Heat resisting plastic optical fibre and method for its manufacture

Abstract: An optical fibre of sheath-core structure has for its core a copolymer which includes an aliphatic N-substituted maleimide as a monomer unit, and preferably methyl methacrylate as a further monomer unit. Such optical fibres have high light transmittance, good mechanical characteristics and an excellent heat-­ resisting durability.

Description: This invention relates to a heat resisting plastic optical fibre with a sheath-core structure. Conventionally, optical fibres are manufactured from quartz glass or a plastic. The quartz glass-based optical fibres have an excellent optical transmittance and are industrially used in long distance communication. However, not only are their workability and flexibility not good but also they are expensive. On the other hand, although plastic-based fibres have an inferior light transmittance, they have the advantages of excellent flexibility, light weight and excellent workability as well as low price, so that they are now increasingly used in short distance communication. As the core component of the plastic-based optical fibres, polymethyl methacrylate is generally used. Although polymethyl methacrylate has not only transparency but also excellent characteristics as regards weather-proofing and mechanical properties, its heat-resistance is not satisfactory. Thus, in applications in which an optical fibre is exposed to a high temperature, the upper limit of the temperature of the optical fibres employing polymethyl methacrylate as the core component is set at about 80°C, so that the demand for improving the heat-resistance is strong. As a method of improving the heat-resistance, the following methods are known: (1) A condensation polymer with a good transparency and a high glass transition point, such as polycarbonate, is used. (2) Methacrylic ester having a voluminous ester group or N-arylmaleimide (Tokkai-sho 61-147203) is copolymerized with the methyl methacrylate. (3) After polymerizing methyl methacrylate, intermolecular cross-linkages are formed by reacting the polymer with an amine or the like. However, although the optical fibres employing the above (1) as the core component have a good heat-resistance, their light transmittance and heat-resisting durability are not satisfactory. This is because it is difficult to eliminate by-products produced in the polymerization reaction and the fibre is coloured by the by-products or by decomposition products of the polymer. The optical fibres employing the above (2) as the core component usually do not have satisfactory heat-resistance because the glass transition point is not sufficiently elevated. In cases where the glass transition point is to be raised by increasing the copolymerization ratio of the monomer or by incorporating an extremely voluminous functional group, satisfactory mechanical characteristics are not obtained or the heat-resisting durability is not good because the voluminous functional group is likely to decompose. Further, N-arylmaleimide is a solid of low volatility, its purification is not easy and the light transmittance is not satisfactory because of the impurities, and the amount of remaining monomers in the polymer is great, which monomers colour the polymer and lower the glass transition point. The optical fibres employing the above (3) as the core component contain a large amount of by-products and are coloured, so that an optical fibre with good light transmittance cannot be obtained. A further proposal for a polymer for optical elements is to be found in EP-A-141 610, where a specific acrylate or methacrylate with a bulky alcohol residue, viz. tricyclo[5.2.1.0 2,6 ]deca-8-yl, is homopolymerized or copolymerized with one or more unsaturated comonomers. Of the possible unsaturated comonomers, the N-substituted maleimides are said to be preferable for improving the heat resistance. The object of the present invention is to provide a polymer for plastic optical fibres, which has high light transmittance, good mechanical characteristics, and an excellent heat-resisting durability. This invention provides an optical fibre with a sheath-core structure, wherein the core component comprises a copolymer of which the monomer components consist of (i) N-isopropyl maleimide and (ii) a comonomer which is methylmethacrylate with the optional addition of styrene and/or methyl acrylate, said monomer components being in the proportion of 5-70 wt % of N-isopropyl maleimide to 30-95 % wt % of comonomer. Preferably, the amount of N-isopropyl maleimide in the copolymer should be 10-50 %. If the amount of N-isopropyl maleimide is lower than 5 %, enough heat resistance cannot be attained, and if it is higher than 70 %, mechanical properties of the fibre are greatly lowered. A preferred material has methylmethacrylate alone as the comonomer. A trace amount of heat stabilizer or antioxidant may be added. All