AN OPTICAL FIBER COATED WITH AN ORGANOPOLYSILOXANE CURABLE WITH ACTINIC RAYS

Abstract: A coated optical fiber is prepared by providing coating a fused quartz glass optical fiber with an ultraviolet-curable organopolysiloxane or a composition comprising the same and irradiating the coating layer with ultra-violet or other actinic radiation. Specifically, the organopolysiloxane is represented by the average unit formula (R¹) a [(R³O) n (R⁴) 3-n-m (R⁵O) m Si-R²-] b SiO (4-a-b)/2 , in which a is a positive number in the range from 1.6 to 2.2 and b is a positive number in the range from 0.02 to 0.5 with the proviso that a+b is in the range from 1.8 to 2.2; n is 1,2 or 3; m is zero, 1 or 2 with the proviso that n+m is 1,2 or 3; the groups denoted by R¹ and R⁴ are each, independently from the others, a substituted or unsubstituted monovalent hydrocarbon group; R² is a divalent hydrocarbon group having 2 to 4 carbon atoms or an oxygen atom; R³ is a monovalent organic group having 4 to 25 carbon atoms and 1 to 3 ethylenically unsaturated groups with the proviso that R³ has at least 2 ethylenically unsaturated groups when n is 1; R⁵ is a monovalent hydrocarbon group free from aliphatic unsaturation having 1 to 18 carbon atoms or an organosilyl group represented by the general formula R⁶ being a monovalent hydrocarbon group having 1 to 9 carbon atoms and p being zero or a positive integer.

Description: The present invention relates to a coated optical fiber or, more particularly, to an optical fiber coated with a layer of a cured organopolysiloxane curable by irradiation with actinic rays and having low water absorption as well as small temperature dependency of the Young's modulus as well as to a method for the preparation of such a coated optical fiber. As is known, various materials are used for optical fibers including fused quartz glass, multi-component glass and plastics. In view of their light weight, low transmission loss, absence of induction, heat resistance and weather resistance as well as large transmission capacity, however, most of th optical fibers in practical service are made of fused quartz glass. It is usual that quartz glass optical fibers are provided with a coating layer of a protecting material since quartz glass fibers are usually very small in diameter and subject to changes of performance with time. Various materials have been proposed for the coating layer on quartz glass optical fibers, including so-called silicones and organic polymers. For example, a typical silicone material for optical fiber coating is an organopolysiloxane composition comprising an organopolysiloxane having vinyl groups bonded to the silicon atoms and an organohydrogen­ polysiloxane having hydrogen atoms directly bonded to the silicon atoms, of which a cured coating layer is formed by the addition reaction or so-called hydrosilation between the silicon-bonded vinyl groups and silicon-bonded hydrogen atoms in the presence of a platinum catalyst. Another curable silicone material for optical fiber coating is an organo­ polysiloxane composition comprising a vinyl-­ containing organopolysiloxane and a mercaptoalkyl-­ containing organopolysiloxane to effect an addition reaction between the vinyl and mercapto groups. Organic polymers include those obtained by use of a polyether urethane acrylate or polybutadiene urethane acrylate. These prior art coating materials for optical fibers have their respective problems and disadvantages. For example, the organopolysiloxane compositions prepared by the hydrosilation reaction have the problem that the organohydrogenpolysiloxane is susceptible to reaction with atmospheric moisture to evolve hydrogen gas which causes increased transmission loss in the optical fiber coated therewith. The organopolysiloxane compositions prepared by the mercapto-vinyl addition reaction are sometimes unacceptable due to the very unpleasant odor of the mercapto groups emitted in the course of the curing reaction. The coating materials based on a polyether urethane acrylate have disadvantages of large water absorption and unduly high rigidity at low temperatures which increase the transmission loss of the coated optical fibers, while those based on a polybutadiene urethane acrylate are disadvantageous in respect of their low curability. Accordingly, it