JPH11337746A - Production of preform for plastic optical fiber - Google Patents

Abstract

PROBLEM TO BE SOLVED: To avoid bubbles or voids in the core without using a special heating device or pressurizing device by heating and polymerizing the core-forming monomers to a temp. higher than the glass transition temp. of the polymer to be obtd. at a specified time in the production process. SOLUTION: When and after the polymn. temp. of core-forming monomers begins to decrease after once the monomer temp. increases by the polymn. reaction heat, namely, after the monomer temp. reaches the peak, bubbles begin to produce. Therefore the heating temp. is increased from the point where the monomer temp. begins to decrease after once it increases, so that the monomer temp. is raised over the glass transition temp. of the polymer to be obtd., preferably >=5 deg.C higher, and more preferably >=10 deg.C higher than the glass transition temp. to continue the polymn. Thus, the core without containing bubbles or voids is formed by controlling the polymn. temp. The upper limit of the polymn. temp. is preferably up to 30 deg.C higher than the glass transition temp.

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 method for manufacturing a preform for a plastic optical fiber in which a core portion does not contain bubbles or voids.

[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.

[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, and 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 WO 93/08488 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. 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. JP-A-61-
In the method described in JP-A-130904, many monomers having a small reactivity ratio are present in the center of the core and remain unreacted due to low reactivity. On the other hand, WO93 / 08
In the method disclosed in U.S. Pat. No. 488, 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 transmission loss can be reduced.

However, in any of the above-mentioned prior art methods, bubbles and voids may be generated when the core-forming monomer is polymerized. Generally, the monomer is
As polymerization reduces the volume, the internal pressure drops,
Bubbles and voids are easily generated. The rate of volume reduction depends on the type of monomer, but is generally 5 to 25%.
It is about. The volume reduction rate of methyl methacrylate, which is often used for plastic optical fibers, is as high as 24%.
Since bubbles and voids generated during the production of the preform do not disappear even when the preform is drawn and optical fibers are used, a preform containing bubbles and voids cannot be used. This is because the light incident on the optical fiber is strongly scattered at the interface between the bubble and the void and the core polymer, and the transmission loss is significantly increased.

Therefore, in order to manufacture a cylindrical optical transmission body (optical fiber) having a low transmission loss, an invention for preventing generation of bubbles and voids during polymerization of a core-forming monomer has been proposed. It is described in the prior art as follows.

Japanese Patent Application Laid-Open No. 10-3010 discloses that a long and thin polymerization vessel is fixed vertically or obliquely and a high-temperature region is sequentially moved from a lower end to an upper end of the polymerization vessel to prevent generation of bubbles and voids. A method and apparatus for manufacturing a plastic optical transmission body is disclosed. In the present invention, since polymerization does not occur in a portion above the heating region, even if volume shrinkage due to polymerization occurs in the heating region, unpolymerized monomers flow to prevent generation of voids due to volume shrinkage.

[0010] Japanese Patent Application Laid-Open No. 9-269424 discloses that a clad pipe is first manufactured in a pressurizable container having a straight pipe as an essential part, and a core liquid is polymerized therein under pressure by an inert gas. Thus, a method of manufacturing a plastic optical transmission body is disclosed. Japanese Patent Application Laid-Open No. 9-304633 discloses a method of manufacturing a clad pipe in a pressurizable container having a straight pipe as an essential part, and then forming a core inside the clad pipe. A method for producing a plastic optical transmission body by polymerizing a liquid under pressure by a piston is disclosed. In each method, generation of air bubbles is prevented by pressurization.

[0011]

However, the inventions disclosed in JP-A-10-3010, JP-A-9-269424 and JP-A-9-304633 are all special heating devices and pressurizing devices. Equipment is required, which complicates manufacturing equipment and processes. An object of the present invention is to provide a method for manufacturing a preform for a plastic optical fiber having no core and no bubbles or voids without using such a special heating device or pressurizing device.

[0012]

In order to solve the above-mentioned problems, the present invention provides a method for filling a core-forming monomer into a hollow portion of a polymer cylinder serving as a clad, and polymerizing the core-forming monomer. In the method for producing a preform for a plastic optical fiber, comprising forming a core portion by allowing the core portion to form a core portion, the polymerization temperature of the core portion forming monomer once rises due to polymerization reaction heat and then begins to decrease, the core portion Provided is a production method characterized in that polymerization is carried out by heating the temperature of the monomer for forming to a temperature higher than the glass transition temperature of the obtained polymer.

[0013]

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.

[0015] 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 portion is formed inside the clad portion. The solution serving as the raw material of the polymer forming the core portion is:
It is prepared by mixing a monomer (usually the same monomer used to form the cladding) with a non-polymerizable compound having a high refractive index. As such a non-polymerizable compound, a compound that is liquid at room temperature or higher, is compatible with the polymer used for the core, has a high boiling point, and is colorless and transparent is used. Examples of such compounds include the following compounds: benzoic acid esters such as benzyl benzoate, sebacic acid esters such as dibutyl sebacate and other alkyl dibasic acid esters, phthalic acid esters such as dimethyl phthalate and dioctyl phthalate. And halogenated compounds such as bromobenzene, and sulfur compounds such as diphenyl sulfide.

