Radiation-curable liquid resin composition and optical fiber

A radiation-curable liquid resin composition contains (A) an organopolysiloxane having a (meth)acryl group at either end of its molecular chain, containing at least 15 mol % of aromatic hydrocarbon groups based on the entire organic groups, and being free of a urethane bond, (B) a compound having at least one ethylenically unsaturated group in a molecule, and (C) a photopolymerization catalyst. The composition has a low viscosity and cures into a product having a low Young's modulus and experiencing a less change of Young's modulus at low temperatures. The composition is useful as a primary coating for optical fibers.

This invention relates to a radiation-curable liquid resin composition
 which has a low viscosity and cures into a product having a low Young's
 modulus and suitable as a primary coating or buffer layer on optical
 fibers. It also relates to an optical fiber covered with a cured product
 of the composition.
 BACKGROUND OF THE INVENTION
 Optical fibers for data communication include a variety of fibers such as
 quartz glass, multi-component glass and plastic fibers. In practice,
 because of their light weight, low loss, durability, and high transmission
 capacity, quartz glass optical fibers are vastly used in a wide range of
 application. Since the quartz glass optical fibers, however, are very thin
 and susceptible to changes by external factors, quartz glass fibers as
 melt spun are generally provided with a primary coating and then with a
 secondary coating for protecting the primary coating. The primary coating
 is formed by applying a liquid curable resin of the type giving a soft
 cured product, followed by curing. The secondary coating is formed by
 applying a liquid curable resin of the type giving a hard cured product,
 followed by curing.
 Properties required for the primary coating material include a low Young's
 modulus and low temperature dependency thereof for preventing microbending
 losses by external stresses or temperature changes, durability in terms of
 heat resistance and water resistance, low water absorption, low hydrogen
 generation, a high refractive index, and a fast-curing ability and low
 viscosity for allowing the drawing speed of optical fibers to be increased
 for improved productivity. To meet these requirements, UV-curable
 compositions based on urethane acrylate were proposed in the past. For
 example, JP-B 1-19694 and Japanese Patent Nos. 2,522,663 and 2,547,021
 disclose liquid UV-curable compositions comprising a urethane acrylate
 oligomer, a reactive monomer, and a polymerization initiator. These
 compositions, however, fail to meet some of the above requirements, that
 is, a low Young's modulus and good low-temperature properties (minimized
 temperature dependency of Young's modulus), low water absorption, and low
 viscosity, because they are based on urethane acrylate oligomers of
 urethane bond-bearing polyethers or polyesters.
 For reducing Young's modulus and improving low-temperature properties, JP-B
 4-29619 corresponding to U.S. Pat. No. 4,496,210 and JP-A 61-21121
 disclose liquid UV-curable compositions using a silicone urethane acrylate
 containing an organic polysiloxane. Urethane bonds are contained likewise.
 Because of the structural factors of urethane bonds (specifically,
 rigidity of the structure and the hydrogen bond in the urethane bond),
 these compositions are not satisfactory to some of the above requirements,
 that is, a low Young's modulus and a low viscosity. The embodiments
 described in these patents suggest that the organic polysiloxane is
 limited to a dimethylsiloxane skeleton, which has a low refractive index
 and is less compatible with reactive monomers. It is then difficult to
 design a liquid UV-curable composition capable of satisfying the required
 values.
 SUMMARY OF THE INVENTION
 An object of the invention is to provide a low-viscosity radiation-curable
 liquid resin composition which cures into a product having a low Young's
 modulus and experiencing a less change of Young's modulus at low
 temperature. Another object of the invention is to provide an optical
 fiber coated with a cured product of this composition.
