U.S. patent application number 11/054437 was filed with the patent office on 2005-07-07 for plastic optical fiber, optical fiber cable, plugged optical fiber cable, and production methods thereof.
This patent application is currently assigned to Mitsubishi Rayon Co., Ltd.. Invention is credited to Akita, Takashi, Fukuba, Yoshinori, Irie, Kikue, Okamoto, Masashi, Okumura, Jun, Sasaki, Shigeaki, Sumi, Toshinori.
Application Number | 20050147368 11/054437 |
Document ID | / |
Family ID | 27334462 |
Filed Date | 2005-07-07 |
United States Patent
Application |
20050147368 |
Kind Code |
A1 |
Fukuba, Yoshinori ; et
al. |
July 7, 2005 |
Plastic optical fiber, optical fiber cable, plugged optical fiber
cable, and production methods thereof
Abstract
The present invention relates to a production method of a
plastic optical fiber which comprises the steps of heat drawing an
undrawn plastic optical fiber obtained by melt spinning and
annealing the drawn fiber at a circumferential velocity ratio
between the front and rear rollers (circumferential velocity of a
rear roller/circumferential velocity of a front roller) of 0.5 to
1.2 under heating conditions which satisfy
4.ltoreq.y.ltoreq.1.5x+330 and (Tgc-5).degree.
C..ltoreq.x.ltoreq.(Tgc+11- 0).degree. C. [Tgc: a glass transition
temperature of a core, x: an annealing temperature (.degree. C.),
and y: an annealing time (seconds)]. According to the present
invention, a plastic optical fiber having a small thermal shrinkage
ratio and excellent heat resistance can be provided.
Inventors: |
Fukuba, Yoshinori;
(Hiroshima, JP) ; Akita, Takashi; (Hiroshima,
JP) ; Irie, Kikue; (Hiroshima, JP) ; Sumi,
Toshinori; (Hiroshima, JP) ; Okamoto, Masashi;
(Hiroshima, JP) ; Okumura, Jun; (Hiroshima,
JP) ; Sasaki, Shigeaki; (Hiroshima, JP) |
Correspondence
Address: |
FITCH, EVEN, TABIN & FLANNERY
P. O. BOX 65973
WASHINGTON
DC
20035
US
|
Assignee: |
Mitsubishi Rayon Co., Ltd.
|
Family ID: |
27334462 |
Appl. No.: |
11/054437 |
Filed: |
February 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11054437 |
Feb 10, 2005 |
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10070616 |
Jul 15, 2002 |
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6871000 |
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10070616 |
Jul 15, 2002 |
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PCT/JP00/06093 |
Sep 7, 2000 |
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Current U.S.
Class: |
385/123 |
Current CPC
Class: |
G02B 6/02033 20130101;
B29C 55/045 20130101; G02B 6/02038 20130101; B29D 11/00721
20130101; G02B 6/105 20130101 |
Class at
Publication: |
385/123 |
International
Class: |
G02B 006/02 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 9, 1999 |
JP |
11-255802 |
Mar 3, 2000 |
JP |
2000-058812 |
Mar 13, 2000 |
JP |
2000-068866 |
Claims
1. A plastic optical fiber having a shrinkage stress occurring
temperature obtained by thermomechanical analysis of not lower than
[(a glass transition temperature of a core)-35].degree. C.
2. The plastic optical fiber as claimed in claim 1, wherein the
core comprises a homopolymer of methyl methacrylate, or a copolymer
comprising a methyl methacrylate unit and another monomer unit.
3. The plastic optical fiber as claimed in claim 1, wherein the
core comprises a homopolymer of methyl methacrylate and has a
birefringence absolute value of not larger than
2.0.times.10.sup.-4.
4. A plastic optical fiber which has a core comprising a
homopolymer of methyl methacrylate and having a birefringence
absolute value of not smaller than 1.5.times.10.sup.-4 and has a
shrinkage stress occurring temperature obtained by thermomechanical
analysis of not lower than [(a glass transition temperature of the
core)-20].degree. C.
5. The plastic optical fiber as claimed in any one of claims 1 to
4, which exhibits a shrinkage ratio of not higher than 2% when
heated at 90.degree. C. for 65 hours.
6. The plastic optical fiber as claimed in claim 4, which exhibits
a shrinkage ratio of not higher than 0.5% when heated at 90.degree.
C. for 65 hours.
7. A plastic optical fiber cable obtained by forming a coating
layer around the plastic optical fiber a: claimed in any one of
claims 1 to 6.
8. A plastic optical fiber cable that has a protective layer
comprising a vinylidene fluoride-tetrafluoroethylene copolymer
formed around the plastic optical fiber as claimed in any one of
claims 1 to 6 having a core-sheath structure in which the sheath
comprises a polymer containing a fluorine-based methacrylate unit
or a vinylidene fluoride unit and that has a coating layer
comprising Nylon 12 formed on the protective layer.
9. A plugged plastic optical fiber cable obtained by attaching a
plug on the tip of the plastic optical fiber cable as claimed in
claim 7 or 8.
Description
TECHNICAL FIELD
[0001] The present invention relates to a plastic optical fiber, an
optical fiber cable and a plugged optical fiber cable which are
excellent in heat resistance, and production methods thereof.
BACKGROUND ART
[0002] Heretofore, as an optical fiber, an inorganic glass optical
fiber capable of excellent optical transmission over a wide
wavelength range has been known and practically used mainly in a
trunk line system. However, the inorganic glass optical fiber is
expensive, has poor workability, and is not so resistant to bending
stress. Therefore, a plastic optical fiber which is less expensive
and in which making diameter large, the work of end faces and
handling are easy has been developed and put to practical use in
some fields of lighting, sensors, and wiring for communication
between OA or FA equipment.
[0003] Generally, a plastic optical fiber (hereinafter referred to
as "POF") is a fiber comprising a core-sheath structure in which as
a core, a polymer having a large refractive index and excellent
light transmittance such as polymethyl methacrylate, polycarbonate,
polystyrene or amorphous polyolefin and a transparent polymer
having a smaller refractive index than that of the core polymer as
a sheath are used.
[0004] As a industrial production process of such a POF, in
general, a core polymer and a sheath polymer are arranged
concentrically by use of a multi-component fiber spinning nozzle
and melt-spun into a fiber, and the fiber is then drawn under
heating for improving mechanical strength.
[0005] Of cores to be used in the POF, polymethyl methacrylate is
used as a core of a high-performance POF on an industrial scale
since it is excellent in transparency, mechanical strength and
weather resistance.
[0006] However, since the glass transition temperature (hereinafter
referred to as "Tg") of polymethyl methacrylate is as high as 100
to 115.degree. C., its application is limited in view of heat
resistance.
[0007] For this reason, Japanese Patent Application Laid-Open No.
18608-1983, for example, proposes that a protective layer is
further provided around a sheath layer to form a structure
comprising three or more layers so as to improve heat
resistance.
[0008] Further, Japanese Patent Application Laid-Open No.
11128-1993 discloses a technique which improves uniformity in the
diameter of a POF by suppressing fluctuations in the diameter of
the POF when it is subjected to heat drawing or an annealing after
the heat drawing.
[0009] Further, Japanese Patent Application Laid-Open No.
16905-1992 discloses a method of improving a transmission loss by
heating a POF having a polycarbonate as a core at 60 to 100.degree.
C. for a long time.
[0010] However, the method disclosed in Japanese Patent Application
Laid-Open No. 16905-1992 cannot improve the heat resistance of a
POF since the annealing temperature is lower than the Tg of the
core by 50.degree. C. or more and the annealing time is short.
Further, the invention disclosed in Japanese Patent Application
Laid-Open No. 18608-1983 has the problem that even if the heat
resistance of a material used in the protective layer is improved,
a core itself is thermally shrunk when the temperature used gets
close to the Tg of the core. Further, Japanese Patent Application
Laid-Open No. 11128-1993 is about the internal structure of a
heating furnace used for the annealing, and since an appropriate
annealing time and tension when a POF is heated at a predetermined
annealing temperature are not set, the effect of reducing the
thermal shrinkage of a POF is not satisfactory.
[0011] In addition, to improve the properties such as heat
resistance of a POF, Japanese Patent Application Laid-Open Nos.
131206-1987, 303304-1988, 68503-1990, 201270-1994, 299912-1987 and
the like disclose a method in which a POF is subjected to a
non-contact annealing in line after the drawing step to maintain
orientation of polymer chains in the POF axial direction which has
been provided in the drawing step as much as possible so as to
suppress the shrinkage of the POF at high temperatures.
[0012] However, since this method cannot remove inner strain of a
POF sufficiently, it cannot improve the heat resistance of the POF
to a satisfactory level. Further, when the temperature of a
non-contact heating furnace is raised to remove the inner strain,
the orientation of polymer chains cannot be maintained, thereby
causing decrease in the mechanical strength of the POF and an
increase in the non-uniformity of the diameter of the POF.
[0013] When these conventional POFs are used in optical
communication or a sensor in a high temperature environment, e.g.,
in an engine room of an automobile or the like or in the interior
of an automobile in mid summer, they are thermally shrunk, thereby
causing decrease in optical transmission properties or trouble in
wiring in a connector or the like.
DISCLOSURE OF THE INVENTION
[0014] An object of the present invention is to provide a plastic
optical fiber, an optical fiber cable and a plugged optical fiber
cable which are excellent in heat resistance, and production
methods thereof.
[0015] The above object can be achieved by the following present
invention.
[0016] The present invention relates to a plastic optical fiber
having a shrinkage stress occurring temperature obtained by
thermomechanical analysis of not lower than [(a glass transition
temperature of a core)-35].degree. C.
[0017] Further, the present invention relates to a plastic optical
fiber cable obtained by forming a coating layer around the plastic
optical fiber of the present invention.
[0018] Further, the present invention relates to a plugged plastic
optical fiber cable obtained by attaching a plug on the tip of the
plastic optical fiber cable of the present invention.
[0019] Further, the present invention relates to a method for
producing a plastic optical fiber which comprises the steps of heat
drawing an undrawn plastic optical fiber obtained by melt spinning
and subjecting the drawn fiber to an annealing at a circumferential
velocity ratio between the front and rear rollers (circumferential
velocity of a rear roller/circumferential velocity of a front
roller) of 0.5 to 1.2 under heating conditions which satisfy
4.ltoreq.y.ltoreq.-1.5x +330 and (Tgc-5).degree.
C..ltoreq.x.ltoreq.(Tgc+110).degree. C. [Tgc: a glass transition
temperature of a core, x: an annealing temperature (.degree. C.),
and y: an annealing time (seconds)].