is desired to develop a novel optical fiber coated with a coating material free from the above described problems and disadvantages in the prior art. In general terms, the invention provides an optical fiber coated with a novel organopolysiloxane coating material. Each of the terminal silicon atoms of the organopolysiloxane (which is preferably linear) has bonded thereto through an oxygen atom one or more ethylenically unsaturated polymerisable groups such that at each silicon atom there are at least two ethylenically unsaturated functional groups. By virtue of this molecular structure, the coating layer may be cured by ultraviolet radiation to give a cured coating. The coated optical fiber of the present invention comprises: (a) a quartz glass optical fiber base; and (b) a coating layer, on the quartz glass optical fiber base, of an organopolysiloxane which is represented by the average unit formula (R¹) a [(R³O) n (R⁴) 3-n-m (R⁵O) m Si-R²-] b SiO (4-a-b)/2 , ...... (I) in which the groups denoted by R¹ and R⁴ are each, independently from the others, a substituted or unsubsti­ tuted monovalent hydrocarbon group, R² is a divalent hydrocarbon group having 2 to 4 carbon atoms or an oxygen atom, R³ is a monovalent organic group having 4 to 25 carbon atoms and 1 to 3 ethylenically unsaturated groups, R⁵ is a monovalent hydrocarbon group free from aliphatic unsaturation having 1 to 18 carbon atoms or an organo­ silyl group of the general formula SiR⁶-OSiR⁶₃ , ...... (II) R⁶ being a monovalent hydrocarbon group having 1 to 9 carbon atoms and p being zero or a positive integer, a is a positive number in the range from 1.6 to 2.2, b is a positive number in the range from 0.02 to 0.5 with the proviso that a+b is in the range from 1.8 to 2.2, n is 1, 2 or 3, m is zero, 1 or 2 with the proviso that n+m is 1, 2 or 3 and R³ has at least two ethyleni­ cally unsaturated groups when n is 1. The present invention also provides a method for the preparation of an optical fiber coated with a layer of a cured organopolysiloxane which comprises coating a quartz glass optical fiber base with an organo­ polysiloxane represented by the above given average unit formula (I) and irradiating the organopolysiloxane with actinic rays such as ultraviolet light, for example. As will be understood from the above given summary of the invention, the most characteristic feature of the inventive coated optical fiber is that the coating layer on the quartz glass optical fiber base is formed by curing a specific organopoly­ siloxane curable by irradiation with actinic rays and represented by the average unit formula (I) given above. The coating material used for forming the coating layer of the inventive coated optical fiber is the above described organopolysiloxane represented by the average unit formula (I) or a composition comprising the same as the principal ingredient. In the formula (I), the groups denoted by the symbols R¹ and R⁴ are each, independently from the others, a monovalent hydrocarbon group generally having 1 to 9 carbon atoms exemplified by alkyl groups, e.g. methyl, ethyl, propyl, butyl and octyl groups, cycloalkyl groups, e.g. cyclohexyl groups, alkenyl groups, e.g. vinyl and allyl groups, aryl groups, e.g. phenyl and tolyl groups, and aralkyl groups, e.g. 3-phenylpropyl group, as well as those substituted groups obtained by the replacement of a part or all of the hydrogen atoms in the above named hydrocarbon groups with substituent atoms and/or groups such as halogen atoms and cyano groups including chloromethyl, 3,3,3-trifluoropropyl and 2-cyanoethyl groups. It is preferable that at least 50% by mole of the groups denoted by the symbols R¹ and R⁴ should be methyl groups and the molar content of phenyl groups, if any, should not exceed 35% of the groups of R¹ and R⁴. When at least a part of R¹ and R⁴ are phenyl groups, the group R² should prefer­ ably be an ethylene group. The group denoted by the symbol R² is, when it is a divalent hydrocarbon group having 2 to 4 carbon atoms, exemplified by ethylene, propylene and butylene groups. The group R² may optionally be an oxygen atom although an oxygen atom as R² is less preferable in respect of the relatively low resistance of the coating layer against water. The group denoted by the symbol R³ is a monovalent hydrocarbon