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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plastic optical fiber having a polymer
core constituted mainly of methyl methacrylate. Particularly, the
invention is directed to a plastic optical fiber having high efficiency of
guiding visible rays of 400 to 650 nm wavelengths and excellent
performance of transmitting signals or optical energy.
2. Description of the Prior Art
For the purpose of producing a plastic optical fiber having high efficiency
of light transmission, the primary object is to produce a core polymer
having a high transparency. U.S. Pat. No. 4,161,500 discloses a process
for spinning fiber which comprises the fractional distillation of a
monomer in a sealed system, charging the refinded monomer through a filter
having a pore size of 0.2 to 1 .mu.m into a cylindrical polymerization
vessel having an inner diameter of 25 to 30 mm, sealing the vessel,
completing the polymerization under specific pressure and temperature
conditions, cooling and withdrawing the resulting solid preform, feeding
it into the barrel of a ram extruder, and co-extruding the fed polymer as
a core material together with a cladding material. It is reported that
light attenuation through the thus produced plastic optical fiber was 274
dB/Km at 656 nm.
U.S. Pat. No. 4,381,269 proposes a polymerization process in a sealed
system which comprises charging a monomer, polymerization initiator, and
chain transfer agent through a distillation step into a polymerization
vessel, bulk polymerizing the monomer to form a core polymer, and
melt-spinning the obtained core polymer. In this process, the monomer is
mixed with 0.1 mol % of an azo bis-t-butane polymerization initiator and
0.3 mol % of an n-butyl mercaptan chain transfer agent, is completely
polymerized and the resulting polymer is extruded through a cock at the
bottom to produce a plastic optical fiber. It is reported that light
attenuations through this optical fiber are 90, 88, and 178 dB/Km at
wavelengths of 523, 568, and 650 nm, respectively. In another example of
this patent, a fiber is produced by similarly polymerizing a methyl
methacrylate monomer using azo bis-t-butane and n-butyl mercaptan, heating
the completely polymerized product to 200.degree. C., and extruding the
polymer from the polymerization vessel by applying pressure with nitrogen
gas. Light attenuations through the obtained optical fiber are confirmed
to be 62, 58, and 130 dB/Km at wavelengths of 516, 566, and 648 nm,
respectively. This patented invention is acceptable to the extent that it
is the first to disclose that an attenuation of 100 dB/Km can be achieved
with a plastic optical fiber, but the production process disclosed for
producing this fiber involves problems when utilized for the manufacturer
of plastic optical fibers which are utilizable in industrial applications.
In contrast to these processes for producing plastic optical fibers in
sealed systems, U.S. Pat. No. 3,993,834 proposes a continuous bulk
polymerization process for producing a core polymer, in which a reaction
mixture of a monomer and--is continuously fed into a polymerization
vessel, thoroughly stirred and kept at a temperature of above 130.degree.
C. and below 160.degree. C. while maintaining polymer content .PHI. in
said reaction mixture substantially constant, so as to satisfy the
following relationship:
50<.PHI.< exp (0.0121T-1.81)
wherein T represents the polymerization temperature in Calsius. Using the
thus produced core polymer, a plastic optical fiber is fabricated.
Japanese patent application Laid-Open No. 104906/82 to proposes a process
for producing a core polymer according to the continuous bulk
polymerization technique of U.S. Pat. No. 3,993,834, except that the
monomer, before being fed into a polymerization vessel, is filtered
through a porous film. According to an example disclosed in this patent
application, a light attenuation of 92 dB/Km at a wavelength of 577 nm is
confirmed. Moreover, Japanese patent application Laid-Open No. 193502/83
proposes a continuous process for producing a plastic optical fiber which
comprises successive removal of dissolved oxygen, monomer peroxide, and
fine particles from a monomer, followed by continuous bulk polymerization
of the purified monomer.
All the above stated prior techniques have been proposed to obtain
high-performance plastic optical fibers, but none of these techniques
produces plastic optical fiber which are satisfactory for practical use
because each of these techniques are connected with the following various
unsolved problems. For example, processes for producing plastic optical
fibers in sealed systems, as proposed in U.S. Pat. Nos. 4,161,500 and
4,381,269, permit a high-degree purification of feedstock, but have the
drawback in that when these processes are utilized, it is extremely
difficult to clean the inner walls of the purification facilities and of
the polymerization vessel to the same level as the level of the purified
raw material. The cleaning of these systems is similarly or more important
and more difficult than the cleaning of the raw materials. Since the
stability of product quality and the economy of production are of extreme
importance to an industrial production process, the cleaning of facilities
becomes an issue in the case of the sealed systems wherein polymerization
initiation is repeated each time and this is undesirable.
