JPH0225481B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing fibers with excellent light transmission properties, that is, optical fibers.

Optical fibers have been known for a long time, and optical fibers using glass and optical fibers using plastic have already been put into practical use. FIELD OF THE INVENTION This invention relates to plastic optical fibers.

Although glass optical fibers have extremely excellent light transmission properties, they have drawbacks such as difficult optical fiber connection methods, heavy weight, poor flexibility, and high cost. On the other hand, optical fibers made of organic polymers have excellent flexibility, are lightweight, and can be manufactured at low cost.
For example, an invention as shown in Japanese Patent Publication No. 53-42261 has been made.

For optical fibers made of organic polymers, it is important that the core material has good light transmittance, and that the core-sheath double structure lowers the refractive index of the sheath by 3% or more than the refractive index of the core. It is.

From this point of view, polymethyl methacrylate and polystyrene are known as organic polymers useful as core components.

The present inventors used polymethyl methacrylate as a core component material to produce an optical fiber with excellent weather resistance, chemical resistance, and light transmission properties.
A detailed study was conducted on the influence of the manufacturing conditions of polymethyl methacrylate and the manufacturing conditions of optical fibers. Optical fibers that use polymethyl methacrylate as a core material by suspension polymerization, which is commonly used in the production of polymethyl methacrylate, are prone to dust and foreign matter in the polymer that adversely affect optical transmission.
It is known that it is not desirable. Also, special public service in 1977-
42261, an optical fiber made of polymethyl methacrylate as a core material produced by the continuous bulk polymerization method shown in Japanese Patent Publication No. 53-42260, has little dust in the polymer and has fairly good optical transmission properties, but It has been found that the optical transmission characteristics are significantly reduced by trace impurities and trace amounts of thermal decomposition products of polymethyl methacrylate.

From this point of view, the present inventors have attempted to reduce the generation of thermal decomposition products of polymethyl methacrylate by employing special polymerization conditions when polymerizing polymethyl methacrylate using a continuous bulk polymerization method in the production of optical fibers. By manufacturing the core component polymer using special continuous bulk polymerization conditions, the core polymer contains fewer impurities. The present invention was achieved by discovering that optical fibers with good heat resistance and excellent light transmission properties can be produced.

That is, in the present invention, the core component material is polymethyl methacrylate or a copolymer mainly composed of methyl methacrylate, and the sheath component material is a fluorine-containing polymer containing at least 20% by weight of fluorine, which has a core-sheath structure. When producing an optical fiber having a core component, a mixture of a monomer, a radical polymerization initiator, and a chain transfer agent is continuously fed into one reaction tank, and the reaction mixture in the reaction tank is heated to a temperature of 90°C or higher and 130°C.
The reaction mixture is stirred and mixed substantially uniformly at a temperature of less than
Polymethyl methacrylate or a copolymer containing methyl methacrylate as the main component produced by polymerizing to satisfy % or more and less than 50%, and then controlling the volatile matter separation step and spinning step at 150°C or more and less than 300°C. This is a method for producing an optical fiber with excellent light transmission properties, which is characterized by using a polymer.

Next, the features of the present invention will be described in detail. The features of the present invention are as follows. In other words, polymethyl methacrylate, which is a core material component, is a polymer that undergoes continuous bulk polymerization using an extremely small amount of initiator at temperatures above 90°C and below 130°C, and has excellent heat resistance. The manufacturing process and the manufacturing of optical fibers are carried out continuously, and the thermal history during the manufacturing process is reduced as much as possible, thereby reducing the formation of color-changing decomposition products in the core material.

According to the results of intensive studies conducted by the present inventors, in order to improve the performance of optical fibers, it is necessary not only to prevent impurities such as dust from being contained in the core component material of optical fibers, but also to ensure that impurities such as dust are not included in the entire process. It is necessary to reduce the thermal history of polymethyl methacrylate.

In optical fibers having a core-sheath structure, light is actually transmitted through the core polymer. At this time, the interfacial state with the sheath component polymer and the transparency of the sheath component polymer itself are important issues in order to prevent light leakage due to light scattering at the core-sheath interface, but there are also important issues to consider. Therefore, it is extremely important to improve the light transmittance of the core component material.

