JPH0140962B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing an optical fiber core having a secondary coating layer made of a thermoplastic resin having a low coefficient of linear expansion and a high modulus of elasticity. When optical glass fibers (hereinafter abbreviated as fibers) are used as is, scratches occur on the fiber surface during the manufacturing process or cable forming process, which becomes a stress concentration source and breaks with extremely low strength. For this reason, a highly reliable transmission line cannot be formed. Conventionally, the following two types of optical fiber manufacturing methods have been proposed in order to prevent a decrease in the fiber's breaking strength, to suppress an increase in transmission loss, or to facilitate handling. One is a method for producing a tight-structured fiber core, in which the fiber is first coated with a material such as a modified silicone resin, a buffer layer is formed on top of that with a material such as a silicone resin, and then a nylon resin is coated on top of that. It is manufactured by applying a secondary coating layer of a material such as. The other is a method for manufacturing loose tube fiber cores, in which the fiber coated with a primary coating or a primary coating and a buffer layer is loosely placed in a protective plastic tube made of a material such as nylon resin or polypropylene resin. It is made by holding it in. Tight structure type fiber cores have the advantage of not requiring high coating uniformity in the secondary coating process because the buffer layer prevents an increase in fiber microbending loss due to uneven coating. . However, the coefficient of linear expansion of conventional secondary coating materials is on the order of 10 ^-4 °C ^-1 , which is much higher than the coefficient of linear expansion of the fiber itself, on the order of 10 ^-7 °C ^-1 . For this reason, the fiber was bent due to expansion and contraction of the secondary coating layer due to temperature changes, resulting in increased loss due to microbending. In order to prevent an increase in microbending loss due to the difference in linear expansion coefficient between the secondary coating material and the fiber, glass fibers are arranged vertically in the fiber length direction on a fiber wire having a silicone buffer layer, and thermosetting resin is A fiber core wire that is cured and fixed with a secondary coating layer has been proposed. The coefficient of linear expansion of the secondary coating layer of this fiber core is on the order of 10 ^-5 °C ^-1 , and the increase in microbending loss is significantly suppressed. However, in this case 2
The low coefficient of linear expansion of the next coating layer is due to the coefficient of linear expansion of glass fiber (on the order of 10 ^-7 °C ^-1 ), and the thermosetting resin itself has a large coefficient of linear expansion (on the order of 10 ^-4 °C ^-1 ). There is no change in the fact that In addition, tight structure type fiber cores require a relatively long cooling process in the secondary coating process. This is done to remove as much as possible the longitudinal orientation of the fibers during the extrusion coating process of the secondary coating material by slow cooling. If the slow cooling is insufficient, the secondary coating layer will shrink even at room temperature due to orientation relaxation or recrystallization, compressive strain will be applied to the fiber, and microbending loss will gradually increase. As the speed of the secondary coating process increases, it is practically impossible to install a sufficient slow cooling process to match the speed increase. For this reason, relaxation of the orientation of the secondary coating layer becomes a problem, and increasing the speed becomes a problem. Loose tube type fiber core wires have the advantage that the increase in microbending loss due to expansion and contraction of the protective plastic, which is the secondary coating, can be alleviated by appropriately providing extra fiber length within the loose tube. . However, since there is a large difference in linear expansion coefficient between the secondary coating layer and the fiber itself, an increase in microbending loss due to expansion and contraction of the secondary coating layer still occurs. In order to prevent an increase in microbending loss due to the difference in linear expansion coefficient between the secondary coating material and the fiber, in the fiber manufacturing process, the loose tube is stretched and oriented in the longitudinal direction in a solid state below its melting point. is proposed. The linear expansion coefficient of the secondary coating layer of this fiber core is 10 ^-5 °C ^-1 or less,
The increase in microbending loss is significantly suppressed. However, in order to produce this drawn and oriented loose tube core wire, a relatively long heating furnace is required for drawing and orienting the loose tube, and the drawn loose tube undergoes heat shrinkage at high temperatures. In order to prevent this, it is necessary to orient the heat treatment furnace after the drawing/heating furnace, which lengthens the production line, requires precise control of the manufacturing process to control excess fiber length, and makes high-speed coating difficult. There were drawbacks such as: The present invention solves these problems caused by the high coefficient of linear expansion found in conventional secondary coating layers.
In the process of continuously coating optical fiber wire by extrusion molding a certain type of thermoplastic resin, the thermoplastic resin in a molten state is almost completely coated with the nozzle of the extruder.
It is characterized by extruding at a shear rate of 10 ³ sec ^-1 or higher to achieve highly oriented molecular orientation in the length direction of the fiber, and then coating it on the optical fiber. An object of the present invention is to provide a fiber core wire having high strength and high strength, and a method for manufacturing the same. Certain crystalline polymers, when heated,
Before melting and becoming a liquid, it may pass through a state that has the anisotropy of a crystal and the fluidity of a liquid. This state is called liquid crystal, and the liquid crystal produced by heating is called thermotropic liquid crystal (liquid crystal can also be seen when it is made into a solution, in which case it is called lyotropic liquid crystal). A polymer in a liquid crystal state when no external force is applied is generally a collection of domains in a certain arrangement order. When an external mechanical force is applied to this system, the domains deform, flow, and even collapse.
It is known that polymer chains are oriented in the direction of flow. Since the liquid crystal is oriented in the direction of flow, the melt viscosity of thermotropic liquid crystal is extremely low, and it is known that under shear flow, the higher the shear rate, the lower the melt viscosity. One way to flow and orient thermotropic liquid crystal is to eject liquid crystal from a small nozzle. That is, by injection molding or, in extrusion molding, by discharging thermotropic liquid crystal from a small die, the polymer chains can be oriented in the injection direction or extrusion direction. Liquid crystals oriented in this way maintain their oriented state even after cooling down. In polymers with molecular chains oriented in this way, the coefficient of linear expansion in the chain axis direction is extremely low, and the modulus of elasticity is high. . Figure 1 shows polyethylene terephthalate (PET)
Thermotropic liquid crystal obtained by treating resin with p-acetoxybenzoic acid (polyethylene terephthalate-p-hydroxybenzoic acid copolymer)
is a diagram showing the relationship between shear rate and linear expansion coefficient in the extrusion direction when extruded from a nozzle at 240°C using a capillary rheometer. As is clear from FIG. 1, as the shear rate increases, the orientation improves and the coefficient of linear expansion decreases. When the shear rate is 10 ³ sec ^-1 or more, the coefficient of linear expansion becomes almost zero. The present invention applies this thermotropic liquid crystal, or a polymer that is oriented in the flow direction under high temperature and flow even though it does not clearly exhibit a liquid crystal state, to the fiber coating. A fiber core wire with high expansion coefficient and high elastic modulus can be easily obtained. Hereinafter, an example of manufacturing the fiber core of the present invention and a fiber core structure will be described. FIG. 2 is an explanatory diagram of an apparatus for manufacturing a fiber core wire by the method of the present invention, in which 1 is a fiber feeder, 2 is a fiber wire, 3 is a crosshead die of an extruder, and 4 is a die. In the straight portion, 5 is a secondary coating resin, 6 is a cooling tank, and 7 is a wire winder. The fiber strand 2 is fed out from the fiber strand feeder 1 and is coated at the portion exiting the crosshead die 3. The secondary coating resin 5 passes through the linear portion 4 of the die, coats the surface of the fiber wire, solidifies in a cooling bath 6, and then is wound into a core wire by a wire winder 7. Shearing stress is applied to the secondary coating resin in a molten state in the die straight section 4, and polymer chains are arranged in the flow direction (extrusion direction). In order to highly orient the polymer chains in the extrusion direction and achieve a low coefficient of linear expansion, the first
As it is clear from the figure, as the shear rate
10 ³ sec ^-1 or more is required. The shearing speed in the die straight section 4 depends on the resin extrusion speed (corresponding to the coating speed), the length of the die straight section, the inner diameter of the resin discharge hole (nipple diameter) and the outer diameter of the resin discharge hole (dice diameter) at the die outlet. It will be decided. Now, the straight part of the die is 4 mm, the nipple diameter is 1.2 mm, and the outer diameter of the core wire is 0.9 mm on the fiber wire with an outer diameter of 0.4 mm.
The shear rate is 10 ³ sec ^-1.
In order to maintain ~10 ⁴ sec ^-1 , the die diameter is approximately 1.5 mm when the wire coating speed is 10 m/min, and 2.0 mm when the wire coating speed is 100 m/min. It was found that when the speed was 1000m/min, the value was 3.0mm. In the case of the die and nipple diameters mentioned above, it is defined as the ratio (S _p /S _i ) of the cross-sectional area of the resin at the die exit (S _p ) and the cross-sectional area of the liquid crystal polymer coating layer (S _i ) after solidification and molding. , the so-called drawdown ratio is when the coating speed is 10m/
1.2 at min, 3.9 at 100m/min, 1000m/min
So it becomes 11.6. However, regardless of the drawing ratio, if the shear rate is approximately 10 ³ sec ^-1 or higher, a liquid crystal polymer coating layer with highly oriented molecules can be formed. FIG. 3 is a cross-sectional view of a fiber core produced by the method of the present invention, in which 8 is a fiber, 9 is a buffer layer, and 5 is a secondary coating layer. As the buffer layer 9, commonly used silicone resin or the like is used. Therefore, the secondary coating layer has a low coefficient of linear expansion (10 ^-6 °C ^-1 order) and a high modulus of elasticity (several Gpa).
Other than the above), there is no difference in structure from the conventional tight structure core wire. Therefore, the applications of conventional tight structure core wires are:
The fiber core produced by the method of the present invention can be applied as is. The secondary coating material in the present invention may be a polymer that exhibits a liquid crystal state in a molten state (thermotropic liquid crystal), or a polymer that orients its molecules in the flow direction under high temperature and flow conditions even if it does not clearly exhibit a liquid crystal state. There are no particular limitations on the materials; specifically, polymers containing chemical structures capable of forming liquid crystals in their side chains or main chains, flexible materials such as polyphosphazene, and polydiethylsiloxane. Examples include certain polymers, or blends of these polymers and other polymers. Transmission loss spectrum and loss-temperature characteristics of fiber core wire manufactured by the method of the present invention using the liquid crystal polymer shown in Figure 1 (temperature ranged from 80°C to -60°C)
℃) are shown in FIGS. 4 and 5. For comparison, FIG. 5 also shows the loss-temperature characteristics of a conventional nylon-coated core wire. Table 1 also shows the physical properties of the coating layer used in the experiment.