[0020] Further, the present invention relates to a method for
producing a plastic optical fiber which comprises the step of
conducting the annealing twice or more under the above heating
conditions in the above production method of the present
invention.
[0021] Further, the present invention relates to a method for
producing a plastic optical fiber which comprises the step of heat
treating a plastic optical fiber obtained by melt spinning at a
temperature from (a glass transition temperature of a core
-5).degree. C. to (a glass transition temperature of the core
+80).degree. C. while a tension of 0.35.times.10.sup.6 to
1.5.times.10.sup.6 Pa is applied to the plastic optical fiber.
[0022] Further, the present invention relates to a method for
producing a plastic optical fiber cable which comprises the steps
of obtaining a plastic optical fiber by the above method of the
present invention and forming a coating layer around the obtained
optical fiber.
[0023] Furthermore, the present invention relates to a method for
producing a plugged plastic optical fiber cable which comprises the
steps of obtaining a plastic optical fiber by the above method of
the present invention and attaching a plug on the tip of the
obtained optical fiber cable.
[0024] According to the present invention, a plastic optical fiber,
an optical fiber cable and a plugged optical fiber cable which have
a small shrinkage ratio and excellent heat resistance can be
provided.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a diagram for illustrating a method of determining
a shrinkage stress occurring temperature of a plastic optical fiber
and a graph indicating changes in stress against temperature and
obtained by thermomechanical analysis.
[0026] FIG. 2 is a diagram showing an example of a DSC curve in the
vicinity of the Tg of a core.
[0027] FIG. 3 is a schematic block diagram showing a production
device 1 used in the present invention.
[0028] FIG. 4 is a schematic block diagram showing a repeated
bending tester.
[0029] FIG. 5 is a schematic block diagram showing a production
device 2 used in the present invention.
[0030] FIG. 6 is a schematic block diagram showing a production
device 3 used in the present invention.
[0031] FIG. 7 is a schematic block diagram showing a production
device 4 used in the present invention.
[0032] FIG. 8 is a schematic block diagram showing a perforated
plate in a production device used in the present invention.
[0033] FIG. 9 is a schematic block diagram showing a production
device used in the present invention which has a perforated plate
disposed therein.
BEST MODE FOR CARRYING OUT THE INVENTION
[0034] Preferred embodiments of the present invention will be
described hereinafter. A glass transition temperature of a core
constituting a POF will be indicated as "Tgc" as appropriate.
[0035] In the present invention, a POF having excellent heat
resistance can be obtained by setting the shrinkage stress
occurring temperature of the POF to be (Tgc-35).degree. C. or
higher (first invention).
[0036] Further, in the present invention, the heat resistance of
the POF of the first invention can be further improved by, in
particular, using a homopolymer of methyl methacrylate as the core
of the POF and setting the absolute value of the birefringence of
the core at 2.0.times.10.sup.-4 or smaller.
[0037] Further, in the present invention, the heat resistance of
the POF of the first invention can be further improved and a
decrease in the mechanical strength of the POF can be reduced by
using a homopolymer of methyl methacrylate as the core of the POF,
setting the shrinkage stress occurring temperature obtained by
thermomechanical analysis of the POF to be (the glass transition
temperature of the core -20).degree. C. or higher and setting the
absolute value of the birefringence of the core to be
1.5.times.10.sup.-4 or larger.
[0038] Further, in the present invention, a POF having excellent
heat resistance can be obtained because an undrawn POF obtained by
melt spinning is heat-drawn and then subjected to an annealing at a
circumferential velocity ratio between the front and rear rollers
(circumferential velocity of a rear roller/circumferential velocity
of a front roller) of 0.5 to 1.2 under heating conditions which
satisfy 4.ltoreq.y.ltoreq.-1.5x+330 and (Tgc -5).degree.
C..ltoreq.x.ltoreq.(Tgc+- 110).degree. C. [Tgc: a glass transition
temperature of a core, x: an annealing temperature (.degree. C.),
and y: an annealing time (seconds)] (second invention).
[0039] Further, in the present invention, in the production method
of a POF of the second invention, a POF having better heat
resistance and mechanical strength can be obtained by using a
homopolymer of methyl methacrylate as the core, carrying out the
above heat drawing such that the absolute value of the
birefringence of the core would become 3.times.10.sup.-4 or larger
and carrying out the above annealing at a circumferential velocity
ratio between the front and rear rollers (circumferential velocity
of the rear roller/circumferential velocity of the front roller) of
1 or less under conditions which satisfy x.ltoreq.(Tgc+20).degree.
C. [Tgc: the glass transition temperature of the core, x: an
annealing temperature (.degree. C.)].
[0040] Further, in the present invention, a POF having further
improved heat resistance can be obtained by carrying out the above
annealing twice or more under the above heating conditions (third
invention).
[0041] Further, in the present invention, a POF having improved
heat resistance can be obtained by heat treating a POF obtained by
melt spinning at temperatures from (Tgc-5).degree. C. to
(Tgc+80).degree. C., preferably after the POF is heat-drawn, while
applying a tension of 0.35.times.10.sup.6 Pa to 1.5.times.10.sup.6
Pa to the POF (fourth invention).
[0042] As the core of the POF of the present invention, a
transparent amorphous polymer is suitable. Particularly, a
homopolymer of methyl methacrylate or a copolymer comprising a
methyl methacrylate unit and another monomer unit (hereinafter
referred to as "MMA polymer" as required) is preferably used. As
other materials usable as the core of the POF, a copolymer
comprising a unit of monomer such as a methacrylic acid ester as
exemplified by cyclohexyl methacrylate, t-butyl methacrylate,
isobornyl methacrylate, adamantly methacrylate, benzyl
methacrylate, phenyl methacrylate or naphthyl methacrylate and a
unit of monomer copolymerizable with these monomers, a
polycarbonate, a polystyrene, a styrene-methacrylate copolymer, or
deuterides of these polymers which are obtained by substituting all
or some of hydrogen atoms of the above polymers with deuterium
atoms can be used. As a matter of course, other transparent
polymers and transparent blends can also be used.
[0043] An MMA polymer, particularly a homopolymer of methyl
methacrylate, has a wider temperature range in which it can remain
in a rubber state than other transparent amorphous materials,
particularly a polycarbonate. Since it never happens that the MMA
polymer becomes stiff and can hardly be drawn at temperatures lower
than a certain temperature or that it becomes suddenly too
stretchable at temperatures higher than a certain temperature, the
MMA polymer can be drawn or annealed in good condition. Further,
since its orientation is not loosened as easily as that of a
polycarbonate in particular, its tension is not lowered by heat
easily. Therefore, even when the annealing is carried out with low
tension, the annealing can be carried out stably and non-uniformity
in the diameter of a fiber is not worsened easily. Further, since
the orientation is not loosened easily, it is not necessary to
adjust an annealing temperature to an excessively low temperature
so as not to loosen the orientation, so that the MMA polymer can be
heat treated in a moderately viscoelastic state. As a result, a
desired annealing effect can be obtained to a sufficient degree
without degrading other properties of a POF such as non-uniformity
in the diameter of the fiber. Particularly, when the annealing is
carried out with low tension as in the fourth invention, a POF
using an MMA polymer as a core is suitable.
[0044] As a copolymer comprising a methyl methacrylate unit and
another monomer unit, a copolymer comprising at least 70% by weight
of methyl methacrylate unit and at most 30% by weight of the other
monomer unit based on 100% by weight of all monomer units is
preferred. Illustrative examples of monomers copolymerizable with
methyl methacrylate include methacrylic acid esters such as
cyclohexyl methacrylate, isobornyl methacrylate, benzyl
methacrylate, phenyl methacrylate and 2,2,2-trifluoroethyl
methacrylate, acrylic esters such as methyl acrylate and ethyl
acrylate, and maleimide compounds intended for improving heat
resistance such as N-cyclohexylmaleimide and
N-isopropylmaleimide.
[0045] A method for producing the core is not particularly limited,
and a known polymerization method is used. However, to prevent
foreign substrates from mixing into the core, a continuous bulk
polymerization method or a continuous solution polymerization
method is preferably used.
[0046] Illustrative examples of materials suitably used in the
sheath of the POF of the present invention from the viewpoint of
transmission characteristics include a homopolymer of a
fluorine-based methacrylate, a copolymer comprising a
fluorine-based methacrylate unit and a methacrylic ester monomer
unit, a copolymer containing a vinylidene fluoride unit as a main
component such as a vinylidene fluoride-tetrafluoroethylene
copolymer, an .alpha.-fluoromethacrylate, and mixtures thereof.
[0047] As the structure of the POF of the present invention, a
known structure can be used. Illustrative examples of a POF having
such a known structure include a POF having a two-layer structure
comprising a core and a sheath, a graded index-type POF whose core
has refractive index distribution, a multi-core POF having a
sea-island structure in which a plurality of island components each
comprising a core and a sheath are disposed in a sea component, a
multi-core POF having a structure in which a plurality of cores are
collected in a sheath, and a multilayer POF whose core or sheath
has a multilayer structure and whose refractive index gradually
changes from the center toward the periphery.
[0048] A protective layer having such properties as solvent
resistance and heat resistance may be formed on the peripheries of
these POFs. As a material (protective material) used in the
protective layer, a material having excellent mechanical strength
is preferably used, and a vinylidene fluoride-tetrafluoroethylene
copolymer is particularly preferably used. The vinylidene
fluoride-tetrafluoroethylene copolymer has high adhesion to a
polymer used as a sheath and comprising a fluorine-based
methacrylate unit or a vinylidene fluoride unit or Nylon 12 which
is suitably used in a coating layer in such an application that
requires heat resistance in particular. Therefore, together with
the characteristic of the POF of the present invention that it has
small shrinkage in a high-temperature environment, a POF cable
obtained by forming a protective layer comprising a vinylidene
fluoride-tetrafluoroethylene copolymer around the POF of the
present invention having a core-sheath structure in which the
sheath comprises a polymer comprising a fluorine-based methacrylate
unit or a vinylidene fluoride unit and by forming a coating layer
comprising Nylon 12 on the protective layer can significantly
reduce the pistoning phenomenon in a high-temperature environment
of the POF cable.
