JPS58193502A - Manufacture of low loss plastic optical fiber - Google Patents

Abstract

PURPOSE:To obtain the entitled optical fiber superior in light transmission characteristics, by continuously polymerizing starting materials pretreated in prescribed manners, forming polymethyl methacrylate having a specified stereoregularity, and using it for a core material. CONSTITUTION:(i) A monomer consisting essentially of methyl methacrylate is degassed in reduced pressure and/or stripped, and heat treated at about 110 deg.C to remove oxygen and peroxide contained, (ii) said monomer, a polymn. initiator composed of an azo compd., and a chain transfer agent are treated by means of distillation, ultrafiltration, etc. to remove ultrafine particles contained, (iii) then, they are fed into a polymerizing reactor, and continuously polymerized to form a polymer having <=1.1 syndiotactic parts/heterotactic parts ratio (area ratio obtd. by measuring an alpha-methyl signal by NMR). (iv) Next, said polymer is used as a core material, and a polymer lower in refractive index than this polymer is used as a sheath material to prepare optical fibers.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a low-loss plastic optical fiber having a core-@ structure, and more particularly, the present invention relates to a method for producing a low-loss plastic optical fiber having a core-@ structure, and more particularly, the present invention relates to a method for producing a low-loss plastic optical fiber having a core structure. The present invention relates to a method for manufacturing a plastic optical fiber whose sheath component is a polymer having a lower refractive index than the other components.

In recent years, demand for plastic optical fibers has increased significantly as their flexibility and workability have been recognized. However, in terms of optical transparency, a sufficiently satisfactory performance has not yet been achieved, and as a result, plastic optical fibers are currently only used for optical transmission over a distance of several tens of meters at most.

The cause of optical loss in plastic optical fibers largely depends on core type coalescence, and in addition to light absorption and Rayleigh scattering inherent to the polymer, there are also losses due to coloring and light scattering caused by the polymer manufacturing process. Quite a lot. In other words, the key to improving the performance of plastic optical fibers is to produce a more optically transparent core-type composite, and methacrylic acid has low light absorption and light scattering for visible light, and also has low optical distortion during spinning processing. It is extremely important to obtain a methyl-based polymer.

By the way, suspension polymerization, bulk polymerization, solution polymerization, and other methods are known as polymerization methods for methyl methacrylate polymers. This is undesirable because foreign substances may remain, and fine dust in the atmosphere may adhere when the beads are dried, resulting in a resin that only scatters a lot of light. Therefore, in order to solve this problem, it is necessary to adopt a bulk polymerization method, and as such a technique, for example, a one-stage complete mixing reactor is used, and radical polymerization is carried out at a polymerization height of 130-160 C. The conditions for the polymerization initiator are the following formula: A polymerization method has been proposed that satisfies the
3-42260).

By adopting this proposal, the amount of impurities mixed into the resulting methyl methacrylate polymer was reduced to some extent, and although the optical performance was the same as above, the light guiding property was still sufficiently satisfactory. It's not a good value.

In view of these circumstances, the inventors of the present invention have conducted intensive research to obtain a low-loss plastic optical fiber with excellent optical performance. After pre-treating the agent, it is charged into a polymerization vessel and polymerized continuously to produce a polymer with a specific degree of stereoregularity, and the resulting polymer and a polymer with a lower refractive index than this. We discovered that if we form a fiber with a core-sheath structure, we can achieve the purpose.
Based on this knowledge, we completed the present invention.In manufacturing plastic optical fibers whose sheaths are made of polymers with a lower refractive index than the core, a process of removing oxygen dissolved in the monomers is required. After removing the peroxide contained in the monomer, a step of removing the peroxide contained in the monomer, and a step of removing the peroxide contained in the monomer and the general
After pretreatment of the raw material, which consists of a step of removing fine particles contained in a polymerization initiator consisting of an azo compound represented by ~8 hydrocarbon groups) and a chain transfer agent, the monomer, A polymerization initiator and a chain transfer agent are charged into a polymerization vessel, and the polymerization is continuously carried out. In the NMR spectrum of the polymer, α to methyl group signals of the methyl methacrylate component are found to be diotactic and heterotactic. The method is characterized by producing a polymer having a ratio of area attributable to a substance of 1.10 or less, and forming the polymer and a polymer having a lower refractive index than this into a fiber having a core-sheath structure. The present invention provides a method for manufacturing a low-loss plastic optical fiber.

