Split type ribbon optical fiber core cable

In a split type ribbon optical fiber core cable capable of being split into cables, there are a plurality of ribbon optical fiber core cable units, each of which are constituted by a plurality of colored optical fiber core cables arranged in a row; a coating resin of an ultraviolet curable resin wholly coats the plurality of colored optical fiber core cables; and a bonding resin of an ultraviolet curable resin bonds the ribbon optical fiber core cable units arranged in a row. In this cable, an adhesion strength between the coating resin and the bonding resin is in the range of 1 to 100 g/cm. Further, the bonding resin after curing has a Young's modulus of from 5 to 100 kg/mm.sup.2. The bonding resin after curing has an elongation coefficient of from 5 to 80%.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a split type ribbon optical fiber core 
cable in which a plurality of optical fiber tape core cables, each of 
which consists of a plurality of colored optical fiber core cables 
arranged in flat arrangement and united with each other, are arranged in a 
row, united with each other and capable of splitting apart from each 
other. 
2. Description of the Related Art 
Unexamined Japanese Patent Publication (kokai) No. Hei. 4-163411 discloses 
a conventional method for producing a split type ribbon optical fiber core 
cable which uses a split unit coated with a soft resin at a split position 
to avoid unnecessary splitting of the split type ribbon optical fiber core 
cable and to be capable of easily finding the split position as seen from 
its exterior. Also, in the method, the split unit and the optical fibers 
are completely coated with a hard resin so that the split position can be 
felt by and incorrect splitting avoided. 
However, the above conventional cable tape has a drawback; splitting the 
cable tape may expose an optical fiber or detach an optical fiber from the 
coating resin. 
These troubles arise because of the poor adhesion in the split position 
between the optical fiber surface and the united coating resin. 
Specifically, when the above split type ribbon optical fiber core cable is 
split, the coating resin peels off the optical fiber leaving a surface 
whose adhesion to the resin is relatively low, resulting in optical fiber 
detachment. 
Also, Unexamined Japanese Patent Publication No. Hei. 4-166808 discloses 
another method in which the thickness of an ribbon optical fiber core 
cable is reduced to a value almost equal to the outer diameter of the 
optical fibers and the adhesion strength between the optical fiber surface 
and the coating resin is regulated to a value in a specific range in terms 
of 90.degree. peel strength so that the ribbon optical fiber core cable 
can be easily split when twisted. 
However, in the above ribbon optical fiber core cable, there is the 
possibility that the cable tape may develop cracks when twisted. 
The cracking occurs because the twisting of the ribbon optical fiber core 
cable imposes a strain on the coating interface within the ribbon optical 
fiber core cable. 
Unexamined Japanese Patent Publication No. Hei. 1-138518 discloses a 
technique in which a bonding resin material having a different elongation 
coefficient is used to bond ribbon optical fiber core cables to each other 
in order to facilitate the splitting of the united ribbon optical fiber 
core cables. 
However, in the above technique, use of a bonding resin having a reduced 
elongation coefficient results in cracking upon application of an external 
force. 
This is because the bonding resin having a reduced elongation coefficient 
is more apt to develop cracks upon bending. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide a split type ribbon 
optical fiber core cable which not only can be split into the ribbon 
optical fiber core cable units without fail, but also does not suffer an 
increase in transmission loss even in a moist-heat or hot-water 
environment. 
A split type ribbon optical fiber core cable capable of being split into 
cables, comprises a plurality of ribbon optical fiber core cable units, 
each unit comprising a plurality of colored optical fiber core cables 
arranged in a row; a wholly coating resin comprising an ultraviolet 
curable resin which wholly coats the plurality of colored optical fiber 
core cables; and a bonding resin comprising an ultraviolet curable resin 
which bonds the ribbon optical fiber core cable units arranged in a row; 
wherein an adhesion strength between the wholly coating resin and the 
bonding resin is in the range of 1 to 100 g/cm. 
According to the present invention, since the adhesion strength between the 
bonding resin and the coating resin is in the given range, the split type 
ribbon optical fiber cable not only can be split into the ribbon optical 
fiber cable units without fail, but does not suffer an increase in 
transmission loss even in a moist-heat or hot-water environment.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention will be explained below in detail. 