percentages given are by weight. As regards the polymerization method of the core component polymer, bulk polymerization or solution polymerization is preferable because they give high light transmittance to the obtained polymer. Examples of the solvent for solution polymerization include hydrocarbons such as toluene and xylene; ester compounds such as butyl acetate; and ether compounds such as dioxane. The solvent is not restricted to these compounds and any solvent which is inactive against the polymerization reaction and which can dissolve the monomers and the polymer may be used. For obtaining uniform quality of the polymer, continuous polymerization is better, but, of course, batch polymerization can be applied. A continuous polymerization in which all of the material component such as monomer, polymerization initiator, molecular weight controlling agent and solvent are continuously fed in a stirred reaction medium is most preferable. 90 - 160°C is preferable as the polymerization temperature and 10 - 500 poise is preferable as the viscosity of the reaction medium. Ordinary polymerization initiators, for example, azo compounds such as azo-t-butane and azo-t-octane, or organic peroxides such as di-t-butylperoxide and di-t-butylperbenzoate are preferably used. As molecular weight controlling agents, mercaptans such as n-propyl-, n-butyl-, n-hexyl-, n-dodecyl-, i-butyl-, i-pentyl-, t-butyl-, and t-hexyl mercaptan can be used; however, molecular weight can also be controlled by changing the amount of the polymerization initiator. Polymerization initiator and molecular weight controlling agent can be added to the monomer or solvent prior to introducing them into the reaction bath. In cases where a polymer is produced by a copolymerization as in the present invention, a solution or bulk polymerization is preferable, such as shown in US-A-4 324 868. However, a solution polymerization process with a high polymerization ratio is better than a bulk polymerization process in which a recovered monomer mixture is subjected to a fractional distillation, since the former has a higher productivity and the amount of the remaining high boiling monomers which are the cause of the lowering of the glass transition point and thermal colouration is smaller in the former process. This is important when a monomer has a boiling point higher than 180°C at atmospheric pressure, or higher than 120°C at 100 mmHg. The preferable amount of the solvent is 10 to 50 wt%, more preferably 15 to 40 wt% based on total weight of the polymerization mixture (including solvent). The thus obtained polymer solution is subjected to a monomer-elimination step to remove volatiles such as unreacted monomers. The volatile component contained in the thus obtained polymer solution should preferably be eliminated to less than 2 wt% because volatile components lower the glass transition temperature of the core. It is preferable that the number average molecular weight of the core component measured by osmotic method is 2 x 10⁴ to 5 x 10⁵. Thereafter, the polymer can be supplied to the core component feed of a composite spinning machine with a sheath-core type spinneret to be joined with a polymer of the sheath component from the sheath component feed to form a plastic optical fiber core-sheath structure. As a sheath component, a polymer having a refractive index which is at least 2% lower than that of the core component is preferable. For example, a polymer or copolymer of fluoroalkylmethacrylate, fluoroalkyl α-fluoroacrylate and fluoroolefin is preferable. For increasing heat resistance, further coating of the sheath-core type optical fibre is effective. As coating materials, polyolefins such as polyethylene, polypropylene, crosslinked polyolefin and polyvinylchloride, polyamides such as Nylon 12 or polyester elastomers such as polyethylene/methylene terephthalate copolymer may be used. Further, direct formation of a concentric three layer structure by a spinning process as disclosed in Laid-open Japanese Patent Publication No. 18608/83 can also be applied. The plastic optical fibre of the present invention has a much better heat-resisting durability than the conventional optical fibre including polymethyl methacrylate as a core component, while keeping light transmittance and mechanical characteristics comparable to those of the conventional optical fibre. Therefore, the plastic optical fibre of the present invention can be used in fields such as the engine compartment of an automobile, in which heat-resistance is required for a long time and in which the conventional optical fibres cannot be used. Thus, the present invention has a great industrial significance and value. In general, the alkyl maleimides have a boiling point higher than 120°C at 100 mmHg. Details of the vapour pressure of various alkyl maleimides are given below with similar details for phenyl maleimide and n-Bornyl methacrylate given by way of