The polymerization initiator used in the method of the present invention is an azoalkane having a structure represented by the general formula (1). The catalyst is active at relatively high temperatures and has not been considered as an initiator for continuous polymerization of methyl methacrylate.However, according to the research conducted by the present inventors,
After removing the oxygen dissolved in the monomer as completely as possible, we further remove as much as possible the peroxide generated by autooxidation of the monomer contained in the monomer, and use it as a raw material. When polymerizing using the above azoalkano catalyst as a polymerization initiator, 600. It has been found that a polymer with low absorption at wavelengths of nm or less can be obtained.

Conventionally, when polymerizing at high temperatures, the amount of oligomers increases, causing coloration, which has become a problem, especially at wavelengths below 600 nm. Surprisingly, when the peroxide produced in An excellent, extremely colorless polymer with no yellowish coloration is obtained.

On the other hand, if the polymer is simply polymerized at a high temperature, only a colored polymer can be obtained, and it cannot be used for practical optical purposes.

It has already been revealed that peroxide is naturally formed in methyl methacrylate monomer. This peroxide exists in the form of monomer peroxides and polymer peroxides, such as eg ', and also in the monomer immediately after leaving the rectification column of the methyl methacrylate monomer plant. These peroxides are included. For example, the presence of about 2Xio=mol/L of peroxide is recognized in the methyl methacrylate monomer immediately after leaving the rectification column.

That is, peroxides in monomers cannot be completely removed by ordinary distillation alone.

The inventors of the present invention have conducted intensive studies on methods to completely remove oxygen and peroxides from monomers on an industrial scale, and have found that dissolved oxygen can be stripped with an inert gas or decompressed under reduced pressure. It has been found that it is preferable to remove as much of the material as possible by air or the like, and then perform heat treatment. This is due to the fact that it is difficult to completely remove oxygen and peroxide in a continuous process, and the purpose is to eliminate residual oxygen by reaction and thermally decompose monomeric peroxide. For this purpose, high temperature treatment of 110° C. or higher is desirable. When treated at temperatures below 100°C, the decomposition reaction is slow and the reaction is not completed even after 6 hours;
The reaction is completed in 2 hours at a temperature of 150°C and within 30 minutes when treated at a temperature of 150°C.

In order to carry out this heat treatment continuously, a polymerization inhibitor with a low vapor pressure, such as N,N'-dinaphthyl-p-phenylenediamine, N,N'-phenyl-p-phenylenediamine, benzophenothiazine, or phenothiazine, is used. It is preferable to add and process.

After this heat treatment, high-boiling substances are subsequently removed by distillation. Even if the monomer that has undergone this series of treatments is heated as is at 100°C for about 7 hours, the polymerization rate remains within 2%, indicating that monomer peroxide and oxygen have been sufficiently removed. It is shown that.

When oxygen and peroxide are completely removed in this manner and polymerization is carried out using an azocalcane catalyst as a polymerization initiator, a polymer with low light absorption at a wavelength of 500 to 600 nm can be obtained even at a high polymerization temperature.

On the other hand, the problem of dust getting mixed into the core type assembly is also a very serious problem, and this mixed dust causes light guide loss due to light scattering in the optical fiber. Since the foreign matter that causes light scattering can be a problem even if it is a fine particle with a wavelength of 1/2 oalff of the light wavelength, it is necessary to remove this foreign matter by a special method.

One method is to remove foreign substances from not only the monomer but also the polymerization initiator and chain transfer agent by distillation, and then charge the mixture into a polymerization vessel. In this method,
It is important to select a polymerization initiator and chain transfer agent that is sufficiently stable at the distillation temperature and has a sufficiently high vapor pressure. As such a polymerization initiator, preferred examples of the azoalkanes of the present invention include azobis-t+9rt-butane, azobisinopropane, azobispropane, etc., and as a chain transfer agent, for example, n- Alkyl mercaptans with a small number of carbon atoms, such as butyl mercaptan, are preferred.

