JP2006084712A - Low heat contraction plastic optical fiber and its manufacturing method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a POF and a POF cable excellent in heat shrinkable characteristic under a high temperature environment of approximately 100-105°C. <P>SOLUTION: On the outer circumference of a core composed of a homopolymer of a methyl methacrylate unit or a copolymer of a methyl methacrylate unit and other copolymerizing monomeric unit, the POF is provided with at least one or more clad layers made of a resin having a lower refractive index than the homopolymer or the copolymer. The POF is also provided with a heat contraction starting temperature of ≥110°C in TMA measurement. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a plastic optical fiber having good optical characteristics and excellent heat shrinkage characteristics in a high temperature environment, and a method for manufacturing the same.

  Conventionally, as an optical fiber, inorganic glass-based optical fibers capable of performing excellent optical transmission over a wide wavelength range are known, and these optical fibers have been put into practical use mainly in trunk systems. There is a disadvantage that it is expensive and has poor processability. Therefore, a plastic optical fiber (hereinafter abbreviated as POF) having advantages such as lower cost, easy end face processing and handling has been developed. For example, OA and FA equipment for lighting, sensor use, or communication use. It has been put to practical use in the field of wiring between.

  POF uses a resin having a large refractive index and excellent light transmittance, such as polymethyl methacrylate (hereinafter abbreviated as PMMA), polycarbonate, polystyrene, or amorphous polyolefin for the core. It is used very frequently because of its excellent mechanical properties such as transparency, weather resistance, mechanical strength, and weather resistance.

  When the POF described above is used as a high-speed communication medium for short / medium-distance communication applications such as indoor wiring or automobile wiring, the POF cable is provided with a coating layer made of a thermoplastic resin on the outer periphery of the POF. It is used in.

  When the POF cable is used as a high-speed communication medium in an automobile, if it is laid in a high-temperature environment such as a ceiling or around the engine where the temperature is high, such as the ceiling or the engine, the POF will be subject to thermal expansion and contraction. When the shape changes and the POF protrudes or retracts (pistoning) from the coating layer, the distance between the light source or the light receiving element and the end face of the POF changes, resulting in coupling loss. In addition to an increase in the amount of light received, the communication may be hindered due to fluctuations in the amount of light received, or a wiring trouble that may result in insufficient connection with the connector portion may occur, resulting in a failure of the system.

  In view of the above circumstances, various methods have been proposed for suppressing heat shrinkage that occurs in POF or pistoning that occurs in POF cables.

  For example, in Patent Documents 1 to 4, the POF is subjected to in-line non-contact heat treatment after the stretching process, thereby maintaining the orientation of the polymer chain in the axial direction of the POF applied in the stretching process as much as possible while maintaining a high temperature environment. Methods have been proposed to reduce the occurrence of thermal shrinkage below.

  However, in the methods described in Patent Documents 1 to 4, heating of POF is insufficient, and the occurrence of heat shrinkage and pistoning cannot be sufficiently suppressed at a high temperature reaching 100 ° C. or higher. Further, when the temperature of the non-contact heat treatment is increased, there is a problem that the unevenness of the POF diameter increases.

Patent Document 5 proposes a method for reducing thermal shrinkage by applying a heat treatment while the stretched POF is wound around a plastic bobbin, but in a high temperature environment reaching 100 ° C. Could not sufficiently reduce heat shrinkage.
JP-A 63-303304 JP-A-2-68503 JP-A-5-11128 JP-A-6-201270 JP 2000-147272 A

  Accordingly, an object of the present invention is to prevent heat shrinkage and pistoning from occurring in POF and POF cables in a high temperature environment of 100 ° C. or higher, particularly about 100 to 105 ° C.

  The present invention has a refractive index higher than that of the homopolymer or copolymer on the outer periphery of a core composed of a homopolymer of methyl methacrylate units or a copolymer of methyl methacrylate units and other copolymerizable monomer units. The present invention relates to a plastic optical fiber having at least one clad layer made of a low resin and having a thermal shrinkage starting temperature of 110 ° C. or higher in TMA measurement.