The present inventors made extensive investigations on the problems 
described above. As a result, they have found that the above described 
problems can be solved by regulating the adhesion strength between a 
wholly coating resin and a bonding resin to a value in a given range, and 
by further regulating the Young's modulus and elongation coefficient of 
the cured bonding resin to values respectively in given ranges. The 
present invention has been completed based on this finding. 
The present invention provides a split type ribbon optical fiber core cable 
capable of being split into cables, said split type ribbon optical fiber 
core cable including ribbon optical fiber core cable units, each unit 
consisting of a plurality of colored optical fiber core cables arranged in 
a row and a wholly coating resin consisting of ultraviolet curable resin 
which wholly coats the plurality of colored optical fiber core cables, and 
a bonding resin consisting of a ultraviolet curable resin which bonds the 
ribbon optical fiber core cable units arranged in a row, wherein an 
adhesion strength between the wholly coating resin and the bonding resin 
is from 1 to 100 g/cm. 
In the split type ribbon optical fiber core cable as described above, the 
bonding resin after curing may have a Young's modulus of from 5 to 100 
kg/mm.sup.2. 
In the split type ribbon optical fiber core cable as described above, the 
bonding resin after curing may have an elongation coefficient of from 5 to 
80%. 
The split type ribbon optical fiber core cable of the present invention 
contains a protective coating layer (bonding resin) as a component 
thereof. This protective coating layer may consist of a first coating 
layer which is a flexible layer formed from an ultraviolet-curable resin. 
Alternatively, the protective coating layer may consists of the first 
coating layer and, formed thereon, a second coating layer which is a rigid 
layer formed likely from an ultraviolet-curable resin. 
The wholly coating resin may be the same resin as any of the 
ultraviolet-curable resins for forming the protective coating layer. 
The bonding resin is desirably selected from the above ultraviolet-curable 
resins so that the given adhesion strength, Young's modulus, and 
elongation coefficient are obtained. 
Examples of the ultraviolet-curable resins include urethane acrylates, 
ester acrylates, epoxy acrylates, butadiene acrylates, and silicone 
acrylates. A suitable ultraviolet-curable resin which gives a rigid or 
soft resin through curing may be selected according to need. 
In the present invention, the adhesion strength between the wholly coating 
resin and the bonding resin should be from 1 to 100 g/cm and is preferably 
from 2 to 70 g/cm, more preferably from 3 to 50 g/cm, when the cable tape 
is split into the individual ribbon optical fiber core cable units each 
comprising colored optical fibers united with each other with the wholly 
coating resin. 
If the adhesion strength between the two resins exceeds 100 g/cm, a 
splitting operation results in the tear of not only the bonding resin but 
also the wholly coating resin. There is hence a fear that the wholly 
coating resin may peel off a colored optical fiber or the colored optical 
fiber may detach from the other parts. 
If the adhesion strength between the two resins is as low as below 1 g/cm, 
partial peeling occurs at the interface between the two resins when the 
split type ribbon optical fiber core cable is exposed to moist heat or 
immersed in hot water. As a result, water penetrates into the resultant 
spaces to cause an increase in transmission loss. 
Consequently, in the case where the adhesion strength between the wholly 
coating resin and the bonding resin is within the range specified above, 
the split type ribbon optical fiber core cable not only can be split into 
the individual ribbon optical fiber core cable units without fail, but 
also is free from an increase in transmission loss even upon exposure to 
moist heat or immersion in hot water. 
In the present invention, the Young's modulus of the bonding resin after 
curing should be from 5 to 100 kg/mm.sup.2, and is preferably from 10 to 
70 kg/mm.sup.2, more preferably from 15 to 60 kg/mm.sup.2. 
If the Young's modulus of the cured bonding resin exceeds 100 kg/mm.sup.2, 
the resin is so rigid that splitting during data transmission results in 
splitting-impulse noises, which cause errors in transmission signals. For 
example, while the signal transmission at 2 Mbps, the code error rate 
which is generally less than 10.sup.-12 instantaneously exceeds 10.sup.-9. 
If the Young's modulus of the cured bonding resin is lower than 5 
kg/mm.sup.2, the resin is so flexible that the split type ribbon optical 
fiber core cable which is being produced may split into the ribbon optical 
fiber core cable units. 
In the present invention, the elongation coefficient of the bonding resin 
after curing should be from 5 to 80%, and is preferably from 10 to 60%, 
more preferably from 20 to 50%. 
