PROCESS FOR PRODUCING PLASTIC OPTICAL FIBERS

Abstract: A process for producing a plastic optical fiber having excellent long-term reliability with respect to resistance to heat and environment is disclosed, which comprises bulk-­ polymerizing a monomer or monomers that provide a non-­ crystalline homo- or copolymer, melt-spinning the resulting homo- or copolymer while removing any residual volatile components therefrom, and heating and/or exposing to radiation the resulting melt-spun fiber, wherein a polyfunc­ tional reactant is incorporated during the step of bulk poly­ merization or melt-spinning.

Description: PROCESS FOR PRODUCING PLASTIC OPTICAL FIBERS FIELD OF THE INVENTION The present invention relates to a process for producing plastic optical fibers. More particularly, the invention relates to a process for producing plastic optical fibers having excellent long-term reliability in such characteristics as resistance to heat and environment. BACKGROUND OF THE INVENTION Plastic optical fibers have many advantages over conventional optical glass fibers such as the ability to be produced in larger diameters with increased numerical aperture, and greater flexibility and lighter weight. The primary uses of plastic optical fibers with jackets are as cords or cables, and as composite cords or cables wherein the fibers are assembled with electric wires. The application of plastic optical fibers to optical data links and photosensors is under review. In these current and projected uses of plastic optical fibers, long-term reliability in such characteristics as resistance to heat and environment is of primary importance. The prior art plastic optical fibers use noncrystalline transparent polymers as core materials and they include polymethyl methacrylates (PMMA, Japanese Patent Publication No. 21660/1978), polystyrenes (PS, Japanese Patent Publication No. 8978/1968) and polycarbonates (PC, Japanese Patent Publication No. 43388/1976). The cladding is made of transparent polymers having lower refractive indices than the core materials. Both the core and cladding in the prior art plastic optical fibers are made of thermoplastic materials. In particular, since the core materials are noncrystalline, drawing is essential for providing optical fibers with goad mechanical properties. However, if the drawn fiber is heated to temperatures close to its glass transition point, the molecular orientation that has been introduced into the core by the drawing operation is relaxed. As-a result, if a plastic optical fiber is heated to temperatures close to or higher than the glass transition point, the fiber either experiences a sudden drop in the mechanical strength or shrinks so greatly that it will no longer be able to maintain its structure as a light guide and will suffer a significant transmission loss Various attempts have been made to improve the thermal properties of plastic optical fibers and the present inventors, too, proposed the following approaches: (l) improvements based on the glass transition point of a core material and the melting point of a cladding material (Utexamined Published Japanese Patent Applications Nos, 34404/1983 and 65402/1983); (2) an optical fiber having a core/cladding structure that has been irradiated with electron beams (Unexamined Published Japanese Patent Application No. 98706/1983); and (3) an optical fiber made of three layers of core, cladding and sheath, with the sheath formed of a UV-radiation cured resin (Unexamined Published Japanese Patent Application No. 178302/1983). SUMMARY OF THE INVENTION The principal object of the present invention is to provide a process for producing a plastic optical fiber having further improved long-term reliability in such characteristics-as resistance to heat and environment. This object of the invention can be achieved by a process for producing a plastic optical fiber comprising the step of bulk-polymerizing a-monomer or monomers that provide a non-crystalline homopolymer or copolymer, the step of melt-spinning the resulting homopolymer or copolymer while removing any residual volatile components therefrom, and the step of heating and/or exposing to radiation the resulting melt-spun fiber, wherein a polyfunctional reactant is incorporated during the step of bulk polymerization or melt-spinning. BRIEF DESCRIPTION OF THE DRAWINGS Fig 1 is a schematic diagram of an apparatus that may be used to implement the process of the present invention. DETAILED DESCRIPTION OF THE INVENTION The core of the plastic optical fiber produced by the process of the present invention is made of a noncrystalline homopolymer or copolymer which is prepared from one or more monomers such as alkyl methacrylates, aryl methacrylates, styrene derivatives, alkyl acrylates, aryl acrylates, and