To form the core portion, a polymerization initiator, a chain transfer agent, and the like are added to a mixture of the above-described monomer and non-polymerizable compound, and the mixture is placed in a polymerization vessel having a clad portion, and heated. Polymerize. At the start of the polymerization, the temperature of the monomer is lower than the heating temperature around the polymerization vessel, but increases due to the heat of polymerization as the polymerization proceeds, and becomes higher than the heating temperature. After a certain time, the heat of polymerization becomes constant, and the temperature of the monomer becomes almost the same as the heating temperature.

The present inventors have confirmed by experiments that:
After the point where the monomer temperature once rises and then begins to fall,
That is, after the monomer temperature reaches the peak, bubbles start to be generated. This is because the polymer volume shrinks due to the decrease in temperature, the internal pressure decreases, and the monomer temperature decreases, the viscosity of the liquid increases, and the volatile substances dissolved in the monomer diffuse. This is considered to be due to the fact that it becomes difficult for bubbles to grow and the like.

In the present invention, the heating temperature is raised from the time when the monomer temperature once rises and then begins to fall, and the monomer temperature is raised to a temperature exceeding the glass transition temperature of the obtained polymer, preferably 5 to less than the glass transition temperature. Over ℃,
More preferably, the temperature is raised to a temperature higher by at least 10 ° C., and the polymerization is continued. By controlling the polymerization temperature in this way,
A core portion containing no bubbles or voids is formed. The upper limit of the polymerization temperature after the temperature rise is not particularly limited, but if the temperature is too high, depolymerization, decomposition, deterioration, deformation and the like of the polymer occur.

The temperature at the start of the polymerization is not particularly limited, but is usually preferably 100 ° C. or lower in order to prevent heat generation and foaming due to rapid polymerization. However, if the temperature is too low, the polymerization takes a long time.
Temperatures in the range of 100 ° C are preferred.

The heating is preferably performed using an oil bath, but other heating means may be used. To control the temperature, attach a temperature sensor to the polymerization vessel and monitor the temperature.
Adjust the temperature of the oil bath. Temperature adjustment can also be performed by automatically controlling the temperature of the heating means based on a signal from the temperature sensor.

[0022]

The present invention will be specifically described below with reference to examples. Example 1 A cylindrical polymerization vessel equipped with a temperature sensor (length: 600 m)
(m, 22 mm in inner diameter) was previously polymerized with methyl methacrylate (MMA) to form a 4 mm-thick cylindrical tube of polymethyl methacrylate (PMMA). The following monomer solution for forming a core portion was injected into the hollow portion of the PMMA cylindrical tube. This monomer solution was prepared by adding MMA monomer to di-t-butyl peroxide 0.1 as an initiator.
3% by weight, 0.2% by weight of n-butyl mercaptan as a chain transfer agent, and 15% by weight of benzyl benzoate as a non-polymerizable compound having a higher refractive index than PMMA. The monomer solution is made of polytetrafluoroethylene (PT) to remove impurities contained in the solution.
After passing through a FE) ultrafiltration membrane, the mixture was injected into a PMMA cylindrical tube.

Both ends of the polymerization vessel into which the monomer solution was injected were sealed without applying pressure, and the whole vessel was immersed in an oil bath in an upright state. Oil bath temperature is 90 ℃
Met. After 3 hours from the immersion, the temperature of the monomer started to decrease, so the temperature of the oil bath was increased to 110 ° C. Polymerization was carried out at that temperature for 20 hours.

The polymerization vessel was taken out of the oil bath, and the preform was taken out of the vessel. The preform is 400 mm shorter in length than the polymerization vessel due to polymerization shrinkage.
But contained no air bubbles. The outer diameter of the preform was 22 mm, and there was no large sink, dent, or flatness.

The obtained preform was drawn to have a diameter of 7
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 54 dB / km.

Examples 2 to 4 The heating temperature after the monomer temperature began to drop was 11
5 ° C. (Example 2), 120 ° C. (Example 3) or 130
A preform was manufactured under the same conditions as in Example 1 except that the temperature was changed to ° C. (Example 4). As in the case of Example 1, a preform containing no air bubbles was obtained. An optical fiber was manufactured from each preform in the same manner as in Example 1, and its transmission loss was measured by the same method as in Example 1.
152 dB / km (Example 2), 160 dB / km
(Example 3) or 165 dB / km (Example 4).

Comparative Example 1 A preform was produced under the same conditions as in Example 1 except that the polymerization was continued at 90 ° C. even after the temperature of the monomers began to drop. The obtained preform contained many air bubbles of a size (about 1 mm or more) that could be visually observed. A long optical fiber could not be obtained from this preform by drawing, and the transmission loss could not be measured.

Comparative Example 2 After the temperature of the monomer started to decrease, the heating temperature was set at 10
A preform was manufactured under the same conditions as in Example 1 except that the temperature was changed to 5 ° C. The obtained preform contained several bubbles of a size (about 1 mm or more) that could be visually observed.
A long optical fiber could not be obtained from this preform by drawing, and the transmission loss could not be measured.

Claims (1)

[Claims] 1. A plastic optical fiber comprising: filling a hollow portion of a polymer cylinder serving as a clad with a core-forming monomer, and polymerizing the core-forming monomer to form a core portion. In the method for producing a preform, at the time when the polymerization temperature of the core-forming monomer once starts to decrease after the polymerization reaction heat, the temperature of the core-forming monomer is calculated from the glass transition temperature of the obtained polymer. A production method characterized by performing polymerization by heating to a high temperature.