The above-mentioned distillation may be carried out for each of the monomers, solvent, polymerization initiator, and chain transfer agent one by one, or a mixture of two or more of these components may be carried out if the vapor pressures and boiling points are close to each other. It can also be distilled as Needless to say, during this distillation, it is necessary to suppress the evaporation rate of the vapor in order to prevent entrainment of fine particles.

Another method for removing foreign matter is a method using a membrane. In this method, in order to remove fine particles that affect light scattering, it is necessary to use an ultrafiltration membrane that can completely remove particles of 0.01 μm or larger. As a result of testing various commercially available ultrafiltration membranes from this point of view, there were concerns about corrosion resistance, but when polyacrylonitrile membranes were used without using adhesives, azobis-tert-butane, n-butyl Shows sufficient corrosion resistance to mercaptan methyl methacrylate solution,
It was found that the membrane strength and separation performance remained almost the same after use for a long time. When using this ultrafiltration membrane, a large filtration differential pressure is unfavorable in terms of durability, and it is usually desirable to keep the differential pressure to less than 0.3 gin/-; It is possible to seal by pressure bonding without using adhesive. However, this method using an ultrafiltration membrane has a small filtration capacity, and is therefore suitable for small amounts of catalysts and other components that are not suitable for distillation. In addition, highly reactive substances such as catalysts can also be filtered as a solution using methyl isobutyrate or the like as a solvent.

The polymerization in the method of the present invention requires a continuous system in order to obtain products of industrially stable quality, and the polymerization temperature is j0
Although a temperature in the range of 0 to 220°C can be adopted, in order to maintain a high proportion of heterotough in the stereoregularity of the polymer, the temperature at the center of polymerization is 165°C or higher, preferably in the range of 180 to 220°C. It is desirable that

In such high temperature polymerization, remarkable features appear in the stereoregularity of the polymer. That is, the syndiotough proportion [S] decreases and the heterotough proportion [111) increases. This toughness ratio [
8]/[H] varies somewhat depending on the polymerization method, but according to experiments by the present inventors, in a one-stage reaction in a complete mixing tank, it is 1 at polymerization temperatures of 135°C, 150°C, 180°C, and 210°C, respectively. The biggest influence on .27.1.20° is that light scattering loss due to plastic anisotropy remaining frozen during the cooling process of the fiber during spinning is reduced, and the toughness is reduced. When the ratio is 1.20, the degree of light scattering is still large, but when the ratio is 1.10
If it falls below, it becomes an acceptable level. Others [8]/[H
] is accompanied by an improvement in the fluidity of the polymer. For example, assuming a constant dimer concentration of 2oooppm, the fluidity index MU of polymethyl methacrylate with a molecular weight of 80,000
(according to ASTM D-1238) [1
/[H] changes from 1.26 to 1.07, M
The r value ranges from 3.8F/10 minutes to 4. ! Improve to M/10 minutes. Such an improvement in fluidity is advantageous for improving fiber diameter fluctuations during spinning and fiber surface smoothness.

In the NMR spectrum of the polymer according to the present invention, the synergy of the α-methyl group signal of the methyl methacrylate component?゛) The ratio of the area attributed to tuffic and heterotough is 1.1 for the reason mentioned above.
It needs to be 0 or less. The area ratio here refers to sample 401n9'iz by NMR (JEOL Model MH100) using deuterated nitromethane as a solvent.
Dissolved in O, 5cc of solvent and measured at 90'C, 'I'
MS as reference material, 0.92 ppm and 1.09
The signals appearing at 1.26 ppm and 1.26 ppm are treated as syndiotactic, heterotactic, and syndiotactic components, respectively, and the slight overlap of each peak is divided vertically at the intersection of the peaks to calculate the area ratio of each. It's what I asked for.

On the other hand, when the polymerization reaction is carried out at such high temperatures, the amount of methyl methacrylate dimers produced increases, but this can be prevented by increasing the amount of polymerization initiator added and shortening the reaction time. The amount produced can be reduced. For example, when continuous bulk polymerization of methyl methacrylate is carried out in one homogeneous reaction tank at 180°C using azobis-tert-butane as a catalyst, the reaction rate is 4.
Azobis-tert-butane sop as 0-65%
pm, the average residence time is 5 hours and the dimer amounts to about 4% of the polymer, but when the azobis-tert-butane loading is 300 ppm, the average residence time is 1
．． At 3 hours, dimer was only 1% of the polymer.

