Coated Optical Fiber

1. Field of the Invention

This invention concerns optical fibers having one or more coated layers thereon.

2. Description of the Prior Art

Optical fibers require protective coatings in order to preserve fiber strength and to protect the fiber from microbending induced optical loss. The coating is generally applied in-line with fiber drawing by passing the fiber through a reservoir containing the coating material and having an exit orifice which has been sized to apply some desired thickness of the material. This technique usually requires the coating material to have a viscosity below approximately 10,000 centipoises at the application temperature if high draw rates, 1 meter/second or higher, are desired. Classes of materials which have been applied to optical fibers by this technique include ultraviolet (UV) curables, thermal curables, solvent-based materials, and hot melts. For an example of the latter, see "Silicone-and-Ethylene-Vinyl-Acetate-Coated Laser-Drawn Silica Fibres with Tensile Strengths >3.5 GN/m.sup.2 (500 kp.s.i.) in >3 km Lengths," T. J. Miller et al, Electronics Letters, Vol. 14, No. 18, pages 603-605 (1978). The "curable" materials are applied as liquids, and solidify by polymerization, which is typically accelerated by ultraviolet radiation or by heating. The solvent-based materials solidify by the removal of the solvent, which again can be accelerated by heating. In contrast, "hot melt" materials have low viscosity at high temperature and solidify upon cooling.

Properties of the coating material which influence the ability to preserve fiber strength are its toughness, abrasion resistance, adhesion to the fiber, and thickness. In general, an increase in any of these properties reduces the susceptibility of the fiber to mechanical damage. In addition, low water absorption by the coating is desirable to preserve fiber strength.

The property of the coating material which correlates most closely to microbending sensitivity is modulus. In general, a reduction in the coating modulus decreases microbending sensitivity. The temperature range over which a low modulus is required depends upon the environment the fiber is expected to experience. For the temperature range -40 to 90 degrees Celsius, the 30 minute tensile relaxation modulus should typically be less than 10.sup.8 dynes/cm.sup.2. This wide operating temperature range for the coated fiber represents expected extremes of service conditions. Many prior art optical fiber coating materials have not obtained good microbending performance over the above wide temperature range.

Thus, in the past, thermally cured silicones with a modulus below 10.sup.7 dynes/cm.sup.2 over the expected operating temperature have been used as coating where low sensitivity to microbending is required. Unfortunately, a low modulus also often means insufficient toughness and abrasion resistance for adequate strength protection of the fiber. Such is frequently the case with the silicones. For this reason, a secondary coating is often placed over the primary silicone for strength protection.

At present, typical silicones also have process limitations in terms of preapplication stability and capability for application at high fiber draw rates, i.e., >1 meter/second, due to their curing behavior. For example, a silicone formulation typically comprises two components that begin to react upon mixing. They further react more rapidly as the hot fiber, drawn at high temperature from a preform, transfers heat to the bulk coating material in the reservoir. As the reaction continues, the viscosity of the silicone increases, ultimately reaching an unusable value if not applied soon enough to the fiber. Thus, the useful lifetime in the applicator is limited, typically being about 2 hours. This typically limits the amount of material that can be maintained in the mixed state, requiring frequent replenishment of the applicator, and sometimes results in an interruption of the coating process.

Another problem concerns the ability to transfer heat to the silicones rapidly enough to cure and harden before the fiber contacts the guide wheel and pulling apparatus. This hardening is necessary to preserve the strength of the fiber and maintain coating quality. Typically with a silicone coating, a furnace is placed below the coating applicator to accelerate the curing process. As draw rates increase, longer furnaces are required, which in turn, require higher draw towers, an expensive solution. The problem is compounded with dual-layer coatings, wherein both coatings must harden in the time and space available, and with the necessity that the first coating harden sufficiently prior to applying the second coating. While ultraviolet-cured silicone materials are currently under investigation by workers in the art, it presently appears that they are not yet ready for commercial application, especially when high cure rates are desired for high-speed fiber production. It is thus desirable to obtain other optical fiber coating materials having good microbending performance over a wide temperature range, and having processing characteristics suitable for high-speed fiber manufacture.

