoxide, and organic azo compounds such as azobiscyanovaleric acid, azobisbutyronitrile, azobis(2,4-dimethyl)valeronitrile and azobis(2-aminopropane) hydrochloride.

[0045] The initiators may be used alone or in admixture of any. The amount of the initiator blended is not critical as long as the secondary coating composition satisfies the desired properties.

[0046] In addition to the above-mentioned components, the resin composition may further contain suitable additives, for example, antioxidants, stabilizers (e.g., UV absorbers), organic solvents, plasticizers, silane coupling agents, coloring pigments, organic and inorganic particles as long as they does not compromise the objects of the invention.

[0047] The resin composition of the invention should desirably have a viscosity of about 1,000 to 10,000 mPa.s at 25° C. from the coating standpoint and more desirably in the range of about 1,000 to 4,000 mPa.s, especially under high-speed production conditions. As are conventional UV-curable compositions, the resin composition of the invention is cured upon exposure to electron beams. The thus cured film should preferably have a Young's modulus of 300 to 900 MPa, which range is desired for the film as the outer cover to protect the underlying optical fiber from external forces.

[0048] The coated optical fiber of the invention is manufactured by applying the above-mentioned resin composition to the primary coated optical fiber and curing it with electron beams accelerated at a low voltage of 50 to 125 kV so that the cured resin composition encloses the primary coating. The coated quartz optical fiber for communications which is now manufactured in a mass scale generally has the structure that a glass fiber in which a quartz core having a diameter of about 10 &mgr;m is covered with a quartz clad to a diameter of about 125 &mgr;m is enclosed with a primary coating to a diameter of about 200 &mgr;m and further enclosed with a secondary coating to a diameter of about 250 &mgr;m. In this fiber structure, if the accelerating voltage of electron beams for curing the secondary coating resin composition exceeds 125 kV, the electron beams can reach and damage the core, resulting in an increased transmission loss. If the accelerating voltage of electron beams is below 50 kV, the electron beams fail to reach the bottom of the secondary coating, leaving a lower portion of the secondary coating uncured. For this reason, the accelerating voltage of electron beams is set to 50 to 125 kV and preferably 60 to 100 kV. The dose of electron beams the secondary coating resin composition absorbs is usually in the range of about 10 to 100 kGy although it varies with the type of the resin composition.

[0049] In this way, a coated optical fiber with a minimized transmission loss is obtained.

EXAMPLE

[0050] Examples of the invention are given below by way of illustration and not by way of limitation. Mn is number average molecular weight and Tg is glass transition temperature.

[0051] Synthesis of Polyurethane Acrylate Oligomer (A)

Synthesis Example 1

[0052] In a nitrogen atmosphere, a liquid mixture of 319.6 g of 2,4-toluene diisocyanate and 502.5 g of polytetramethylene ether glycol having an average molecular weight of 1,000 was subjected to reaction at a temperature of 70 to 80° C. for 3 hours. The reaction mixture was then cooled to 30° C., and the reactor interior was purged with dry air. With 0.4 g of a polymerization inhibitor, 2,6-di-tert-butylhydroxytoluene added, 310.1 g of 2-hydroxyethyl acrylate was added dropwise so that the reaction mixture was kept below 50° C. Next, the temperature was gradually raised, and reaction was effected for 6 hours at a temperature of 60 to 70° C. until the absorption peak attributable to isocyanate (NCO) group became undetectable by IR spectroscopy. In this way, a polyurethane acrylate oligomer I was obtained.