[0049] A POF melt-spun by use of a material as described above is
preferably heat-drawn for the purpose of improving its mechanical
strength. As a drawing device, one having a heating furnace
disposed between a front roller and a rear roller can be used, for
example. Although an appropriate heating temperature (drawing
temperature). at the time of drawing is set according to the
physical properties of a core, the temperature is preferably
(Tgc+5).degree. C. to (Tgc+65).degree. C., more preferably
(Tgc+10).degree. C. to (Tgc+60).degree. C. When the drawing
temperature is out of the above temperature range, that is, lower
than (Tgc+5).degree. C., the POF may become difficult to stretch.
On the other hand, when the drawing temperature is higher than
(Tgc+65).degree. C., sufficient mechanical strength may not be
imparted to the POF by drawing. When the core is a PMMA, the
drawing temperature is preferably 115.degree. C. or higher, more
preferably 120.degree. C. or higher, much more preferably
125.degree. C. or higher, also is preferably 185.degree. C. or
lower, more preferably 180.degree. C. or lower, much more
preferably 175.degree. C. or lower.
[0050] The drawing ratio in the heat drawing is preferably 1.1 to
3.5 times (the circumferential velocity ratio between the front and
rear rollers (circumferential velocity of the rear
roller/circumferential velocity of the front roller) is 1.1 to
3.5), more preferably 1.5 to 3.3 times, much more preferably 1.5 to
3.0 times. When the drawing ratio is out of the above range, that
is, smaller than 1.1 times, sufficient mechanical strength may not
be imparted to the POF by drawing. On the other hand, when the
drawing ratio is larger than 3.5 times, it may become difficult to
carry out uniform drawing.
[0051] Thus, in the production of the POF, drawing is generally
carried out for improving the mechanical strength of the POF.
However, while the molecules of the POF are oriented by drawing the
POF, stress (inner strain) different from orientation remains and
is fixed in the POF. Therefore, as the temperature of the POF is
increased to a temperature close to the Tgc of the POF, the fixed
residual stress (inner strain) is released, thereby causing the POF
to shrink significantly.
[0052] The degree of such stress (inner strain) remaining in a POF
can be known by means of thermomechanical analysis. That is, when a
POF is subjected to thermomechanical analysis as the temperature of
the POF is increased at a fixed speed, fixed residual stress is
released at a given temperature, thereby causing shrinkage stress.
A POF having less residual stress has a shrinkage stress occurring
temperature shifted to a higher temperature side. Thus, a POF
having a high shrinkage stress occurring temperature is a POF
having a small thermal shrinkage ratio and excellent heat
resistance.
[0053] In the present invention, a POF is heat-treated under given
conditions to remove residual stress from the POF, thereby reducing
the thermal shrinkage of the POF and improving the heat resistance
of the POF.
[0054] The POF of the present invention has a shrinkage stress
occurring temperature obtained by thermomechanical analysis (TMA)
of not lower than (Tgc-35).degree. C. (first invention). Such a POF
has a low thermal shrinkage ratio and exhibits excellent heat
resistance accordingly. The higher the shrinkage stress occurring
temperature the better it is. However, it is generally lower than
or equal to the Tgc. When a POF is applied to such an application
as interior wiring of an automobile, it is required to be heat
resistant at least at 90.degree. C. A POF which has excellent heat
resistance, that is, exhibits a thermal shrinkage ratio of not
higher than 2% in a heat resistance test conducted at 90.degree. C.
for 65 hours can be obtained by setting the shrinkage stress
occurring temperature of the POF to be not lower than
(Tgc-35).degree. C. as in the present invention.
[0055] The shrinkage stress occurring temperature can be determined
from the shrinkage stress curve shown in FIG. 1 as described later.
Further, since the result of the thermomechanical analysis is apt
to slightly change according to measurement conditions, the
thermomechanical analysis is carried out at a temperature rising
rate of 5.degree. C./min under a constant load of
2.2.times.10.sup.5 Pa in the present invention.
[0056] A thermal shrinkage ratio which is an indicator of the heat
resistance of the POF of the present invention is a thermal
shrinkage ratio in the longitudinal direction of the POF. A POF
having a smaller thermal shrinkage ratio has better heat
resistance. The measurement of the thermal shrinkage ratio is
carried out in the following manner. A POF is marked at an interval
(L.sub.0) of 50 cm in advance, and the POF is hung in a hot air
dryer set at 90.degree. C. without contacting walls, shelves and
the like in the hot air dryer. After 65 hours, the POF was taken
out of the dryer and left standing to cool to room temperature
(20.degree. C.). Then, an interval (L.sub.1) between the marks is
measured, and a thermal shrinkage ratio is calculated from the
L.sub.0 and L.sub.1 by use of the following equation.
[0057] Thermal shrinkage Ratio
(%)=[(L.sub.0-L.sub.1)/L.sub.0].times.100
[0058] Further, in the present invention, a POF having a reduced
thermal shrinkage ratio and exhibiting excellent heat resistance
accordingly can be obtained by using a homopolymer of methyl
methacrylate as a core of the POF and setting the birefringence
absolute value of the core to be not larger than
2.0.times.10.sup.-4.
[0059] As described above, heat drawing is carried out for
imparting mechanical strength to a POF. An example of an indicator
for molecular orientation of a core which is caused by stretch of a
POF is birefringence of the core. The value of the birefringence
may be positive or negative depending on a material used as a core.
The larger the absolute value of the birefringence, the higher the
degree of molecular orientation and the mechanical strength of a
POF. However, as the absolute value of the birefringence becomes
larger, residual stress (inner strain) fixed in a POF which
degrades the heat resistance of the POF also increases. In the
present invention, the birefringence of a core of a POF may be
adjusted so as to relax the residual inner strain in the POF and
improve the heat resistance of the POF. In the case of a POF having
a core comprising a homopolymer of methyl methacrylate, when the
absolute value of birefringence becomes 2.0.times.10.sup.-4 or
smaller, its thermal shrinkage ratio becomes small and heat
resistance improves. Therefore, when prime importance is placed on
the heat resistance of the POF of the present invention which has a
core comprising a homopolymer of methyl methacrylate, the
birefringence absolute value of the core is preferably not larger
than 2.0.times.10.sup.-4, more preferably 0.5.times.10.sup.-4 to
2.0.times.10.sup.-4. When the birefringence absolute value of the
core becomes smaller than 0.5.times.10.sup.-4, its mechanical
strength may be degraded.
[0060] A POF which has a core comprising a homopolymer of methyl
methacrylate and having a birefringence absolute value of not
larger than 2.0.times.10.sup.-9, though depending on the physical
properties of the used homopolymer of methyl methacrylate, can be
obtained by subjecting the POF to the annealing to be described
later. The birefringence is measured in the manner to be described
later.
[0061] As described above, the mechanical strength of a POF can be
improved by drawing the POF to impart molecular orientation so as
to increase the absolute value of the birefringence of a core.
However, at the same time, the degree of its thermal shrinkage also
becomes greater. In the present invention, the birefringence of a
core and the thermal shrinkage stress occurring temperature of a
POF can be set to be within predetermined ranges so that thermal
shrinkage and a decrease in mechanical strength can be reduced. For
example, in the case of a POF having a core comprising a
homopolymer of methyl methacrylate, the POF preferably has a
birefringence absolute value of not lower than 1.5.times.10.sup.-4
and a shrinkage stress occurring temperature obtained by
thermomechanical analysis of not lower than (Tgc-20).degree. C.
since sufficient mechanical strength and anti thermal shrinkability
can be imparted to the POF. More preferably, the absolute value of
the birefringence is not lower than 2.3.times.10.sup.-4 and the
shrinkage stress occurring temperature is not lower than
(Tgc-15).degree. C. When the absolute value of the birefringence is
too high, it is difficult to have a shrinkage stress occurring
temperature obtained by thermomechanical analysis of not lower than
(Tgc-20).degree. C. Therefore, the absolute value of the
birefringence is preferably not higher than 5.times.10.sup.-4. Such
a POF can exhibit a thermal shrinkage ratio of not higher than 0.5%
in a heat resistance test conducted at 90.degree. C. for 65 hours,
and the number of repeated bending counted for a POF cable which is
obtained by forming a coating layer composed of a polyethylene
around the POF and has an outside diameter (thickness of the
coating layer: 0.6 mm) of 2.2 mm can be at least 10,000 times.
Further, the number of repeated bending counted for a POF cable
which is obtained by forming a coating layer composed of Nylon 12
around the POF and has an outside diameter (thickness of the
coating layer: 0.6 mm) of 2.2 mm can be at least 20,000 times. The
coating layer composed of Nylon 12 can be coated twice or more as
required. The number of repeated bending is counted by a method to
be described later. A POF with a larger number of repeated bending
has better mechanical strength.
[0062] In the production method (second invention) of a POF of the
present invention, a drawn POF is heat-treated at a circumferential
velocity ratio between the front and rear rollers (circumferential
velocity of a rear roller/circumferential velocity of a front
roller) of 0.5 to 1.2 under heating conditions which satisfy
4.ltoreq.y.ltoreq.-1.5x+330 and (Tgc-5).degree.
C..ltoreq.x.ltoreq.(Tgc+110).degree. C. Thereby, internal stress
(inner strain) fixed and remaining in the POF is partially relaxed,
and a POF having a smaller thermal shrinkage ratio and better heat
resistance than a POF not subjected to such an annealing can be
obtained. By the production method of the present invention, a POF
having a shrinkage stress occurring temperature obtained by
thermomechanical analysis of not lower than (Tgc-35).degree. C. can
be obtained, and a POF which exhibits a thermal shrinkage ratio of
not higher than 2% in a heat resistance test conducted at
90.degree. C. for 65 hours. Further, when a homopolymer of methyl
methacrylate is used as a core, a POF which has a core having a
birefringence absolute value of not higher than 2.0.times.10.sup.-4
and has excellent heat resistance can be obtained. Further, when a
homopolymer of methyl methacrylate is used as a core, a POF which
has a core having a birefringence absolute value of not lower than
1.5.times.10.sup.-4 and has a shrinkage stress occurring
temperature obtained by thermomechanical analysis of not lower than
(Tgc-20).degree. C., that is, a POF having excellent heat
resistance and mechanical strength can be obtained. In this case,
the absolute value of the birefringence of the core is preferably
not lower than 2.3.times.10.sup.-4, and the shrinkage stress
occurring temperature obtained by thermomechanical analysis is
preferably not lower than (Tgc-15).degree. C.