In this sense, we investigated the manufacturing method of polymethyl methacrylate, which is a core component material, and found that polymethyl methacrylate with excellent heat resistance was produced under special polymerization conditions during continuous bulk polymerization, and the thermal history was reduced. If this polymer is used as a core component material under the following conditions, it will have a much superior optical transmittance compared to polymethyl methacrylate produced by conventional methods such as continuous bulk polymerization and suspension polymerization. It became clear that fibers could be obtained.

The polymethyl methacrylate used in the present invention is produced as follows. Manufactured at 90℃ or higher
It is carried out in two steps: a continuous bulk polymerization step at less than 130° C., followed by a continuous separation step of volatiles, mainly residual unreacted monomers, designed to reduce the thermal history.

In the continuous bulk polymerization process, one reaction vessel is used, and a methyl methacrylate monomer mixture containing 1.0 mol% or less and 0.0005 mol% or more of a radical polymerization initiator and 0.01 mol% or more and less than 10 mol% of mercaptans is used. The reaction mixture in the reaction tank is continuously supplied to the reaction tank, and the reaction mixture in the reaction tank is mixed and stirred substantially uniformly at a temperature of 90°C or higher and lower than 130°C, and the polymer content (wt%) φ of the reaction mixture is 30%. Continuous polymerization is carried out by continuously taking out the polymer while maintaining a substantially constant value of less than 50%. From the reaction mixture containing the polymer thus produced, the polymer is produced by continuously separating and removing volatile substances mainly consisting of unreacted monomers.

Examples of the radical polymerization initiator used in the production of the polymer include di-tert-butyl peroxide, methyl ethyl ketone peroxide,
tert-butyl perphthalate, tert-butyl perbenzoate, tert-butyl peracetate, methyl isobutyl ketone peroxide, lauroyl peroxide, cyclohexanone peroxide, tert-butyl peroctanoate, tert-
Organic peroxides such as butyl perisobutyrate, tert-butyl peroxyisopropyl carbonate, di-isopropyl peroxy-dicarbonate, and dimethyl 2,2' azobisisobutyrate, 1,1'-azobiscyclohexane, carbonitrile, 2-phenylazo-2,4-dimethyl-
4methoxyvaleronitrile, 2-carbamoylazoisobutyronitrile, 2,2'-azobis2,4
Examples include azo compounds such as -dimethylvaleronitrile and 2,2'azobisisobutyronitrile.

Such a radical polymerization initiator must be selected in an amount that can be appropriately decomposed depending on a predetermined polymerization temperature. As claimed in Japanese Patent Publication No. 53-42261, the product of the square root of the number of moles of initiator used and the reciprocal of the square root of the half-life of the radical initiator at the polymerization temperature must satisfy the following formula (1). .

20≧A ^1/2・B ^-1/2 ×10 ³ ...(1) However, A = number of moles of radical initiator in 100 g of monomer feed B = half-life (time) of radical polymerization initiator at polymerization temperature ) In other words, the right-hand side of equation (1) indicates the amount of radicals generated, and in normal suspension polymerization, it takes a fairly large value, but when manufacturing optical fibers by continuous bulk polymerization, the amount of radicals generated is However, in order to reduce the heat resistance, it is necessary to reduce the amount of generated radicals as much as possible and allow the polymerization to proceed by chain transfer reaction, thereby strengthening the heat resistance of the polymer terminal and preventing the formation of thermal decomposition products. Therefore, the reaction temperature may be set in accordance with the decomposition temperature of the radical polymerization initiator, and the amount of the radical polymerization initiator may be determined so as to match equation (1).

Mercaptans used in the present invention include n-propyl, n-butyl, isobutyl, n-
-pentyl, isopentyl, neopentyl, n-
Primary mercaptans such as hexyl, isohexyl, n-heptyl, n-octyl, n-decyl, n-dodecyl, n-tetradecyl, n-hexadecyl, n-octadecyl, or isopropyl,
sec-butyl, sec-pentyl, sec-hexyl,
sec-heptyl, sec-octyl, sec-nonyl,
Secondary mercaptans such as sec-decyl, or
tert-butyl, tert-hexyl, tert-heptyl,
Tertiary mercaptans such as tert-octyl, tert-nonyl, tert-dodecyl, or aromatic mercaptans such as phenyl mercaptan, 4-tert-butyl-O-thiocresol, 4-tert-butylthiophenol, or trimethylol. Examples include polyfunctional mercaptans such as propane trithioglycolate, trimethylolpropane trismercaptopropionate, pentaerythritol tetrathioglycolate, and the like. Among these mercaptans, tert-butyl and n-octyl mercaptan are particularly preferred.