On the other hand, the continuous system is favorable for industrial
production. However, when a monomer in the liquid state is filtered
through separator films having pore sizes of 500 to 2000 .ANG. as
described in Japanese patent application Laid-Open No. 104906/82, fine
particles which remain in the monomer would have a significant effect so
that a high-performance plastic optical fiber cannot be obtained. When a
monomer in the vapor state is filtered, pores the of the filter tend to be
clogged with polymeric matter so that a stable operation cannot be
continued for long period of time. When methyl methacrylate or a monomer
mixture composed mainly thereof is filtered through such ultrafilters with
pore sizes of scores of angstroms capable of filtering off human albumin
in a separation efficiency of at least 90% as described in Japanese patent
application Laid-Open No. 193502/83 (filed by the present inventors), the
polymer that formed therefrom increases with time passage and is caught by
the filters, which gradually leads to their pores being clogged so that
long-term continuous stable operation of such equipment is impossible.
Further, in order to distill a monomer (methyl methacrylate or a monomer
mixture composed mainly of it) in the absence of oxygen as described in
Japanese patent application Laid-Open No. 193502/83, the monomer peroxide
contaminating the monomer should be completely decomposed in advance by
heat-treatment. Otherwise the distillate monomer will readily polymerize.
In any case, difficulties in long-term operation are connected with the
processes described above.
SUMMARY OF THE INVENTION
An object of the invention is to provide a plastic optical fiber having a
polymer core constituted mainly of methyl methacrylate, through which
light attenuation will be minimized over a visible ray region as wide as
from 400 to 650 nm.
Another object of the present invention is to provide a process for
producing such a plastic optical fiber.
To achieve the above objects, the present inventors have made intensive
studies, and as a result, have found that the process described in detail
hereinafter gives an unprecedently and unexpectedly high-performance
plastic optical fiber, in particular through which light attenuation is
very slight over a wide range of wavelengths. Based on this finding, the
present invention has been accomplished.
According to an embodiment of the present invention, there is provided a
plastic optical fiber comprising a core polymer constituted mainly of
methyl methacrylate and a clad polymer having a lower refractive index
than that of the core, characterized in that the core consists of a methyl
methacrylate homopolymer or a copolymer constituted of at least 95% by
weight of methyl methacrylate and less than 5% by weight of (i) methyl
acrylate, (ii) ethyl acrylate, or (iii) a mixture of both acrylates, that
the weight-average molecular weight of the core polymer is from 80,000 to
200,000, and that light attenuations through the fiber are up to 250, 130,
80, and 130 dB/Km at wavelengths of 400, 450, 570, and 650 nm,
respectively.
According to another embodiment of the present invention, there is provided
a process for producing a core-cladding structure, which comprises;
(A) the steps of
distilling a monomer in the presence of oxygen, removing dissolved oxygen
from the distillate, and continuously charging the purified monomer into a
polymerization vessel, and
(B) on the other hand, the steps of
diluting each of a chain transfer agent and a polymerization initiator or
their mixture with a purified solvent, either (i) filtering the solution
through an ultrafilter constructed of hollow fibers having dense walls
with a pore size of 100 .ANG. or less, and removing dissolved oxygen from
the filtrate, or (ii) removing said oxygen and then filtering the
resultant, and continuously charging the purified filtrate into the
polymerization vessel,
(C) followed by the steps of
continuous solution polymerization of the charged monomer, removing
volatile matter from the polymerization product in a degasifier, forming a
core fiber from the polymer product, and cladding the core fiber with a
polymer having a lower refractive index than that of the core polymer.
BRIEF DESCRIPTION OF THE DRAWING
FIG. 1 shows a typical spectrum of light attenuation through the optical
fiber obtained in Example 1 of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The core polymer constructing the high-performance plastic optical fiber of
the present invention consists of a methyl methacrylate homopolymer or a
copolymer constituted of at least 95% by weight of methyl methacrylate and
less than 5% by weight of (i) methyl acrylate, (ii) ethyl acrylate, or
(iii) a mixture of both acrylates. The limitation of the methyl
methacrylate content to at least 95% by weight is for the purpose of
securing a plastic optical fiber resistant to temperatures of up to at
least 80.degree. C. The molecular weight of core polymer is desirably from
80,000 to 200,000, preferably from 90,000 to 120,000. If the molecular
weight is below 80,000, the mechanical strength will be unsatisfactory. If
the molecular weight exceeds 200,000, smooth spinning of such a core
polymer would be impossible. The content of volatile matter in the core
polymer is desirably up to 1%, preferably up to 0.5%, by weight so as to
ensure the reliability of the plastic optical fiber for long-term use. In
addition, the plastic optical fiber fabricated by cladding the core
polymer needs to exhibit attenuations of up to 250, 130, 80, and 130 dB/Km
at wavelengths of 400, 450, 570, and 650 nm, respectively. The core
polymer exhibiting such high performance has not been obtained up to the
present time by a prior art method. It has for the first time been
obtained by the present inventors. In order to obtain such a core polymer
for a high-performance plastic optical fiber; mere simple high-degree
purification of the raw materials is not sufficient. It has been
discovered that a process must be established and employed in which the
resulting polymer is produced from purified raw materials which will not
undergo any new contamination. One new contamination source is derived
from the low degree of cleaning of the whole production system for raw
material purification, raw material feeding, polymerization, and spinning,
which the materials must pass through. It has been ascertained that
several days are required for a complete purging of the fine particles
which are adhered to the whole inner wall of the production system. For
this purpose, it is important to operate the production process steadily
over a long term, and the amelioration of raw material purification steps
in particular has been investigated. Although preferable, the high degree
purification of raw materials is not acceptable if it is excessive and
complicated and does not permit a long-term stable operation. The present
invention has optimized both the degree of raw material purification and
the stability of operation.