Synthesis Example 2

[0053] In a nitrogen atmosphere, a liquid mixture of 14.04 g of hexamethylene diisocyanate, 385.5 g of polyoxypropylene ether glycol having an average molecular weight of about 3,000 (OH number=34 mg KOH/g), 149.5 g of polytetramethylene ether glycol (OH number=37.5 mg KOH/g) and 0.5 g of a reaction catalyst, dibutyltin dilaurate was subjected to reaction at a temperature of 70 to 80° C. for 4 hours. The disappearance of the absorption peak attributable to isocyanate (NCO) group was confirmed by IR spectroscopy. The reaction mixture was then cooled to 30° C., after which 369.3 g of 2,4-toluene diisocyanate was added to the mixture, which was reacted for 2 hours at a temperature of 60 to 70° C. The reaction mixture was then cooled to 30° C., and the reactor interior was purged with dry air. With 0.44 g of a polymerization inhibitor, 2,6-di-tert-butylhydroxytoluene added, 472.4 g of 2-hydroxyethyl acrylate was added dropwise so that the reaction mixture was kept below 50° C. Next, 0.9 g of a reaction catalyst, dibutyltin dilaurate was added, the temperature was gradually raised, and reaction was effected for 6 hours at a temperature of 60 to 70° C. until the absorption peak attributable to isocyanate (NCO) group became undetectable by IR spectroscopy. In this way, a polyurethane acrylate oligomer II was obtained.

Examples 1-4 and Comparative Examples 1-6

[0054] As shown in Table 1, electron beam-curable resin compositions of Examples 1-4 and Comparative Examples 1-6 were prepared by mixing the above-synthesized polyurethane acrylate oligomer I or II with a compound having an ethylenically unsaturated group. Then the physical properties of these compositions were measured by the following methods. Separately, a quartz fiber was coated with a primary coating composition and then with the composition of Example 3, which was cured by means of an electron beam emitting equipment. The thus coated optical fiber was measured for transmission loss. The results are shown in Table 1.

[0055] Evaluation Methods:

[0056] (1) Preparation of Cured Film

[0057] The electron beam-curable resin composition was applied onto a glass plate to a thickness of 50 to 60 &mgr;m. The coating was cured by accelerating electron beams at a voltage of 30 to 100 kV and directing the electron beams to the coating in nitrogen so as to give a dose of 100 kGy. A cured film was obtained.

[0058] (2) Measurement of Young's Modulus

[0059] The cured film was conditioned for 24 hours at 25° C. and relative humidity 50% before 2.5% tensile modulus was measured under conditions: a gage mark distance of 25 mm and a pulling rate of 1 mm/min.

[0060] (3) Measurement of Tensile Strength and Elongation at Break

[0061] The cured film was conditioned for 24 hours at 25° C. and relative humidity 50% before tensile strength and elongation at break were measured under conditions: a gage mark distance of 25 mm and a pulling rate of 50 mm/min.

[0062] (4) Cure

[0063] The electron beam-curable resin composition was applied onto a glass plate to a thickness of 50 to 60 &mgr;m. The coatings were cured by accelerating electron beams at a voltage of 100 kV and directing the electron beams to the coating in nitrogen so as to give a dose of 20, 30 and 100 kGy. The cured films were measured for Young's modulus.

[0064] (5) Measurement of Young's Modulus Ratio and Tg

[0065] The cured film was conditioned for 24 hours at 25° C. and relative humidity 50% before Young's modulus was measured at −40° C. and 25° C. using an instrument of determining a viscoelastic behavior, Rheometrics Solids Analyzer RSAII (Rheometrics Scientific F. E.), from which Young's modulus ratio was calculated. Changes of tans with temperature were measured, and the temperature giving the maximum change is Tg.

[0066] (6) Preparation of Coated Optical Fiber and Measurement of Transmission Loss

[0067] A quartz glass fiber as spun was coated with a primary coating composition (as formulated below) so as to give a diameter of 200 &mgr;m, which was cured in a UV irradiation equipment. Then the fiber was coated with the resin composition of Example 3 (see Table 1), which was cured by means of an electron beam emitting equipment such that electron beams accelerated at the indicated voltage were simultaneously irradiated to the coating from three directions in a dose of 100 kGy. In this way, a coated optical fiber of 3,000 m long was manufactured. Using OTDR, the coated optical fiber was measured for transmission loss with light having a wavelength of 1,550 nm.