 The invention addresses a radiation-curable liquid resin composition
 comprising a (meth)acryl group-bearing oligomer or polymer as a main
 component. The inventor has found that an organopolysiloxane having a
 (meth)acryl group at each end of its molecular chain, containing at least
 15 mol % of aromatic hydrocarbon groups based on the entire organic groups
 attached to silicon atoms, and being free of a urethane bond within the
 molecule is fully compatible with a reactive monomer and when it is used
 as the main component, the resulting radiation-curable liquid resin
 composition has a low viscosity and cures into a product having a low
 Young's modulus and a high refractive index. Especially when a monomer
 based on an organosiloxane skeleton and having an ethylenically
 unsaturated group is used as the reactive monomer, the cured product
 experiences a minimal change of Young's modulus at low temperatures.
 The invention provides a radiation-curable liquid resin composition
 comprising (A) an organopolysiloxane having a (meth)acryl group at either
 end of its molecular chain, containing at least 15 mol % of aromatic
 hydrocarbon groups based on the entire organic groups attached to silicon
 atoms, and being free of a urethane bond within the molecule, (B) a
 compound having at least one ethylenically unsaturated group in a
 molecule, and (C) a photopolymerization catalyst.
 This composition is effective for the coating of optical fibers. Therefore,
 an optical fiber covered with a cured product of the composition is also
 contemplated herein.
 DESCRIPTION OF THE PREFERRED EMBODIMENTS
 (A) Urethane Bond-free, (meth)Acryl Group-bearing Organopolysiloxane
 The first component of the radiation-curable liquid resin composition
 according to the invention is an organopolysiloxane which has a
 (meth)acryl group at either end of its molecular chain, contains at least
 15 mol % of aromatic hydrocarbon groups based on the entire organic
 substituents attached to silicon atoms, and is free of a urethane bond
 within the molecule; especially a linear diorganopolysiloxane containing
 at least 15 mol % of aromatic hydrocarbon groups based on the entire
 organic substituents (that is, substituted or unsubstituted monovalent
 hydrocarbon groups) attached to silicon atoms, excluding the (meth)acryl
 group-bearing organic groups attached to the silicon atoms at both ends of
 the molecular chain. In the specification, (meth)acryl group means acryl
 group and/or methacryl group.
 This organopolysiloxane is the base polymer of the liquid resin composition
 according to the invention and is basically a linear diorganopolysiloxane
 whose backbone consists of recurring diorganosiloxane units. The number of
 silicon atoms in the molecule (or the degree of the polymerization) is
 generally about 80 to about 1,200, preferably about 100 to about 1,000.
 Preferably the (meth)acryl group is attached to the silicon atom at each
 end of the molecular chain as a (meth)acryloxyalkyl group.
 Typically, the organopolysiloxane is represented by the following general
 formula (1).
 ##STR1##
 Herein R.sup.1 is a hydrogen atom or a methyl group, R.sup.2, which may be
 the same or different, is a substituted or unsubstituted monovalent
 hydrocarbon group having 1 to 10 carbon atoms, R.sup.2 contains at least
 15 mol % of aromatic hydrocarbon groups, m is an integer of 1 to 5, and n
 is an integer of 80 to 1,200, and preferably 100 to 1,000.