The monomer purification method adopted in the present invention comprises
a distillation of the monomer in the present of oxygen to remove fine
particles, followed by an immediate elimination of the dissolved oxygen
from the distillate monomer. The distillation in the presence of oxygen,
as compared with a distillation in the absence of oxygen, results in the
introduction of an extremely minute amount of oxygen and monomer peroxide
in the monomer but avoids problems, i.e. polymerization occurring during
the distillation step. The amount of oxygen in the distillation step is
required to be of such an order so as to dissolve at least 50 ppm,
preferably from 100 to 5000 ppm of the monomer liquid, in excess of the
previously dissolved oxygen present in the monomer liquid that consists
mainly of methyl methacrylate. The newly supplied oxygen may be in the
form of air, plain oxygen gas, or oxygen diluted with an inert gas. For
the purpose of preventing the coloration due to the methyl methacrylate
monomer peroxide, it is desirable that the distillation be conducted at a
low temperature under reduced pressure and that the distillate be cooled
to a low temperature, so that the formation of monomer peroxide from the
monomer and oxygen would be inhibited to the extent possible. Dissolved
oxygen is required to be removed immediately from the monomer liquid
obtained by distillation, without withdrawing the monomer from the system.
This is due to methyl methacrylate peroxide resulting from the contact of
the monomer with oxygen. The presence of this peroxide in the monomer,
similarily to the presence of oxygen, is responsible for the coloration of
the polymer. A most effective method for this removal is to immediately
feed the distillate monomer into a stripping column without stagnation,
and bring the fed distillate monomer into contact with a countercurrently
blowing inert gas such as nitrogen gas, thereby quickly removing dissolved
oxygen. After this oxygen removal step, the material is kept at a
temperature of up to 15.degree. C., preferably up to 10.degree. C. The
efficiency of dissolved oxygen removal from the monomer is determined by
using a dissolved oxygen analyzer comprising a polyarographic type sensor
for nonaqueous solvent purposes. This measurement is conducted as follows:
Methyl methacrylate is exposed to the atmosphere at 10.degree. C. for a
sufficient time; the thus equilibrated methyl methacrylate is used as a
calibration liquid; the amount of dissolved oxygen therein is measured by
taking the reading on the analyzer; then the amount of dissolved oxygen in
a sample monomer, which had been subjected to the oxygen removal
treatment, is measured at 10.degree. C. by taking the reading on the
analyzer. The ratio (%) of the latter reading to the former is calculated
as the percentage of the remaining dissolved oxygen. Since plastic optical
fibers prepared by using monomers of high percentages of remaining
dissolved oxygen give large attenuations in the wavelength region of from
400 to 450 nm, the percentage of remaining dissolved oxygen should be
controlled to 3% or less, preferably 1% or less. The inert gas used for
countercurrent contact with the distilled monomer in the stripping column
must be previously filtered, of course, through an ultrafilter or the
like. To minimize the content of impurity oxygen in the inert gas, it is
desirable to use a commercially available high purity nitrogen gas of 0.1
ppm oxygen concentration or a similar gas purified through a column to
adsorb traces of oxygen. By such a treatment, dissolved oxygen can be
removed sufficiently without any contamination of the monomer. In the
above monomer purification, no problem arises when the monomer to be
treated is methyl methacrylate along or a mixture thereof with methyl
acrylate or with ethyl acrylate. The purified monomer is then continuously
into pumped a polymerization vessel.
In the next place, a polymerization initiator and a chain transfer agent
are purified and charged in the following manner: These materials are
dissolved separately or together in a purified solvent. After the
continuous removal of fine particles and dissolved oxygen, these solutions
are continuously charged into the polymerization vessel.