group having 4 to 25 carbon atoms and having, in a molecule, 1 to 3 ethylenically unsaturated functional groups, e.g. CH₂=CHCOO-, CH₂=C(CH₃)COO- and CH₂=CHCO- exemplified by CH₂=CHCOOCH₂CH₂-, [CH₂=C(CH₃)COOCH₂]₃C-CH₂- and (CH₂=CHCOOCH₂)₂C(C₂H₅)CH₂-. The group denoted by the symbol R⁵ is, when it is a monovalent hydrocarbon group free from aliphatic unsaturation having 1 to 18 carbon atoms, exemplified by alkyl groups, e.g. methyl, ethyl, propyl, butyl, pentyl and neopentyl groups. Alterna­ tively, the group denoted by R⁵ may be an organosilyl group represented by the general formula (II) given above, in which R⁶ has the same meaning as R¹ defined above and the subscript p is zero or a positive integer. Examples of such organosilyl groups include methyl ethyl phenyl silyl, trimethyl silyl, dimethyl phenyl silyl and vinyl dimethyl silyl groups with p equal to zero and organosiloxy-substituted silyl groups. The meaning of each of the subscripts a, b, m and n is defined already. When n is equal to 1, the group R³ should preferably have more than one ethylenically unsaturated functional groups. Since the group R³0 necessarily contained in the organopolysiloxane is sensitive to actinic rays, the organopolysiloxane can be readily cured by irradiating with actinic rays such as ultraviolet light. The organopolysiloxane should preferably have a viscosity in the range from 1,000 to 20,000 centipoise at 25°C. When the viscosity of the organopolysiloxane is too low, the quartz glass optical fiber base is poorly wettable with the organopolysiloxane and it is difficult to form a coating layer thereof having an adequate thickness. When the viscosity of the organopolysiloxane is too high, on the other hand, disadvantages are caused in the difficult removal of air bubbles from the coating layer as well as in respect of the difficulties in obtaining an adequate thickness of the coating layer and decreased workability. The organopolysiloxane represented by the average unit formula (I) can be prepared by a dehydrochlorination reaction between an organopolysiloxane of a corresponding molecular structure having chlorine atoms directly bonded to the silicon atoms and an ethylenically unsaturated compound having an active hydroxy group in the molecule. The above mentioned chlorine-containing organopolysiloxane is exemplified by those compounds expressed by the following structural formulas, in which methyl and phenyl groups are denoted by the symbols Me and Ph, respectively: The hydroxy-containing ethylenically unsaturated compound to be reacted with the above described chlorine-containing organopolysiloxane is exemplified by 2-hydroxyethyl (meth)acrylate, trimethylolpropane di(meth)acrylate and pentaerithritol tri(meth)acrylate and the like, of which pentaerithritol derivatives are advantageous since the resultant organopolysiloxane of the average unit formula (I) should preferably have two or more of the ethylenically unsaturated groups bonded to the same silicon atom. In the above described dehydrochlorination reaction, not all of the chlorine atoms in the chlorine-containing organopolysiloxane need be reacted with the ethylenically unsaturated hydroxy-containing compound. Namely, it is optional with the object of adequately controlling the viscosity of the organopolysiloxane or hardness of the cured product thereof that a part of the functional groups may be alkoxy groups or organosiloxy groups introduced by the reaction with an alcohol such as methyl alcohol, ethyl alcohol, propyl alcohol, butyl alcohol, 2-methylpropan-1-ol, 2-methylpropan-2-ol and 2,2-dimethylpropan-1-ol and the like, for example, or organosilanol compound such as vinyl dimethyl silanol and the like, for example. Although the coated optical fiber of the invention can be obtained by coating a quartz glass optical fiber base with the above described organopolysiloxane alone followed by irradiation with actinic rays to effect curing thereof, the organo­ polysiloxane may optionally be admixed with various kinds of additives. For example, physical properties of the cured coating layer can be modified by admixing the organopolysiloxane with an acrylic monomer or an oligomer thereof. Various kinds of actinic rays are effective to cure the coating layer of the organopolysiloxane or a composition comprising the