JP2000171642A - Production of preform for plastic optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a producing method of a preform for a plastic optical fiber having excellent heat resistance. SOLUTION: A hollow part of a polymer cylindrical body as a clad is filled with a mixture of core forming monomers and a nonpolymerizable compd. having a higher refractive index than the refractive index of the polymer of the core forming monomers. Then the core forming monomers are polymerized to form the core having the distribution of refractive index and to produce a preform for a plastic optical fiber. In this method, as for the nonpolymerizable compd., a copolymer of the core forming monomers and a copolymerizable compd. having a higher refractive index than the refractive index of the polymer of the core forming monomers is used.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a preform for a plastic optical fiber, and more particularly, to a preform for a plastic optical fiber having improved heat resistance and a refractive index distribution coefficient which does not increase even when heated. It relates to a manufacturing method.

[0002]

2. Description of the Related Art As a method of manufacturing a preform for a plastic optical fiber, Japanese Patent Application Laid-Open No. Sho 61-130904 discloses a method in which a polymer cylindrical tube itself is formed as a clad, and a monomer solution serving as a core is polymerized therein. A method of solidifying to produce a preform for a plastic optical fiber is disclosed. The feature of this method is that the solution used for forming the core is, for example, methyl methacrylate (MMA).
When the monomer is the main component, MMA monomer and MMA
The point is that a mixed solution of a monomer having a lower reactivity ratio than a monomer and a higher refractive index than polymethyl methacrylate (PMMA) is used.

JP-A-5-173026 discloses a method for producing a preform for a plastic optical fiber by producing a polymer cylindrical tube which itself becomes a clad, and polymerizing and solidifying a monomer solution which becomes a core portion therein. Has been disclosed. The point that the polymerization proceeds from the inner surface of the hollow tube toward the center to form a continuously changing refractive index distribution is described in
This is the same as the method of JP-A-1-190404, but is characterized in that a polymerizable compound having a larger molecular size than the first component monomer is used as the second component for forming the refractive index distribution. In the methods disclosed in the above two publications, since the core is formed from a copolymer, microphase separation occurs in accordance with the chain distribution of the two components, thereby increasing transmission loss. In particular, in the method described in JP-A-61-130904, a large amount of a monomer having a small reactivity ratio is present at the center of the core and remains unreacted due to low reactivity.

[0004] WO 93/08488 also discloses a method of forming a refractive index distribution by promoting polymerization of a monomer from the inner wall of a cylindrical container made of a polymer. Specifically, a PMMA tube is used as a cylindrical container, and a core is formed in a hollow portion of the PMMA tube. This PMMA tube is produced by heating and polymerizing the monomer while rotating the glass tube containing the MMA monomer solution.The core is formed by injecting the MMA monomer solution containing the non-polymerizable compound into the PMMA tube and rotating the core. It is formed by heat polymerization, whereby a preform having a refractive index distribution can be produced. The obtained preform is drawn by heating and melting to obtain an optical fiber having a predetermined diameter.

[0005] The method disclosed in WO93 / 08488 is a method of producing a core having a continuously changing refractive index inside a polymer tube which itself becomes a clad. Although they are common, they differ in that a non-polymerizable compound is used to form a refractive index distribution in the core. When a monomer solution as a raw material of a core is injected into a cylindrical polymer tube, the raw material monomer partially dissolves the inner wall surface of the polymer tube. The polymerization proceeds from the inner wall surface having increased viscosity toward the center of the cylinder due to the gel effect, so that the concentration of the non-polymerizable compound having a large refractive index increases toward the center. Therefore, a continuous refractive index distribution is formed. Here, the polymer forming the cylindrical tube serving as the clad is a polymer obtained by polymerizing the same monomer as a part or most of the polymer serving as the core, provided that the polymer is soluble in a monomer solution which is a raw material of the core. is there.

The point that the method disclosed in WO 93/08488 is superior to the method described in the above two publications is that the transmission loss of the finally obtained optical fiber is small. As described above, according to the methods disclosed in the above two publications, microphase separation occurs according to the chain distribution of two components, and transmission loss increases. On the other hand, according to the method disclosed in WO93 / 08488, since the polymer forming the core is a homopolymer containing a non-polymerizable compound, microphase separation due to chain distribution does not occur, and thus the transmission loss can be reduced. Can be.

The difference in refractive index (Δn) between the core and the clad depends on the concentration of the non-polymerizable compound contained in the monomer for forming the core. That is, to increase the refractive index difference, the concentration of the non-polymerizable compound may be increased. However, when a liquid non-polymerizable compound is used, the glass transition point of the obtained polymer decreases with increasing concentration, and the low-molecular-weight non-polymerizable compound diffuses in the core,
The refractive index distribution in the core is likely to be blunt. That is, an ideal refractive index distribution in which the refractive index difference increases as approaching the center cannot be obtained.