comparison. The vapour pressure figures in column 2 are taken from references, those at 120°C are extrapolated values, as are the boiling point figures. It has been mentioned above that solution polymerization is particularly satisfactory in such cases with a preferred solvent content of 10 to 50 wt%. The use of solvent enables the viscosity of the polymerization mixture to be kept low, and a high polymerization ratio can be attained. This results in a drastic decrease of remanent monomer which must be removed prior to spinning, high purity being absolutely important in this field. It follows of course that the method which has been particularly referred to above is equally applicable to other plastics for optical fibres, where one of the monomers has a boiling point higher than 120°C at 100 mmHg. In this aspect of the invention, therefore, we provide a method of making a elastic optical fibre by copolymerizing in solution N-isopropyl maleimide with methyl methacrylate, with the optional addition of styrene and/or methyl acrylate, the quantity of solvent being 10 to 50 wt% based on the total weight of the polymerization mixture, evaporating volatile components from the polymer thus formed and spinning as the core component in a composite spinning machine with a sheath-core spinneret. The present invention will now be described more concretely by way of the examples thereof. The light transmittance was evaluated in the examples as follows: A light beam from a tungsten lamp was divided by a diffraction grating and was unified by a lens. The light was introduced into the end of sample optical fibre with a length of 10 m - 30 m of which both ends had been polished, and the light from the other end of the fibre was detected as a photoelectric power by a photodiode. The light was measured employing a so called cut back method in which the fibre was cut at about 2 m from the light-impinging end of the fibre to give a reference fibre and the measurement of the light from the other end of the fibre was repeated. The loss of light transmittance was calculated according to the following equation: Loss (dB/km) = (Pr - Ps)/(Ls - Lr).1000 wherein L: length of fibre in metres P: photoelectric power (dBm) s: sample fibre r: reference fibre The heat-resistant durability was evaluated as follows: The optical fibre subjected to the above-mentioned measurement was heated in a hot blast drier for a prescribed time. Thereafter, the loss of light transmittance was determined according to the above method and the loss of light transmittance before and after the heat treatment were compared. Example 1 A mixture of N-isopropyl maleimide 200 wt parts Methyl methacrylate 466.7 wt parts n-butyl mercaptan 0.82 wt parts azo-t-butane 0.87 wt parts was prepared after distilling each of the components, and the mixture was fed into a polymerization bath after filtering through a Teflon filter with a pore diameter of 0.05 µm. After polymerizing the mixture at 130°C for 16 hours under nitrogen pressure, the mixture was gradually heated to 180°C and was held at this temperature for 16 hours to complete the polymerization and to completely decompose the polymerization initiator. The mixture was further heated to 230°C and was retained at this temperature for 1 hour. Thereafter, the mixture was gradually introduced into a monomer-eliminating step under nitrogen pressure to remove unreacted monomers to obtain a polymer. Then the vessel was connected to a composite spinning machine having a sheath-core type spinneret. On the other hand, as a sheath component, tetrafluoropropyl α-fluoroacrylate/trifluoroethyl α-fluoroacrylate copolymer (85/15 by wt) was melted at 210°C and was supplied to the spinneret. The mixture was spun at a spinning temperature of 230°C, at a take up speed of 5 m/min. and was drawn 2.0 times of the original length at 160°C, to obtain a sheath-core type fibre with a core radius of 980 µm and a sheath thickness of 10 µm. This fibre was made into a cord by coating the fibre with polypropylene. The polymer after the monomer-eliminating step had remanent monomers in an amount of 0.18 wt% as determined by GC measurement, and the glass transition point as determined by DSC measurement was 136°C. The loss of light transmittance at 25°C of the optical fibre made into a cord was 210 dB/km at 660 nm, and that after a heat treatment at 125°C for 1000 hours was 220 dB/km at 660 nm, - thus the loss of light transmittance was not changed significantly. Further, the flexibility of the fibre was so great that the fibre was able to be satisfactorily wound around a rod with a diameter of 1 mm. Thus a plastic optical fibre having an excellent heat-resistance while keeping the good light transmittance and mechanical characteristics, which are comparable to those of methyl methacrylate, was obtained. Example 2 Optical fibres were obtained in the same manner as Example 1 except that the monomer composition was changed