If the elongation coefficient of the bonding resin is lower than 5%, the 
bonding resin develops cracks when the split type ribbon optical fiber 
core cable is bent by a roller or the like at the time of producing a 
cable, resulting in poor handleability. 
If the elongation coefficient of the bonding resin exceeds 80%, the bonding 
resin is difficult to tear, making the cable difficult to split. 
The present invention will be explained below in more detail by reference 
to the following embodiments, but the scope of the invention should not be 
construed as being limited by these embodiments. 
FIG. 1 is a sectional view illustrating the sectional structure of a 
representative split type ribbon optical fiber core cable according to the 
present invention. In this embodiment, two ribbon optical fiber core 
cables are arranged in a row and wholly covered with a bonding resin. 
In FIG. 1, numeral 1 denotes a glass fiber; 2, a protective coating layer; 
3, a colored layer; 4, a wholly coating resin; 5, an ribbon optical fiber 
core cable; 6, a bonding resin; and 7, a split type ribbon optical fiber 
core cable. 
FIG. 2 is a sectional view illustrating the sectional structure of another 
split type ribbon optical fiber core cable according to the present 
invention. In this embodiment, two or more ribbon optical fiber core 
cables are arranged in a row and bonded to each other with a bonding resin 
applied in a given width. 
Optical fibers were produced which each consisted of a single-mode glass 
fiber 1 coated with a protective coating layer 2, as shown in FIG. 1. The 
protective coating layer had a two-layer structure composed of a flexible 
layer as a first coating layer and a rigid layer as a second coating 
layer, which were formed from ultraviolet-curable urethane acrylate 
resins. 
Colored optical fibers having a colored layer 3 were produced from the 
above-obtained optical fibers by applying an ultraviolet-curable ink 
containing an epoxy acrylate or a urethane acrylate as the main ingredient 
to the optical fibers and curing the ink. 
Four such colored optical fibers were arranged in parallel on the same 
plane, and united with each other by means of a wholly coating resin 4 
consisting of an ultraviolet-curable urethane acrylate resin. Thus, two 
ribbon optical fiber core cables 5 were produced. Here, five types of the 
ribbon optical fiber core cable cable in which 2.0%, 1.0%, 0.5%, 0.3% and 
no silicone oils are mixed with the wholly coating resin 4 were produced. 
The ribbon optical fiber core cables 5 were arranged in parallel on the 
same plane, and bonded to each other with a bonding resin 6 consisting of 
an ultraviolet-curable urethane acrylate resin having the properties shown 
in Table 1. Thus, a split type ribbon optical fiber core cable 7 was 
produced, which is shown in FIG. 1. 
(Evaluation Test) 
The adhesion strength between the ultraviolet-curable bonding resin 6 and 
the wholly coating resin 4 was measured and charted as follows. 
The wholly coating resin was applied to a polyester sheet in a thickness of 
from 40 to 50 .mu.m, and then cured by irradiation with ultraviolet in an 
irradiation dose of 500 mJ/cm.sup.2. Thereafter, the bonding resin was 
applied to the cured wholly coating resin in a thickness of from 40 to 50 
.mu.m, and then cured by irradiation with ultraviolet in an irradiation 
dose of 500 mJ/cm.sup.2. All the above procedure was conducted in a 
nitrogen atmosphere. 
The resultant film composed of the wholly coating resin and the bonding 
resin was peeled from the polyester sheet. The wholly coating resin was 
fixed, and the bonding resin was pulled upward at a rate of pulling of 200 
mm/min, with the angle between the two resins being 180.degree. 
(180.degree. peeling). 
The maximum value found on the resultant chart was taken as the adhesion 
strength between the wholly coating resin and the bonding resin. A 
silicone oil was mixed with the wholly coating resin. Then, the adhesion 
strengths of the ribbon optical fiber core cable having the wholly coating 
resin containing 2%, 1.0%, 0.5%, 0.3% and no of silicone oils were 0.5 
g/cm, 2 g/cm, 50 g/cm, 70 g/cm and 150 g/cm, respectively. 