The polymerization initiator and the chain transfer agent may be treated
separately or in combination. Fine particles in the solution(s) of both
materials need to be removed by using ultrafilters capable of removing
fine particles having sizes of 100 .ANG. or more. As an example, U.S. Pat.
No. 3,871,950 describes such ultrafilters, which are constructed of a
dense layer having pore sizes of 100 .ANG. or less. Desirably, ultrafilter
films used herein are made of polyacrylonitrile in view of the corrosion
resistance thereof to the solvent, the polymerization initiator, and the
chain transfer agent (a mercaptan). Ultrafilters formed of hollow
polyacrylonitrile fibers are particularly suitable from the viewpoint of
the fineness of removable particles in which these fibers are capable of
removing and the corrosion resistance to the polymerization initiator, the
chain transfer agent, and the solvent. Separation filters favorable in
filtration capability are available which are capable of separating human
albumin (molecular weight 50,000) in 90% or more efficiency (e.g. hollow
polyacrylonitrile fiber HH-1, supplied by Asahi Chemical Industriesl Co.,
Ltd.). More desirable separation filters are capable of separating
cytochrome C (molecular weight 13,000) in 90% or more efficiency (e.g.
hollow polyacrylonitrile fiber HC-5, supplied by Asahi Chemical Industries
Co., Ltd.). The efficiency of separating human albumin or cytochrome C is
defined herein as follows: Human albumin or cytochrome C is dissolved in
physiological saline solution (buffered to pH 7 with a 0.15 mol/1
phosphate solution) to a concentration (C.sub.1) of 0.025% by weight. The
solution is passed in a hollow fiber for ultrafiltration at a velocity of
1 m/sec. Then, the concentration (C.sub.2) of human albumin or cytochrome
C in the outflow is determined by measuring the ultraviolet absorbance at
280 nm. The thus determined value [(C.sub.1 -C.sub.2).times.100/C.sub.1 ]
is defined as the separation efficiency.
It should be noted that common ultrafilter modules commercially available
cannot be used as such, since these modules, for fixing hollow fibers,
comprise adhesives and housing materials which are readily attacked by the
monomer, the polymerization initiator, and the chain transfer agent.
For the purpose of fixing hollow polyacrylonitrile fibers, it is advisable
to insert stainless steel tubes having an outer diameter slightly larger
than the inner diameter of the hollow fiber into the hollow fiber. The
housing may be constructed of stainless steel or some other
corrosion-resistant material.
The solvent used herein is charged along with the polymerization initiator
and the chain transfer agent into the polymerization vessel. Hence, it is
necessary to choose, as the solvent, a liquid having no adverse effect on
the quality of the resulting polymer. Such suitable liquids include
ethylbenzene, methyl isobutyrate, and toluene which have been treated with
activated alumina. In particular, ethylbenzene when used gives
high-performance plastic optical fibers. Other effects of the use of a
solvent are that minute mounts of the polymerization initiator and chain
transfer agent can be more quantitatively supplied by dilution with the
solvent and hence the resulting polymer has definite quality and that the
presence of the solvent in the polymerization system prevents local high
concentration of solids in the system. The solvent is not satisfactorily
purified by distillation alone; adsorption treatment with activated
alumina is necessary. A more favorable method for solvent purification is
to combine activated alumina treatment with distillation. To the
polymerization initiator and chain transfer agent diluted with such
solvents, ultrafilters comprising hollow polyacrylonitrile fibers
withstand as long as 90 days or more.
Dissolved oxygen in solutions of the polymerization initiator and of the
chain transfer agent is removed in the same manner as for the oxygen
removal from the monomer. The step of oxygen removal and the step of fine
particle removal on an ultrafilter may be operated in reverse order. The
polymerization initiator and the chain transfer agent are added an amount
sufficient to make the concentration of the solvent to the monomers
desirably 5 to 30% by weight. While a large amount of solvent is used for
the production of a core polymer of specially high molecular weight, too
large amounts of solvent are not advisable since the amount of fine
particles increases as the amount of solvent is increased. Thus the
polymerization initiator and the chain transfer agent in the form of
solution are free of fine particles and oxygen, are ready to be charged
into the polymerization vessel.
The polymerization can be continuously carried out completely in a mixing
single-stage type reaction system, or a multistage reaction system, or
completely in a mixing vessel and plug flow type reactor combined system,
or a plug flow type of reaction system. The polymer content in the
reaction mixture relates to the contamination (degradation) caused by the
stagnation in dead spaces in the equipment during passage of the reaction
mixture from the polymerization vessel to a degasifying stage. When the
polymer content is high, the temperature of the pipe for passing the
polymerization product mixture is required to be increased and this tends
to develop colored matter. Therefore it is desirable not to much raise the
polymer content in any large extent. Particularly preferred polymer
contents are from 30 to 60% by weight.