 Formula (1) is described in detail. R.sup.1 is a hydrogen atom or a methyl
 group, although the hydrogen atom is preferred when the curing rate of the
 composition upon exposure to radiation is taken into account. R.sup.2
 represents substituted or unsubstituted monovalent hydrocarbon groups
 having 1 to 10 carbon atoms, preferably substituted or unsubstituted
 monovalent hydrocarbon group having no aliphatic unsaturated bonds, for
 example, straight, branched or cyclic alkyl groups of 1 to 10 carbon
 atoms, especially 1 to 6 carbon atoms, aryl groups of 6 to 10 carbon
 atoms, and aralkyl groups of 7 to 10 carbon atoms. Exemplary groups of
 R.sup.2 are alkyl groups such as methyl, ethyl, propyl, isopropyl, butyl,
 isobutyl, tert-butyl, pentyl, hexyl, cyclohexyl, octyl, nonyl and decyl;
 aryl groups such as phenyl, tolyl, xylyl, and ethylphenyl; aralkyl groups
 such as benzyl, .beta.-phenylethyl, and .alpha.-methyl-.beta.-phenylethyl;
 and substituted ones of these groups wherein some of the hydrogen atoms
 are replaced by halogen atoms (e.g., F, Cl and Br), typically
 halo-substituted alkyl groups such as chloromethyl, bromoethyl, and
 3,3,3-trifluoropropyl. Methyl and phenyl groups are preferable from the
 commercial aspect. For increasing the refractive index of the inventive
 composition, it is desired to increase the compatibility of the
 organopolysiloxane (A) with the monomer having at least one ethylenically
 unsaturated bond in a molecule (B) as the second component of the
 composition, especially an acrylic compound. To this end, aromatic
 hydrocarbon groups are contained in an amount of at least 15 mol %,
 typically 15 to 50 mol %, especially 15 to 30 mol %, based on the R.sup.2
 groups. Exemplary aromatic hydrocarbon groups are aryl groups such as
 phenyl, tolyl, xylyl, and ethylphenyl, and aralkyl groups such as benzyl,
 .beta.-phenylethyl, and .alpha.-methyl-.beta.-phenylethyl, with the aryl
 groups such as phenyl being preferred. Letter m is an integer of 1 to 5,
 especially 1 to 3.
 The organopolysiloxane (A) preferably has a degree of polymerization (n) of
 about 80 to about 1,200, especially about 100 to about 1,000. With n of
 less than 80, the cured product of the composition has a high Young's
 modulus and a low elongation. With n of more than 1,200, the composition
 has a high viscosity and the organopolysiloxane becomes less compatible
 with the monomer (B). Most preferably, the degree of polymerization (n) is
 in the range of about 150 to about 500.
 The organopolysiloxanes (A) can be synthesized by well-known acid
 equilibration reaction. More particularly, they are synthesized through
 acid equilibration reaction between a hexaorganodisiloxane having a
 (meth)acryl group with various cyclic polysiloxanes. Of these disiloxanes,
 bis(acryloxymethyl)-tetramethyldisiloxane is preferable for ease of
 synthesis. Of the cyclic polysiloxanes, octamethylcyclotetrasiloxane,
 hexamethylcyclotrisiloxane,
 1,1-diphenyl-3,3,5,5-tetramethylcyclotrisiloxane, and
 1-phenyl-1,2,2,3,3-pentamethylcyclotrisiloxane are used for ease of acid
 equilibration reaction.
 (B) Ethylenically Unsaturated Group-bearing Compound
 Component (B) of the inventive composition is a compound having at least
 one ethylenically unsaturated group in a molecule. Component (B) has a
 function that it allows the composition to crosslink or cure by reacting
 with the (meth)acryl group-bearing organopolysiloxane of component (A),
 and thus is a component which is often referred to a reactive monomer or
 reactive diluent. Illustrative are N-vinyl compounds and compounds of the
 structure wherein (meth)acrylic acid is attached to compounds having an
 amino or hydroxyl group by amidation reaction or esterification reaction.
 For example, the following monofunctional, difunctional and polyfunctional
 compounds can be used.