The amount of mercaptan used is preferably 0.1 mol% or more and less than 10 mol% based on the monomer. When the amount of mercaptan used is less than 0.1 mol%, the degree of polymerization increases and the polymer undergoes a high thermal history after the continuous degassing step. If it is added in an amount of 10 mol % or more, the degree of polymerization will decrease and the fiber strength will decrease, which is not preferable. The polymerization degree of the polymer is 100~
The amount of mercaptan is determined to be 2000, preferably in the range of 800 to 1500.

The polymerization reaction is carried out by substantially uniformly mixing and stirring the reaction mixture at a temperature of 90°C or higher and lower than 130°C. If the temperature of the reaction mixture is 90° C. or lower, the viscosity of the reactant becomes high, making it difficult to control the reaction. Moreover, if the temperature of the reaction mixture is 130° C. or higher, side reactions will be produced, which will adversely affect the performance of the optical fiber. For example, when methyl methacrylate is polymerized at a high temperature, a dimer is produced, and as shown in FIG. 1, the amount of the dimer produced increases as the polymerization temperature increases, degrading the performance of the optical fiber. Therefore, the polymerization temperature is 90℃ or higher.
The temperature is preferably less than 130°C, preferably 110°C or more and 120°C or less.

In the reaction vessel, the polymerization temperature is controlled by conventional jacket heating. At this time, adhesion of polymers can be prevented by cooling the upper part of the tank.

The polymer content of the reaction mixture must be controlled within the range of 30 weight percent or more and less than 50 weight percent. In the range of 50% by weight or more, the viscosity of the polymer mixture increases, making it difficult to control the polymer content. On the other hand, if the amount is less than 30% by weight, the cost of separating volatiles mainly composed of unreacted monomers increases and the industrial merit decreases.

In the volatile matter separation process, which consists mainly of unreacted monomers, a continuously fed reaction mixture with a predetermined polymerization rate is heated to 150 to 300°C under pressure to continuously remove most of the volatile matter. Separate and remove. The equipment used for volatiles separation is generally a pent extruder, but in order to improve the performance of optical fibers, it is important to prevent the generation of thermal decomposition products of polymers, and for this purpose, the volatiles separation process is must be processed at the lowest possible temperature and in the shortest possible time. Specifically, a reaction mixture with a predetermined polymerization rate is
Pressurized to 100Kg/cm ² G, 150 to 300℃, preferably
Volatiles separation is carried out by raising the temperature to 180-220°C and blowing into a pent extruder. Incidentally, FIG. 2 shows the state of coloration when the polymer mixture is heated under predetermined temperature conditions. The higher the temperature and the longer the residence time, the greater the degree of color banding, which deteriorates the performance of the optical fiber. The color band was measured as a 10% by weight chloroform solution using the yellow index based on JISK7103.

As described above, in the present invention, in the production of the core component polymer, a relatively small amount of catalyst and a considerably large amount of mercaptan are used compared to this amount of catalyst,
After polymerizing continuously at a relatively low reaction rate to obtain a polymer with good heat resistance, unreacted monomers are continuously removed at a relatively low temperature to produce a polymer. By guiding the coalescence into a spinning machine and making it into fibers, we can produce a polymer with extremely high heat resistance and less contamination of foreign substances such as thermal decomposition products and dust.
The method is characterized in that it uses the polymer as a core material to produce optical fibers with excellent light transmission properties.

The above-described method for polymerizing methyl methacrylate, which is the basis of the present invention, can be applied not only to producing polymethyl methacrylate but also to producing a copolymer containing at least 80% by weight of methyl methacrylate units. The copolymerization component is selected from alkyl acrylates or alkyl methacrylates (excluding methyl methacrylate) having an alkyl group having 1 to 18 carbon atoms, such as methyl, ethyl,
Examples include alkyl acrylates or alkyl methacrylates having alkyl groups such as n-propyl, n-butyl, 2-ethylhexyl, dodecyl, and stearyl. Alternatively, esters of acrylic acid or methacrylic acid containing a benzene nucleus, such as benzyl methacrylate, can also be used as a copolymerization component.