[0063] When the circumferential velocity ratio (circumferential
velocity of the rear roller/circumferential velocity of the front
roller) is smaller than 1, the obtained POF is shrunk. In view of
the possibility that too large shrinkage may cause a decrease in
molecular orientation imparted by drawing and in turn a degradation
in the mechanical strength of the POF, the possibility that
productivity may be decreased since it takes long time to have
large shrinkage, and the possibility that the mechanical strength
of the POF may be further degraded since the POF is heated at high
temperatures not lower than (Tgc-5).degree. C. during the long time
period, a circumferential velocity ratio of not smaller than 0.5 is
used. On the other hand, when the circumferential velocity ratio
exceeds 1.2, an effect of improving heat resistance according to
the present invention is not exhibited to a sufficient degree.
[0064] Further, in the present invention, the annealing is carried
out under heating conditions which satisfy 4.ltoreq.y.ltoreq.-1.5x
+330 and (Tgc-5).degree. C..ltoreq.x.ltoreq.(Tgc+110).degree. C.
When an annealing temperature x is out of the above temperature
range, an effect of improving heat resistance by the annealing
becomes less sufficient as the annealing temperature becomes lower
than (Tgc-5).degree. C., while the annealing temperature is larger
than (Tgc+110).degree. C., mechanical strength imparted by drawing
may be degraded. The annealing temperature x is preferably
(Tgc+90).degree. C. or lower, more preferably (Tgc+75) or lower.
Also, the annealing temperature x is preferably Tgc .degree. C. or
higher, more preferably (Tgc+15).degree. C. or higher. When a core
is polymethylmethacrylate, the annealing temperature x is
preferably 110.degree. C. or higher, more preferably 115.degree. C.
or higher. Also, the annealing temperature x is preferably
200.degree. C. or lower, more preferably 190.degree. C. or lower.
Meanwhile, when the annealing time y is less than 4 seconds, an
effect of improving heat resistance by the annealing becomes
insufficient, while when the annealing time y is more than (-1.5x
+330) seconds, mechanical strength imparted by drawing is degraded.
In the present invention, the annealing time is calculated from the
following equation:
[0065] The annealing time (min)=2s/(v.sub.1+v.sub.2) In the above
equation, s indicates a furnace length (m) of a heating furnace,
v.sub.1 a circumferential velocity (m/min) of a front roller, and
v.sub.2 a circumferential velocity (m/min) of a rear roller.
[0066] As a heating furnace used in conducting the annealing, a
non-contact heating furnace is preferably used for protecting a POF
from being damaged. A heating furnace using hot air as a heating
medium or a moist heating furnace using pressurized steam as a
heating medium can be used.
[0067] Further, in the production method (second invention) of the
present invention, a homopolymer of methyl methacrylate is
preferably used as a core, a POF is preferably heat-drawn such that
the absolute value of the birefringence of the core would be not
smaller than 3.times.10.sup.-4, and the subsequent annealing is
preferably carried out at a circumferential velocity ratio between
the front and rear rollers (circumferential velocity of a rear
roller/circumferential velocity of a front roller) of not larger
than 1 and an annealing temperature of not higher than
(Tgc+20).degree. C. The heat drawing is preferably carried out such
that the absolute value of the birefringence of the core would be
not larger than 6.times.10.sup.-4 in view of a load to be imposed
on the subsequent annealing. When the core of the heat-drawn POF
has a birefringence absolute value of not smaller than
3.times.10.sup.-4, molecular orientation caused by heat drawing can
be imparted to a sufficient degree. When the circumferential
velocity ratio between the front and rear rollers at the time of
the annealing conducted after the heat drawing is not larger than
1, a shrinkage stress occurring temperature can be increased and
thermal shrinkage can be reduced. When the annealing temperature is
set to be relatively low, i.e., not higher than (Tgc+20).degree.
C., a decrease in molecular orientation caused by the annealing can
be minimized. That is, by carrying out heat drawing and the
annealing under such conditions, sufficient mechanical strength and
anti thermal shrinkability can be imparted to a POF, whereby a POF
which has a shrinkage stress occurring temperature obtained by
thermomechanical analysis of not lower than (Tgc-20).degree. C. and
has a core having a birefringence absolute value of not lower than
1.5.times.10.sup.-4 can be obtained. Further, by producing the POF
under the above conditions, the thermal shrinkage ratio of the POF
can be 0.5% or smaller, the number of repeated bending counted when
a coating layer composed of a polyethylene is formed around the POF
can be at least 10,000 times, and the number of repeated bending
counted when a coating layer composed of Nylon 12 is formed around
the POF can be at least 20,000 times as described above. A
plurality of coating layers can be coated as required.
[0068] In the present invention, a POF is preferably heat-treated
while a tension of 0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa,
preferably 0.35.times.10.sup.6 to 1.35.times.10.sup.6 Pa is applied
to the POF. When the tension is smaller than 0.35.times.10.sup.6
Pa, the POF makes contact with an annealing device at the time of
the annealing, so that the annealing may not be able to be carried
out stably. On the other hand, when it is larger than
1.5.times.10.sup.6 Pa, an effect of improving heat resistance by
the annealing may be insufficient. The tension at the time of the
annealing can be adjusted by setting an annealing temperature, a
flow rate of a heating medium used in the annealing and a
circumferential velocity ratio between the front and rear rollers
(circumferential velocity of a rear roller/circumferential velocity
of a front roller) of a heating furnace at the time of the
annealing to be proper values.
[0069] Further, in the production method (second invention) of the
present invention, when the annealing is carried out while tension
is applied to a POF as described above, the tension is preferably
not larger than 1.35.times.10.sup.6 Pa, an annealing temperature is
preferably (Tgc-5).degree. C. to (Tgc+15).degree. C., and the
annealing time preferably satisfies 10.ltoreq.y.ltoreq.1.5x+330 [x:
an annealing temperature (.degree. C.), and y: an annealing time
(seconds)]. By carrying out the annealing under such conditions, a
POF having a birefringence absolute value of not lower than
2.3.times.10.sup.-4 and a shrinkage stress occurring temperature of
not lower than (Tgc-15).degree. C. can be obtained.
[0070] In the above production method, a drawing step and an
annealing step may be carried out in a continuous process.
Alternatively, a drawn POF may be wound around a bobbin, and then
the POF may be heat-treated.
[0071] Further, in the production method of a POF according to the
present invention, heat resistance can be improved by carrying out
the annealing twice or more under the above heating conditions of
the above production method (second invention) (third invention).
Thereby, inner strain (internal stress) fixed and remaining in a
POF is partially relaxed, and a POF having a smaller thermal
shrinkage ratio and better heat resistance than a POF not subjected
to such an annealing or a POF subjected to the annealing only once
can be obtained. In this case, an annealing temperature x is
preferably not lower than Tgc .degree. C. When a POF having a
shrinkage stress occurring temperature of not lower than
(Tgc-35).degree. C. is produced by the method (third invention) of
the present invention, a thermal shrinkage ratio of not higher than
2%, or even not higher than 1.5%, can be achieved in a heat
resistance test conducted at 90.degree. C. for 65 hours. Further,
when a homopolymer of methyl methacrylate is used as a core, a POF
with a core having a birefringence absolute value of not larger
than 2.0.times.10.sup.4 can be obtained.
[0072] The annealing which satisfies the above heating conditions
is preferably carried out twice to six times. When the annealing is
carried out at least twice, heating can be done to a sufficient
degree without increasing a heating furnace length. Further, when
the annealing is carried out six times at the maximum, the
constitution of the device becomes relatively simple. This
annealing may be repeated under the same conditions or different
conditions. A heating furnace used in the annealing and heating
conditions at the time of the annealing are the same as those of
the above production method (second invention) and are set as
appropriate according to the number of times the annealing is
carried out.
[0073] In the production method of the present invention, a drawing
step and the annealing may be carried out in a continuous process.
Alternatively, after the drawing step and a first annealing are
carried out in a continuous process, the subsequent annealing may
be carried out after a drawn POF may be wound around a bobbin.
Alternatively, a drawn POF may be wound around a bobbin, and then
the POF may be subjected to the annealing. Alternatively, a method
in which a drawn POF is caused to move back and force in an
annealing device can also be used.
[0074] In the present invention (fourth invention), a POF obtained
by melt spinning is heat-treated at temperatures ranging from
(Tgc-5).degree. C. to (Tgc+80).degree. C. while a tension of
0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa, preferably
0.35.times.10.sup.6 to 1.35.times.10.sup.6 Pa, is applied to the
POF, thereby removing inner strain fixed in the POF and obtaining a
POF having excellent heat resistance. According to this method, the
POF of the first invention, that is, a POF having a shrinkage
stress occurring temperature obtained by thermomechanical analysis
of not lower than (Tgc-35).degree. C. can be obtained. Further, a
POF which exhibits a thermal shrinkage ratio of not higher than 2%
in a heat resistance test conducted at 90.degree. C. for 65 hours
can be obtained. Further, when a homopolymer of methyl methacrylate
is used as a core, a POF with a core having a birefringence
absolute value of not larger than 2.0.times.10.sup.-4 can be
obtained. Further, when a homopolymer of methyl methacrylate is
used as a core, a POF which has a core having a birefringence
absolute value of not smaller than 1.5.times.10.sup.-4 and has a
shrinkage stress occurring temperature obtained by thermomechanical
analysis of not lower than (Tgc-20).degree. C. can be obtained.
[0075] The above annealing is preferably carried out after an
undrawn POF obtained by melt spinning is subjected to heat
drawing.
[0076] In the above annealing, when the tension is smaller than
0.35.times.10.sup.6 Pa, the POF may make contact with an annealing
device at the time of the annealing, so that the annealing may
become unstable, while when it is larger than 1.5.times.10.sup.6
Pa, an effect of improving heat resistance by the annealing is
insufficient. The tension at the time of the annealing can be
adjusted by setting an annealing temperature, a flow rate of a
heating medium used in the annealing and a circumferential velocity
ratio between the front and rear rollers (circumferential velocity
of a rear roller/circumferential velocity of a front roller) of a
heating furnace at the time of the annealing to be proper
values.
[0077] The annealing temperature is (Tgc-5).degree. C. to
(Tgc+80).degree. C., preferably (Tgc-5).degree. C. to
(Tgc+60).degree. C., more preferably (Tgc-5).degree. C. to
(Tgc+30).degree. C., particularly preferably Tgc .degree. C. to
(Tgc+15).degree. C. When the annealing temperature is out of the
above range, that is, lower than (Tgc-5).degree. C., an improving
effect caused by the annealing is insufficient. On the other hand,
when the annealing temperature is higher than (Tgc+80).degree. C.,
mechanical properties may be degraded due to relax of orientation
imparted by drawing.