4 This dimer can be further degassed together with unreacted monomer. As the deaerator, a screw extruder, rotating disk type deaerator, or slit type deaerator can be used, and if the final deaeration condition Th is 1 TOrr or less, the dimer concentration t 2000 can be easily achieved at 190 to 230°C.
It can be made less than ppm.

By the way, the degassing process also has another important meaning. It is a problem of residual initiator, and azobis-ter
Since a portion of the high temperature active catalyst such as t-butane remains undecomposed, it is necessary to completely remove it in the degassing step.

The degree of degassing can be determined by the remaining amount of dimer, but 2000] Dp! 11 or less preferably 11000pp
The following should complete it.

The polymer thus degassed and purified was extremely thermally stable. This was a surprising result, overturning the conventional assumption that it would be easily thermally decomposed under high-temperature polymerization, and showing superior thermal stability to commercially available methyl methacrylate.

In the method of the present invention, the polymerization conditions should be selected such that the dimer production rate is not too high and a polymer with excellent heat decomposition resistance is obtained. Such conditions include, for example, azobis-tert- When the reaction is carried out in a homogeneous stirring tank at 180° C. using butane as a catalyst, the amount of catalyst added can be selected within the range of 15 to soo ppm and polymerization time of 1 to 5 hours.

The above catalyst, azobis-tert-phytane, has the following Arrhenius formula for its thermal decomposition: = 10 exp (-428
00/RT), it is active at high temperature and is a preferred catalyst for high temperature polymerization.

The polymerization reaction in the method of the present invention is carried out as continuous bulk polymerization.
In addition to a single-stage homogeneous mixing reaction tank, a combination of a homogeneous mixing reaction tank and a plug flow reactor, or a plug flow reactor 75) can be selected.

The resulting core-shaped composite is then molded into a core-sheath structure with a polymer having a lower refractive index than this polymer. Examples of the polymer used as the sheath material include Japanese Patent Application Laid-Open No. 52-15464.
There is a polymer mainly composed of vinylidene fluoride as described in Japanese Patent No. 3. As the polymer mainly composed of vinylidene fluoride, for example, vinylidene fluoride is
A copolymer of vinylidene sulfuride and tetrafluoroethylene containing 99% by weight, a copolymer consisting of 75 to 95% by weight of vinylidene fluoride, 4 to 20% by weight of tetrafluoroethylene, 1 to 10% by weight of hexafluoropropene, and a force ≧ Examples include copolymers consisting of 75 to 95% by weight of vinylidene fluoride, 4 to 20% by weight of tetrafluoroethylene, and 1 to 5% by weight of vinyl fluoride; Copolymers with monomers such as , fluoroalkyl vinyl ether, methacrylic ester, acrylic ester, and vinyl acetate may also be used.

However, although such crystalline polymers are suitable as sheath materials for fibers for extremely short distance signal transmission, when high performance is required, fluorinated alkyl methacrylates,
It is better to use a copolymer of fluorinated alkyl acrylate, etc., with methacrylic ester, acrylic ester, etc.

As the spinning method in the present invention, for example, composite spinning using a spinneret, a method of spinning a core-type combination and then coating with a sheath polymer can be adopted to obtain a core-sheath structure.

Next, the present invention will be explained in more detail with reference to Examples.

Example 1 All of these operations were performed in the absence of light.

Methyl methacrylate with N, N'-dinaphthyl-p
- After adding 50 ppm of phenylene diamine and stirring, the mixture was supplied to a stirring vessel at an average residence time of 30 minutes and a liquid temperature of 150°C, and then simple distillation was performed at 760 Torr. The obtained fraction is further fed to the bottom of the rectification column 45
Distillation was carried out at a reflux ratio of 1 under a reduced pressure of Torr. All distillation column vents were sealed with high purity nitrogen. The purified methyl methacrylate thus obtained had a polymerization rate of 2 even when heated to 100°C for 7 hours without adding a catalyst.
% scratches, oxygen and peroxides were sufficiently removed.