deuterium-substituted products or fluorine compounds thereof. Polymers mainly composed of an alkyl methacrylate and copolymers mainly composed of an alkyl methacrylate and a styrene derivative are preferably used for the core material, with the alkyl methacrylate being preferably methyl methacrylate. Further, an alkyl acrylate (e.g., methyl acrylate, ethyl - acrylate, propyl acrylate, butyl acrylate, pentyl acrylate, hexyl acrylate, etc.) is preferably used as a comonomer in a small amount, preferably 1 to 5 voIZ based on the total volume of the monomers. When electron bean irradiation is performed in the later step, an alkyl acrylate having the alkyl moiety of 4 or more carbon atoms is preferably used as the comonomer The polyfunctional reactan which can be used in the present invention includes those having functional groups such as -C=C , -CC EMI4.1 -N=C=O, EMI4.2 EMI4.3 and the like. The polyfunctional reactant preferably has a molecular weight of 100 to 1,000. Preferred examples include ally compounds such as diethylene glycol diallyl carbonate and triallyl isocyanurate, acrylate and methacrylate compounds such as trimethylolpropane triacrylate, diethylene glycol dimethacrylate, diethylene glycol diacrylate, neopentyl glycol diacrylate, pentaerythritol pentacrylate and glycidyl acrylate, and deuterium-substituted products or fluorides thereof. The polyfunctional reactant is generally added in an amount of 0.5 to 50 parts by weight, preferably 1 to 20 parts by weight, per 100 parts by weight of the core material. When the amount is less than 0.5 parts by weight, heat resistance of the resulting plastic optical fiber is slightly improved, and when it is more than 50 parts by weight, flexibility of the fiber is deteriorated. The cladding is made of any of the polymers that are substantially transparent and have lower refractive indices than the core materials five., core-forming polymers). Illustrative cladding polymers include a copolymer of vinylidene fluoride and tetrafluoroethylene, a terpolymer of vinylidene fluoride, tetrafluoroethylene and hexafluoropropylene, a fluorine-containing methacrylate polymer , a copolymer of fluorine-containing methacrylate and methyl methacrylate, silicone resin and ethylene-vinyl acetate copolymer. The process of the present invention is explained in more detail with reference to Fig. 1. The monomers in tanks 1, 1' are driven by respective transfer pumps 2, 2' into a-bulk polymerization reactor 3. The monomers that have been polymerized in reactor 3 to a predetermined degree of polymerization (preferably several thousands to several hundreds thousand) are driven by a transfer pump 4 into the second reactor 5, wherein substantially complete polymerization of the monomers occurs. A polyfunctional reactant in a tank 6 is driven by a transfer pump 7 into the polymer or copolymer formed in the second reactor 5. The unreacted monomers and any other residual volatile components are removed from the polymer or copolymer by passage through a vacuum line 8. The clean polymer or copolymer is heated to melt and the molten stream is fed into a spinning head 9 where it is covered with a molten cladding that is obtained by the passage through an extruder 11 of a cladding material supplied from a tank 10. The core and cladding materials combined in a concentric fashion are spun into a fiber 12 as they pass through the head 9. The fiber 12 is heated in a heater t3 so as to polymerize the polyfunctional reactant in the fiber. It is then taken up by rollers 14 and accumulated on a winding machine 15. The process of the present invention is by no means limited to the particular embodiment described above. Instead of being incorporated in the polymer in the second reactor 5, the polyfunctional reactant may be incorporated in the polymer after it has been freed of any volatile components and while it is being subjected to the melt-spinning step. With reference to Fig. 1, the polyfunctional reactant in a tank 16 may be driven by a transfer pump 17 into the polymer just before transfer to the spinning head 9. The heater 13 may be replaced by an infrared radiation generator; alternatively, a generator of X-rays, ss-rays or ultraviolet radiation may be used instead of the heater 13. Fur-ther, both heating and exposing to radiation may also be performed. In the step of heating and/or exposing to radiation the drawn fiber to polymerize the polyfunctional reactant in the fiber as described above, the heating temperature varies depending on kind and molecular weight of the core material, and it is preferably high as long as the draw fiber does not-change in diameter or deform since the heating time can be shortened. For the purpose, the heating temperature is preferably within the range of from glass transition point (Tg) of