Coated optical fiber

A coated optical fiber comprising an optical fiber for transmitting information having coated thereon a ring-opened (co)polymer of a norbornene derivative having a polar group. Due to the excellent moldability of the polymer, the coated optical fiber can be produced with high productivity. Even when the coated optical fiber is produced at high speeds, the molding strain on the coating is small, and the coated optical fiber has a reduced heat shrinkage.

FIELD OF THE INVENTION 
This invention relates to a coated optical fiber with an amorphous resin 
having a high modulus of elasticity, excellent resistance to fracture upon 
impact and a reduced molding strain. 
Commercialization of optical fibers as a high-performance transmission 
system is now being extensively studied. Since optical fibers are glass 
fibers composed mainly of silica, the optical fibers have a low elongation 
and are weak to impact and torsion and susceptible to considerable 
fracture by an external force which may be exerted thereon during 
production or installation or in an environment in which the optical 
fibers are used. Hence, the optical fibers must be protected by coating. 
BACKGROUND OF THE INVENTION 
Coated optical fibers have various structures which are roughly classified, 
for example, into (1) a three-layer structure, (2) a loose structure, (3) 
an FRP structure, and (4) a tape structure. 
The coated optical fiber of the three-layer structure is obtained by 
coating a varnish such as a silicone varnish, on an optical fiber having a 
diameter of 100 to 150 microns, forming a cushion layer of a silicon 
rubber having a thickness of 100 to 150 microns thereon as a primary 
coating, and further providing a secondary coating of a thermoplastic 
resin such as nylon-12, and has an outside diameter of about 1 mm. 
However, since nylon-12 attains a temperature of 200.degree. C. or higher 
during molding, the optical fiber may be deteriorated. Furthermore, 
because the resin is crystalline, it shrinks greatly during or after 
cooling and solidification and strains occur as microbending in the 
optical fiber to degrade its transmission characteristics. 
The FRP coated optical fiber is obtained by using glass fibers as a coating 
on an optical fiber and solidifying the glass fibers by a thermosetting 
resin. Since the FRP coated optical fiber can be formed at low 
temperatures, there is no likelihood of deteriorating the optical fiber 
during the coating step. It is difficult, however, to procure a resin 
having both excellent rigidity and excellent strength against fracture by 
bending, and moreover, the FRP coated optical fiber has a low strength 
against an external impact. 
The coated optical fiber of the tape structure is obtained by holding an 
optical fiber coated with a curable resin between coating films, and then 
bonding them by heating or by using an adhesive. When the bonding is 
effected by heating, the shrinkage of the resin becomes a problem. When an 
adhesive is used, a drying step is required, and this reduces the 
productivity. 
Resins which can be used for coating optical fibers must have a high 
modulus of elasticity, excellent impact strength, resistance to cracking 
against bending stress, a low shrinkage after melt-shaping, moldability at 
low temperatures, a reduced molding strain even upon high-speed extrusion 
and good heat aging resistance. No resin meeting all of these requirements 
is presently available, and nylon-12 is employed although it gives rise to 
some problem in regard to strains during molding or moldability at low 
temperatures. 
As a result of extensive investigations in order to provide a resin which 
meets these requirements, it has been found that a ring-opened polymer or 
ring-opened copolymer of a norbornene derivative having a polar 
substituent is suitable for coating optical fibers which require 
thermoplasticity due to a high modulus of elasticity, excellent impact 
strength, substantially low molding strains (since it is an amorphous 
resin), and low-temperature moldability. 
SUMMARY OF THE INVENTION 
An object of this invention is to provide a coated optical fiber comprising 
a glass fiber having coated thereon a ring-opened polymer or copolymer of 
a norbornene derivative having a polar substituent. 
By coating such a specified resin, there can be obtained a coated optical 
fiber which is prevented from fracture by an external force which may be 
exerted thereon during production or installation and in an environment in 
which it is used. Furthermore, since molding strains are small even when 
it is produced at high speeds at low temperatures, the light transmitting 
characteristics of the optical fiber are not reduced.