[Table] As is clear from FIG. 4, there is no difference in the loss spectra before and after coating with liquid crystal polymer. In other words, it can be seen that there is no increase in loss due to coating. Also the fifth
As is clear from the figure, the liquid crystal polymer coated core wire is 80
There is no increase in loss in the temperature range from ℃ to -60℃. This is because, as shown in Table 1, the coefficient of linear expansion of the coating layer was extremely low, and therefore no increase in fiber microbending loss occurred due to expansion and contraction of the coating layer. On the other hand, in the case of nylon-coated core wire, fiber microbending loss increases at low temperatures. As can be seen from Table 1, this is due to the large coefficient of linear expansion of the nylon coating layer. As shown above, in the case of a core wire coated with a liquid crystal polymer, there is not only no increase in loss due to coating, but also no change in loss due to temperature change. In other words, since it has a liquid crystal polymer secondary coating layer that has a low coefficient of linear expansion and a high modulus of elasticity due to highly oriented molecular orientation in the length direction of the fiber, there is no increase in transmission loss due to changes in operating temperature over a long length, and It has the advantage of high strength. Furthermore, the fiber core wire of the present invention has the advantage of being suitable for high-speed coating. In other words, in the conventional tight structure fiber core wire, as mentioned above,
A slow cooling step is necessary to remove as much as possible the molecular orientation during extrusion coating of the secondary coating layer. For this reason, it had the disadvantage of poor high-speed coating properties. On the other hand, since the secondary coating layer in the present invention is produced by actively orienting the molecules during extrusion coating, there is no need for an annealing step to relax the molecular orientation. As shown in FIG. 6, this is because the oriented secondary coating layer of the present invention maintains its orientation extremely stably even at high temperatures, compared to the secondary coating layer of conventional tight-structured fiber cores. , this is because orientation relaxation does not occur. As described above, in the secondary coating process of the present invention, since the slow cooling process is unnecessary (a rapid cooling process is sufficient), the resin melt viscosity is low, and this has the advantage of excellent high-speed coating properties. There is.