same as the principal ingredient including ultraviolet light, far-ultraviolet light, electron beams, X-rays, γ-rays and the like, of which ultraviolet is preferred in respect of the simplicity and handiness of the apparatus. The ultraviolet lamps suitable as the irradiation source include high- and low-pressure mercury lamps, xenon lamps, hydrogen discharge tubes and the like. The irradiation with the actinic rays can be performed in any desired gaseous atmosphere such as air or inert gases. It is sometimes advantageous, depending on the type of the actinic rays for the irradiation of the coating layer, that the organopolysiloxane is admixed with a photosensitizer or photopolymerization initiator such as benzophenone compounds, e.g. benzophenone, benzoin ether compounds, e.g. benzoin isobutyl ether, ketal compounds, e.g. acetophenone diethyl ketal, thioxanthone compounds, acetophenone compounds and the like in an amount not exceeding 20% by weight or, preferably, in the range from 1 to 5% by weight based on the organopolysiloxane. In the following, the coated optical fiber of the invention is described in more detail with reference to examples preceded by a description of the preparation procedure of the organopolysiloxanes as the coating material on the optical fiber base. Preparation 1. Into a flask of 1 liter capacity equipped with a stirrer, reflux condenser, dropping funnel, thermometer and gas inlet tube were introduced 452 g of a chlorine-­ containing organopolysiloxane 1 expressed by an average structural formula Cl₂MeSi-CH₂CH₂-SiMe₂O-SiMe₂O-SiMe₂― -CH₂CH₂-SiMeCl₂ and 300 g of diisopropyl ether followed by the addition of 14.6 g of triethyl amine and 54 g of pentaerithritol triacrylate having a hydroxy value of 130.5 and contain­ ing 0.2% of dibutyl hydroxytoluene as a polymerization inhibitor to form a reaction mixture. The reaction mixture was heated under agitation at 70 °C for 7 hours and then cooled to room temperature followed by filtration to remove the precipitated salt and distillation of the filtrate at 65 °C under a reduced pressure to remove the solvent. The filtrate freed from the solvent was filtered again under pressurization. The thus obtained product was a clear and oily organo­ polysiloxane expressed by the formula [(CH₂=CHCOOCH₂)₃CCH₂O]₂MeSi-CH₂CH₂-SiMe₂O-SiMe₂ -O-SiMe₂-CH₂CH₂-SiMe[OCH₂C(CH₂OCOCH=CH₂)₃]₂. This product is referred to as Organopolysiloxane I hereinbelow. Preparation 2. Into the same reaction vessel as used in Preparation 1 were introduced 350 g of the chlorine-containing organopolysiloxane 2 shown by the formula given below and 300 g of toluene followed by the addition of 32 g of triethyl amine and 22 g of 2-hydroxyacrylate contain­ ing 0.2% by weight of dibutyl hydroxytoluene and the mixture was heated at 70 °C under agitation. After 2 hours of agitation at the same temperature, the reaction mixture was admixed with 8.5 g of trimethyl silanol and further agitated for additional 3 hours to complete the reaction. The reaction mixture cooled to room tempera­ ture was filtered and the filtrate was freed from the solvent by distillation under reduced pressure at 100 °C followed by further filtration to give a product, which is shown by the formula given below and referred to as Organopolysiloxane II hereinbelow. Preparation 3 to 6. The synthetic procedure in each of these Prepara­ tions was substantially the same as in Preparation 2 described above except that the starting chlorine containing organopolysiloxane 3, 4, 5 or 6, amount thereof, the ethylenically unsaturated compound and amount thereof, the amount of triethyl amine added to the reaction mixture and the organopolysiloxane product, referred to as Organopolysiloxane III, IV, V or VI hereinbelow, were as shown in the following by the respective structural formula, when appropriate. In Preparation 3, 2.8 g of methyl alcohol were additionally added to the reaction mixture. The organopolysiloxane VI was prepared for comparative purpose. (Preparation 3) Chlorine-containing organopolysiloxane 3: 424 g Pentaerithritol triacrylate: 34 g Triethyl amine: 18 g Organopolysiloxane III: (Preparation 4) Chlorine-containing organopolysiloxane 4: 448 g Trimethylolpropane dimethacrylate: 45 g Triethyl amine: 18 g Organopolysiloxane IV: (Preparation 5) Chlorine-containing