DETAILED DESCRIPTION OF THE INVENTION 
The norbornene derivative having a polar substituent which can be used in 
this invention is norbornene derivatives having an ester group, a carboxyl 
group, a nitrile group, an amide group, an imide group, a halogen atom and 
an acid anhydride group. Norbornene derivatives represented by the 
following formula (I) 
##STR1## 
wherein X represents --CH.sub.2).sub.n COOR.sup.3, --CH.sub.2).sub.n 
OCOR.sup.4, --CH.sub.2).sub.n COOM, --CH.sub.2).sub.n CN, 
--CH.sub.2).sub.n CONR.sup.5 R.sup.6 or --CH.sub.2).sub.n Z (in which n 
represents an integer of 0 to 17, each of R.sup.3, R.sup.4, R.sup.5 and 
R.sup.6 represents a hydrocarbon group having 1 to 20 carbon atoms, M 
represents a hydrogen atom, an alkali metal, or an alkaline earth metal, 
and Z represents a halogen atom); each of R.sup.1, R.sup.2 and Y 
represents a hydrogen atom or a hydrocarbon group having 1 to 20 carbon 
atoms; provided that X and Y, taken together, may represent an imide 
residue or an acid anhydride residue represented by 
##STR2## 
(in which R.sup.7 represents a hydrocarbon group having 1 to 20 carbon 
atoms), are generally used. 
The norbornene derivatives having these polar substituents can be ring-open 
(co)polymerized using the metathesis catalyst described, for example, in 
Japanese Patent Application (OPI) No. 77999/74 (the term "OPI" as used 
herein refers to a "published unexamined Japanese patent application"). 
The norbornene derivatives may be copolymerized with monomers 
copolymerizable therewith. Preferred copolymerizable monomers include 
cycloolefins such as cyclopentene, cyclooctene, 1,5,9-cyclododecatriene, 
1,5-cyclooctadiene, dicyclopentadiene and norbornene. 
Preferred norbornenes are those represented by the following formula 
##STR3## 
wherein each of R.sup.17, R.sup.18, R.sup.19 and R.sup.20 represents a 
hydrogen atom or a hydrocarbon residue having 1 to 20 carbon atoms, 
provided that R.sup.18 and R.sup.19, taken together, may form a saturated 
or unsaturated cyclic hydrocarbon. 
The mole ratio of the norbornene derivative to the cycloolefin to be 
copolymerized is from 100:0 to 40:60, preferably from 100:0 to 50:50. If 
the proportion of the cycloolefin exceeds 60%, the resulting polymer has a 
reduced modulus of elasticity and becomes unsuitable as a coating for 
optical fibers. 
Preferred ring-opened (co)polymers are the norbornene derivatives having a 
ester group or a carboxylic acid and its ester group. Examples of the 
(co)polymers of the norbornene derivatives having ester groups or 
carboxylic acid and their ester groups are polymers consisting essentially 
of a structural unit (A) represented by the following formula (II) 
##STR4## 
and a structural unit (B) represented by the following formula (III) 
##STR5## 
wherein R.sup.12 represents a hydrogen atom, an alkyl group or a phenyl 
group, R.sup.13 represents a hydrogen atom or an alkyl group, and R.sup.14 
represents an alkyl group. 
The content of the structural unit (A), in terms of the mole ratio of 
(A)/(A)+(B), is from 0 to 0.85, preferably from 0.005 to 0.70. 
A ring-opened copolymer comprising a mixture of the structural unit (A) of 
the formula (II) and/or the structural unit (B) of the formula (III), and 
a unit (D) of a norbornene derivative having a nitrile group can be used. 
The structural unit (D) of the norbornene derivative having a nitrile group 
is represented by the following formula (IV) 
##STR6## 
wherein R.sup.15 represents a hydrogen atom, an alkyl group having 1 to 20 
carbon atoms, or a phenyl group, and R.sup.16 represents a hydrogen atom 
or an alkyl group. 
The mole ratio of (D)/(A)+(B)+(D) is in the range of 0.05 to 0.90 and 
preferably the mole ratio of (A)/(A)+(B) is from 0 to 0.8. 
A copolymer comprising the structural unit (A) of formula (II) and/or the 
structural unit (B) of formula (III), and a norbornene unit (E) of formula 
(V) described below or ring-opened cycloolefine unit (F) of formula (VI) 
described below can also be used. 