 Monofunctional compounds:
 Exemplary N-vinyl compounds are N-vinylpyrrolidone, N-vinylcaprolactam,
 N-vinylacetamide, and N-vinylformamide. Exemplary compounds of the
 structure wherein (meth)acrylic acid is attached to compounds having an
 amino or hydroxyl group by amidation reaction or esterification reaction
 are methoxyethylene glycol (meth)acrylate, methoxypolyethylene glycol
 (meth)acrylate, nonylphenoxyethyl (meth)acrylate, nonylphenoxypolyethylene
 glycol (meth)acrylate, nonylphenoxypolypropylene glycol (meth)acrylate,
 3-chloro-2-hydroxypropyl (meth)acrylate, phenoxyethyl (meth)acrylate,
 phenoxypolypropylene glycol (meth)acrylate, butoxypolyethylene glycol
 (meth)acrylate, alkyl (meth)acrylates, cyclohexyl (meth)acrylate,
 tetrahydrofurfuryl (meth)acrylate, isobornyl (meth)acrylate, benzyl
 (meth)acrylate, cumylphenol (meth)acrylate, cumylphenoxypolyethylene
 glycol (meth)acrylate, cumylphenoxypolypropylene glycol (meth)acrylate,
 2-hydroxypropyl (meth)acrylate, 2-ethylhexyl (meth)acrylate,
 dicyclopentadiene (meth)acrylate, 2-hydroxy-3-phenoxypropyl
 (meth)acrylate, 2-(meth)acryloyloxyethyl-2-hydroxyethylphthalic acid,
 3-acryloyloxyglycerin mono(meth)acrylate, 2-hydroxybutyl (meth)acrylate,
 2-hydroxy-1-(meth)acryloxy-3-(meth)acryloxypropane, polypropylene glycol
 mono(meth)acrylate, polyethylene glycol mono(meth)acrylate,
 poly-.epsilon.-caprolactone mono(meth)acrylate, dialkylaminoethyl
 (meth)acrylates, glycidyl (meth)acrylate,
 mono[2-(meth)acryloyloxyethyl]-acid phosphate, trichloroethyl
 (meth)acrylate, 2,2,3,3-tetrafluoropropyl (meth)acrylate,
 2,2,3,4,4,4-hexafluorobutyl (meth)acrylate, perfluorooctylethyl
 (meth)acrylate, dicyclopentenyl (meth)acrylate, dicyclopentenyloxyalkyl
 (meth)acrylates, tricyclodecanyl (meth)acrylate, tricyclodecanyloxyethyl
 (meth)acrylate, tricyclodecanyloxyethyl (meth)acrylate, isobornyloxyethyl
 (meth)acrylate, and morpholine (meth)acrylate.
 As the monofunctional compound having an ethylenically unsaturated group,
 an acrylate compound containing a straight or branched organosiloxane
 skeleton (that is, organic silicon compound) represented by the following
 general formula (2) may be used. The use of this acrylate compound is
 desirable because of the reduced change of Young's modulus at low
 temperatures.
 ##STR2##
 Herein R.sup.1 is a hydrogen atom or a methyl group, and a is equal to 0 or
 1. Although R.sup.1 is a hydrogen atom or a methyl group, the hydrogen
 atom is preferred when the curing rate of the composition upon exposure to
 radiation is taken into account.
 Difunctional compounds:
 Exemplary difunctional compounds are di(meth)acrylate of
 2,2-dimethyl-3-hydroxypropyl-2,2-dimethyl-3-hydroxypropionate, ethylene
 glycol 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, di(meth)acrylate of ethylene oxide adduct of
 bisphenol A, di(meth)acrylate of propylene oxide adduct of bisphenol A,
 2,2'-di(hydroxyethoxyphenyl)propane di(meth)acrylate, tricyclodecane
 dimethylol di(meth)acrylate, dicyclopentadiene di(meth)acrylate, pentane
 di(meth)acrylate, and (meth)acrylic acid adduct of
 2,2-bis(glycidyloxyphenyl)propane.
 Polyfunctional compounds:
 Exemplary polyfunctional compounds are trimethylolpropane
 tri(meth)acrylate, trimethylolpropane trioxyethyl(meth)acrylate,
 pentaerythritol tetra(meth)acrylate, dipentaerythritol hexa(meth)acrylate,
 tris(acryloxymethyl) isocyanurate, tris(acryloxyethyl) isocyanurate,
 tris-(acryloxypropyl) isocyanurate, triallyl trimellitic acid, and
 triallyl isocyanurate.