[0008]

DISCLOSURE OF THE INVENTION The present invention is directed to an increase in transmission loss due to microphase separation that occurs when a conventional copolymer is used for a core portion, or to use a conventionally known non-polymerizable compound for a core portion. An object of the present invention is to provide a method for producing a preform for a plastic optical fiber, which can prevent a decrease in a glass transition point of a core portion caused when added, can improve heat resistance of a core portion, and can prevent diffusion of a non-polymerizable compound. It is.

[0009]

SUMMARY OF THE INVENTION In order to solve the above-mentioned problems, the present invention provides a method for forming a core-forming monomer and a polymer of the core-forming monomer in a hollow portion of a polymer cylinder serving as a clad. A preform for a plastic optical fiber, comprising filling a mixture of non-polymerizable compounds having a refractive index higher than the refractive index of, and polymerizing a core-forming monomer to form a core having a refractive index distribution. Wherein the non-polymerizable compound is a copolymer of a core-forming monomer and a copolymerizable compound having a refractive index higher than the refractive index of the polymer of the core-forming monomer. To provide a method.

[0010]

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the production method of the present invention, first,
A cylindrical clad is prepared. The method of forming the cladding is not particularly limited, but usually, the monomer is put into a cylindrical polymerization vessel, and the monomer is polymerized while rotating horizontally while keeping the polymerization vessel horizontal. The polymerization vessel is usually made of glass, but may be made of other materials, such as metal. The size of the polymerization container may be appropriately selected according to the size of the preform to be manufactured. The cylindrical clad portion can also be produced by hollowing out a cylindrical clad polymer mass. Alternatively, the polymer may be extruded to produce a cylindrical shape.

As the polymer forming the clad portion,
A colorless and highly transparent plastic conventionally used for a plastic optical fiber can be used. Examples of the monomer that provides such a plastic include methacrylates, styrene compounds, fluorinated acrylates, fluorinated methacrylates, and the like as follows: (a) methacrylate and acrylic acid Ester Methyl methacrylate, ethyl methacrylate, isopropyl methacrylate, t-butyl methacrylate, benzyl methacrylate, phenyl methacrylate, cyclohexyl methacrylate, diphenylmethyl methacrylate, etc .; methyl acrylate, ethyl acrylate, t-butyl acrylate (B) styrene-based compounds styrene, α-methylstyrene, chlorostyrene, bromostyrene, dichlorostyrene, dibromostyrene, etc .; (c) fluorinated acrylate 2,2,2-trifluoro B ethyl acrylate; (d) a fluorinated methacrylate ethyl 1,1,2-trifluoroethyl methacrylate.

To such a monomer, a polymerization initiator and a desired additive (for example, a chain transfer agent) are added, and an appropriate amount is placed in a polymerization vessel. While rotating the polymerization vessel with a motor,
The cladding is formed by heating with a heating device to polymerize the monomer on the inner wall of the polymerization vessel. In order to adjust the refractive index of the clad portion, a non-polymerizable compound having a high refractive index such as that added to the core portion may be added. The amount of monomer is
What is necessary is just to determine from the magnitude | size of a superposition | polymerization container, and the thickness of a clad part. Alternatively, an excessive amount of monomer may be added, polymerization may be stopped when a clad portion having a desired thickness is formed, and unnecessary monomer may be discharged from the polymerization container.

Next, a core is formed inside the clad. The liquid as the raw material of the copolymer for forming the core portion is formed by adding the monomer for forming the core portion and the monomer for forming the core portion to the monomer for forming the core portion (usually the same monomer used for forming the clad portion). It is prepared by mixing a copolymer with a copolymerizable compound having a refractive index higher than that of the coalesced compound.

In the present invention, an aromatic compound having an ethylenically unsaturated group (for example, a vinyl group, an allyl group, etc.) is preferably used as such a copolymerizable compound.
Examples of such aromatic compounds include vinyl esters of aromatic carboxylic acids, aromatic vinyl and allyl ether, and specific examples thereof include vinyl benzoate, phenyl vinyl ether and allyl phenyl ether. These compounds, for example, have a large difference in reactivity ratio r ₁ , r ₂ with respect to methyl methacrylate, and thus are alternately copolymerized with methyl methacrylate. Therefore, the obtained copolymer has high compatibility with the polymer forming the core portion,
Does not cause microphase separation. Further, since it is not a low molecular compound, it does not diffuse in the core and does not impair the heat resistance of the core. The r ₁ and r ₂ of vinyl benzoate, phenyl vinyl ether and allyl phenyl ether are as follows. Vinyl benzoate: r ₁ = 20.3; r ₂ = 0.0
7 Phenyl vinyl ether: r ₁ = 140; r ₂ = 0.13 Allyl phenyl ether: r ₁ = 66; r ₂ = 0.06

The ratio of the core-forming monomer to the copolymerizable compound is 8 to 20 parts by weight with respect to 1 part by weight of the former. A radical polymerization initiator is added to the monomer mixture, and the mixture is heated to a temperature corresponding to the type of the initiator and copolymerized. The polymerization time is usually 3 to 10 hours. After the polymerization, unreacted monomers are distilled off under reduced pressure to obtain a copolymer for adjusting the refractive index. The weight average molecular weight of the copolymer is usually 100 to 3
000, preferably 500-2000.