(When purified without heat treatment, the polymerization rate was 9%). Purified methyl methacrylate is distilled under reduced pressure into a raw material supply tank until a specified amount is reached, and then switched.
I did □. azobis-tert-butane and n-
A predetermined amount of butylmerkafrin was sucked into an evaporator at 45 TOrr and 35° C. from a container in which nitrogen bubbling was performed, evaporated until dryness, and then supplied to a raw material supply tank. Two raw material supply tanks were installed and used alternately every 4 hours. In this way, a stock solution of methyl methacrylate, 53 ppm of azobis-tert-butane, 12,000 ppm of n-butyl mercaptan, 1:1 m of n-butyl mercaptan was continuously supplied at 200 f/hr to a polymerization vessel that was uniformly stirred at an internal temperature of 180°C. . The polymerization vessel was used in a full state, and polymerization was carried out without supplying fresh nitrogen gas from the outside. A polymerization rate of 54% was achieved at an average residence time of 460 hours, and dimer formation was 2.9% based on the polymer. Subsequently, the weight average molecular weight of the polymer containing 0.2% dimer and 0.2% monomer was 80,000, and the ratio of weight average molecular weight to number average molecular weight was MW. /MN was 1.92.

Furthermore, the ratio of the heterotough area and the syndiotough area determined from the area of the α-methyl group was measured by NMR (Model MH100 manufactured by JEOL Ltd.) using a deuterated nitromethane solvent at 90°C. It was 1.06.

On the other hand, as a sheath material polymer, a monomer mixture consisting of 65 parts by weight of octafluoropentyl methacrylate and 35 parts by weight of methyl methacrylate, tert-butylperoxy-2-ethylhexane-)t-polymerization initiator, and octyl mertabutane were added. was added as a molecular weight regulator to make the pore size 0.2 μm.
After passing through a Teflon filter into an ampoule with a chromium-plated inner surface and completely degassing, polymerization was carried out in a closed system at 60°C for 15 hours, and then for 10 hours up to iso'C, and the sheath material was removed. Obtained. The refractive index of this polymer was 1.42.

After degassing the core polymer in an extruder, it is subsequently fed to a composite spinner, and the polymer for the sheath material is also fed in a molten state from an ampoule to the composite spinner using nitrogen gas.
An optical fiber having a core-sheath structure as shown in FIG. 1 and having a diameter of 1 was obtained by spinning at 0 tl. Using this fiber 5()m as a sample, we measured the light guide loss using a fiber loss spectrometer.As shown in Figure 2, there were windows at 52 Onmb, 570 r+m and 650 nm, and the light guide losses were 107 dB/Km and 88 dB/Km, respectively.
B/Km, 165dB/.

Example 2 Instead of preparing the catalyst by distillation in Example],
Azobis-tert-butane 0.21 as catalyst preparation liquid
%, n-butyl mercaptan4. ξ% of purified methyl methacrylate solution was prepared by the method described by feeding it through a hollow fiber for ultrafiltration membrane made of polyacrylonitrile, and the filtration limit molecular weight was 45,000 (approximately 3
OA). A syringe needle was inserted into this hollow fiber with an inner diameter of 0.8 mm, and a method was adopted in which it was crimped with a Teflon gasket, and a filtration device was assembled without using adhesive. This filtration device consists of one ultrafiltration membrane hollow fiber with a length of 15.0 crn, and the differential pressure is adjusted in the range of 0.2 to 0.3 Kf/ci, and the pressure is adjusted from the inside of the membrane to the outside. The amount of permeation of the catalyst preparation liquid was set to 5.0 f/hr. The catalyst solution treated in this way contains monomer 1 purified in the raw material supply tank.
95? /hr, polymerized in the same manner as in Example 1, and then subjected to composite spinning under the same conditions as in Example 1 to obtain a diameter of 1
Obtained Dark Optical Fiber. The light guide loss of this fiber is 110 dB/Km190 dB/Km11 at 520 nm, 570 nm% and 650 nm, respectively.
66dB/! It was 11.

Example 3 A monomer mixture consisting of 99 parts by weight of methyl methacrylate and 1 part by weight of ethyl methacrylate was purified in the same manner as in Example I. The amount of catalyst added was 300 ppm of azobis-tert-butane, n-
Butyl mercaptan at 900 ppm s residence time 1.
The reaction was allowed to take place for 25 hours. At this time, the dimer production rate was 1.1% based on the polymer, and the polymerization reaction rate was 52%. Subsequently, volatile components were removed in a degassing step to reduce the dimer concentration to 800 p'pm. The weight average molecular weight of this polymer was 102,000, and the ratio of syndiotuffic and heterotoughtic peak areas measured by NMR was 1.
It was 06.