[0078] To improve heat resistance significantly without degrading
mechanical strength in the annealing, it is preferable to use a
polymer containing a methyl methacrylate unit in an amount of at
least 70% by weight as a core of a POF. Particularly, when a POF
with a core comprising a homopolymer of methyl methacrylate is
heat-treated at temperatures ranging from (Tgc-5).degree. C. to
(Tgc+30).degree. C., preferably temperatures not higher than
(Tgc+15).degree. C. while a tension of 0.35.times.10.sup.6 to
1.5.times.10.sup.6 Pa, preferably a tension of not larger than
1.35.times.10.sup.6 Pa is applied to the POF, a POF which has a
core having a birefringence absolute value of not lower than
1.5.times.10.sup.-4, preferably not lower than 2.3.times.10.sup.-4
and has a shrinkage stress occurring temperature obtained by
thermomechanical analysis of not lower than (Tgc-20).degree. C.,
preferably not lower than (Tgc-15).degree. C., that is, a POF
having mechanical strength and heat resistance can be obtained.
Further, by producing the POF under the above conditions, the
thermal shrinkage ratio of the POF can be 0.5% or smaller, the
number of repeated bending counted when a coating layer composed of
a polyethylene is formed around the POF can be at least 10,000
times, and the number of repeated bending counted when a coating
layer composed of Nylon 12 is formed around the POF can be at least
20,000 times as described above. A plurality of coating layers can
be coated as required.
[0079] For the annealing of a POF, a known method such as a hot air
method, a moist heating method or a hot water method can be used.
When hot air is used in the annealing, the higher the velocity of
the hot air the more preferable it is, from the Viewpoint of
heating efficiency. However, the velocity of the hot air is
generally set to be 2 to 10 m/s. When the velocity of the hot air
is lower than 2 m/s, heating efficiency becomes insufficient,
whereby the heat resistance of the POF may not be able to be
improved to a sufficient degree. On the other hand, when the
velocity of the hot air is higher than 10 m/s, the POF sways in a
heating furnace, whereby the quality of the POF may be degraded. In
this case, the sway of the POF can be prevented effectively by
coinciding the heading direction of the hot air with the heading
direction of the POF.
[0080] Tension applied to a POF at the time of the annealing varies
according to the temperature of the POF at the time of the
annealing. The temperature of the POF is determined by an annealing
temperature, a flow rate of a heating medium and the like. When the
temperature of the POF at the time of the annealing is increased,
tension tends to increase since the POF is shrunk. However, when
the temperature is too high, orientation is further relaxed,
thereby causing the tension to lower.
[0081] This annealing may be a fixed-length treatment
(circumferential velocity ratio of rollers is about 1) or an
alleviation treatment (circumferential velocity ratio of rollers is
less than 1). However, to carry out the annealing under a tension
of 0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa, the annealing is
preferably the alleviation treatment. The alleviation treatment is
preferably carried out at a circumferential velocity ratio between
the front and rear rollers (circumferential velocity of a rear
roller/circumferential velocity of a front roller) of 0.5 to 0.98,
more preferably 0.7 to 0.95, particularly preferably 0.8 to 0.95.
When the circumferential velocity ratio is much smaller than the
above range, mechanical properties of a POF may be degraded, while
when the circumferential velocity ratio is much larger than the
above range, it becomes difficult to have a tension applied to the
POF of 0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa.
[0082] Further, in the present invention, the annealing can be
carried out by use of a heating furnace into which a POF is
introduced horizontally. However, when the annealing is carried out
under a tension of 0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa,
the POF may hang down in the furnace by its own weight and make
contact with the heating furnace in some cases. To prevent this, a
POF to be heat-treated is introduced into an annealing zone
substantially vertically to a horizontal plane and heat-treated. To
carry out the annealing in this manner, an annealing device may be
disposed perpendicularly to the ground, for example.
[0083] Further, when a POF is heat-treated under a tension of
0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa by use of a heating
furnace disposed substantially horizontally, the annealing can be
carried out with the POF to be heat-treated supported by a heating
medium which provides buoyancy to the POF to prevent the POF from
making contact with the heating furnace. For example, a POF is
heat-treated with the POF supported by a heating medium such as hot
air or pressurized steam blown out of pores of a porous plate such
as a porous metal plate, porous heat-resistant plastic or porous
sintered metal plate disposed in a heating furnace. In that case,
to prevent the POF to be heat-treated from swaying, a wedge-shaped
or concave path for passing the POF may be formed on the porous
plate so that the heating medium can also be blown out from the
path. The shape of the pores of the porous plate may be circular or
rectangular but is not limited to these. When the pore is circular,
its diameter is set as appropriate according to the annealing
conditions and the size of a POF but is preferably 0.1 to 3 mm.
When the diameters of the pores are too small, effects of the
annealing and supporting a POF by a heating medium may be
insufficient, while when the diameters of the pores are too large,
the flow rate of a heating medium may be insufficient, thereby
making it difficult to support a POF.
[0084] A POF obtained by carrying out the annealing at temperatures
ranging from (Tgc-5).degree. C. to (Tgc+80).degree. C. under a
tension of 0.35.times.10.sup.6 to 1.5.times.10.sup.6 Pa can achieve
a thermal shrinkage ratio after heated at 90.degree. C. for 65
hours of not higher than 0.5%. It is desirable to set the above
conditions as appropriate in order to obtain such a POF.
[0085] Further, in the present invention, measurement of tension is
carried out near an outlet of a heating furnace. For example, in
the case of the production device shown in FIG. 5, the measurement
is carried out between the roller 16 and the heating furnace 18,
that is, at the point represented by reference numeral 20; in the
case of the production device shown in FIG. 6, the measurement is
carried out between the roller 22 and the heating furnace 23, that
is, at the point represented by reference numeral 25; and in the
case of the production device shown in FIG. 7, the measurement is
carried out between the guide 29 on the side of outlet and the
heating furnace 28, that is, at the point represented by reference
numeral 31. The constitutions of the devices are not limited to
these.
[0086] The heat resistance of a POF obtained by the above-described
production method (second, third or fourth invention) of a POF can
be further improved by subjecting the POF to the annealing,
preferably a batch-type fixed-length annealing, at temperatures not
higher than (Tgc+8).degree. C.
[0087] The annealing in this case can be carried out by a known
method such as a hot air method, a vacuum heating method or a moist
heating method can be used. An annealing temperature is set as
appropriate according to a material of POF to be heat-treated and
is preferably (Tgc-30).degree. C. or higher, more preferably
(Tgc-25).degree. C. or higher, much more preferably
(Tgc-15).degree. C. or higher, also is preferably (Tgc+8).degree.
C. or lower, more preferably (Tgc+5).degree. C. or lower, much more
preferably (Tgc+3).degree. C. or lower. Particularly, when a
homopolymer of methyl methacrylate is used as a core, the annealing
temperature is preferably 85.degree. C. or higher, more preferably
90.degree. C. or higher, much more preferably 100.degree. C. or
higher, also is preferably 123.degree. C. or lower, more preferably
120.degree. C. or lower, much more preferably 118.degree. C. or
lower. When the annealing is carried out at an excessively high
temperature, e.g., at a temperature higher than a Tg by 10.degree.
C. or more, over several hours, mechanical strength imparted by
drawing may be further degraded as the temperature and time of the
annealing increase. On the other hand, when the annealing
temperature is too low, an effect of improving heat resistance may
become insufficient.
[0088] The annealing time is selected as appropriate. When an
annealing temperature is low, it may take a very long time, e.g.,
two to three days, to obtain an effect of annealing. When the
annealing temperature is close to a glass transition temperature,
the effect is exhibited even after a half day.
[0089] The above-described POF (first invention) of the present
invention and POFs produced by the above-described production
methods (second, third and fourth inventions) of a POF of the
present invention can be used as POF cables by forming a coating
layer around these POFs. As a material constituting the coating
layer, a nylon resin such as conventionally used Nylon 12, a
polyvinyl chloride, a polychlorotrifluoroethylene copolymer, a
polyethylene, a polyurethane, perprene or the like can be used.
[0090] In addition, the POF cable can be used as a plugged POF
cable by attaching a plug on the tip of the POF cable. A known plug
can be used as the plug and attached by a known method.
EXAMPLES
[0091] The present invention will be described in more detail with
reference to examples hereinafter. Evaluation methods and
production devices used in the examples are as follows. As for the
horizontal and vertical axes shown in FIG. 1, temperature and
stress increase in the directions of corresponding arrows.
[0092] <Thermomechanical analysis (TMA)>
[0093] TMA-SS100 manufactured by Seiko Instruments Inc. was used as
a measuring device. The temperature rising rate was set at
5.degree. C./min. And a Shrinkage stress under a static load of
2.2.times.10.sup.5 Pa was measured. A point of intersection of a
horizontal tangent to a point where stress starts to be on the
increase after the decrease and a tangent to a point where the
inclination of stress becomes maximum after it has been on the
increase was taken as shrinkage stress occurring temperature as
shown in FIG. 1.
[0094] <Measurement of Glass Transition Temperature>
[0095] DSC-220 manufactured by Seiko Instruments Inc. was used. A
sample was heated to 200.degree. C. at a temperature rising rate of
10.degree. C./min and kept at the temperature for 10 minutes to be
melted, and then quenched to 20.degree. C. and heated at a
temperature rising rate of 10.degree. C./min again. From a DSC
curve indicating exothermic and endothermic behaviors at that time,
as shown in FIG. 2, a point of intersection of a straight line
obtained by extending a base line at lower temperature side than a
stepwise transition portion where glass transition occurs to higher
temperature side and a tangent to a point of the DSC curve where
the inclination of the stepwise transition portion of the DSC curve
becomes maximum was taken as the glass transition temperature
(refer to page 54 of "NEW BASICS AND APPLICATIONS OF THERMAL
ANALYSIS" (1989), edited by Japan Thermal Measurement Institute,
published by REALIZE CO., LTD., for example).
[0096] <Measurement of Birefringence>
[0097] A POF was immersed in a solvent such as dimethyl sulfoxide
to remove a sheath layer and a protective layer so as to expose a
core. The birefringence of the core was measured at room
temperature by use of a polarizing microscope (polarizing
microscope OPTIPHOTI-POL of Nikon Corporation) and a Senarmont
compensator. When a vinylidene fluoride-tetrafluoroethylene
copolymer is used in a sheath layer or a protective layer, the
sheath layer and the protective layer can be removed with ease by
immersing a POF in dimethyl sulfoxide at room temperature
(20.degree. C.) for about 10 seconds.