Desirable polymerization initiators are azoalkane catalyst such as
azobisoctane represented by the formula
##STR1##
and azo bis-t-butane represented by
##STR2##
the former being preferable. The use of azonitrile compounds and peroxides
result in core polymers which give large light attenuations at wavelengths
of up to 450 nm. The polymerization initiator, for example an azoalkane
catalyst, when added in a large amount, results in large attenuations at
wavelengths of up to 450 nm and hence is required to be added in an amount
of up to 0.01 mol %, preferably up to 0.005 mol %, based on the monomer.
From this viewpoint, it is undesirable that the polymerization initiator
be added in large amounts for the purpose of raising the polymerization
yield. Specially, in the method of completing polymerization in a sealed
system, the amount of polymerization initiator is too large for achieving
small attenuations at wavelengths of 400 to 450 nm. This is in contrast
with the present continuous polymerization method, in which the amount of
polymerization initiator can be decreased.
Suitable chain transfer agents for use herein include n-butyl mercaptan,
t-butyl mercaptan, n-propyl mercaptan, and n-octyl mercaptan. The amount
of chain transfer agent added governs the molecular weight of the
resulting core polymer. Favorable molecular weights of the core polymer
are from 80,000 to 200,000, particularly from 90,000 to 120,000, in terms
of weight-average molecular weight as measured by gel permeation
chromatography (GPC). When the molecular weight is less than 80,000, the
mechanical strength of the polymer is unsatisfactory. The molecular weight
exceeding 200,000 makes it difficult to spin the polymer smoothly. The
molecular weight depends chiefly on the type and amount of chain transfer
agent though influenced also by the type and amount of solvent used in the
polymerization. When the amount of solvent is large, a polymer of the
intended molecular weight can be obtained with a somewhat less amount of
chain transfer agent. When an alkyl mercaptan as cited above is used in
amounts of about 0.22 to 0.07 mol %, the weight-average molecular weight
becomes from 80,000 to 200,000. The molecular weight measured by GPC is
the value based on a calibration chart made from an elution curve which
has been obtained by using standard monodisperse polystyrenes of known
different molecular weights and tetrahydrofuran as solvent.
The core polymer of the present invention may be a methyl methacrylate
homopolymer or a copolymer constituted of 95% by weight of methyl
methacrylate and either methyl acrylate or ethyl acrylate. This limitation
of the methyl methacrylate content to at least 95% by weight is for the
purpose of securing a plastic optical fiber resistant to temperatures of
up to 80.degree. C. at least. The polymerization product mixture composed
of the polymer, unreacted monomer, solvent, and polymerization initiator,
in the continuous-polymerization vessel, is then fed into a degasifier,
wherein volatile matter is expelled from the mixture. Suitable degasifiers
for use herein include a vent-type extruder, a flush tank in which the
heated product mixture is fed through a slit to flow down and meantime
volatile matter is expelled from the mixture, or an arrangement combining
such a flush tank with a vent-type extruder. The volatile matter content
in the core polymer should be decreased to 1% or less, preferably 0.5% or
less, to ensure the reliability of the plastic optical fiber for long-term
use. The core polymer free from volatile matter is then fed into a
composite-spinning die, and spun along with a cladding polymer fed from
another extruder, thereby fabricating a plastic optical fiber of
core-cladding structure.
For the high-performance plastic optical fiber of the present invention,
the core polymer is specially important while the cladding polymer also
plays an important role. A property necessary for the cladding polymer is
a sufficiently lower refractive index than that of the core polymer. The
refractive index (n.sub.D.sup.20) of cladding polymer is desirably up to
1.43, preferably up to 1.415. As the refractive index lowers, the maximum
possible light incident angle increases. Additional properties necessary
for the cladding polymer are high transparency, mechanical strength, heat
resistance, and adhesiveness to the core. At present, however, no perfect
cladding polymer is found and polymer well-balanced, as a whole, in
properties are chosen as cladding materials.
In the process of the present invention, the polymer used for the cladding
to provide superior attenuation characteristics can be selected from the
group comprising;
(1) copolymers of at least one of the following group (A) monomers and at
least one of the following group (B) monomers;
(A) group monomers: CH.sub.2 .dbd.C.multidot.CH.sub.3 COOCH.sub.2
(CF.sub.2).sub.m H, wherein m is 1 or 2,
(B) group monomers: CH.sub.2 .dbd..multidot.CH.sub.3 COOCH.sub.2
(CF.sub.2).sub.n F, wherein n is 1 or 2,
(2) copolymers of at least one of the group (A) monomers, at least one of
the group (B) monomers, and methyl methacrylate, and
(3) copolymers of at least one of the groups (A) and (B) monomers and
methyl methacrylate.