  Further, the present invention provides a refractive index from the homopolymer or copolymer on the outer periphery of a core composed of a homopolymer of methyl methacrylate units or a copolymer of monomer units that can be copolymerized with methyl methacrylate units. A plastic optical fiber having at least one clad made of a low-resin resin in a heated steam atmosphere having a temperature of Tgc-30 ° C. or higher and Tgc ° C. or lower (Tgc: glass transition temperature of the core) and a relative humidity of 70% or higher. The present invention relates to a method for producing a plastic optical fiber, characterized in that heat treatment is performed at a constant length below.

  The present invention also relates to a plastic optical fiber cable in which a coating layer made of a thermoplastic resin is formed on the outer periphery of the plastic optical fiber.

  According to the present invention, it is possible to provide a POF and a POF cable with extremely small thermal shrinkage and pistoning in a high temperature environment of about 100 to 105 ° C. without causing deterioration of optical characteristics.

  Hereinafter, the POF of the present invention and the production method thereof will be specifically described.

  The POF of the present invention has a core-clad structure having one or more clad layers made of a resin having a refractive index lower than that of the core on the outer periphery of the core.

  As the core, an amorphous transparent polymer that is excellent in mechanical properties, such as PMMA, or a copolymer composed of methyl methacrylate (MMA) and other copolymerizable monomer units is used. . When the core is the latter copolymer, from the viewpoint of maintaining transparency and mechanical properties, the total of all monomer units of the copolymer is 100% by weight, the MMA unit is 70% by weight or more, The amount of other monomer units copolymerizable with the MMA unit is preferably 30% by weight or less.

  Examples of monomers copolymerizable with MMA include methacrylic acid esters such as cyclohexyl methacrylate, isobornyl methacrylate, benzyl methacrylate, phenyl methacrylate, 2-2-2trifluoroethyl methacrylate, and acrylic acid. For the purpose of improving the heat resistance, acrylic esters such as methyl and ethyl acrylate, and monomers such as maleimide compounds such as N-cyclohexylmaleimide and N-isopropylmaleimide.

  The above (co) polymer used for the core can be produced by a known polymerization method. However, since it is necessary to sufficiently consider the surface of foreign matters from the viewpoint of sufficiently reducing POF transmission loss, It is preferable to produce by bulk polymerization or continuous solution polymerization.

  The POF having the above-described core-cladding structure is usually subjected to a heat treatment for removing strain remaining in the POF after undergoing a melt compound spinning process and a stretching process in the manufacturing process. Even if the applied POF is placed in a high temperature environment of about 105 ° C. for a long time, it contracts.

  Therefore, the POF of the present invention is obtained by subjecting the POF after stretching treatment to a heated steam atmosphere having a temperature of Tgc-30 ° C. or higher and Tgc ° C. or lower (Tgc: glass transition temperature of the core material) and a relative humidity of 70% or higher. The heat shrinkage starting temperature is raised to 110 ° C. or higher by heat treatment in FIG. Such POF can suppress the occurrence of heat shrinkage even in a high temperature environment of about 100 to 105 ° C.

  This is because the copolymer consisting of PMMA or MMA unit and other copolymerizable monomer units constituting the core has a saturated water absorption of about 2%, which is relatively large among general-purpose resins. Since the glass transition temperature is lowered and the heat treatment is performed under the heating steam as described above, the mobility of the molecular chain is enhanced, and the residual strain inside the POF is easily removed as compared with the case where the heat treatment is performed in a dry heat environment. Because.

  The temperature of the heated steam atmosphere is preferably higher, but if it is set to a temperature higher than Tgc ° C., the molecular orientation originally imparted to the POF by the stretching treatment and the entanglement between the polymers are alleviated too much, and the mechanical strength is increased. Problems such as reduction and fusion between POFs occur. On the other hand, when the temperature is lower than Tgc-30 ° C., the heat treatment takes too much time, and the productivity is lowered. For the above reasons, the temperature of the heated steam atmosphere needs to be in the range of Tgc-30 ° C. or higher and Tgc ° C. or lower, and more preferably Tgc-15 ° C. to Tgc-5 ° C.

  When the relative humidity of the heated steam atmosphere is lower than 70%, sufficient moisture does not penetrate into the core, so that the effect of removing the distortion cannot be obtained sufficiently. Further, when the treatment is performed in 100% relative humidity, that is, in warm water, the effect of the heat treatment is great, but distortion occurs in the fine structure of the POF, resulting in a reduction in transmission loss. Therefore, the relative humidity of the heated steam atmosphere needs to be 70% or more, and 72% or more is preferable in order to more reliably remove the strain remaining in the POF.