[0098] <Production Device 1>
[0099] An example of a production device used in the present
invention is shown in FIG. 3. In FIG. 3, reference numerals 1, 2, 3
and 4 denote nip rollers which have the function of delivering or
withdrawing a POF denoted by reference numeral 7 at a fixed speed.
Reference numerals 5 and 6 denote a heating furnace in which a POF
is heated by use of hot air. A melt-spun undrawn POF is drawn at a
given circumferential velocity ratio set by adjusting the
circumferential velocities of rollers 1 and 2 and then heat treated
between the rollers 3 and 4. Although this production device
comprises a first drawing device and a second annealing device
arranged in series, these devices may be arranged separately.
Further, when the annealing is carried out twice or more, other
annealing devices may be disposed after the annealing device.
Reference numeral 20 denotes a point where tension is measured.
[0100] <Count of Number of Repeated Bending>
[0101] The number of repeated bending was counted by means of a
repeated bending tester shown in FIG. 4. A POF cable 8 having a
diameter of 2.2 mm is hung between two rollers 9 disposed with a
spacing of 2.3 mm therebetween and each having a radius of 15 mm. A
weight 10 having a weight of 0.5 kg is attached to one end of the
POF cable 8, and the other end is held by holding hardware 13. The
holding hardware 13 is attached to an arm which moves the holding
hardware 13 along a virtual semicircle having the center at a point
.largecircle. shown in FIG. 4 and formed over the two rollers 9.
The point .largecircle. is a point located at a point of
intersection of a straight line connecting the tops of the two
rollers to each other and the symmetry axis of the two rollers. To
be more specific, the arm falls down to one of the rollers 9 from a
vertical direction to a horizontal direction at the center
.largecircle., returns to the vertical direction, falls down to the
other roller 9, and returns to the vertical direction again. The
arm repeated this reciprocating action. The reciprocating action is
counted as one action. The POF cable is repeatedly bent 30 times
per minute. A light source is disposed at one end of the POF cable
8, and a photodetector is disposed at the other end. An LED 11
having a wavelength of 660 nm is used as the light source, and an
optical power meter 12 is used as the photodetector. The POF cable
8 is disposed such that the distance from the optical power meter
12 to the roller 9 would be 1.5 m and the distance from the roller
9 to the LED 11 would be 3.5 m. The number of repeated bending in
this test is the number of bendings of the POF cable when a
quantity of light detected by the optical power meter is lowered
from an initial value by 1 dB. In each example, an average of the
results of five tests is shown as the number of repeated
bending.
[0102] <Production Device 2>
[0103] An example of a production device used in the present
invention is shown in FIG. 5. In FIG. 5, reference numerals 14, 15
and 16 denote nip rollers which have the function of delivering or
withdrawing a POF denoted by reference numeral 19 at a fixed speed.
Reference numerals 17 and 18 denote a heating furnace in which a
POF is heated by use of hot air. A melt-spun undrawn POF is drawn
at a given circumferential velocity ratio set by adjusting the
circumferential velocities of rollers 14 and 15 and then heat
treated between the rollers 15 and 16. Although this production
device comprises a first drawing device and a second annealing
device arranged in series, these devices may be arranged
separately. Further, when the annealing is carried out twice or
more, other annealing devices may be disposed after the annealing
device.
[0104] <Production Device 3>
[0105] An example of a production device used in the present
invention is shown in FIG. 6. In FIG. 6, reference numerals 21 and
22 denote nip rollers which have the function of delivering or
withdrawing a POF denoted by reference numeral 24 at a fixed speed.
Reference numeral 23 denotes a heating furnace in which a POF is
heated by use of hot air. The present production device is
constituted such that hot air is blown into the heating furnace
from its center and the hot air blown into the heating furnace
returns to a hot air generator from both sides of the heating
furnace. The POF is drawn or heat treated by adjusting the
circumferential velocities of the rollers 21 and 22. Reference
numeral 25 denotes a point where tension is measured.
[0106] <Production Device 4>
[0107] An example of a vertically disposed production device used
in the present invention is shown in FIG. 7. In FIG. 7, reference
numerals 26 and 27 denote nip rollers which have the function of
delivering or withdrawing a POF denoted by reference numeral 30 at
a fixed speed. Reference numeral 28 denotes a heating furnace in
which a POF is heated by use of hot air. The present production
device is constituted such that hot air is blown into the heating
furnace from its bottom (POF outlet). and the hot air blown into
the heating furnace returns to a hot air generator from the top
(POF inlet) of the heating furnace. Reference numeral 29 denotes a
guide. The POF is heat treated by adjusting the circumferential
velocities of the rollers 26 and 27. Reference numeral 31 denotes a
point where tension is measured.
[0108] <Measurement of Tension>
[0109] Tension at the time of the annealing was measured by use of
a DTMB-2.5B tension meter manufactured by NIDEC-SHIMPO corporation
at the point denoted by the reference numeral 20 between the roller
16 and the heating furnace 18 in the case of the production device
2, at the point denoted by the reference numeral 25 between the
roller 22 and the heating furnace 23 in the case of the production
device 3, and at the point denoted by the reference numeral 31
between the guide 29 and the heating furnace 28 in the case of the
production device 4.
Comparative Example 1
[0110] Polymethylmethacrylate obtained by continuous bulk
polymerization was used as a core. The Tg of the core was
112.degree. C. (DSC method, temperature rising rate: 10.degree.
C./min). A copolymer of vinylidene
fluoride/tetrafluoroethylene=80/20 (mol %) was used as a
sheath.
[0111] An undrawn POF comprising a core and a sheath was prepared
from these polymers by means of a melt spinning method. The
obtained undrawn POF was drawn at a roller circumferential velocity
ratio (circumferential velocity of the roller 2/circumferential
velocity of the roller 1) of 2.0 by means of the drawing device
(first device shown in FIG. 3) comprising the non-contact heating
furnace 5 set at 150.degree. C. and having a furnace length of 3 m
and the rollers 1 and 2 so as to obtain a POF having a fiber
diameter of 750 .mu.m and a core-sheath structure.
[0112] This POF was subjected to TMA measurement to determine
shrinkage stress occurring temperature. The thermal shrinkage ratio
and shrinkage stress occurring temperature of the obtained POF are
shown in Table 1. (Examples 1 to 7 and Comparative Examples 2 to 5)
After a POF drawn by means of the first drawing device shown in
FIG. 3 in the same manner as in Comparative Example 1 was wound
around a bobbin, the POF was heat treated at a roller
circumferential velocity ratio (circumferential velocity of the
roller 4/circumferential velocity of the roller 3) of 1.0 and at a
temperature shown in Table 1 for a duration shown in Table 1 by
means of the annealing device (second device shown in FIG. 3)
comprising the non-contact heating furnace 6 having a furnace
length of 3 m and the rollers 3 and 4 so as to obtain a POF having
a fiber diameter of 750 .mu.m and a core-sheath structure. The
thermal shrinkage ratio and shrinkage stress occurring temperature
of the obtained POF are shown in Table 1.
[0113] In Table 1, the symbol .largecircle. is given to examples in
which annealing conditions satisfy 4.ltoreq.y.ltoreq.-1.5x+330 and
(Tgc-5).degree. C..ltoreq.x.ltoreq.(Tgc+5).degree. C. [Tgc: a glass
transition temperature of a core, x: an annealing temperature
(.degree. C.), and y: an annealing time (seconds)], and the symbol
X is given to examples in which the annealing conditions do not
satisfy these equations.
[0114] POFs obtained by the method (second invention) of the
present invention, that is, POFs which satisfy the above annealing
conditions, have a shrinkage stress occurring temperature of at
least (Tgc-35).degree. C., that is, at least 77.degree. C., and a
thermal shrinkage ratio of not higher than 2% which is lower than
those of the POFs of Comparative Examples. Therefore, they are
excellent in heat resistance.
1 TABLE 1 Annealing Shrinkage Conditions Thermal Stress Temp. Time
Shrinkage Occurring Annealing (.degree. C.) (sec) Ratio (%) Temp.
(.degree. C.) Conditions Comp. Ex. 1 -- -- 2.4 73 -- Comp. Ex. 2
145 3 2.1 76 X Example 1 145 6 2.0 79 .largecircle. Example 2 145
10 1.7 80 .largecircle. Example 3 145 15 1.2 84 .largecircle.
Example 4 145 30 1.0 85 .largecircle. Comp. Ex. 3 165 3 2.2 76 X
Example 5 165 6 1.9 77 .largecircle. Example 6 165 9 1.5 80
.largecircle. Comp. Ex. 4 185 3 2.3 76 X Example 7 185 5 1.5 78
.largecircle. Comp. Ex. 5 190 3 2.2 74 X
Comparative Example 6
[0115] A POF having a fiber diameter of 1,000 .mu.m and a
core-sheath structure was produced in the same manner as in
Comparative Example 1 except that the roller circumferential
velocity ratio (circumferential velocity of the roller
2/circumferential velocity of the roller 1) was set to be 3.0. The
thermal shrinkage ratio of the obtained POF is shown in Table
2.
Examples 8 to 13
[0116] The POF obtained in Comparative Example 6 was heat treated
at a temperature of the non-contact heating furnace 6 having a
furnace length of 3 m of 165.degree. C., a circumferential velocity
ratio (circumferential velocity of the roller 4/circumferential
velocity of the roller 3) of 1.0, and roller circumferential
velocities which were adjusted such that an annealing time would be
10 seconds, and the POF was then wound around a bobbin (Example 8).
Then, the POF obtained in Example 8 was heat treated under the same
conditions as used in Example 8 (using only the second device shown
in FIG. 3), and the resulting POF was wound around a bobbin
(Example 9). Thus, annealing of the POF was repeated under the same
conditions as used in Example 8 for the numbers of times shown in
Table 2 to obtain POFs which undergone the annealing for a
different number of times (Examples 10 to 13).
[0117] The thermal shrinkage ratios of the obtained POFs are shown
in Table 2. As is obvious from Table 2, POFs having smaller thermal
shrinkage ratios and excellent heat resistance could be obtained by
repeating the annealing twice or more.
2 TABLE 2 Number of Repeated Thermal Shrinkage Annealing Ratio (%)
Comp. Ex. 6 -- 2.1 Example 8 1 1.6 Example 9 2 1.3 Example 10 3 1.2
Example 11 4 1.0 Example 12 5 0.9 Example 13 6 0.8
Examples 14, 15 and 16
[0118] The POF obtained in Example 8 was subjected batch-type
fixed-length annealing at 90.degree. C. for 20 hours by using a
commercially available hot air dryer in a similar manner (Example
14). Similarly, the POFs obtained in Examples 10 and 13 were
subjected the batch-type fixed-length annealing (Examples 15 and
16). The results are shown in Table 3. POFs having further smaller
thermal shrinkage ratios and excellent heat resistance could be
obtained by the batch-type fixed-length annealing.