organopolysiloxane 5: 344 g Pentaerithritol triacrylate: 200 g Triethyl amine: 50 g Organopolysiloxane V: (Preparation 6) Chlorine-containing organopolysiloxane 6: 380 g ClSiMe₂-OSiMe₂-OSiMe₂Cl 2-Hydroxyethyl acrylate: 12 g Triethyl amine: 11 g Organopolysiloxane VI: Examples 1 to 5 and Comparative Examples 1 to 3. A curable organopolysiloxane composition was prepared by uniformly blending 2 parts of benzophenone with 100 parts of one of the Organopolysiloxanes I to VI obtained in the above described Preparations 1 to 6. The Organopolysiloxanes I to V were for Examples 1 to 5, respectively, and the Organopolysiloxane VI was for Comparative Example 1. An optical fiber base of fused quartz glass having a diameter of 125 µm and obtained by spinning at a velocity of 30 meters/minute was coated with the composition in a coating thickness of 50 µm and irradiated with ultraviolet light from a high-pressure mercury lamp having a linear output of 80 watts/cm at a distance of 10 cm for 0.4 second to give a dose of 100 mJ/cm² so that it was found that the coating layers in Examples 1 to 5 had been fully cured into a uniform coating film having a smooth and uniform surface without stickiness. On the other hand, curing of the coating layer in Comparative Example 1 was not complete leaving considerable stickiness on the surface. For further comparison, the Organopolysiloxanes I to VI and two non-silicone coating materials conven­ tionally used in optical fiber coating (Comparative Examples 2 and 3) with admixture of 2% by weight of benzophenone were each shaped into a sheet of 1 mm thick­ ness, which was irradiated with a high-pressure mercury lamp having a linear output of 80 watts/cm to give a dose of 300 mJ/cm². The coating materials used in Comparative Examples 2 and 3 were a polyether urethane acrylate and a polybutadiene urethane acrylate, repec­ tively, expressed by the following formulas, in which (C₆H₃Me) is a 1,4-tolylene group. (Comparative Example 2) (Comparative Example 3) These ultraviolet-cured sheets were examined for the surface condition and subjected to the determination of water absorption and temperature dependency of the Young's modulus at low temperatures to give the results shown in the table below. In the table, the results of the evaluation are given by rating in two, four or three grades for the surface condition, water absorption and temperature dependency of Young's modulus according to the following criteria. The water absorption was deter­ mined in the following manner. Thus, a cured sheet of 0.5 mm by 50 mm by 50 mm which was immersed for 24 hours in pure water at 25 °C and the weight W₁ was determined after being freed from water on the surface. Thereafter, the test piece was kept in a desiccator of calcium chloride until the test piece lost no additional weight to have a constant weight W₂. The water absorption in % was given by (W₁ - W₂)/W₂ x 100. Surface condition A: The sheet was uniformly cured without surface tackiness. B: The surface has tackiness to retain finger prints by touching with a finger tip. Water absorption A: less than 1.0% B: 1.0 to 2.0% C: larger than 2.0% Temperature dependency of Young's modulus A: small temperature dependency at -30 °C or below B: small dependency at -30 to -10 °C C: relatively large at -10 °C or higher It will be appreciated from the foregoing that we have provided organopolysiloxane-based coatings for optical fibers in which the organopolysiloxane is readily cured by irradiation with actinic rays such as ultraviolet so that the process of forming the coating layer on the optical fiber base can be performed with a high efficiency and the thus radiation-cured coating layer of the organopolysiloxane has outstandingly low water absorption and temperature dependency of Young's modulus at low temperatures of, for example -10 to -30°C. Needless to say, the coating material is free from the problem of any unpleasant odor. Optical fiber coated therewith has excellent resistance against heat and water and low water absorption of the coating layer in comparison with conventional coated optical fibers in addition to the outstandingly low temperature dependency of the Young's modulus. Accordingly, the coated optical fiber is serviceable over a long period of time under severest environmental conditions.