The time required for the above heat treatment needs to be adjusted as appropriate because it depends on the POF stretching ratio during the POF production, but it is preferable to adjust the time to be approximately 100 hours or less in consideration of productivity.

  The POF that has been heat-treated as described above has a heat shrinkage start temperature of 110 ° C. or higher in thermal stress strain measurement (TMA measurement) described later, and the POF when heat-treated for 24 hours under 105 ° C. dry heat. The heat shrinkage rate can be suppressed to a very small value of 1% or less.

  If the heat shrinkage start temperature of POF is 110 ° C or higher and the heat shrinkage rate is 1% or less, the connector and fiber end face when used as a POF cable in a high temperature environment such as a car interior ceiling or an engine room in summer. It is difficult to cause problems such as a gap between them, and the coupling loss can be kept very small.

  In addition, in order to obtain POF with more excellent mechanical strength, the above heat treatment is preferably performed at a constant length. Specifically, a method of leaving the POF in a constant temperature bath set in the above-described temperature and relative humidity conditions in a wound-up state or in a state wound around a plastic bobbin can be mentioned.

  In addition, it is preferable that the stretch rate of POF before heat processing exists in the range of 1.3-3, and if it is 1.4-2.1, it is more preferable. When the stretch ratio is less than 1.3, the mechanical strength of the POF is insufficient, and when the POF is bent, it tends to break. If the stretch ratio is greater than 3, the shrinkage of the POF during heat treatment increases, and particularly when the heat treatment is performed with the POF wound on a plastic bobbin, the surface of the POF is damaged or the transmission loss of the POF deteriorates. In addition, it takes a very long time to sufficiently remove the strain, and it is necessary to perform heat treatment many times, so that productivity tends to decrease.

  By the way, when a constant length heat treatment is performed by winding POF on a plastic bobbin, when the POF contracts, the cladding surface tends to be damaged or the transparency is lowered, so that the POF transmission loss tends to increase.

  When such a problem occurs, the outer peripheral portion of the POF, that is, the outermost layer of the clad, contains at least a tetrafluoroethylene (TFE) unit and has a heat of crystal melting in differential scanning calorimetry (DSC) of 40 mJ / mg or less. It is preferable to use an olefin resin. Even if it is said resin, while being low in crystallinity, it can maintain a high optical characteristic even if it exposes to high temperature for a long time, without impairing the other characteristic as a cladding material of POF. .

  When the heat of crystal fusion is greater than 40 mJ / mg, the resin has high crystallinity and tends to become cloudy in a high-temperature environment, the initial transmission layer loss of POF increases, or POF has a high (wet) heat. When left in the environment for a long time, the transmission loss tends to increase remarkably. In order to prevent an increase in transmission loss more reliably, the heat of crystal fusion is preferably 15 mJ / mg or less.

Specifically, as the fluorine-containing olefin resin containing the TFE unit, a binary copolymer comprising 60 to 90% by mass of vinylidene fluoride (VdF) units and 10 to 40% by mass of TFE units,
A terpolymer comprising 10 to 60% by mass of VdF units, 20 to 70% by mass of TFE units, and 5 to 35% by mass of hexafluoropropylene (HFP) units;
A terpolymer comprising 5 to 25% by mass of VdF units, 50 to 80% by mass of TFE units, and 5 to 25% by mass of perfluoro (fluoro) alkyl vinyl ether units;
A terpolymer comprising 5 to 60% by mass of ethylene units, 25 to 70% by mass of TFE units, and 5 to 45% by mass of HFP units,
A quaternary copolymer comprising 10 to 30% by mass of VdF units, 40 to 69% by mass of TFE units, 21 to 40% by mass of HFP units, and 1 to 15% by mass of perfluoro (fluoro) alkyl vinyl ether units,
A binary copolymer comprising 40 to 90% by mass of TFE units and 10 to 60% by mass of perfluoro (fluoro) alkyl vinyl ether units;
Examples thereof include binary copolymers composed of 30 to 75% by mass of TFE units and 25 to 70% by mass of HFP units.