the core material to Tg + 300C, and the heating time is within several seconds to several minutes. In order to complete the reaction of the polyfunctional reactant with the fiber, the heat treatment may be continued for additional several tens minutes. When the drawn fiber is exposed to electron beam, the exposure amount is preferably not more than 15 Mrad and more preferably 10 Mrad or less. If it is too large, the mechanical strength and the transmission loss characteristic of the fiber are deteriorated. The plastic optical fiber produced by the process of the present invention has the crosslinked or intertwined structure that has been introduced into the core forning polymer or copolymer as a result of its reaction with the polyfunctional reactant following the spinning and drawing operations. Because of this crosslinked or intertwined structure, the molecular orientation in the core polymer will not be lost at all or is only lost with difficulty even if the core is heated to temperatures higher than its glass transition point. As a result, a plastic optical fiber having good thermal properties as characterized by little or no heat shrink can be obtained. The structural integrity of the fiber as a light guide is maintained and its transmission loss characteristics have a rery small temperature dependency. In addition, the cress-linked or intertwined structure in the core polymer provides high mechanical properties for the fiber, especially high tensile strength. A further advantage results from the fact that by localizing the presence of the polyfunctional reactant around the core, a refractive index distribution can be introduced into the core, thus providing a plastic optical fiber having-improved light transmission. The following Examples are provided for further illustration of the claimed process and are by no means to be construed as limiting the invention. Example 1 A substantially oxygen-free pure monomer composition was prepared in a monomer tank, which was composed of two monomers, methyl methacrylate (98 vol%) and n-butyl acrylate (2 viol%), and 0.01 molt of tertiary butyl peroxide (polymerization initiator) and 0.3 molt of n-butyl mercaptan (chain transfer agent). This monomer composition was transferred to a bulk polymerization reactor where polymerization was conducted for about 3 hours at a temperature of 120 - 1300C and a pressure of 2 - 3 kg/cm2. The reaction product was transferred to the second reactor held at 1800C. As the polymerization of the monomers proceeded in the second reactor, any volation components were removed by passage through a vacuum line held at 5 - 10 mmHg. The clean polymer was fed into a spinning head as a purified trimethyloipropane triacrylate (polyfunctional reactant) in a separate tank was added in an amount of 3 parts by weight per 100 parts by weight of the polymer. At the same time, a cladding material made of a vinylidene fluoride/tetrafluoroethylene copolymer was ed into the spinning head (220- C) where it was combined with the molten core material and coextruded as a fiber having a core-cladding structure. Immediately after the coextrusion, the fiber was passed through a heating unit having an internal temperature of 1500C so as to draw down the fiber at a draw ratio of 2 to 3 and polymerize the polyfunctional reactant. The fiber obtained by winding at 12 m/min had the characteristics summarized in Table below. Example 2 A plastic optical fiber was prepared as in Example 1 except that the polyfunctional reactant was polymerized by irradiating the drawn fiber with electron beams. The characteristics of the fiber are shown in the following Table. Comparative Example 1 A plastic optical fiber was prepared as in Example 1 except that no polyfunctional reactant was used. The characteristics of the fiber are shown in the following Table. Comparative Example 2 A plastic optical fiber was prepared as in Example 2 except that no polyfunctional reactant was used. The characteristics of the fiber are shown in the following Table. Comparative Example 3 A plastic optical fiber was prepared as in Example 1 except that commercial PMMA resin pellets, rather than the reaction product of the bulk polymerization of monomers, were supplied into the second reactor. The characteristics of the fiber are shown in the following Table. Comparative Example 4 A plastic-optical fiber was prepared as in Example 2 except that commercial PMMA resin pellets, rather than the reaction product of the bulk polymerization of monomers, were supplied into the second reactor. The characteristics of the-fiber are shown in the following Table. Example 3 A plastic optical fiber was prepared as in Example 1 except that diethylene glycol diallyl carbonate was used as the polyfunctional reactant. The characteristics of the