Before transferring the polymer to the spinning process, it is possible to take it out in the form of pellets and heat it again to melt it and spin it, but this increases the chance that foreign matter such as dust gets mixed in during the process, and it is difficult to heat and melt the polymer. By repeating this process, there are many opportunities to generate thermal decomposition products of the polymer. Therefore, it is preferable that the core polymer degassing extrusion step and the spinning step are carried out continuously. Furthermore, although the core polymer has sufficient heat resistance to be used as a normal molding material resin, from the point of view of optical fiber performance, it may become discolored as shown in Figure 2. Heating operations should be omitted as much as possible. Also, if the polymer is left in the air in the form of pellets,
Small particles floating in the air may adhere to the resin, and small particles may also adhere to the resin during the process of drying the pellets in a vacuum dryer. Such dust greatly reduces the performance of optical fibers. Therefore, compared to conventional resin, fiber shaping, and spinning technologies, the production of optical fibers requires advanced dust-proofing equipment and deaeration equipment to suppress the incorporation of foreign substances such as dust. be. However, it is necessary to carry out process control with equipment that can satisfy these industrial requirements, which means that a fairly sophisticated management system is required in terms of productivity and equipment maintenance. This method, which directly connects spinning equipment to continuous bulk polymerization that can sufficiently remove dust without installing dust-proof equipment and reduce thermal history, is suitable for industrial optical fiber production. This is an advantageous method.

In the present invention, a fluorine-containing polymer containing at least 30% by weight of fluorine and having a refractive index of 1.43 or less is used as the sheath-forming polymer for the optical transmission fiber having a core-sheath structure.

A specific example of such a fluorine-containing polymer is the general formula (Here, m is an integer from 1 to 5, n is an integer from 1 to 10,
X is F or H or Cl, Z is H or CH ₃ ) or (Here, Y is H or CF ₃ ) Polymers of fluorine-containing acrylic esters or methacrylic esters, vinylidene fluoride, tetrafluoroethylene, hexafluoropropylene, etc. alone or copolymers, preferably copolymers. Polymers such as coalescence can be mentioned.

These sheath component polymers can be produced by conventionally known methods. In the case of sheath component polymers, the manufacturing method does not have as much of an effect on optical transmission as in the case of core component polymers, so it is best to manufacture sheath component polymers by preventing the contamination of foreign substances such as dust. .

The production of the optical transmission fiber having a core-sheath structure according to the present invention can be broadly classified into the following two methods.

The first method is to attach a spinning machine with a core-sheath spinneret to the final outlet of the polymer in the continuous polymerization process, and use the polymer as the core component and the previously manufactured fiber as the sheath component. This is a method in which a melt of a polymer containing 100% carbon is extruded and continuous composite spinning is performed. Here, the easiest method is to store the sheath component polymer in advance as pellets and then remelt it as necessary. A method can be used in which polymerization is carried out and the polymer is continuously transferred to a spinning process.

In the second method, a spinning machine with a conventional spinneret is attached to the final outlet of the polymer in the continuous polymerization step, and the core component polymer is first spun alone, cooled, and stretched in some cases. Thereafter, a concentrated solution of the sheath component polymer is continuously coated on the fibrous core material, and then the solvent of the sheath component polymer is removed to obtain a light transmission fiber.

The melt spinning temperature varies somewhat depending on the properties of the core and sheath polymers, but is usually 180 to 280°C.
Preferably it is 200-265°C. When employing such a composite spinning method using a core-sheath spinneret, it is important to bring the melt viscosities of the core component polymer and the sheath component polymer as close as possible in order to produce uniform fibers. Generally speaking, methods for changing the polymer and melt viscosity include changing the molecular weight of the polymer and forming a copolymer.

Composite spun optical fibers are heated at 100 to 160℃ with the main purpose of imparting toughness against curling.
Stretched at an appropriate temperature. In order to provide sufficient mechanical performance without deteriorating optical performance, the elongation ratio is 1.3 times or more, preferably 1.5 to 2.5.
A double stretching process is applied.