Further, eleven kinds of bonding resins were applied to a glass with the 
thickness of 40 to 50 .mu.m and then cured by irradiation with ultraviolet 
in an irradiation dose of 500 mJ/cm.sup.2. The cured sheet-like samples 
were measured. Their 2.5% Young's Modulus was measured as the rate of 
straining per elapsed time of 1 mm/min. and the elongation coefficient at 
the breakage point was measured as the rate of straining per elapsed time 
of 50 mm/min. by using JIS 2nd dumbbell based on JIS K 7113. Further, by 
using the eleven kinds of bonding resins, split type ribbon optical fiber 
core cables were produced and subjected to various kinds of evaluations. 
Further, used wholly coating resins and boding resins were changed to 
produce Comparative Example 1 which had small adhesion strength of 0.5 
g/cm, Comparative Example 2 which had large adhesion strength of 120 g/cm, 
Comparative Example 3 which had small adhesion strength and small Young's 
Modulus of 3 kg/cm.sup.2 and Comparative Example 4 which had large 
adhesion strength, large Young's Modulus and small elongation coefficient 
of 3%. 
Respective Examples and Comparative Examples were made to be a bundle 
having the length of 700 m and the periphery length of 2 m and immersed in 
water for two weeks. Then, the transmission loss at the wavelength of 1.55 
.mu.m was measured by OTDR. In this measurement, if the transmission loss 
was less than 0.05 dB/km, the result was judged as very good (A); if from 
0.05 to 0.10 dB/km, the result was judged as good (B); and if more than 
0.10 dB/km, the result was judged as bad (C). The measured transmission 
losses of Comparative Examples 1 and 3 were measured as 0.13 dB/km and 
0.22 dB/km, respectively. Accordingly, optical cables according to 
Comparative Examples 1 and 3 has a problem in actual use because there is 
a problem of the reliability if the outer coating of the cable is injured 
and the like. 
The reason for the transmission loss of these Comparative Examples is 
considered to be that water which permeated from the surface of the 
bonding resin generated a liquid bubble at the interface between the 
bonding resin and the wholly coating resin where the adhesion strength was 
relatively weak. Thus, a difference was generated in the side pressure 
applied to the optical fibers, thereby increasing the transmission loss. 
The split ability was measured by using a jig 20 which cut a right and a 
left ribbon optical fiber core cables in the vertical direction as shown 
in FIG. 7A. As shown in FIG. 7B, the both ribbon optical fiber core cables 
were successively split from each other without breaking was judged as 
success. On the contrary, as shown in FIG. 7C, if the outer coating of one 
of ribbon optical fiber core cables were broken, it was judged as a 
failure. In each Examples and Comparative Examples, the split tests were 
repeated 100 times with respect to 5 m samples. According to the results 
of this tests, if no failure occurred, it was judged as good (A), and if 
at least one failure occurred, it was judged as bad (C). These judgement 
are shown in Table 1. 
Examples 1 to 11 exhibited good split ability. However, although Example 11 
could be split, large force was necessary to split. This is because the 
elongation coefficient after curing of the bonding resin was an large as 
100% and had viscosity. 
Consequently, by setting the adhesion strength in the range of 1 to 100 
g/cm, the split type ribbon optical fiber has a reliability necessary for 
an optical fiber cable and an excellent split ability for a terminal. 
On the other hand, for actual use, in order to increase the core cable use 
efficiency of the split type ribbon optical fiber cable, the right and the 
left ribbon optical fiber core cable may be branched off at different 
positions. The split type ribbon optical fiber is necessary to be split 
even when one of the right and left ribbon optical fiber core cable is in 
use. During this splitting, it is not desirable that the communication by 
the ribbon optical fiber core cable in use occurs transmission error. 
Accordingly, a transmission path in which four optical fiber cores in the 
right ribbon optical fiber core cable are continuously coupled was 
produced. While transmitting test signal of wavelength of 1.55 .mu.m and 
transmission ratio of 2 Mbps, the split type ribbon optical fiber core 
cable is split and the variation of the code error rate was estimated. For 
this estimation, the input strength of the optical signal was adjusted to 
be low and the code error rate before working was adjusted to be 
10.sup.-12. The split working was performed five times. If the maximum 
code error rate exceeded 10.sup.-9, it was judged as passable (B) but care 
needs to be taken during split working. If it did not exceed 10.sup.-9, it 
was judged as good (A). As the result of this estimation, the transmission 
error occurred in Example 9. Accordingly, in the case of a high Young's 
Modulus, it is understood that the split force is more directly 
transmitted to the optical fiber core cable causing a transmission error. 