fiber are shown in the following Table. Example 4 A plastic optical fiber was prepared as in Example 2 except that diethylene glycol diallyl carbonate was used as the polyfunctional reactant. The characteristics of the fiber are shown in the following Table. Examples 5 and 6 A plastic optical fiber was prepared as in Example 4 except that the amount of diethylene glycol diallyl carbonate (polyfunctional reactant) was increased to 5 parts by weight (Ex. 5) or 10 parts by weight (Ex. 6) and that the temperature of the coextrusion spinning head was reduced to 2100C (Ex. 5) or 1900C (Ex. 6). The characteristics of the fibers are shown in the following Table. Table Examples 1 2 3 4 5 6 Core material methyl methacrylate polymer(polymerized Same as the Same as the Same as the Same as the Same as the and spun within left left left left left closed space) Cladding vinylidene fluoride/ Same as the Same as the Same as the Same as the Same as the material tetrafluoroethylene left left left left left copolymer Polyfunctional reactant trimethylol- diethylene type trimethylolpropane Same as the Same as the Same as the propane tri- glycol diallyl triacrylate left left left acrylate carbonate amount(parts by weight per 3 3 3 3 5 10 100 parts by weight of core material) method of poly- electron electron electron electron heat heat merization beam beam beam beam Fiber dimensions core inside 0.46 0.48 0.47 0.48 0.46 0.45 diameter(mm) cladding outside 0.50 0.50 0.51 0.50 0.49 0.49 diameter(mm) Transmission loss 0.4 0.6 0.3 0.6 0.4 0.7 at 660 nm(dB/m) Resistance to heat shrinkage 92 96 90 97 95 98 (percent retention, %) for 120 C x 10 min Table Comparative Examples 1 2 3 4 Core material methyl methacrylate methyl methacrylate commercial PMMA commercial PMMA polymer (polymerized pellets pellets and spun within and spun within closed space) closed space) Cladding material vinylidene fluoride/ vinylidene fluoride/ vinylidene fluoride/vinylidene fluoride/ tetrafluoroethylene tetrafluoroethylene tetrafluoroethylene tetrafluoroethylene copolymer copolymer copolymer copolymer Polyfunctional reactant type none none trimethylol- trimethylolpropane propane tri- triacrylate acrylate amount (parts by weight per 100 parts by 0 0 3 3 weight of core material) method of polyheat electron beams heat electron beams merization Fiber dimensions core inside 0.46 0.47 0.47 0.47 diameter(mm) cladding outside 0.50 0.50 0.50 0.50 diameter (mm) Transmission loss 0.3 0.5 1.1 2.0 at 660 nm(dB/m) Resistance to heat shrinkage 45 65 89 94 (percent retention, %) for 120 C x 10 min While the invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.

Claims: Claims: 1. A process for producing a plastic optical fiber comprising the step of bulk-polymerizing a monomer or monomers that provide a non-crystalline homopolymer or copolymer, the step of melt-spinning the resulting homopolymer or copolymer while removing any residual volatile components therefrom, and the step of heating and/or exposing to radiation the resulting melt-spun fiber, wherein a polyfunctional reactant is incorporated during the step of bulk polymerization or melt-spinning. 2. A process for producing a plastic optical fiber of claim 1, wherein said radiation is electron beam. 3. A process for producing a plastic optical fiber of claim 1, wherein said heating is performed by irradiation of ultraviolet ray. 4. A process for producing a plastic optical fiber of claim 1, wherein said non-crystalline homopolymer or copolymer is a polymer mainly composed of an alkyl methacrylate or a copolymer mainly composed of an alkyl methacrylate and a styrene derivative. 5-. A process for producing a plastic optical fiber of claim 4, wherein said alkyl methacrylate is methyl methacrylate. 6. A process for producing a plastic optical fiber of claim 4, wherein said polymer mainly composed of an alkyl methacrylate or said copolymer mainly composed of an alkyl methacrylate and a styrene derivative contains 1 to 5 volt of alkyl acrylate. 7. A process for producing a plastic optical fiber of claim 6, wherein said alkyl acrylate has the alkyl moiety of 4 or more carbon atoms. 8. A process for producing a plastic optical fiber of claim 1, wherein said polyfunctional reactant is incorporated in an amount of 1 to 20 parts by weight per 100 parts by weight of the homopolymer or copolymer. 9. A process for producing a plastic optical fiber of claim 8, wherein said polyfunctïonal reactant is selected from the group consisting of polyfunctional allyl compounds, polyfunctional acrylate compounds and polyfunctional methacrylate compounds.

Inventors: MATSUMIYA, NORIFUMI C/O OSAKA WORKS, UEBA, YOSHINOBUC/O OSAKA WORKS
Applicants: SUMITOMO ELECTRIC INDUSTRIES LIMITED
Application year: 1985
Main classification: B29D11
IPC codes: B29D11, G02B6, D01F1