 Of these, the monofunctional compounds are preferred since the composition
 of the invention is especially suited as the low Young's modulus primary
 coating on optical fibers.
 The amount of the compound having at least one ethylenically unsaturated
 group in a molecule (B) blended is determined in accordance with the type
 of (meth)acryl group-bearing organopolysiloxane (A) and compound (B), the
 desired viscosity of the resin composition, and the desired physical
 properties of a cured product thereof. For example, a choice may be made
 in the range of about 5 to 200 parts, preferably about 10 to 150 parts,
 more preferably about 20 to 100 parts by weight per 100 parts by weight of
 (meth)acryl group-bearing organopolysiloxane (A).
 (C) Photopolymerization Initiator
 Any of well-known photopolymerization initiators may be used. Examples
 include 1-hydroxycyclohexyl phenyl ketone,
 2,2-dimethoxy-2-phenylacetophenone, phenylacetophenone diethyl ketal,
 alkoxyacetophenones, benzyl methyl ketal, benzophenone and benzophenone
 derivatives such as 3,3-dimethyl-4-methoxybenzophenone,
 4,4-dimethoxy-benzophenone, 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-dimethy
 lamino-1-(morpholinophenyl)-butanone-1, and phosphine oxide derivatives
 such as 2,4,6-trimethyl-benzoyldiphenylphosphine oxide and
 bis(2,6-dimethoxy-benzoyl)-2,4,4-trimethylpentylphosphine oxide.
 Of these photopolymerization initiators, the phosphine oxide derivatives
 are preferred for fast curing. The initiators may be used alone or in
 admixture of two or more. The amount of the initiator used is usually
 about 0.01 to 15 parts, preferably about 0.1 to 10 parts by weight per 100
 parts by weight of components (A) and (B) combined.
 In the resin composition of the invention, various additives, for example,
 stabilizers such as antioxidants and UV absorbers, organic solvents,
 plasticizers, surfactants, silane coupling agents, titanium coupling
 agent, coloring pigments, and organic or inorganic particles may be used
 if desired and insofar as the objects of the invention are not impaired.
 The resin composition of the invention is prepared by blending the
 above-described components and agitating and mixing them. The composition
 is preferably adjusted to a viscosity of about 500 to about 10,000
 centipoise at 25.degree. C. from the working standpoint for adapting
 itself to usual manufacturing conditions of optical fiber cores and
 especially about 500 to about 4,000 centipoise at 25.degree. C. for
 adapting itself to high-speed manufacturing conditions.
 Like conventional UV-curable compositions, the liquid resin composition of
 the invention cures upon exposure to radiation, typically UV. The thus
 cured coating should desirably have a Young's modulus of up to 0.1
 kgf/mm.sup.2 in order to protect cores from microbending by external
 forces and temperature changes. The type of radiation with which the
 inventive composition is curable includes IR, visible rays, and UV as well
 as ionizing radiation such as x-rays, electron beams, .alpha.-rays,
 .beta.-rays and .gamma.-rays.
 The radiation-curable liquid resin composition of the invention is not only
 useful as optical fiber coatings, but also finds many other applications,
 for example, as mold release coatings, water-repellent coatings,
 protective coatings, various types of ink and paint.
 The radiation-curable liquid resin composition of the invention is
 especially useful as a primary coating on optical fibers. It is directly
 applied to optical glass fibers to form a primary coating, over which a
 secondary coating having a high Young's modulus is applied. The secondary
 coating is typically a urethane acrylate composition which is a UV-curable
 resin composition. The composition of the invention is also applicable as
 a buffer or filler for water-proof fiber cables and submarine cable
 optical fiber units.

EXAMPLE
 Examples of the invention are given below by way of illustration and not by
 way of limitation. All parts are by weight.