To form the core, a polymerization initiator and a chain transfer agent are added to a mixture of the monomer for forming the core and the copolymer for controlling the refractive index. And polymerize. The polymerization temperature may be appropriately adjusted according to the type of the core-forming monomer. The amount of the refractive index adjusting copolymer may be appropriately selected according to the desired refractive index distribution in the core portion, the type of the copolymerizable compound, the molecular weight of the refractive index adjusting copolymer, and the like.

[0017]

The present invention will be specifically described below with reference to examples. Example 1 Methyl methacrylate and vinyl benzoate in a weight ratio of 1:
Azobisisobutyronitrile (0.2% by weight) as a polymerization initiator was added to the mixture.
To carry out polymerization. After 5 hours, stop heating,
Once cooled with liquid nitrogen, the remaining monomers were removed under reduced pressure of 1 mmHg while thawing. The residue was used as a copolymer for adjusting the refractive index of the core in the following step.
In a cylindrical polymerization vessel (length 500 mm, inner diameter 20 mm)
Benzoyl peroxide (polymerization initiator) 0.1% by weight and n
-Methyl methacrylate (MMA: refractive index 1.49) added with 0.2% by weight of butyl mercaptan (chain transfer agent)
2) was added, the polymerization vessel was held horizontally, and polymerization was carried out at 80 ° C. for 20 hours while rotating at 2000 rpm to form a cylindrical tube of polymethyl methacrylate. In the hollow portion of this cylindrical tube, 20% by weight of the previously prepared copolymer for adjusting the refractive index, 0.01% of t-butyl peroxide (polymerization initiator)
% By weight and n-butyl mercaptan (chain transfer agent)
After injecting MMA containing 0.2% by weight and sealing the tube, the polymerization vessel was kept horizontal and rotated at 5 ° C. at 90 ° C. for 4 hours.
Polymerization was carried out for 0 hour to form a core portion. The glass transition point of this core was 78 ° C.

The obtained preform is drawn to have a diameter of 7 mm.
An optical fiber of 50 μm was manufactured. When the transmission loss of this optical fiber was measured by a cutback method of cutting from 5 m to 1 m, the transmission loss was 1 at a wavelength of 650 nm.
It was 98 dB / km. The refractive index distribution coefficient was 2.3. The refractive index distribution coefficient is the refractive index distribution of the core portion expressed by the following formula:

[Number 1] _{n (r) = n 0 {} 1-2Δ (r / a) α} 1/2 [ wherein, n (r) is the refractive index at distance r from the core part center, n ₀ is the core portion refractive index at the center (r = 0) of, delta is ^{_{(n o 2 -n 1 2)}} / 2n 0, n 1 is the refractive index of the cladding, a is representative of the radius of the core portion. ], The coefficient α becomes an ideal smooth refractive index distribution when α is 2, and when α becomes infinite, the refractive index distribution becomes step-like. When this optical fiber was heated at 70 ° C. for 1,000 hours, the transmission loss was 201 dB / km and hardly deteriorated, and the refractive index distribution coefficient was still 2.3.

Comparative Example 1 Benzyl benzoate 1 was used as a core-forming monomer mixture.
Using MMA containing 7% by weight, this monomer mixture is
90 ° C. while keeping the polymerization tube horizontal and rotating at 5 rpm
A preform was prepared in the same manner as in Example 1 except that the polymerization was carried out for 40 hours. The glass transition point of the core is 62 ° C
Met. The obtained preform is drawn to a diameter of 75.
An optical fiber of 0 μm was manufactured. When the transmission loss of this optical fiber was measured in the same manner as in Example 1, the transmission loss was 162 dB / km at a wavelength of 650 nm. The refractive index distribution coefficient was 2.3. When this optical fiber was heated at 70 ° C. for 1000 hours, the transmission loss increased to 192 dB / km, and the refractive index distribution coefficient also increased to 3.5.

Claims (4)

[Claims] 1. A mixture of a core-forming monomer and a non-polymerizable compound having a refractive index higher than that of a polymer of the core-forming monomer in a hollow portion of a polymer cylinder to be a clad. And forming the core portion having a refractive index distribution by polymerizing the core portion forming monomer, wherein the non-polymerizable compound comprises a core portion forming monomer And a copolymer having a refractive index higher than the refractive index of the polymer of the core-forming monomer. 2. The copolymer has a weight average molecular weight of 100.
The production method according to claim 1, wherein the number is from 3,000 to 3,000. 3. The copolymerizable compound having a refractive index higher than the refractive index of the polymer of the core-forming monomer is an aromatic compound having an ethylenically unsaturated group in the molecule. The manufacturing method as described. 4. The method according to claim 3, wherein the aromatic compound having an ethylenically unsaturated group in the molecule is vinyl benzoate, phenyl vinyl ether or allyl phenyl ether.