When coating a core polymer with a sheath polymer solution, it is generally preferred to use the sheath polymer solution as a concentrated solution. The fibrous core component polymer that has passed through a concentrated solution of the sheath component polymer stored in the upper part of the die is continuously taken out through the die, where the core component polymer is coated with the sheath component polymer at a constant thickness. be done. Thereafter, the adhering solvent is removed by an appropriate method (for example, heating to a constant temperature) to produce an optical transmission fiber having a core-sheath structure.

When preparing a concentrated solution of the sheath component polymer, a solvent that dissolves the sheath component polymer but does not dissolve the core component polymer, and a low boiling point solvent is particularly preferable, but a solvent that dissolves the core component polymer is particularly preferred. Even if a solvent is used, if a concentrated solution of the sheath component polymer is used, modification of the core component by the solvent can be almost eliminated.

The solvent for dissolving the sheath component polymer that can be used in the present invention varies slightly depending on the sheath component polymer used, but generally halogenated solvents such as 1.1.2-trifluoro-1.2.2-trichloroethane are used. hydrocarbons, ketones such as acetone, methyl ethyl ketone, and methyl butyl ketone, amide solvents such as dimethylformamide and dimethyl acetamide, acetate esters such as methyl acetate, ethyl acetate, propyl acetate, and butyl acetate, or fluorine-containing acrylic acid. Alternatively, the methacrylic acid ester monomer itself can be mentioned. The concentration of polymer in solutions of sheath component polymers using these solvents is usually 10
~60% by weight, preferably 20-50% by weight.

The present invention will be explained in more detail with reference to Examples below.

Example 1 A core component polymer was produced using an apparatus system in which a storage tank, a continuous supply pump, a reaction tank equipped with a paddle spiral stirrer, a reactant take-off pump, and a volatile matter separator were connected in series. The internal volume of the reaction tank was 40, and a single screw vent extruder was used as the volatile matter separator.

A monomer mixture consisting of 100 parts of methyl methacrylate, 0.37 parts of tert-butyl mercaptan, and 0.0030 parts of di-tert-butyl peroxide was prepared in a nitrogen atmosphere, and filtered through a Teflon filter (Fluoropore FP, manufactured by Sumitomo Electric Industries, Ltd.) with a pore size of 2.5 μm. /hr was continuously supplied to the reaction vessel. The reaction tank was filled with nitrogen and the internal pressure was set to 7Kg/ ^cm2 gauge pressure, and the polymerization temperature was
The temperature was adjusted to 120℃. The stirring rotation speed was set to 80 r.pm to ensure sufficient mixing, and after about 12 hours the feeding rate was reduced to 5/5.
Transitioned to continuous operation as hr. The average residence time of the reaction mixture in the tank was 6 hours, and the average polymerization rate was
It was 45% by weight. The residence time of the reaction mixture from the reaction tank to the volatile matter separator is about 5 minutes, and the temperature of the vent extruder is 200°C in the vent part, 200°C in the extrusion part,
The degree of vacuum at the vent part was 5 mmHg.

The polymer produced from the vent extruder is guided to a spinning head directly connected thereto, and the low refractive index polymer is transferred from another extruder arranged in parallel, where it is composite-spun. A composite fiber with a uniform core-sheath composition.

A polymer of 2.2.2-trifluoroethyl methacrylate was used as the sheath polymer. The refractive index of this polymer was 1.41. The spinning head temperature was 210°C, and the core-sheath polymer blending ratio was 90:10 by weight.

The optical transmission properties of the optical fiber obtained in this way are
It showed extremely excellent performance of 189 dB/Km at a wavelength of 646 nm and 97 dB/Km at a wavelength of 577 nm.

The degree of polymerization of the core component polymer obtained in this example was 900, and the amount of residual monomer in the core component polymer was 0.1% by weight or less.

Example 2 Instead of performing composite spinning in Example 1, a one-filament extrusion accessory was attached to the spinning head, and the polymer continuously fed from the vented extruder was fed into the coating pot. It was coated continuously through the refractive index polymer solution. A 2.2.2-trifluoropropyl methacrylate polymer was used as the low refractive index polymer solution, and coating treatment was performed with a 30% methyl acetate solution, and the coating thickness was adjusted to 2% of the fiber diameter. After coating, it was heated in an air heating oven at 140°C to remove the coating solvent methyl acetate. The optical fiber thus obtained had an extremely excellent optical transmission property of 188 dB/Km at a wavelength of 646 nm and 99 dB/Km at a wavelength of 577 nm.