In addition, as said perfluoro (fluoro) alkyl vinyl ether unit (henceforth "FVE"), following general formula (I)
CF ₂ = CF— (OCF ₂ CF (CF ₃ )) _a O—R _f2 (I)
(In the formula, R _f2 represents an alkyl group having 1 to 8 carbon atoms, a fluoroalkyl group, an alkoxylalkyl group or a fluoroalkoxylalkyl group, and a is an integer of 0-3.)
The structural unit shown by can be mentioned.
Furthermore, the FVE unit is preferably a compound unit represented by any one of the following general formulas (II) to (V).
_{CF 2 = CFO (CF 2)} n -OCF 3 (II)
(Where n is an integer from 1 to 3)
CF ₂ = CF (OCF ₂ CF (CF ₃ )) _m O (CF ₂ ) _{n ′} CF ₃ (III)
(Wherein, m is an integer from 0 to 3, and n ′ is an integer from 0 to 3)
CF ₂ = CFO (CH ₂ ) _l (CF ₂ ) _{n ″} CF ₃ (IV)
(Wherein l is an integer of 1 to 3, n ″ is an integer of 0 to 3)
CF ₂ = CFO (CH ₂ ) _{l ′} CH ₃ (V)
(Wherein l 'is an integer of 0 to 3)

More specific FVE _{unit, CF 2 = CFOCF 3, CF} 2 = CFOCF 2 CF 3, CF 2 = CFOCF 2 CF 2 CF 3, CF 2 = CFOCH 2 CF 3, CF 2 = CFOCH 2 CF 2 CF 3 A unit of at least one compound selected from the group consisting of CF ₂ = CFOCH ₂ CF ₂ CF ₂ CF ₃ , CF ₂ = CFOCH ₃ , CF ₂ = CFOCH ₂ CH ₃ and CF ₂ = CFOCH ₂ CH ₂ CH ₃ It is preferable from the viewpoint of low cost of the raw material.

  When the clad is composed of a plurality of layers, a known material can be used for the clad of the inner layer, but a homopolymer of a fluorinated methacrylate unit, a copolymer of a fluorinated methacrylate unit and a methacrylic ester monomer unit Good transmission characteristics using a copolymer mainly composed of vinylidene fluoride units, such as a copolymer of vinylidene fluoride units and tetrafluoroethylene units, α-fluoromethacrylate resins, and mixtures thereof It is preferable at the point which can obtain POF which has this.

  The POF of the present invention is usually used in the form of a POF cable in which a coating layer made of a thermoplastic resin is disposed on the outer periphery of the POF so as to improve flex resistance and moist heat resistance.

  Examples of the thermoplastic resin used for the coating layer include conventionally used polyamide resins, polyvinyl chloride resins, polyethylene resins, polyurethane resins, polychlorotrifluoroethylene copolymers, and the like.

  In particular, when a POF cable is used for in-car communication, it is preferable to use a polyamide-based resin for the coating layer from the viewpoint of heat resistance, flex resistance, chemical resistance, and impact resistance. Examples of the polyamide resin include nylon 66, nylon 6, nylon 11, nylon 12, nylon 612, nylon 621, various copolymer nylons including these structures, nylon elastomers, copolymers thereof, and mixtures thereof. Among these, nylon 11 or nylon 12 or a copolymer formed by combining monomer units forming these polymers is preferable. These resins have low temperature conditions at the time of coating, have good moldability in the coating process, and do not easily cause thermal and mechanical damage to POF. Furthermore, since the dimensional stability in a high temperature environment is excellent, the occurrence of pistoning can be effectively prevented.

  In order to prevent external light from entering the POF, the thermoplastic resin may contain a light shielding agent such as carbon black. Moreover, in order to improve the discriminability and designability of the POF cable, a chromatic colorant can be contained in the covering material. As the colorant, known dyes and inorganic ones are used, but inorganic pigments are preferably used from the viewpoint of heat resistance.

  In addition, in order to impart flame retardancy to the coating layer, a flame retardant may be included in the thermoplastic resin in the previous period. As the flame retardant, known flame retardants such as metal hydroxides, phosphorus compounds, and triazine compounds can be used. When a polyamide resin is used as the main component of the coating layer, a triazine compound is preferable, and melamine cyanurate is particularly preferable.

  The coating layer is not limited to a single layer, and may be formed from a plurality of layers by further coating a resin having desired performance such as cable identification and flame retardancy.

