Note: Descriptions are shown in the official language in which they were submitted.
8~4
PROCESS AND APPAE~ATUS FOR PRODUC~ION Ol~
PLASTIC OPTICAL FIBER
FIELD OE` THE INVENTION
The present invention relates to a process and an
apparatus for the production of a plastic optical fiber
having low attenuation of light transmission and good
mechanical properties.
B~CKGROUND OF ~E INVENTION
U.S. Patent No. 3,993,834 and Japanese Patent
Publication No. 42261/1978 disclose processes for producing
a plastic optical fiber comprising spinning the fiber by
means of a screw extruder. In these processes, a polymer
which has been continuously bulk polymerized is extruded by
means of a twin-screw extr~der to form pellets or fibers
while removing the unreacted monomer and volatile materials
contained in the polymer. The optical fiber obtained by the
processes is, however, contaminated with impurities such as
metals resulting from the abrasion of the screw metal.
Thus, the o~tained optical fiber has a absorptivity coeffi-
cient of 1.4 to 4 x 10 3 cm l, namely attenuation as high as
about 600 to 1,700 dB/Km.
For the production of a plastic optical fiber
having low attenuation, it is proposed to polymerize a
monomer in the form of a preform and then to spin it by
means of an extruder.
,
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~ lowever, the preform made by the polymerization of
the monomer is contaminated with dust or other Eoreign
particles since it is once removed from a polymerization
reactor into air before it is spun. It is extremely diffi-
cult to prevent such contamination of the preform. For
Example, U.S. Patent No. 4,161,500 discloses a process
comprising forming a rod or a preform of polymethyl metha-
cxylate (hereinafter referred to as "PMMA") in a gold-plated
cylinder and then spinning it by means of a ram extruder and
simultaneously extruding a cladding material by means of
another extruder to form a cladding on the periphery of the
PM~ fiber core to obtain an optical fiber. In this pro-
cess, the preform should be, however, removed into air from
the reactor, and the produced optical fiber has attenuation
of liqht transmission not lower than 300 dB/Km. A process
disclosed in U.S. Patent No. 4,138,194 provides an optical
fiber made of deuterized polymethyl methacrylate (herein-
after referred to as "P~MMA-d8)") having attenuation not
lower than 225 dB/Km.
As a process in which the preform is not exposed
to air, there is known, for example, a process disclosed in
Japanese Patent Publication (unexamined) No. 84403/1982. In
the process, polymerization of monomers is completed in a
reactor, and the prepared pGlymer is spun from one end of
the reactor by applying internal pressure with gas without
using any extruder. Although the optical~fiber obtained by
the process has low attenuation of 55 to 125 dBlKm for P~A
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and of 20 dB/Km for P(~A-d8), the mechanical properties of
the optical fiber is not satisfactory since the polymer is
spun directly rom the cylinder by the application of the
internal pressure to the cylinder and therefore the vis
cosity of the polymer cannot be made so high, and further it
is spun without stretching.
The present invention has been developed in order
to overcome the drawbacks of the above described conven-
tional processes and to provide a process and an apparatus
for the production of a plastic optical fiber having low
attenuation of light transmission and good mechanical
properties.
SUMMARY OF THE INVENTION
According to the first aspect of the invention,
there is provided a process for producing a plastic optical
fiber which comprises riltering a monomer composition
comprising a purified monomer, a polymerization initiator
and a chain transfer agent through a filter made o~ a porous
material; polymerizing the filtered monomer composition
s~lbstantially free from optically foreign substances in an
aimosphere of inert gas in a cylinde.r with stirring; conti-
nuing the polymerization without stirring after viscosity of
the polymeri~ation mixture reaches to a certain value to
complete the polymeri~ation; heating the polymer at a
temperature not lower than its softening poir.t with removing
volatile components; transferring the polymer in a spinning
head connected with one end of the cylinder to form a fiber
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core and simultaneously forming a cladding on the periphery
of the core; and drawing the opt:ical fiber.
According to the second aspect of the invention,
there is provided an apparatus for producing a plastic
optical fiber which comprises a cylinder; a monomer charging
inlet connected with the cylinder; a filter made of a porous
material connected with the monomer charying inlet; a
spinning head connected with one end of the cylinder by
which a core of the optical fiber is formed;.a stirrer which
stirs a monomer composition in the cylinder and can be
removed from the cylinder; means for transferring a polymer
produced in the cylinder to the spinning head; means for
forming.a cladding on the periphery of the core which is
connected with the spinning head; and means for drawing the
produced optical fiber.