Example 3 Using the same equipment as in Example 1, the monomer composition was changed to 100 parts of methyl methacrylate, 0.25 parts of n-octyl mercaptan, and 0.007 parts by weight of dimethyl 2,2'azobisisobutyrate, Polymerization was carried out in the same manner as in Example 1. Polymerization temperature is 120
The average residence time of the reaction mixture in the tank was 5.5 hours, and the average polymerization rate was 40% by weight.

The reaction mixture is led to the vented extruder, and the vented part
The temperature was 190°C, the extrusion part was 200°C, and the degree of vacuum in the vent part was 5 mmHg.

The polymer discharged from the vent extruder was made into a core-sheath composite fiber in the same manner as in Example 1. However, here, vinylidene fluoride-tetrafluoroethylene copolymer (7:3 molar ratio) was used as the low refractive index polymer, and the spinning head temperature was 210°C.

The optical transmission properties of the obtained optical fiber were 230 dB/Km at a wavelength of 646 nm and 230 dB/Km at a wavelength of 577 nm.
It was 190dB/Km.

Example 4 In Example 2, instead of applying the coating treatment, the core polymer was obtained in the form of pellets, and after drying the polymer in a vacuum dryer at 90°C for 12 hours, using a composite spinning machine, vinylidene fluoride-tetra An optical fiber was obtained by composite spinning using a fluoroethylene (70:30 molar ratio) copolymer as a sheath component.
This optical transmission performance is achieved at a wavelength of 646ηm.
It was 271dB/Km, and 238dB/Km at a wavelength of 577ηm.

Comparative Example 1 A methyl methacrylate polymer was produced according to a commonly used methyl methacrylate suspension polymerization method. 100 parts of methyl methacrylate, 150 parts of water, n
- Using 0.22 parts of octyl mercaptan, 0.1 part of azobisisobutyronitrile, and 0.05 parts of partially saponified polymethyl methacrylate as a dispersant, at 80°C.
Suspension polymerization was carried out for 2.5 hours. The obtained polymer was washed, dried, and pelletized through a devolatilizing extruder. The amount of methyl methacrylate remaining in the pellets was 0.1% by weight.

The pellets thus obtained were used as a core polymer, and the vinylidene fluoride-tetrafluoroethylene copolymer (7:3 molar ratio) used in Example 3 was used as a sheath polymer for composite spinning. The spinning temperature is
The temperature was 190℃.

The optical transmission performance of the obtained optical fiber with a diameter of 1.0 m/m was 890 dB/Km at a wavelength of 646 nm and 1060 dB/Km at a wavelength of 577 nm, which was quite poor.
One reason for this is that impurities and decomposed products in water are adsorbed during suspension polymerization and cannot be removed, and the other reason is that the produced methyl methacrylate polymer has poor heat resistance. This is thought to be because thermal decomposition products were generated despite devolatilization and shaping at low temperatures, causing scattering and absorption of transmitted light.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the heating time and the amount of methyl methacrylate dimer produced using the polymerization temperature of the core polymer as a parameter. FIG. 2 is a diagram showing the relationship between the heating conditions of the core polymer and the coloration state of the core polymer itself.

Claims (1)

[Claims] 1 The core component material is polymethyl methacrylate or a copolymer mainly composed of methyl methacrylate, and the sheath component material is at least 20% by weight of fluorine.
When manufacturing an optical fiber having a core-sheath structure as a fluorine-containing polymer, the manufacturing process of the core component material is a continuous bulk polymerization process followed by a continuous separation process of volatiles mainly consisting of residual unreacted monomers. The process consists of continuously feeding a mixture of a monomer, a radical polymerization initiator, and a chain transfer agent as the core component material into one reaction tank, and adding 90% of the reaction mixture in the reaction tank.
The polymer content (wt%) of the reaction mixture at a temperature of ℃ or higher and lower than 130℃ is 30% or higher and 50%
It is characterized by using polymethyl methacrylate or a copolymer mainly composed of methyl methacrylate, which is produced by polymerizing to satisfy the following conditions and then controlling the volatile matter separation process at 150°C or higher and lower than 300°C. A method for manufacturing optical fibers with excellent optical transmission properties.