[0068] Primary Coating Composition

[0069] A reactor was charged with 407.3 g of polypropylene glycol having a Mn of about 4,000 (Sanix PP400, OH number=27.6, by Sanyo Chemical Industries, Ltd.) and 52.2 g of 2,4-tolylene diisocyanate, which were reacted in a nitrogen stream for 2 hours at 70 to 80° C. Thereafter, the reaction temperature was lowered to 50 to 60° C., and 0.6 g of dibutyltin dilaurate, 0.15 g of 2,6-di-tert-butylhydroxytoluene and 23.2 g of 2-hydroxyacrylate were added. Reaction was effected for 5 hours, yielding a polyether polyurethane acrylate oligomer having a Mn of about 8,900. The primary coating composition was obtained by mixing 60 parts by weight of this oligomer, 20 parts by weight of Aronix M-113 (Toagosei Co., Ltd.) 10 parts by weight of lauryl acrylate, 10 parts by weight of N-vinylcaprolactam and 3 parts by weight of Irgacure 1700 (Ciba Specialty Chemicals). 1 TABLE 1 Example Comparative Example Component (pbw) 1 2 3 4 1 2 3 4 5 6 Polyurethane acrylate oligomer I 70 70 70 70 70 70 Polyurethane acrylate oligomer II 65 65 65 65 Monoacrylate compound N-vinylpyrrolidone (Tg* = 175° C.) 10 10 10 10 N-vinylcaprolactam (Tg* = 165° C.) 10 10 N-vinylformamide (Tg* = 126° C.) 10 10 Isobornyl acrylate (Tg* = 90-100° C.) 20 20 25 30 20 20 25 Dicyclopentenyl acrylate (Tg* = 95° C.) 25 25 Dicyclopentenyloxyethyl acrylate 35 (Tg* = 12° C.) Young's modulus (MPa) @25° C. EB accelerating voltage 30 kV 606 367 630  635  641  617  EB accelerating voltage 50 kV 792 801 819 799 762 522 EB accelerating voltage 100 kV 803 810 830 812 788 543 Elongation at break (%) @25° C. 65 66 65 64 66 67 Tensile strength (MPa) @25° C. 41 45 3.9 3.8 4.1 3.1 Cure in terms of Young's modulus (MPa) @25° C. Dose 20 kGy 494 656 570 552 251 184 Dose 30 kGy 692 722 668 651 552 370 Dose 100 kGy 803 810 830 812 788 543 Young's modulus ratio (60° C./25° C.) 2.7 1.7 1.8 2.0 3.4 12 Tg (tan&dgr; peak temperature, ° C.) 86 110 118 115 81 66 Optical fiber transmission loss (dB/m) @1550 nm wavelength EB accelerating voltage 50 kV 0.19 EB accelerating voltage 100 kV 0.18 EB accelerating voltage 125 kV 0.19 EB accelerating voltage 150 kV 2.21 *glass transition temperature of homopolymer

[0070] Japanese Patent Application No. 2000-328129 is incorporated herein by reference.

[0071] Although some preferred embodiments have been described, many modifications and variations may be made thereto in light of the above teachings. It is therefore to be understood that the invention may be practiced otherwise than as specifically described without departing from the scope of the appended claims.

Claims

1. A coated optical fiber comprising a quartz glass fiber, a primary coating thereon, and a secondary coating on the primary coating, wherein

said secondary coating is formed by curing a resin composition comprising (A) 20 to 90% by weight of a polyurethane (meth)acrylate oligomer having a number average molecular weight of up to 20,000 and (B) 80 to 10% by weight of a mono-ethylenically unsaturated compound containing at least one N-vinyl compound, whose homopolymer has a glass transition temperature of at least 50° C., with electron beams accelerated at 50 to 125 kV.

2. The coated optical fiber of claim 1 wherein said N-vinyl compound is N-vinylpyrrolidone, N-vinylcaprolactam or N-vinylformamide.