However, it is desirable to use such a copolymer as defined below, for the
purpose of producing a plastic optical fiber having not only super
anti-attenuation characteristics but also high mechanical strength, heat
resistance, and long-term stability under harsh environmental conditions.
That is a copolymer which comprises
(a) 40 to 80% by weight of 2-(perfluorooctyl)ethylmethacrylate represented
by the formula
CH.sub.2 .dbd.C.multidot.CH.sub.3 .multidot.COO(CH.sub.2).sub.2
(CF.sub.2).sub.7 CF.sub.3 (I),
(b) 15 to 50% by weight of at least one monomer selected from the group
consisting of short-chain fluoroalkyl methacrylates represented by the
formula
CH.sub.2 .dbd.C.multidot.CH.sub.3 COOCH.sub.2 (CF.sub.2).sub.n X (II),
wherein n is an integer of 1 to 4, and
(c) 0 to 20% by weight of methyl methacrylate, said copolymer exhibiting a
melt flow index of 10-200 g/10 min as measured under the conditions
(230.degree. C., 3.8 Kg load, orifice diameter 2.0955 mm) defined in ASTM
D-1238, a refractive index (n.sub.D.sup.20) of 1.39 to 1.42, and a Vicat
softening temperature (ASTM D1525-76) of 50.degree. to 85.degree. C.,
preferably 60.degree. to 85.degree. C. Preferably, this type copolymer has
at least 5% by weight of difluoroethyl methacrylate or tetrafluoropropyl
methacrylate as the short-chain fluoroalkyl methacrylate. These copolymers
may further contain up to 1% by weight of a copolymerizable monomer, e.g.
acrylic acid, acrylic ester, methacrylic acid, or methacrylic ester.
Care must be taken in measuring light attenuations through plastic optical
fibers since the value varies with the measurement conditions. In the
present invention, conditions of measuring spectra of attenuation through
optical fibers with a spectrophotomer are as follows:
A monochromatic light beam whose half breadth is 2.5 nm from a
monochrometer is converged to give a range of incident angles of 0.15
radian and a beam diameter of less than 0.2 mm at an end surface of the
test optical fiber to enter the fiber. The spectrophotomer is provided
with a chopper and a lock-in amplifier so as not to be affected by other
incident rays. The element to detect the light leaving the fiber is an
Si-PIN photodiode. Since the attenuation at 650 nm is affected by the
degree of moisture absorption in the sample fiber, it is conditioned by
drying in a hot-air over at 70.degree. C. for 5 to 24 hours before
measurement. The sample fiber is cut to a length of 52 m and both the ends
are pressed against a hot plate to be mirror-finished. One end of the
fiber is fixed on a minutely shiftable table positioned on the light
source side and the other end of the fiber on a minutely shiftable table
positioned on the light-detector side. The positions of the fiber ends are
adjusted by manipulating the minutely shiftable tables to maximize the
light energy transmitted by the fiber. After measurement of this
transmitted light energy (P.sub.1) in the wavelength range of 400 to 650
nm, the sample fiber is cut and removed but 2 m of one end portion thereof
is left with the end fixed as such and the other end of the portion is
fixed anew. This sample is similarly measured for the transmitted light
energy (P.sub.2) in the wavelength range of 400 to 650 nm. The attenuation
is determined from the following equation:
dB/Km=10.times.log (P.sub.2 /P.sub.1).times.(1000/(52-2))
The reproducibility of this measuring method is as good as the variation in
the found value is .+-.1 dB/Km at a wavelength of 450 nm or longer and up
to .+-.5 dB/Km at 400 nm. The spectrophotomer is previously calibrated for
wavelengths by using standard light sources.
In the present invention, the attenuation is measured in principle on bared
plastic optical fibers of core-cladding structure but also may be
evaluated on cords fabricated by coating such bared fibers with
polyethylene since the attenuation in this case is practically not
altered.
The following examples illustrate the present invention.
EXAMPLE 1
A monomer mixture of 99.5 wt % of methyl methacrylate containing no
polymerization inhibitor and 0.5 wt % of methyl acrylate is continuously
fed at a rate of 3.5 Kg/hr into a still under operation at 100 torr.
Continuous distillation is effected while blowing air at a rate of 5N 1/hr
into the bottom and withdrawing bottoms at a rate of 0.1 Kg/hr. The
obtained distillate is fed into the top of a stripping column of 2 m
packing height and 40 mm inner diameter packed with 3 mm diameter glass
beads. High purity nitrogen gas of 0.1 ppm oxygen concentration is
filtered in two stages through ultrafilters (HC-5, supplied by Asahi Chem.