3TABLE 3 Thermal Shrinkage Ratio (%) Remarks Example 14 0.5 POF of
Example 8 was subjected to the fixed-length annealing at 90.degree.
C. for 20 hours. Example 15 0.4 POF of Example 10 was subjected to
the fixed-length annealing at 90.degree. C. for 20 hours. Example
16 0.3 POF of Example 13 was subjected to the fixed-length
annealing at 90.degree. C. for 20 hours.
[0119] As described above, POFs obtained by the method (second
invention) of the present invention, that is, POFs which satisfy
the above annealing conditions, have a shrinkage stress occurring
temperature of at least (Tgc-35).degree. C., that is, at least
77.degree. C., and a thermal shrinkage ratio of not higher than 2%
which is lower than those of the POFs of Comparative Examples.
Therefore, POFs having excellent heat resistance could be obtained.
Further, it is understood that POFs having smaller thermal
shrinkage ratios and excellent heat resistance could be obtained by
carrying out the fixed-length annealing.
Examples 17 and 18
[0120] Polymethyl methacrylate obtained by continuous bulk
polymerization was used as a core. The Tg of the core was
112.degree. C. (DSC method, temperature rising rate: 10.degree.
C./min). A copolymer of 51 parts by weight of 2,2,2-trifluoromethyl
methacrylate, 30 parts by weight of
1,1,2,2-tetrahydroperfluorodecyl methacrylate, 18 parts by weight
of methyl methacrylate and 1 part by weight of methacrylic acid was
used as a sheath. A copolymer of vinylidene
fluoride/tetrafluoroethylene=80/20 (mol %) was used as a material
for a protective layer.
[0121] An undrawn POF comprising a core, a sheath and a protective
layer was prepared from these polymers by means of a melt spinning
method. The obtained undrawn POF was drawn at a roller
circumferential velocity ratio (circumferential velocity of the
roller 2/circumferential velocity of the roller 1) of 2.7 by means
of the drawing device (first device shown in FIG. 3) comprising the
non-contact heating furnace 5 set at 160.degree. C. and having a
furnace length of 2.5 m and the rollers 1 and 2 so as to obtain a
POF having a fiber diameter of 1,000 .mu.m and a
core-sheath-protective-layer structure.
[0122] After wound around a bobbin, the obtained POF was heat
treated at annealing temperatures and roller circumferential
velocity ratios (circumferential velocity of the roller
4/circumferential velocity of the roller 3) shown in Table 4 for
annealing times shown in Table 4 by use of the second device shown
in FIG. 3, i.e., the annealing device comprising the non-contact
heating furnace 6 having a furnace length of 2.5 m and the rollers
3 and 4, to obtain POFs having a fiber diameter of 1,000 .mu.m and
a core-sheath-protective-layer structure. The shrinkage stress
occurring temperatures, birefringence absolute values and thermal
shrinkage ratios of the obtained POFs are shown in Table 4.
[0123] As is clear from Table 4, POFs having birefringence absolute
values of not larger than 2.0.times.10.sup.-4 and excellent heat
resistance could be obtained.
4 TABLE 4 Annealing Conditions Circumferential Circumferential
Shrinkage Velocity of Velocity of Roller Annealing Birefringence
Thermal Stress Roller 3 Roller 4 Circumferential Temp. Time
Absolute Shrinkage Occurring (m/min) (m/min) Velocity Ratio
(.degree. C.) (sec) Value (.times.10.sup.-4) Ratio (%) Temp.
(.degree. C.) Ex. 17 2.5 2.5 1.0 145 60 1.240 0.4 100 Ex. 18 5 4.25
0.85 145 32 0.854 0.2 102
Comparative Example 7
[0124] A POF having a fiber diameter of 1,000 .mu.m and a
core-sheath structure was obtained in the same manner as in
Comparative Example 1 except that the non-contact heating furnace
was set to be 135.degree. C. and the roller circumferential
velocity ratio (circumferential velocity of the roller
2/circumferential velocity of the roller 1) was set to be 2.5 for
drawing. The birefringence and thermal shrinkage ratio of the
obtained POF are shown in Table 5.
Examples 19 to 21
[0125] A POF drawn in the same manner as in Comparative Example 7
by use of the first drawing device shown in FIG. 3 was heat treated
at annealing temperatures and roller circumferential velocity
ratios (circumferential velocity of the roller 4/circumferential
velocity of the roller 3) shown in Table 5 for annealing times
shown in Table 5 by use of the annealing device comprising the
non-contact heating furnace 6 having a furnace length of 3 m and
the rollers 3 and 4, to obtain POFs having a fiber diameter of
1,000 .mu.m and a core-sheath structure. The shrinkage stress
occurring temperatures, birefringence absolute values and thermal
shrinkage ratios of the obtained POFs are shown in Table 5.
[0126] As is clear from Table 5, POFs having birefringence absolute
values of not larger than 2.0.times.10.sup.-4, small thermal
shrinkage ratios and excellent heat resistance could be
obtained.
Examples 22 and 23
[0127] After wound around a bobbin, a POF drawn in the same manner
as in Comparative Example 7 was then heat treated at annealing
temperatures and roller circumferential velocity ratios
(circumferential velocity of the roller 4/circumferential velocity
of the roller 3) shown in Table 5 for annealing times shown in
Table 5 by use of the annealing device shown in FIG. 3 comprising
the non-contact heating furnace 6 having a furnace length of 3 m
and the rollers 3 and 4, to obtain POFs having a fiber diameter of
1,000 .mu.m and a core-sheath structure. The shrinkage stress
occurring temperatures, birefringence absolute values and thermal
shrinkage ratios of the obtained POFs are shown in Table 5.
[0128] As is clear from Table 5, POFs having birefringence absolute
values of not larger than 2.0.times.10.sup.-4, small thermal
shrinkage ratios and excellent heat resistance could be
obtained.
5 TABLE 5 Annealing Conditions Circumferential Circumferential
Shrinkage Velocity of Velocity of Roller Annealing Birefringence
Thermal Stress Roller 3 Roller 4 Circumferential Temp. Time
Absolute Shrinkage Occurring (m/min) (m/min) Velocity Ratio
(.degree. C.) (sec) Value (.times.10.sup.-4) Ratio (%) Temp.
(.degree. C.) Comp. Ex. 7 -- -- -- -- -- 4.952 1.4 -- Example 6.8
6.8 1.0 160 26 1.360 0.34 99 19 Example 6.8 6.46 0.95 160 27 1.256
0.26 99 20 Example 6.8 6.12 0.9 160 28 1.139 0.24 101 21 Example 16
14.4 0.9 170 12 1.864 0.622 93 22 Example 20 18 0.9 180 10 1.850
0.58 94 23
Comparative Example 8
[0129] A POF having a fiber diameter of 1,000 .mu.m and a
core-sheath structure was obtained in the same manner as in
Comparative Example 1 except that the non-contact heating furnace 5
was set to be 135.degree. C. and the roller circumferential
velocity ratio (circumferential velocity of the roller
2/circumferential velocity of the roller 1) was set to be 2.7 for
drawing. The thermal shrinkage ratio of the obtained POF is shown
in Table 6.
Examples 24 to 25
[0130] After wound around a bobbin, a POF drawn in the same manner
as in Comparative Example 8 by means of the first drawing device
shown in FIG. 3 was heat treated at annealing temperatures and
roller circumferential velocity ratios (circumferential velocity of
the roller 4/circumferential velocity of the roller 3) shown in
Table 6 for annealing times shown in Table 6 by means of the
annealing device comprising the non-contact heating furnace 6
having a furnace length of 3 m and the rollers 3 and 4. The thermal
shrinkage ratios of the obtained POFs are shown in Table 6.
6 TABLE 6 Annealing Conditions Circumferential Circumferential
Roller Velocity Velocity Circumerential Annealing Thermal of Roller
of Roller Velocity Temp. Time Shrinkage 3 (m/min) 4 (m/min) Ratio
(.degree. C.) (sec) Ratio (%) Comp. -- -- -- -- -- 1.6 Ex. 8
Example 3.4 3.06 0.9 115 56 0.5 24 Example 3.4 3.06 0.9 110 56 0.4
25
Examples 26 to 28
[0131] The same polymers as those used in Example 17 were used in a
core, a sheath and a protective layer, and an undrawn POF having a
core-sheath-protective-layer structure was prepared by a melt
spinning method.
[0132] The obtained undrawn POF was drawn by means of the
production device 1 shown in FIG. 3 at a temperature of the first
non-contact heating furnace 5 of 135.degree. C. and a roller
circumferential velocity ratio (circumferential velocity of the
roller 2/circumferential velocity of the roller 1) of 2.7 so as to
obtain a POF having a birefringence absolute value of
5.912.times.10.sup.-4. This POF was continuously fed into the
non-contact heating furnace 6 of the second annealing device of the
production device 1 of FIG. 3 and heat treated at a temperature of
the non-contact heating furnace 6 having a furnace length of 3 m of
130.degree. C., a roller circumferential velocity ratio
(circumferential velocity of the roller 4/circumferential velocity
of the roller 3) of 0.7 and a circumferential velocity of the
roller 3 of 3.5 m/min to obtain a POF having a
core-sheath-protective-layer structure (Example 26).
[0133] Further, a POF was drawn under the same conditions as those
used in Example 26 by means of the first drawing device of the
production device 1 shown in FIG. 3, and the drawn POF was wound
around a bobbin. This POF was heat treated by means of the second
annealing device of the production device 1 shown in FIG. 3 at a
temperature of the non-contact heating furnace 6 having a furnace
length of 3 m of 125.degree. C., a roller circumferential velocity
ratio (circumferential velocity of the roller 4/circumferential
velocity of the roller 3) of 0.7 and a circumferential velocity of
the roller 3 of 3.5 m/min to obtain a POF having a
core-sheath-protective-layer structure (Example 27).
[0134] Further, a POF was drawn under heating by means of the
production device 1 shown in FIG. 3 at a temperature of the first
non-contact heating furnace 5 of 140.degree. C. and a roller
circumferential velocity ratio of 3.3 and then wound around a
bobbin. A POF having a birefringence absolute value of
5.833.times.10.sup.-4 was obtained. The obtained POF was heat
treated at an annealing temperature of the non-contact heating
furnace 6 having a furnace length of 3 m in the second annealing
device shown in FIG. 3 of 115.degree. C., a roller circumferential
velocity ratio (circumferential velocity of the roller
4/circumferential velocity of the roller 3) of 0.9 and a
circumferential velocity of the roller 3 of 3.5 m/min to obtain a
core-sheath-protective-layer structure (Example 28).