I have invented an optical fiber coating material based on a block copolymer rubber. The coating material is typically applied to the fiber at an elevated temperature. The end-use properties are attained on cooling.

The material can be used alone as a single-layer coating for optical fibers or as the inner coating of a dual-coated fiber. It can be applied at draw rates in excess of 1 meter/second, and typically has a pot life of several hours at elevated temperatures.

The material according to the invention is a mixture of:

(X) An ASTM type (according to ASTM D-2226 classification) 103, 104A, or 104B, or mixture thereof, napthenic or paraffinic oil having a minimum specific gravity of 0.860, and a maximum pour point ASTM D-97 of approximately -4 degrees Celsius (25 degrees F);

(Y) A styrene-rubber-styrene block copolymer having a styrene-rubber ratio of approximately 0.2 to 0.5 and preferably about 0.4, with the rubber midblock typically being ethylene butylene and the styrene end-block being substituted or unsubstituted styrene; and optionally

(Z) A resin which associates with the styrene end-blocks in the block copolymer, such as coumarone-indene copolymer or vinyl toluene -.alpha. methyl styrene copolymer, and having a ring and ball softening point, ASTM E-28, between 100 and 160 degrees Celsius.

The material further optionally comprises a wax. In the case of a single-layer or outer-layer coating, this wax is preferably a microcrystalline wax, with a melting point, ASTM D-127, above 70 degrees Celsius. In the case of an inner layer coating, the wax can be microcrystalline, paraffinic, polyethylene, etc. The material further comprises a thermal oxidative stabilizer such as tetrakis [methylene 3-(3',5'-di-tert-butyl-4'-hydroxyphenyl)proprionate] methane (Ciba-Geigy's Irganox 1010), or other suitable thermal oxidative stabilizer known in the art.

The amounts of the foregoing ingredients are typically combined in the proportions described below. Other styrene block copolymers with a saturated rubber midblock, such as hydrogenated polyisoprene or hydrogenated polybutadiene, are also satisfactory for (Y).

FIG. 1 shows a ternary composition diagram giving the composition range of one embodiment of the fiber coating material of the invention (the area ABCD);

FIG. 2 shows a similar diagram showing a presently preferred composition range for both the single coating and dual coating cases (the area A'B'C'D').

In FIGS. 1 and 2, the XZ axis indicates the total parts oil and wax. All components in the FIGS. are specified in terms of weight percentage of the total mixture exclusive of the thermal oxidative stabilizer.

The following description relates to an optical fiber having an inventive coating that exhibits low modulus down to approximately -40 degrees Celsius, yet has significantly greater toughness than typical silicones, and significantly alleviates the process limitations previously discussed. The coating material is applied at an elevated temperature to the fiber, typically between 150 to 230 degrees Celsius. One component of the coating material is a block copolymer.

In a block copolymer molecule, two or more polymer segments of different chemical composition are attached end-to-end. This is in contrast to random copolymers where the basic monomer units are randomly polymerized to form the polymer molecule.

In the styrene-ethylene butylene-styrene (SEBS) block copolymer used in this work, the styrene and ethylene-butylene blocks are incompatible and therefore form separate phases or domains. The styrene domains are rigid and act as physical crosslinks up to the glass transition temperature of polystyrene, about 90 degrees Celsius. The ethylene-butylene phase is flexible down to its glass transition temperature, about -65 degrees Celsius. The SEBS material therefore behaves as a crosslinked rubber between -65 and 90 degrees Celsius. Also, since the midblock is saturated, the SEBS molecule possesses good thermal oxidative stability at elevated temperature. While the styrene end-blocks of the SEBS material used in the Examples herein are unsubstituted styrene, substituted styrene end-blocks can alternately be used. For example, poly(.alpha.-methyl styrene) has a glass transition temperature (T.sub.g) of 441 degrees K. (168 degrees Celsius) and hence gives a higher temperature limit to block copolymer material made therefrom. Other high T.sub.g substituted styrenes are also possible; see Polymer Handbook, Second Edition, J. Brandrup and E. H. Immergut, Editors, John Wiley and Sons, Inc. (1975) at pages III-152 through III-154. As used hereafter, the term "styrene" includes both substituted and unsubstituted styrene, when referring to the block copolymer.