[Brief explanation of drawings]

Fig. 1 is a diagram showing the relationship between coefficient of linear expansion and shear rate, Fig. 2 is an explanatory diagram of an apparatus for producing a cored optical fiber by the method of the present invention, and Fig. 3 is an illustration of the optical fiber produced by the method of the present invention. A cross-sectional view of the core wire, FIG. 4 is a loss spectrum diagram of the optical fiber core wire of the present invention,
FIG. 5 is a loss-temperature characteristic diagram of the liquid crystal polymer coated core wire and the nylon coated core wire, and FIG. 6 is a high temperature shrinkage characteristic diagram of the secondary coating layer. DESCRIPTION OF SYMBOLS 1... Fiber strand feeding machine, 2... Fiber strand, 3... Extruder cross head die, 4... Straight section of extruder head die, 5... Thermoplastic resin for secondary coating, 6... Cooling Tank, 7... Core wire winding machine, 8...
...Faiba, 9...Batsuhua layer.

Claims (1)

[Scope of Claims] 1. An optical fiber core wire comprising an optical fiber, a buffer layer disposed to cover the optical fiber, and a liquid crystal polymer coating layer thereon, and having a tight structure. 2 Consisting of an optical fiber, a buffer layer arranged to cover the optical fiber, and a liquid crystal polymer coating layer on top of the optical fiber, when manufacturing an optical fiber core wire with a tight structure, continuous extrusion molding of the liquid crystal polymer is performed. During the coating process, the extruder nozzle pumps the liquid crystal polymer in a molten state at approximately 10 ³ sec ^-1
1. A method for producing a cored optical fiber, which comprises extruding at a shearing rate above and coating a buffer layer-coated optical fiber with a liquid crystal polymer.