  The covering layer can be formed by a method generally used as a POF cable forming method. However, the method of forming a covering layer using a crosshead die sufficiently achieves the effects of the present invention. This is preferable because an expressed POF cable can be obtained.

  Further, by providing a plug for connection at one or both ends of the POF cable, it can be used as a POF cable with a plug, a point sensor, or the like. In addition, what is necessary is just to use what is normally used as a plug of a POF cable as the plug used according to the objective.

  EXAMPLES Hereinafter, although an Example demonstrates this invention in detail, this invention is not limited by these Examples. In addition, the evaluation method and manufacturing apparatus used in the Example are as follows.

[Heat shrinkage start temperature]
A thermal stress strain measuring device (TMA measuring device) (Seiko Instruments Inc., TMA / SS120) was used for the measurement. The measurement conditions were a temperature rise rate of 5 ° C./min and a thermal shrinkage rate when a constant load of 9.23 mN / m ² was applied to the sample. In addition, as shown in FIG. 1, the intersection of the line which extended the base line before shrinkage | contraction start to the high temperature side, and the tangent drawn by the point where inclination becomes the maximum after shrinkage | contraction start was made into thermal shrinkage start temperature.

[Glass transition temperature (Tg), heat of crystal melting]
A differential scanning calorimeter (DSC) (manufactured by Seiko Instruments Inc., DSC-220) was used for the measurement. The sample was heated up to 200 ° C. at a heating rate of 10 ° C./min, held and melted for 5 minutes, then cooled down to 0 ° C. at a cooling rate of −10 ° C./min, and then heated again at 10 ° C./min. The glass transition temperature and the crystal melting heat at this time were determined.

[Heat shrinkage]
The POF marked at intervals of 60 cm was placed in a dryer set at 105 ° C. At this time, the POF was hung in the dryer so that the measurement unit did not touch the inner member surface such as the wall surface or shelf of the dryer body. After 24 hours, the length (cm) of the measurement part was measured, and the thermal contraction rate was calculated by the following formula.
Thermal contraction rate (%) = ((60− (measured part length after 24 hours)) / 60) × 100

[Transmission loss]
A transmission loss at a wavelength of 650 nm at an incident NA = 0.1 was measured by a cut-back method of 25-5 m.

[Pistoning evaluation]
A POF cable having a test length of 200 mm was heat-treated at 105 ° C. for 24 hours, the difference between the length of the cable after the treatment and the strand was measured, and the amount of pistoning was obtained.

[Comparative Example 1]
As shown in Table 1, the core is PMMA, the first cladding is 51 parts by weight of 2,2,2-trifluoroethyl methacrylate (3FM), 1,1,2,2-tetrafluoroperfluorodecyl methacrylate (17FM) ) 30 parts by weight, 18 parts by weight of MMA, 1 part by weight of methacrylic acid (MAA) copolymer, 48 parts by weight of VdF, 43 parts by weight of TFE, and 9 parts by weight of HFP in the second cladding, respectively. Was fed to a concentric composite nozzle and melt-spun into a three-layer fiber of core / first clad / second clad at 220 ° C. Next, the undrawn yarn was passed through a heating drawing furnace and drawn twice while heating to prepare a POF having a core diameter of 970 mm and a cladding thickness of 15 mm. Table 2 shows the results of various evaluations of the POF thus obtained.

[Example 1]
The POF obtained in Comparative Example 1 was allowed to stand for 96 hours in a thermostatic bath set at a temperature of 85 ° C. and a relative humidity of 95%, and was subjected to heat treatment. After the heat treatment, POF was dried at 50 ° C. for 50 hours in order to sufficiently remove the moisture present inside. Table 2 shows the results of various evaluations of the POF thus obtained.

[Example 2]
The POF obtained in Comparative Example 1 was subjected to heat treatment and drying treatment under the same conditions as in Example 1 except that the relative humidity was set to 85%. Table 2 shows the results of various evaluations on the obtained POF.

Example 3
The POF obtained in Comparative Example 1 was subjected to heat treatment and drying treatment under the same conditions as in Example 1 except that the relative humidity was set to 75%. Table 2 shows the results of various evaluations on the obtained POF.