Ind. Co., Ltd.) formed of 1.4 mm inner diameter hollow polyacrylonitrile
fibers (separation efficiency: at least 90% for cytochrome C (M.W. 13,000,
calculated particle size 30 .ANG. or less)), and fed at a rate of 1N
m.sup.3 /hr into the bottom of the stripping column. In a glass cell
placed in the course of the monomer mixture effluent from the bottom, the
mixture is irradiated with an He-Ne laser beam to check the presence of
shining fine particles. Over 90 days' observation, there is not found
shining particle in the path of the laser beam or enlargement of the laser
beam width.
The amount of oxygen dissolved in the monomer is determined by using an
oxygen analyzer model 2713 mfd. by Orbisphere Laboratories. A sensor
attached to said analyzer is called as model 2110. For the determination
of the oxygen amount dissolved in the monomer, however, among parts of
said sensor, parts setting up by Delurin.RTM. and Viton.RTM. are replaced
by those made of Teflon.RTM. (a registered trademark of Du Pont) material
due to their non-anticorrosive properties against the monomer.
The operation of the determination is conducted as mentioned below:
In order to remove the oxygen dissolved in electrolyzates of said sensor
completely before the determination, said sensor is kept soaked in a
saturated aqueous sulfurous acid for one hour with keeping the switch of
said analyzer on until the reading of the meter indicated less than 1 ppb.
After the confirming that, said sensor is placed in a closed chamber
containing the air of atmospheric pressure whose relative humidity is
adjusted to 100%. Thereafter, the interior temperature of the chamber is
measured. The meter of said sensor is adjusted to indicate the reading
corresponding to the amount of oxygen dissolved in water having the same
temperature as that determined above. That is, if the temperature is
23.degree. C., the meter indication should be adjusted to 8.55 ppm.
In this respect, it should be kept in mind that the amount of oxygen
dissolved in the monomer is given only as a relative figure while that of
oxygen dissolved in water is given as an absolute one. Therefore, in order
to know to what extent the oxygen dissolved in the monomer is removed, a
ratio of a reading for the amount of oxygen dissolved in the monomer after
the removal to that of oxygen dissolved in the monomer under atmospheric
pressure of the air was determined in the manner mentioned below:
After bubbling the air into methacrylate stored in a tank kept at
10.degree. C. under atmospheric pressure so as to dissolve oxygen at the
saturated concentration therein, the resulting methacrylate was supplied
to the sensor of model 2110 of said analyzer at the rate of 3 1/hr.
Usually, the reading of the meter is around 8.94 (R.sub.0). Then, the
monomer from which dissolved oxygen had been removed according to the
present process is supplied at the same rate to the sensor. Usually, the
reading of the meter is about 0.063 (R.sub.1). That is, the ratio of the
remaining oxygen in the monomer which has been subjected to the removal
procedure of the present invention can be given by the equation:
R.sub.1 .div.R.sub.0 .times.100,
and usually, the ratio is about 0.7% as given by
0.063.div.8.94.times.100=0.7%.
This monomer is fed at a rate of 3 Kg/hr into a polymerization vessel. On
the other hand, azobisoctane, as a polymerization initiator, and n-butyl
mercaptan, as a chain transfer agent, are each dissolved in ethylbenzene,
which had been passed in advance through a glass column packed with an
activated alumina (neutral, Grade I) supplied by Woelm Co. to remove
impurities by adsorption. This alumina column on adsorption of impurities
develops a yellow absorption band. Hence the column is exchanged with a
fresh one before the yellow band would extend to the bottom of the column.
Further, the ethyl benzene is distilled to remove fine particles.
Concentrations of azobiscotane and n-butyl mercaptan in the ethylbenzene
solutions are 0.072% and 2.935%, respectively, by weight. These solutions
are each fed at a rate of 0.17 Kg/hr to the next purification stage. Each
solution is filtered in two stages through ultrafilters (HC-5, supplied
by Asahi Chem. Ind. Co., Ltd.) formed of hollow polyacrylonitrile fibers.
Then the filtrate is fed into a column of 1 m packing height and 15 mm
inner diameter, packed with 2 mm-diameter glass beads wherein the same
purity nitrogen gas as used for removal of dissolved oxygen from the
monomer mixture is flowed countercurrently at a rate of 80N 1/hr to expel
oxygen. The thus purified polymerization initiator and chain transfer
agent solutions are associated with the monomer flow immediately before
entering the polymerization vessel, and the mixture is fed thereinto
continuously.