as shown in Table 1. The properties are shown in Table 1. Comparative Examples 1 and 4. Optical fibres were obtained in the same manner as Example 1 except that the monomer composition of the sheath was changed to tetrafluoropropyl α-fluoroacrylate and methyl methacrylate (85/15 in weight). Example 3 Optical fibres were obtained in the same manner as Example 1 except that the monomer composition of the core was changed as shown in Table 1 and solution polymerization was conducted after adding the monomer mixture to toluene at a monomer concentration of 70 wt% with respect to the whole mixture. Comparative Examples 2, 5 and 6 Optical fibres were obtained in the same manner as Example 3 except that the monomer composition of the core was changed as shown in Table 1 and the monomer composition of the sheath was changed to tetrafluoropropyl α-fluoroacrylate and methyl methacrylate (85/15 in weight), and in Comparative Examples 2 and 6, n-butyl mercaptan was not included. Example 4 A mixture of N-isopropyl maleimide 24.5 wt% Methyl methacrylate 45.5 wt% azo-t-octane 0.052 wt% toluene 30.0 wt% was fed to a polymerization bath at a rate of 5 kg/hour while filtering the mixture through a Teflon filter with a pore diameter of 0.1 µm. The polymerization temperature was 135°C, and the liquid level was controlled so as to obtain an average dwelling time of 4 hours. The reacted polymer solution was discharged by a metering pump at a rate of 5 kg/hour. This solution was supplied to an extruder with a vent, and the unreacted monomers and the solvent were removed at 190°C - 250°C, at 250 - 2 torr. The resultant was then introduced into a sheath-core type fibre spinning machine as a core component. After this, the procedure in Example 1 was followed to obtain an optical fibre. The reaction ratio of the polymer from the polymerization bath was 90 wt% as determined by GC measurement, and the copolymer contained 33 wt% of N-isopropyl maleimide as determined by elementary analysis. Comparative Example 3 Optical fibres were obtained in the same manner as in Example 4, except that the monomer composition of the core was changed as shown in Table 1, the content of azo-t-octane was adjusted to 0.026 wt%, and the monomer composition of the sheath was changed to tetrafluoropropyl α-fluoroacrylate and methyl methacrylate (85/15 in weight ratio). The polymerization ratio of the monomers was 90 wt% and the composition of the copolymer was 21.5:34.0:44.5 (cyclohexyl maleimide:methyl methacrylate: styrene) by weight.

Claims: A heat resisting plastic optical fibre with a sheath-core structure, characterized in that the core component comprises a copolymer of which the monomer components consist of (i) N-isopropyl maleimide and (ii) a comonomer which is methylmethacrylate with the optional addition of styrene and/or methyl acrylate, said monomer components being in the proportion of 5-70 wt % of N-isopropyl maleimide to 30-95 wt % of comonomer. A heat resisting plastic optical fibre according to claim 1, wherein the said monomer components are N-isopropyl maleimide and methylmethacrylate. A heat resisting plastic optical fibre according to claim 1 or claim 2, wherein the proportion of N-isopropyl maleimide in the said copolymer is 10 - 50 wt %. A heat resisting plastic optical fibre according to any preceding claim, wherein the optical fibre contains a stabilizer or an antioxidant. A heat resisting plastic optical fibre according to any preceding claim, wherein the sheath component is a copolymer of tetrafluoropropyl α-fluoroacrylate and methylmethacrylate. A heat resisting plastic optical fibre according to any of claims 1 to 4, wherein the sheath component is a copolymer of tetrafluoropropyl α-fluoroacrylate and trifluoroethyl α-fluoroacrylate. A method of making heat resisting plastic optical fibre by copolymerizing in solution N-isopropyl maleimide with methyl methacrylate, with the optional addition of styrene and/or methyl acrylate, the quantity of solvent being 10 to 50 wt % based on the total weight of the polymerization mixture, evaporating volatile components from the polymer thus formed and spinning said polymer as the core component together with a sheath component in a composite spinning machine with a sheath-core spinneret. A method of making heat resisting plastic optical fibre according to claim 7, wherein hydrocarbon is used as a solvent of the solution copolymerization. A method of making heat resisting plastic optical fibre according to claim 7, wherein at least one compound selected from the group consisting of toluene, xylene, butyl acetate and dioxane is used as a solvent of the solution copolymerization.

Inventors: KOBAYASHI HISAAKI, MATSUNAGA TADAYO, SUGANUMA HEIROKU, TANEICHI SHOSHIRO, KOBAYASHI,HISAAKI, MATSUNAGA, TADAYO, SUGANUMA, HEIROKU, TANEICHI, SHOSHIRO
Applicants: TORAY INDUSTRIES, TORAY INDUSTRIES, INC.
Application year: 1987
Main classification: G02B6
IPC codes: D01F8