Example 4
The outer periphery of the POF obtained in Example 1 was coated with a polyamide 12 resin (manufactured by Daicel Degussa, Daiamide-L1640) with a thickness of 250 μm using a crosshead cable coating apparatus, and a POF having an outer diameter of 1.5 mm. Created a cable. The transmission loss of this POF cable was 137 dB / km. Moreover, the size of the pistoning after heat-treating this POF cable at 105 ° C. for 24 hours was 30 μm.

[Comparative Example 2]
The POF obtained in Comparative Example 1 was subjected to heat treatment and drying treatment under the same conditions as in Example 1 except that the temperature was set to 75 ° C. Table 2 shows the results of various evaluations on the obtained POF.

[Comparative Example 3]
Table 2 shows the results of various evaluations of the POF obtained in this manner, after the POF obtained in Comparative Example 1 was allowed to stand for 95 hours and heat-treated in an oven at 85 ° C. under dry conditions. Show.

[Comparative Example 4]
The POF obtained in Comparative Example 1 was immersed in warm water at 85 ° C. for 95 hours, and then dried at 50 ° C. for 50 hours. Table 2 shows the results of various evaluations of the POF thus obtained.

[Comparative Example 5]
A POF was produced in the same manner as in Comparative Example 1 except that a copolymer comprising 80 parts by weight of VdF and 20 parts by weight of TFE was used for the second cladding of POF. Table 2 shows the results of various evaluations on the obtained POF.

Example 5
The POF obtained in Comparative Example 5 was heat-treated under the same conditions as in Example 2. Table 2 shows the results of evaluating various characteristics of the obtained POF.

Example 6
The POF obtained in Comparative Example 5 was heat-treated under the same conditions as in Example 1. Table 2 shows the results of evaluating various characteristics of the obtained POF.

  As is apparent from Table 2, the POFs of Examples 1 to 3 had good transmission characteristics (136 to 137 dB / km or less), and the thermal shrinkage rate under a high temperature environment was 1.0% or less. On the other hand, the POFs of Comparative Examples 1 to 3 had a low heat shrinkage start temperature and a large heat shrinkage rate. In addition, the POF obtained in Comparative Example 4 showed a significant decrease in transmission loss.

The POFs of Examples 5 to 6 had good heat shrinkage characteristics, but a slight decrease in transmission loss was observed.

表１中の略号は下記の化合物を示す。
ＶｄＦ：フッ化ビニリデン
ＴＦＥ：テトラフルオロエチレン
ＨＦＰ：ヘキサフルオロプロピレン
ＰＭＭＡ：ポリメタクリル酸メチル
ＭＭＡ：メタクリル酸メチル
ＭＡＡ：メタクリル酸
３ＦＭ：２，２，２−トリフルオロエチルメタクリレート
１７ＦＭ：２−（パーフルオロオクチル）エチルメタクリレート

The abbreviations in Table 1 indicate the following compounds.
VdF: vinylidene fluoride TFE: tetrafluoroethylene HFP: hexafluoropropylene PMMA: polymethyl methacrylate MMA: methyl methacrylate MAA: methacrylic acid 3FM: 2,2,2-trifluoroethyl methacrylate 17FM: 2- (perfluorooctyl ) Ethyl methacrylate

It is a graph which shows the measurement result of the thermal contraction start temperature by a thermal stress strain measuring device (TMA measurement).

Claims (3)

  It consists of a resin having a refractive index lower than that of the homopolymer or copolymer on the outer periphery of the core consisting of a homopolymer of methyl methacrylate units or a copolymer of methyl methacrylate units and other copolymerizable monomer units. A plastic optical fiber having at least one clad layer and having a thermal shrinkage starting temperature of 110 ° C. or higher in TMA measurement.   A plastic optical fiber cable, wherein a coating layer made of a thermoplastic resin is formed on the outer periphery of the plastic optical fiber according to claim 1. It consists of a resin having a refractive index lower than that of the homopolymer or copolymer on the outer periphery of the core consisting of a homopolymer of methyl methacrylate units or a copolymer of methyl methacrylate units and other copolymerizable monomer units. A plastic optical fiber having at least one clad layer is heat-treated at a constant length in a heated steam atmosphere having a temperature of Tgc-30 ° C. or higher and Tgc ° C. or lower (Tgc: core glass transition temperature) and a relative humidity of 70% or higher. A method for producing a plastic optical fiber, characterized in that:

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