##STR7## 
wherein each of R.sup.17, R.sup.18 R.sup.19 and R.sup.20 represents a 
hydrogen atom or a hydrocarbon group having 1 to 20 carbon atoms. 
EQU --CR.sup.21 .dbd.CR.sup.22 --CR.sup.23 R.sup.24).sub.n (VI) 
wherein each of R.sup.21, R.sup.22, R.sup.23 and R.sup.24 represents a 
hydrogen atom or a hydrocarbon residue having 1 to 10 carbon atoms, 
preferably 1 to 3 carbon atoms, and n represents an integer of 2, 3, or 5 
to 10. 
The mole ratio of (E) and (or) (F)/(A)+(B)+(E)+(F) is from 0.01 to 0.6, 
preferably from 0.05 to 0.4 and preferably the mole ratio of (A)/(A)+(B) 
is from 0 to 0.85. 
When the ring-opened (co)polymer of the norbornene derivative is used as a 
secondary coating of a coated optical fiber, it preferably has high 
rigidity. Desirably, it has a trans-form double bond content of at least 
40%, preferably at least 45%. 
A stabilizer, a plasticizer, or an inorganic or organic filler may be added 
to the ring-opened (co)polymer. 
In the case where a coated optical fiber is produced using the 
above-described ring-opened copolymer, the coated optical fiber having a 
three-layer structure is formed. 
As shown in FIG. 1, a coated optical fiber 1 of the three-layer structure 
is formed by first forming an anchor coat layer 3 such as a silicone 
varnish on the surface of an optical fiber 2 as required, pre-coating an 
elastomer such as a silicone rubber on the outside to form a primary 
coating 4 and further coating the ring-opened (co)polymer of the invention 
on the outside of the layer 4 to form a secondary coating 5. 
The coated optical fiber 1 of the three-layer structure can be produced by 
the method shown in FIG. 2. 
A preform 6 as a matrix of of the optical fiber 2 is melted in a heating 
furnace 7 and then drawn into a fine filamentary optical fiber 2. The 
optical fiber 2 is passed through a precoating vessel 8 holding a 
precoating material 4a and a heating furnace 9 and precoated to form a 
primary coating 4. 
The precoating can be effected by applying an elastomer such as a silicone 
rubber to the surface of the glass fiber after, as required, an anchor 
coat 3 such as a silicone varnish is applied to it by a coater (not 
shown). 
The reference numeral 10 represents a fiber diameter measuring device, and 
11, a fiber diameter control circuit. By adjusting the rotating speed of a 
capstan 12, the fiber diameter is adjusted to a constant value. 
The pre-coated optical fiber 2 is fed to a crosshead 14 of an extruder for 
melt-extruding the ring-opened (co)polymer of the invention. The polymer 
is thus coated to form a coated optical fiber 1 which is wound up by a 
winder 15. 
Several coated optical fibers of the three-layer structure so produced are 
bundled with a tension member 16 placed at the center as shown in FIG. 3. 
The periphery of the bundle is wrapped with a cushioning material 17 such 
as a plastic cushioning material. Several such bundles are bundled with a 
second tension member 18 placed at the center, and the bundle is covered 
with a plastic tape 19, an aluminum laminate tape 20, a polyethylene resin 
21, etc. to form an optical fiber cable 22 which is used for light 
transmission systems. 
The coated optical fiber 1 of the invention may also have the tape 
structure shown in FIG. 4. 
The coated optical fiber of the tape structure has the structure shown in 
FIG. 4 in which a first sheath 24, a second sheath 25 and a third sheath 
26 are applied through fibrous or tape-like reinforcing materials 27 and 
28 to the outside of a ribbon-like tape 23 formed by heat-bonding a 
plurality of glass fibers 2. The ring-opened (co)polymer of the invention 
is used in the ribbon-like tape 23, the first sheath 24, the second sheath 
25 and the third sheath 26. 
The coated optical fiber of the invention may also have a loose structure 
as shown in FIG. 5. A coating 29 is formed on the outside of an optical 
fiber 2 and the resulting coated fiber is inserted in an outer cylinder 
30. The ring-opened (co)polymer can be used as a material for the coating 
29 and the outer cylinder 30. 