The polymerization vessel is divided into a portion for complete mixing and
a portion for plug flow, said portions having internal capacities of 9.5
and 1.7 Kg, respectively. The polymerization is conducted at temperatures
of 135.degree. to 140.degree. C. in the complete mixing portion and at
temperatures gradient from 140.degree. to 170.degree. C. in the plug flow
portion. The polymer content in the product mixture is 42 wt %.
This crude product mixture is then continuously fed into a degasifier to
expel volatile matter, yielding a core polymer. The weight-average
molecular weight of this polymer is 98,000 as measured by GPC. The GPC is
conducted by using a gel permeation chromatograph (LC-1, supplied by
Shimazu Co., Ltd.) provided with columns HSG-20 and HSG-50, which was
calibrated by using standard polystyrenes as molecular weight standards.
Tetrahydrofuran is used for the solvent.
The clad polymer is formed of a transparent polymer constituted of 40 wt %
of 2-(perfluorooctyl)ethyl methacrylate, 30 wt % of tetrafluoropropyl
methacrylate, 20 wt % of trifluoroethyl methacrylate, and 10 wt % of
methyl methacrylate. This copolymer is found to have a Vicat softening
temperature (ASTM D-1525-76) of 70.degree. C., refractive index
n.sup.D.sub.20 of 1.410, and melt flow index (ASTM D1328, 230.degree. C.,
3.8 Kg load) of 35 g/10 min.
The core and clad polymers are fed into a composite spinning die to spin a
fiber of core-cladding structure, which was then stretched and
heat-treated to yield a plastic optical fiber of 0.98 core diameter and
1.00 outer diameter.
To measure light attenuations through this optical fiber, it is conditioned
by drying in a hot-air oven at 70.degree. C. for 5 hr. Then, this fiber is
cut to a length of 52 m to prepare a test specimen.
The determination of the attenuations is carried out by using a fiber loss
spectrometer model FP-889 mfd. by Oplex Corp. The lamp used for this
determination is a quartz halogen lamp JC12V50W. The half breadth of
spectrum diffracted by diffraction grating is 2.5 nm. A monochromatic
light beam from a diffraction grating type of light source is converged to
give an incident angle range of 0.15 radian and incident on an end surface
of the sample fiber, said light source being previously calibrated for
wavelengths by using standard light sources emitting severally rays of
wavelengths 404, 546, and 632.8 nm. The light transmitted by the test
specimen 52 m long is detected with an Si-Pin photodiode and outputs
(P.sub.1) therefrom are read in the wavelength range of 400 to 660 nm.
Then the specimen is cut at a position 2 m distant from the light-incident
end and removed except this 2-m long portion with this end left being
fixed as such. The other end of the 2-m long fiber is properly fixed and
then the intensity of the light transmitted by this short specimen was
similarly measured, where the outputs (P.sub.2) are read in the wavelength
range of 400 to 660 nm. Attenuations through the plastic optical fiber is
determined from the equation dB/Km=10.times.log (P.sub.2
/P.sub.1).times.(1000/(52-2)). The found attenuations were 197, 103, 66,
and 124 dB/Km at wavelengths of 400, 450, 570, and 650 nm, respectively.
The spectrum of the determined attenuations is shown in FIG. 1.
Each attenuation is an average of many found values, in which variation is
very small. The attenuation measurement was continued over 60 days.
During those periods, as one of the favorable date, there are observed the
attenuations of 183, 98, 62 and 119 dB/Km at wavelengths of 400, 450, 570
and 650 nm, respectively.
EXAMPLE 2
The procedure of Example 1 is followed except that the amount of the chain
transfer is altered, that is, a solution of 2.38 wt % of n-butyl mercaptan
in ethylbenzene is fed at a rate of 0.17 Kg/hr to the polymerization
vessel. The core polymer degasified through the vent-type extruder is
found to have a weight-average molecular weight of 120,000. A core fiber
formed from this polymer is coated with the same cladding polymer as used
in Example 1. Light attenuations through the thus obtained plastic optical
fiber are 240, 120, 70, and 128 dB/Km at wavelengths of 400, 450, 570, and
650 nm, respectively.
EXAMPLE 3
A monomer change test was conducted in the course of the polymerization of
Example 1. That is, a monomer mixture of 99 wt % of methyl methacrylate
and 1.0 wt % of ethyl acrylate, for exchange, is fed into the still.
Thereafter, the procedure of Example 1 was followed and light attenuations
through the obtained plastic optical fiber are measured.
The found value at 650 nm was 170 dB/Km after 1 day from the change of
monomer, 150 dB/Km after 2 days, 135 dB/Km after 4 days, and 126 dB/Km
after 6 days, nearly the same transmission efficiency as that of the fiber
of Exampl | | |