BRIEF DESCXIPTION OF THE DRAWINGS
Fig. 1 shows a schematic view of an apparatus
according to the invention in the step of polymerization of
the monomer composition, and
Fig. 2 shows a schematic par~ial view of the
apparatus of Fig. 1 in the steps of forming the core and
cladding of the optical fiber and drawing it.
DETAILED DESCRIPTION OF THE DRAWINGS
Reference is made to Figs. 1 and 2 which illus-
trate one embodiment of the apparatus according to the
invention for producing the plastic optical fiber.
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The monomer composition comprising the monomer,
the polymerization initiator ancl the chain transer agent is
prepared in a monomer purifying vessel 1, in which the
monomer composition is purified by distillation under
S reduced pressure or by mixing the components which have been
purified separately. The pllrified monomer composition is
transferred to a monomer tank 2. Then, a predetermined
amount of the monomer composition is charged in a cylinder 4
through a monomer charging inlet 3, during which the monomer
composition is filtered through a filter 13 made of a porous
material and positioned between the monomer tank 2 and the
inlet 3 in order to make it substantially free from opti-
cally foreign substances. Before the charge of the monomer
composition, the c~linder is evacuated, and after the
- 15 monomer composition is transferred to a reaction chamber 5
of the cylinder 4, the cylinder is flashed with inert gas
such as nitrogen gas. In Figs. 1 and 2, numeral 6 desig-
nates the monomer composition in the reaction chamber 5, in
which a stirrer 7 is immersed. One end of the cylinder 4 is
sealed with a ram 8, and the other end is connected with a
cpinning head 9 and sealed with a valve (not shown) provided
therein.
_ The purified monomer composition 6 is polymerized
in an atmosphere of inert gas under pressure of from atmos-
pheric pressure to 10 Kg/cm2, during which the composition
is heated to a temperature of 80 to 150C by an outer heater
10 provided around the reaction chamber 5 of the cylinder 4.
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When the polymerization proceeds to a certain
degree and the reaction mixture becomes viscous, the stirrer
7 is removed from the reaction chamber 5 and the polymeri-
zation is continued to complete the polymerization. The
stirrer 7 may be removed at a viscosity at which the reac~
tion mixture is not throughly stirrred by the stirrer. If
necessary, the stirrer 7 may be removed before the reaction
mixture becomes as viscous as described above. Further, the
time of the removal of the stirrer is not uniformly deter-
iO mined based on the viscosity of the reaction mixture sincethe effect of stirring depends on the type of the stirrer,
the volume of the cylinder and the polymerization tempera-
ture.
After the completion of the polymer.ization, the
unreacted monomer and other volatile materials are reved
by diminishing the interior pressure of the cylinder 4, and
simultaneously the thus obtained polymer is heated to a
temperature not lower than its softening point, for example,
230 to 250C.
Thereafter, by forwarding the hydraulic ram 8
toward the reaction chamber 5, the melted polymer is trans-
ferred to the spinning head 9 under constant pressure to
form a fiber core. As means for transferring the polymer to
the spinning head, not only the ram 8 but also pressurized
gas may be used. With the spinning head 9, cladding means
11 such as a screw or a ram extruder is connected. As soon
as the core of the optical fiber is formed by the spïnning
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head, a cladding material is cladded on the peripher~ of the
core. Preferably, a gear pump 12 provided at the tip of the
extruder 11 stabilizes the delivery rate. The cladding
means may be means for coating the cladding material on the
periphery of the core.
After extruded from the spinning head 9, the
plastic optical fiber comprising the core and the cladding
cladded on the periphery of the core is drawn by drawing
means (not shown~. The draw rate depends on-the extrusion
rates of the ram 8 and the extruder ll and is usually 1 to
40 m/min., preferably, 1 to 5 m/min When it is less than 1
m/min., the production efficiency of the optical fiber is
decreased, and when it is higher than 40 m/min., irregulari-
ties such as surge appear on an interface between the core
and the cladding. Particularly, when the draw rate is less
than 5 m/min., such irregularities hardly appearsu
Specific examples of the monomer to be used as the
core according to the invention are those can afford
transparent amorphous polymers such as methacrylates 5eg.
methyl methacrylate, phenyl methacrylate, isobornyl metha-
crylate, etc.), styrene and its derivatives (eg. p-tert-
butylstyrene, etc.). These monomers include ones in that
all or a part of hydrogen atoms are substituted with deute-
rium atoms~
Specific examples o~ the polymerization initiator
are azoalkanes teg. azomethane, azopropane, azo-t-butane,
azobisisobutyronitrile, etc.) and peroxides (eg.