When the ring-opened (co)polymer is coated by melt-molding, its suitable 
intrinsic viscosity is 0.2 to 4.0, preferably 0.2 to 2.0, more preferably 
0.25 to 1.0. If the intrinsic viscosity is less than 0.2, the polymer does 
not have sufficient resistance to fracture by impact. If the intrinsic 
viscosity exceeds 4.0, the polymer cannot be molded at low temperatures, 
and strains become great during molding. 
An alternative method of producing a coated optical fiber comprises 
dissolving the ring-opened (co)polymer in a solvent such as acetone or 
tetrahydrofuran, dipping an optical fiber in the solution to coat its 
surface, and then drying the coated optical fiber. 
The ring-opened (co)polymer of the norbornene derivative can be used in 
combination with another monomer because the (co)polymer has excellent 
impact strength and contains a double bond and a polar functional group. 
According to this method, the ring-opened (co)polymer of the norbornene 
derivative is mixed with a polymerizable monomer such as styrene, methyl 
(meth)acrylate, ethyl (meth)acrylate, butyl (meth)acrylate, an epoxy resin 
or an oligoamide; a crosslinking agent such as a difunctional or higher 
mercaptan; a resin precursor; a radical initiator such as 
azobisisobutyronitrile; a light sensitizer such as acetophenone or 
benzophenone; and as required a solvent (for example, an aromatic solvent 
such as benzene, toluene or xylene; a ketone such as acetone or methyl 
ethyl ketone; or tetrahydrofuran) to form a solution of the ring-opened 
(co)polymer, coating the solution on an optical fiber, and curing the 
coating by light or heat. 
The ring-opened (co)polymer used in this method has an intrinsic viscosity 
of 0.1 to 2.0, preferably 0.1 to 1.0. The concentration of the ring-opened 
(co)polymer in the solution is 2 to 90% by weight, preferably 5 to 60% by 
weight. 
A coated optical fiber having improved tensile strength can be produced by 
providing reinforcing glass fibers along an optical fiber, coating the 
fiber assembly with the aforesaid polymer solution, and solidifying the 
coating. 
Since the ring-opened (co)polymer of the norbornene derivative shrinks 
little during molding, light losses due to the occurrence of strain can be 
prevented. Furthermore, since it has high rigidity and high impact 
strength, it is possible to prevent an accident of fracture of the optical 
fiber which may be caused by the cracking of the coating, resulting in 
stress concentration. 
The coated optical fiber of this invention may further be coated with 
another resin such as nylon-12. When the ring-opened (co)polymer is used 
as an internal coating it preferably has a high tensile elongation. 
Desirably, the ring-opened (co)polymer has a trans-form double bond 
content of not more than 90%, preferably not more than 80%. 
The following Examples illustrative the present invention more 
specifically. 
EXAMPLE 1 
The basic properties of a polymer of methyl 5-norbornene-2-carboxylate 
(intrinsic viscosity 0.35, measured at 30.degree. C. in tetrahydrofuran) 
are shown in Table 1. This resin showed a three-point flexural modulus of 
elasticity of 21,500 kg/cm.sup.2 which is nearly twice as high as that of 
nylon-12. Furthermore, the polymer was amorphous and had a small strain 
during molding. This was due to the fact that the resin had resistance to 
the occurrence of microbending. 
The resin was extrusion-coated by an extruder (50 mm) at 150.degree. C. at 
a line speed of 30 m/min. on an optical fiber having a diameter of 125 
microns, and the coated optical fiber was wound up on a rotating drum. The 
optical fiber as a core was removed, and the secondary coating composed of 
the above resin was examined for molding strain and heat aging resistance. 
The results are shown in Table 2. It was seen that the resin did not 
undergo molding shrinkage, and had good heat aging resistance. 
EXAMPLE 2 
A coated optical fiber was produced by the same manner as in Example 1 
except that the ring-opened polymer used in Example 1 was hydrolyzed to an 
extent of 30 mole% to form a copolymer of methyl 
5-norbornene-2-carboxylate and 5-norbornene-2-carboxylic acid. The resin 
extruding temperature was changed to 190.degree. C. 
The data of molding strain and heat aging resistance obtained as same as in 
Example 1 are shown in Table 2. These properties of the coating were both 
good. 