Claims: 1. A coated optical fiber which comprises: (a) a fused quartz glass optical fiber base; and (b) a coating layer on the optical fiber base formed of a coating material comprising, as the principal ingredient thereof, an organopolysiloxane represented by the average unit formula (R¹)a[(R³O)n(R⁴)3-n-m(R⁵O)mSi-R²-]bSiO(4-a-b)/2, in which a is a positive number in the range from 1.6 to 2.2 and b is a positive number in the range from 0.02 to 0.5 with the proviso that a+b is in the range from 1.8 to 2.2; n is 1,2 or 3; m is zero, 1 or 2 with the proviso that n+m is 1,2 or 3; the groups denoted by R¹ and R⁴ are each, independently from the others, a substituted or unsubstituted monovalent hydrocarbon group; R² is a divalent hydrocarbon group having 2 to 4 carbon atoms or an oxygen atom; R³ is a monovalent organic group having 4 to 25 carbon atoms and 1 to 3 ethylenically unsaturated groups with the proviso that R³ has at least 2 ethylenically unsaturated groups when n is 1; R⁵ is a monovalent hydrocarbon group free from aliphatic unsaturation having 1 to 18 carbon atoms or an organosilyl group represented by the general formulaSiR⁶₂-OSiR⁶₃, R⁶ being a monovalent hydrocarbon group having 1 to 9 carbon atoms and p being zero or a positive integer. 2. A coated optical fiber as claimed in claim 1, wherein in the organopolysiloxane at least 50 mole % of the groups which are R¹ or R⁴ groups are methyl groups and/or the proportion of such groups which are phenyl does not exceed 35 mole %. 3. A coated optical fiber as claimed in claim 1 or claim 2 wherein in the organopolysiloxane at least some R¹ groups or R⁴ groups are phenyl and R² is ethylene. 4. A coated optical fiber as claimed in claim 1 or claim 2 wherein the organopolysiloxane R² is a divalent group having 2 to 4 carbon atoms. 5. A coated optical fiber as claimed in any one of the preceding claims, wherein in the organopolysiloxane R¹ is CH₂=CHCOOCH₂-CH₂-, [CH₂=C(CH₃)COOCH₂]₃C-CH₂- or (CH₂=CHCOOCH₂)₂C(C₂H₅)-CH₂-. 6. A method for the preparation of a coated optical fiber which comprises: (a) coating a fused quartz glass optical fiber base with a coating material as defined in claim 1; and (b) irradiating the resultant coating layer with ultraviolet light or other actinic radiation to cure the organopolysiloxane. 7. An organopolysiloxane as defined in claim 1. 8. A method for making an organopolysiloxane as defined in claim 1, comprising subjecting to a dehydrochlorination reaction a compound of the formula R³OH wherein -OH is an active hydroxy group and the corresponding chloro-organopolysiloxane in which chlorine atoms to be replaced by the group R³O- are bonded to the silicon atoms. 9. A method as claimed in claim 6 or claim 8 wherein the organopolysiloxane is as further defined in any one of claims 2 to 5. 10. An organopolysiloxane as claimed in claim 7 wherein the organopolysiloxane is as further defined in any one of claims 2 to 5 and/or has a viscosity at 25°C of from 1,000 to 20,000 centipoise.

Inventors: HIDA, YOSHINORI, KATAYAMA, SEIJI, KOZAKAI, SHOHEI, MEGURIYA, NORIYUKI
Applicants: SHIN-ETSU CHEMICAL CO., LTD.
Application year: 1986
Main classification: C03C25
IPC codes: C08G77, C09D183, C03C25, G03F7, C09D5, C08F290, C08F299, G02B6