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di-t-butylperoxide, t-butyl peracetate, etc.). Speciic
examples of the chain transfer agent are mercaptans such as
methylmercaptan, ethylmercaptan, propylmercaptan,
butylmercaptan, etc.
The cladding material to be used according to the
invention should have a refractive index lower than that of
the polymer of the core. Its examples are a homopolymer of
fluorine-containing methacrylate, copolymers of fluorine-
containing methacrylate with, for example, methyl methacry-
late, silicone resins, polyvinylidene fluoride, vinylidene
fluoride/vinyl acetate copolymer, etc.
The porous material from which the filter is made
should be inactive to the monomer composition for the core
and not be swelled by the composition. Specific examples of
the porous material are those made of polyolefins such as
polypropylene, fluororesins such as polytetrafluoroethylene
and copolymers of tetrafluoroethylene, ceramics, etc. The
pore size of the porous material is selected so as to remove
any size of the optically foreign substances that disturb
the transmission of the light in the core. Generally, the
average pore size is no larger than 1,000 A, preferably not
larger than 800 A. Generally, the average pore size is not
less than 50 A, preferably not less than 100 A, more prefe-
rably not less than 150 A. When the pore size is too small,
the filtration rate of the monomer composition is too low,
which makes the whole process long.
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_ 9 _
The present invention will be hereinafter
explained further in detail by following Example,
Example
An apparatus described in Figs. 1 and 2 was used.
A purified monomer composition 1800 ml) of methyl
methacrylate (hereinafter referred to as "MMA") containing
as a polymerization initiator, di-tert-butylperoxide (0.01 %
by mole) and as a chain transfer agent, t-butylmercaptan
~0.3 % by mole) was filtered through a filte~ made of a
polypropylene having an average pore size of 20~ A and then
charged in the reaction chamber of the super patented SUS
304 made cylinder. Then, nitrogen gas was flashed in the
interior of the cylinder to keep the interior pressure to 5
to 8 Kg/cm2, and the monomer composition was polymerized at
15 a temperature of fxom 120 to 130C for about three hours
with stirring till the composition became viscous.
After removing the stirrer from the reaction
chamber to stop stirring, the polymerization was continued.
After polymerizing for eight hours, the pressure of the
cylinder was reduced to remove volatile materials including
the unreacted MMA from PMMA for 4 hours, while the tempera-
ture was elavated to 240C to melt PMMA. After replacing
the atmosphere with nitrogen gas, melt PMMA was transferred
to the spinning head by forwarding the ram in the cylinder.
PM~ was formed in a fiber core having a diameter of O . 30 mm
by the spinning head. A copolymer of octafluoropentyl
methacrylate and MMA in the weight ratio of 1:1 (n=1.45) as
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the cladding material was extruded from the screw extruder
connected with the spinning head on the periphery of the
just Pormed fiber core to form the cladding having a thick-
ness of 0.05 mm. The optical fiber comprising the core
S extruded from the spinning head and the cladding surrou~ding
the core was drawn at a rate of 4 m/min.
The properties of the thus produced optical ~iber
were examined. Attenuation of light transmission of the
optical fiber was 80 d~/Km for a wave length of 570 nm, and
its tensile strength was 10 Kg/mm2, both of which were
satisfactory.
Since, according to the invention, the fiber core
is formed without using any screw extruder, it is not
contaminated with metal particles due to the abrasion of the
screw and the cylinderO Since the polymer is polymerized
and spun in a closed system and thereby it is not exposed to
air in the form of a preform, it is not contaminated with
dust or other foreign particles, and its attenuation is
lower than lO0 ds/Km.
In addition, since the polymer is extruded from
the cylinder to the spinning head by the hydraulic ram, the
polymer having large degree of polymerization can be ext-
_ ruded, and therefore, the optical fiber having excellent
mechanical properties is produced.