On the other hand, the sheath material was made of 81 parts by weight of vinylidene fluoride, 15 parts by weight of tetrafluoroethylene, and 4 parts by weight of hexafluoropropene, and had a refractive index of 1.392 and a melt viscosity η. Composite spinning was performed at 2301: using a polymer having a poise of 26,000 at 230°C to obtain an optical fiber having a diameter of 1 mm. The light guiding loss of this fiber is 5
20 nm s 570 nm % 650 nm respectively] 40 dB/Kns 120 dB
/Kms 190 dB/-.

Comparative Example 1 A monomer mixture consisting of 99 parts by weight of methyl methacrylate and 1 part by weight of ethyl acrylate was placed in a container, the gas phase was replaced with nitrogen, and then 15 parts by weight of di-tert-butyl peroxide was added.
1 ppm 1 octyl mer-butane 2100 ppm was added, and polymerization was carried out at 150° C. for an average residence time of 4 hours using a homogeneous stirring reactor. Next, deaeration was performed according to a conventional method. When the toughness of this polymer was measured by NMR, the ratio of syndiotough to heterotough was 1.20. Subsequently, composite spinning was performed at 230° C. using the same sheath material as in Example 3. The light guide loss of this plastic optical fiber is 520 nm, 56
360 at 8 nm and 650 nm, respectively.
dB/Km.

255 dB/Km% 275 dB/Km.

Comparative Example 2 Thermal decomposition of the polymer was measured. Sample A as sample
(Azobis-tert-butane 53 ppm% n-butyl mercaptan t200 based on methyl methacrylate
After bubbling the mixture to which ppm was added with high-purity nitrogen for 1 hour, it was continuously fed to a polymerization vessel without any special treatment, and the polymerization was carried out in the same manner as in Example 1. and Sample B (pellets commercially available as polymethyl methacrylate molding material) 80N) f Sample C (
The thermal decomposition properties of the three core polymers obtained in Example 1 were compared.

Thermal decomposition was measured using a detector in which a certain amount of polymer was coated on a platinum plate in a thermal decomposition furnace set at a temperature of 270° C. in a nitrogen gas flow, and the results are shown in FIG. 2
The decomposition rate for 5 minutes was 6.6% for sample A and 3.6% for sample B.
, in Sample C of the present invention, it was only 2.2%.

[Brief explanation of the drawing]

Fig. 1 is a cross-sectional view of the plastic optical fiber according to the present invention, and Fig. 2 is a graph showing the results of the optical transmission characteristics in the visible light range of the low-loss plastic optical fiber containing the methyl methacrylate polymer as a core component according to the present invention. , FIG. 3 is a graph comparing the thermal decomposition properties of various methyl methacrylate combinations. The symbols in the figure have the following meanings. 1... Optical fiber, 2... Core, 3... Sheath Patent applicant's agent Akira Agata

Claims (1)

[Scope of Claims] 1. In producing a plastic optical fiber having a core made of a polymer mainly composed of methyl methacrylate and a sheath made of a polymer having a low refractive index, a step of removing oxygen contained in the monomer, a step of removing peroxide contained in the monomer after removing oxygen, and a step of removing the peroxide contained in the monomer and the general formula R, -N==N-R, (formula middle R
1 and Rt are hydrocarbon groups having 1 to 8 carbon atoms. Thereafter, the monomer, a polymerization initiator, and a chain transfer agent are charged into a polymerization vessel and polymerized continuously. A fiber having a core-sheath structure is produced by producing a polymer having an area ratio of 1.10 or less attributable to an area attributable to a heterotough material and a polymer having a lower refractive index than this. A method for producing a low-loss plastic optical fiber characterized by molding it into. 2. The manufacturing method according to claim 1, wherein the method for removing oxygen and peroxides is a method of retaining the monomer under a temperature condition of 110° C. or higher. 3. The manufacturing method according to claim 1, wherein the method for removing fine particles is a distillation method. 4. The manufacturing method according to claim 1, wherein the method for removing fine particles is passing through an ultrafiltration membrane capable of removing particles with a particle size of 0.01 μm or more.