 Synthesis Example 1
 Synthesis of Acryl Group-bearing Organopolysiloxane (A)
 A reactor was charged with 100 parts of
 1,3-bis(acryloxymethyl)-tetramethyldisiloxane, 1,960 parts of
 octamethylcyclotetrasiloxane, and 4,582 parts of
 1,1-diphenyl-3,3,5,5-tetramethylcyclotrisiloxane. At 60.degree. C. , 7
 parts of trifluoromethanesulfonic acid was added and equilibration
 reaction effected for 24 hours. The reaction mixture was neutralized with
 sodium bicarbonate, treated with activated carbon, and filtered. Volatiles
 were distilled off at 150.degree. C. and 5 mmHg, yielding acryl
 group-bearing organopolysiloxane (A) as shown below. It had a viscosity of
 6,610 centipoise at 25.degree. C. and a refractive index of 1.4835.
 Acryl-bearing organopolysiloxane (A):
 ##STR3##
 It is noted that
 ##STR4##
 units and
 ##STR5##
 units are randomly distributed in the molecular chain.
 Synthesis Example 2
 Synthesis of Acryl Group-bearing Organopolysiloxane (B)
 An acryl group-bearing organopolysiloxane (B) as shown below was
 synthesized as in Synthesis Example 1, but using 100 parts of
 1,3-bis(acryloxymethyl)-tetramethyldisiloxane, 3,430 parts of
 octamethylcyclotetrasiloxane, 2,291 parts of
 1,1-diphenyl-3,3,5,5-tetramethylcyclotrisiloxane, and 6 parts of trif
 luoromethanesulfonic acid. It had a viscosity of 2,150 centipoise at
 25.degree. C. and a refractive index of 1.4448.
 Acryl-bearing organopolysiloxane (B):
 ##STR6##
 It is noted that
 ##STR7##
 units and
 ##STR8##
 units are randomly distributed in the molecular chain.
 Synthesis of Organosiloxane Compound (C) (a compound having an
 organosiloxane skeleton and containing an ethylenically unsaturated group)
 A reactor was charged with 652 parts of water, 327 parts of isopropyl
 alcohol and 47 parts of 36% hydrochloric acid and cooled below 5.degree.
 C. To this mixture, a mixture of 218 parts of
 acryloxypropylmethyldimethoxysilane and 434 parts of trimethylchlorosilane
 w as added dropwise while maintaining the reaction solution below
 15.degree. C. After the completion of addition, the reaction solution was
 stirred for 2 hours. The upper layer was separated off. The solution was
 then washed with water, neutralized, dried over anhydrous sodium sulfate,
 filtered, and distilled, yielding an organosiloxane compound (C) shown
 below as a colorless, clear liquid. It had a refractive index of 1.4185.
 Organosiloxane compound (C):
 ##STR9##
 Examples 1-9 and Comparative Examples 1-2
 Radiation-curable resin compositions of Examples 1-9 and Comparative
 Examples 1-2 were prepared by mixing an acryl group-bearing
 organopolysiloxane, an ethylenically unsaturated group-bearing compound
 and a photo-polymerization initiator as shown in Table 1. The compositions
 were examined for physical properties by the following tests.
 Each resin composition was applied onto a glass plate to a build-up of a
 thickness of 200 .mu.m. UV radiation having a wavelength of 350 nm was
 irradiated to the coating in a dose of 500 mJ/cm.sup.2, obtaining a cured
 film.
 Young's modulus:
 After the cured film was conditioned for 24 hours at 25.degree. C. and RH
 50%, a 2.5% tensile modulus was measured under conditions: a gage mark
 distance of 25 mm and a pulling rate of 1 mm/min.
 Tensile strength and Elongation at rupture:
 After the cured film was conditioned for 24 hours at 25.degree. C. and RH
 50%, measurement was made under conditions: a gage mark distance of 25 mm
 and a pulling rate of 50 mm/min.
 With respect to outer appearance, the cured film was visually observed and
 rated "O" when it was clear and "X" when it was white, opaque and
 delaminated.
 TABLE 1