[0135] TMA and measurements of birefringences and thermal shrinkage
ratios were conducted on these POFs, and a molten polyethylene was
continuously applied to the peripheries of these POFs by use of a
cable production device to produce POF cables, and the numbers of
repeated bending of these POF cables were counted. The results are
shown in Table 7. These POFs were excellent in mechanical
properties and heat shrinkability because they had birefringence
absolute values of not smaller than 1.5.times.10.sup.-4 and
shrinkage stress occurring temperatures of not lower than
(Tg-20).degree. C.
7TABLE 7 Shrinkage Number of Stress Birefringence Thermal Repeated
Occurring Absolute Shrinkage Bending Temp. (.degree. C.) Value
Ratio(%) (times) Example 26 98 2.157 .times. 10.sup.-4 0.46 12350
Example 27 100 1.880 .times. 10.sup.-4 0.37 10240 Example 28 99
3.270 .times. 10.sup.-4 0.47 18310
Examples 29 to 30
[0136] The same polymers as those used in Example 17 were used in a
core, a sheath and a protective layer, and an undrawn POF having a
core-sheath-protective-layer structure was prepared by a melt
spinning method.
[0137] The obtained undrawn POF was drawn under heating by means of
the production device 2 shown in FIG. 5 at a temperature of the
first non-contact heating furnace 17 of 140.degree. C. and a roller
circumferential velocity ratio (circumferential velocity of the
roller 15/circumferential velocity of the roller 14) of 2.7 to
obtain a POF having a birefringence absolute value of
4.36.times.10.sup.-4. Except that this POF was continuously fed
into the second annealing device shown in FIG. 5 and heat treated
at a temperature of the non-contact heating furnace 18 having a
furnace length of 2 m of 130.degree. C., a hot air velocity of 7
m/sec, a roller circumferential velocity ratio (circumferential
velocity of the roller 16/circumferential velocity of the roller
15) of 0.83 and a circumferential velocity of the roller 15 of 6.8
m/min, a POF having a core-sheath-protective-layer structure was
obtained in the same manner as in Example 17 (Example 29).
[0138] Further, except that a POF drawn in the same manner as in
Example 29 by use of the first production device shown in FIG. 5
was wound around a bobbin and then drew under heating by means of
the second annealing device shown in FIG. 5 comprising the
non-contact heating furnace 18 having a furnace length of 2.5 m and
the rollers 15 and 19 at an annealing temperature of 115.degree.
C., a hot air velocity of 7 m/sec, a roller circumferential
velocity ratio (circumferential velocity of the roller
16/circumferential velocity of the roller 15) of 0.95 and a
circumferential velocity of the roller 15 of 2.0 m/min, a POF
having a core-sheath-protective-layer structure was obtained in the
same manner as in Example 17 (Example 30).
[0139] TMA and measurements of birefringences and thermal shrinkage
ratios were conducted on these POFs, and molten Nylon 12 was
continuously applied to the peripheries of these POFs to form two
layers of Nylon 12 on the POFs by use of a cable production device
so as to produce POF cables, and the numbers of repeated bending of
these POF cables were counted. The results of these measurements
and tensions at the time of annealing are shown in Table 8. These
POFs were excellent in mechanical properties and heat shrinkability
because they had birefringence absolute values of not smaller than
1.5.times.10.sup.-4 and shrinkage stress occurring temperatures of
not lower than (Tg-20).degree. C.
8 TABLE 8 Shrinkage Stress Number of Occurring Birefringence
Thermal Repeated Tension Temp. Absolute Shrinkage Bending (Pa)
(.degree. C.) Value Ratio (%) (times) Ex- 1.47 .times. 10.sup.6 99
2.03 .times. 10.sup.-4 0.45 27150 ample 29 Ex- 1.31 .times.
10.sup.6 99 2.78 .times. 10.sup.-4 0.41 49320 ample 30
Comparative Example 9
[0140] The same polymers as those used in Comparative Example 1
were used in a core and a sheath.
[0141] These polymers were fed into a multi-component fiber
spinning nozzle heated at 230.degree. C., and an undrawn POF was
prepared by a melt spinning method. The obtained undrawn POF was
drawn by use of a drawing device comprising the rollers 21 and 22
and non-contact heating furnace 23 shown in FIG. 6. The drawing was
carried out at a furnace length of the non-contact heating furnace
of 3 m, a hot air velocity of 6 m/sec, a hot air temperature
(annealing temperature) of 135.degree. C. and a roller
circumferential velocity ratio (circumferential velocity of the
roller 22/circumferential velocity of the roller 21) of 2.7. A POF
having a diameter of 1,000 .mu.m and a core-sheath structure was
obtained. The Tg of the core of the POF was 112.degree. C. (DSC
method, temperature rising rate: 10.degree. C./min). The thermal
shrinkage ratio of the obtained POF is shown in Table 9.
Comparative Example 10
[0142] A POF having a diameter of 1,000 .mu.m and a core-sheath
structure was obtained in the same manner as in Comparative Example
9 except that the velocity of hot air at the time of drawing was
changed to 9 m/sec. The thermal shrinkage ratio of the obtained POF
is shown in Table 9.
Example 31
[0143] After wound around a bobbin, the POF obtained in Comparative
Example 9 was then heat treated by use of the production device 4
shown in FIG. 7 which had a furnace length of 2 m. The annealing
was conducted at a hot air velocity of 6 m/sec, a circumferential
velocity of the roller 26 of 3.4 m/min, and a heating furnace
temperature and roller circumferential velocity ratio
(circumferential velocity of the roller 27/circumferential velocity
of the roller 26) shown in Table 9. Tensions at the time of the
annealing, thermal shrinkage ratios, shrinkage stress occurring
temperatures and birefringence absolute values before and after the
annealing of the obtained POF are shown in Tables 9 and 10.
Example 32
[0144] After wound around a bobbin, the POF obtained in Comparative
Example 10 was then heat treated by use of the production device 3
shown in FIG. 6 which had a furnace length of 3 m. The annealing
was conducted at a velocity of hot air from the hot air generator
of 9 m/sec, a circumferential velocity of the roller 21 of 3.4
m/min, and a heating furnace temperature and roller circumferential
velocity ratio (circumferential velocity of the roller
22/circumferential velocity of the roller 21) shown in Table 9.
Tensions at the time of the annealing, thermal shrinkage ratios,
shrinkage stress occurring temperatures and birefringence absolute
values before and after the annealing of the obtained POF are shown
in Tables 9 and 10.
Comparative Example 11
[0145] Polymethyl methacrylate obtained by continuous bulk
polymerization was used as a core. A copolymer of 51 parts by
weight of 2,2,2-trifluoromethyl methacrylate, 30 parts by weight of
1,1,2,2-tetrahydroperfluorodecyl methacrylate, 18 parts by weight
of methyl methacrylate and 1 part by weight of methacrylic acid was
used as a sheath. A copolymer of vinylidene
fluoride/tetrafluoroethylene=80/20 (mol %) was used as a material
for a protective layer.
[0146] These polymers were fed into a multi-component fiber
spinning nozzle heated at 230.degree. C., and an undrawn POF was
prepared by a melt spinning method. The obtained undrawn POF was
drawn by use of the production device 3 comprising the rollers 21
and 22 and non-contact heating furnace 3 shown in FIG. 6. The
drawing was carried out at a furnace length of the non-contact
heating furnace of 2.5 m, a hot air velocity of 6 m/sec, a hot air
temperature (annealing temperature) of 140.degree. C. and a roller
circumferential velocity ratio (circumferential velocity of the
roller 22/circumferential velocity of the roller 21) of 2.7. A POF
having a diameter of 1,000 .mu.m and a core-sheath-protective-layer
structure was obtained. The Tg of the core of the POF was
112.degree. C. (DSC method, temperature rising rate: 10.degree.
C./min). The thermal shrinkage ratio of the obtained POF is shown
in Table 9.
Example 33
[0147] As shown in FIG. 8, a stainless steel porous plate 33 having
a plurality of pores each having a diameter of 0.5 mm all over its
surface and a wedge-shaped path for passing a POF on the surface
was disposed in the production device 3 having a furnace length of
2 m shown in FIG. 6 in the manner shown in FIG. 9 to prepare an
annealing device. Arrows in FIGS. 8 and 9 indicate directions of
hot air. Further, FIG. 8(a) is a plan view of the porous plate 33
with a POF 24 placed in the wedge-shaped path, FIG. 8(b) is a cross
sectional view of the POF of FIG. 8(a) in the longitudinal
direction, and FIG. 8(c) is a cross sectional view of the POF of
FIG. 8(a) in a direction perpendicular to the longitudinal
direction. Using this annealing device, the POF of Comparative
Example 11 which had been wound around a bobbin was heat treated at
a velocity of hot air from the hot air generator of 6 m/sec, a
circumferential velocity of the roller 21 of 2 m/min, and a hot air
temperature and roller circumferential velocity ratio
(circumferential velocity of the roller 22/circumferential velocity
of the roller 21) shown in Table 9. Tensions at the time of the
annealing, thermal shrinkage ratios, shrinkage stress occurring
temperatures and birefringence absolute values before and after the
annealing of the obtained POF are shown in Tables 9 and 10.
9 TABLE 9 Annealing Conditions Tension at Thermal Circumferential
Annealing Shrinkage Temp. (.degree. C.) Velocity Ratio (Pa) Ratio
(%) Comp. Ex. 9 -- -- -- 2.2 Comp. Ex. 10 -- -- -- 1.6 Example 31
115 0.9 0.43 .times. 10.sup.6 0.4 Example 32 110 0.9 1.2 .times.
10.sup.6 0.4 Comp. Ex. 11 -- -- -- 1.3 Example 33 115 0.93 0.81
.times. 10.sup.6 0.3
[0148]
10 TABLE 10 Birefringence Absolute Shrinkage Value Stress before
after Occurring Annealing Annealing Temp. (.degree. C.) Example 31
5.88 .times. 10.sup.-4 2.31 .times. 10.sup.-4 99 Example 32 4.33
.times. 10.sup.-4 2.45 .times. 10.sup.-4 99 Example 33 4.35 .times.
10.sup.-4 2.33 .times. 10.sup.-4 101
* * * * *