If it is also considered that the split working while a part of optical 
fibers are in use is necessary, the bonding resin preferably has a Young's 
Modulus of 5 to 100 kg/mm.sup.2 after curing. 
Next, with regard to the manufacture of the optical cable, the necessity of 
splitting the optical cable during production may be a problem in the 
split type ribbon optical fiber core cable. FIG. 6 is a diagrammatic view 
illustrating the appearance of the twisting apparatus used for an 
experiment. By using this apparatus, the split type ribbon optical fiber 
core cable was applied the weight 14 of 500 g which was the maximum load 
during the general production and given a twist of a rotational angle of 
900.degree. until a pitch became 200 mm. This test was subjected to eleven 
kinds of Examples of the split type ribbon optical fiber core cable 11. If 
it was split, it was judged as passable (B) but care needs to be taken 
during split working. If it was not split, it was judged as good (A). As 
the result, the split type ribbon optical fiber core cable of Example 8 
split twice. In Example 8, when the load (weight 14) was reduced to 350 g, 
the split type ribbon optical fiber core cable was not split even if it 
was twisted. Since Young's Modulus of the bonding resin of Example 8 was 
as small as 3 kg/mm.sup.2, the optical cable broke. Namely, Young's 
Modulus of the bonding resin after curing is desiraby more than 5 
kg/cm.sup.2 in view of handling during production. 
Further, during production, the split type ribbon optical fiber core cables 
are guided by many rollers to produce an optical cable. Accordingly, it is 
desirable to have resistance to an unexpected split due to bending by the 
roller or the like. Accordingly, the split type ribbon optical fiber core 
cable was wound around mandrels having the outer diameters of 50 mm and 
100 mm fifty times to estimate whether or not it is split. As the result, 
when the split type ribbon optical fiber core cable of Example 10 was 
wound around the mandrel having outer diameter of 50 mm, the split 
occurred three times. Accordingly, the split type ribbon optical fiber 
core cable of Example 10 which should be used with care with regard to 
bending was judged as passable (B). This is because the elongation 
coefficient of the bonding resin is as small as 3%. Accordingly, the resin 
is easily cracked. Therefore, it is desirable for the elongation 
coefficient of the bonding resin is desirable more than 5%. Remaining 
Examples did not split and were judged as good (A). 
TABLE 1 
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Adhesion Young's 
Elongation 
Trans. loss Resistant 
strength modulus 
Coefficient 
after Immersion 
Split 
Trans. error 
to twist 
Resistant 
(g/cm) (kg/mm.sup.2) 
(%) in hot water 
ability 
during split 
test to bend 
__________________________________________________________________________ 
Example 1 
50 50 50 A A A A A 
Example 2 
2 50 50 A A A A A 
Example 3 
70 50 50 A A A A A 
Example 4 
50 10 50 A A A A A 
Example 5 
50 70 50 A A A A A 
Example 6 
50 50 10 A A A A A 
Example 7 
50 50 60 A A A A A 
Example 8 
50 3 50 A A A B A 
Example 9 
50 120 50 A A B A A 
Example 10 
50 50 3 A A A A B 
Example 11 
50 50 100 A A A A A 
Comp. Ex. 1 
0.5 50 50 C A 
Comp. Ex. 2 
120 50 50 A C 
Comp. Ex. 3 
0.5 3 50 C A 
Comp. Ex. 4 
120 120 3 A C 
__________________________________________________________________________ 
As described above, the split type ribbon optical fiber core cable of the 
present invention has the following effects. 
By using a combination of a wholly coating resin and a bonding resin in 
which the adhesion strength between the two resins is in the specific 
range of from 1 to 100 g/cm, the split type ribbon optical fiber core 
cable not only can be split into the ribbon optical fiber core cable units 
without fail, but does not suffer an increase in transmission loss even in 
a moist-heat or hot-water environment. The split type ribbon optical fiber 
core cable retains the intact suitability for splitting even when the 
adhesion strength between the wholly coating resin and the bonding resin 
is any value within that specific range. 
By regulating the Young's modulus of the bonding resin to a value in the 
specific range of from 5 to 100 kg/mm.sup.2, transmission signal errors 
during splitting and unexpected splitting during cable fabrication can be 
prevented. 
By regulating the elongation coefficient of the bonding resin to a value in 
the specific range of from 5 to 80%, unexpected splitting can be prevented 
and the force necessary for splitting can be reduced.