EXAMPLE 3 
A coated optical fiber was produced by the same manner as in Example 1 
except that instead of the ring-opened polymer used in Example 1, a 
copolymer of methyl 5-norbornene-2-carboxylate and cyclopentene 
(cyclopentene content 15 mole%, intrinsic viscosity 0.9) was used, and the 
extruding temperature was changed to 180.degree. C. The molding strain and 
aging resistance of the clad layer were measured and the results are shown 
in Table 2. Both of these properties were found to be good. 
EXAMPLE 4 
A coated optical fiber was produced by the same manner as in Example 1 
except that a ring-opened polymer of 5-norbornene-2-nitrile (intrinsic 
viscosity 0.65) was used instead of the ring-opened polymer used in 
Example 1, and the extruding temperature was changed to 180.degree. C. The 
molding strain and aging resistance of the clad layer were measured, and 
the results are shown in Table 2. Both of these properties were found to 
be good. 
As stated hereinabove, by using the ring-opened (co)polymer of a polar 
norbornene derivative as a coating material, a coated optical fiber could 
be obtained at temperatures lower than 200.degree. C. Since the coating 
has a reduced molding strain and a high modulus of elasticity, light 
transmission losses due to microbending are small. 
TABLE 1 
__________________________________________________________________________ 
Measuring 
Example Comp. 
Item Unit method 
1 2 3 4 Example 1 
__________________________________________________________________________ 
Three-point bending 
Kg/cm.sup.2 
D790 21500 
22500 
19500 
20000 
11000 
modulus of elasticity 
Tensile strength 
Kg/cm.sup.2 
D638 340 390 310 500 450 
Tensile elongation 
% D638 180 150 230 170 100 
Izod impact strength 
Kg cm/cm(*) 
D256 &gt;420 
&gt;420 
&gt;420 
&gt;420 
6 
Heat distortion 
.degree.C. 
D648 58 77 52 90 50 
temperature (18.6 Kg 
load) 
Coefficient of linear 
.times.10.sup.-5 cm/cm/.degree.C. 
D696 6 6 8 6 10 
expansion 
Water absorption 
% D570 0.4 0.6 0.5 0.6 1.4 
Molding shrinkage 
% 0.5 0.5 0.5 0.5 1.2 
Hardness R-Scale D785 105 108 103 118 106 
__________________________________________________________________________ 
(*)Sample thickness 2 mm 
TABLE 2 
______________________________________ 
Molding strain (*1) 
Heat aging 
(%) resistance (*2) (%) 
______________________________________ 
Example 1 0 -6 
Example 2 0 -5 
Example 3 0 -10 
Example 4 0 -6 
Comp. -1 -32 
Example 1 
______________________________________ 
(*1): The molding shrinkage was determined as follows: The secondary 
coating left after removing the core was cut to a length of 50 mm, and 
maintained at 100.degree. C. for 1 hour. Then, a change in size of the 
sample was measured by slide calipers. 
(*2): The heat aging resistance was determined as follows: The sample 
described above was treated at 80.degree. C. for 5 days, and the tensile 
elongation of the sample was compared with that before the treatment 
EXAMPLE 5 
100 parts by weight of a polymer of methyl 5-norbornene-2-carboxylate 
(intrinsic viscosity 0.22, measured at 30.degree. C. in tetrahydrofuran) 
was put in 250 parts by weight of tetrahydrofuran and heated to form a 
viscous solution. 20 parts by weight of pentaerythritol 
tetrakis(mercaptopropionate) and 1 part by weight of benzophenone were 
then added to form a solution. 
An optical fiber having a primary coating of silicone rubber was passed 
through the solution to coat it and the coated fiber was then dried. 
The coated fiber was passed through a 1 KW high-pressure mercury lamp for 3 
seconds to cure the coating by UV irradiation to form a secondary coating. 
The coated fiber was wound up on a rotating drum. The molding shrinkage 
was 0%, and the heat-aging resistance was -10%. 
While the invention has been described in detail and with reference to 
specific embodiments thereof, it will be apparent to one skilled in the 
art that various changes and modifications can be made therein without 
departing from the spirit and scope thereof.