I have determined that the above combination of properties, when combined with the other components described below, makes the SEBS thermoplastic rubber a useful component for an optical fiber coating material, both for processing and end-use properties. Other thermoplastic styrene-rubber-styrene block copolymers can also be used. Typical examples include those having hydrogenated polyisoprene or hydrogenated polybutadiene as the rubber midblock.

One can choose additives which are compatible with the styrene blocks or additives compatible with the rubber block. Compatibility may be preferential; that is, an additive can be compatible with both but prefer one. Not all additives work the same; e.g., xylene will weaken the styrene domains while coumarone-indene resins can strengthen them. In the case of the rubber midblock, one can add oils to reduce the modulus or polycrystalline materials, such as waxes, to raise the modulus. The preferred coating material herein maintains or strengthens the styrene domains, while lowering the modulus.

The proportions of the presently preferred components of the material which can be used as an inner coating are indicated in FIG. 1 in the area bounded by ABCD. A subregion of this area, providing a presently preferred material for use as either a single-layer coating, or as the inner layer of a dual-layer coating, is shown in FIG. 2 bounded by A'B'C'D'. The preferred components are listed in Table I below:

The following properties are desirable for a dual-layer or single-layer coating material:

Viscosity at 190 degrees Celsius--Less than approximately 10,000 centipoises for high draw rate application capability.

Ring and Ball Softening Point (ASTM E-28)--Greater than approximately 100 degrees Celsius, to ensure no significant flow at 90 degrees Celsius.

Tensile Relaxation Modulus at 30 Minutes and -40 degrees Celsius--Not greater than 1.0.times.10.sup.8 dynes/cm.sup.2 for protection from microbending-induced losses.

In addition, the following requirements apply to single-layer coatings:

Blocking--No substantial blocking of coated fiber as wound on a reel. ("Blocking" as used herein refers to sticking of the coated fiber to previously wound fibers that is sufficient to cause damage to the coating when unwound.)

Adhesion to Glass at Room Temperature--A minimum T peel bond strength of approximately 1.0 kg/cm of width where a 0.25 mm thickness of the coating material is bonded between two 0.25 mm thick layers of oriented poly(ethylene terephthalate) film (OPET). Although adhesion of this type of hot melt to glass is not the same as to OPET, the polar nature of both surfaces makes relative comparisons possible. Therefore, to simplify the test procedure, OPET was chosen.

Toughness at Room Temperature--A minimum of approximately 500 psi, taken from the area under a stress-strain curve run per ASTM D-1708 at 225 percent/minute, to help ensure protection of the fiber from mechanical damage.

The properties noted above are attained in the present formulations, as follows: A rubber/oil ratio of 36/74 produces approximately a 10,000 cps viscosity for a rubber viscosity of 1190 cps, according to the test given below. This ratio is point A in FIG. 1. The oil is preferably a refined mineral oil to prevent discoloration of the coated layer, typically desirable when a coloring substance is added to the formulation.

The substitution of 9 parts of the microcrystalline wax for 9 parts of the oil increases the viscosity the same as increasing the rubber content by about 1.5 parts. Since this is not a significant change, the wax and oil are considered as a single entity in FIGS. 1 and 2.

Substitution of the coumarone-indene resin for the oil significantly increases the viscosity. The effect is greater than would be expected by simply changing the rubber/oil ratio. For every 2 parts of the resin that is added, approximately 1 part of the rubber must be removed to maintain a constant viscosity. This relationship is reflected in the slope of lines AB and A'B' in FIGS. 1 and 2, respectively. The points B and B' are fixed by compatibility limitations described in the next section.

The viscosity of the blends depends not only on the relative composition of the materials, but also on the molecular weight of the thermoplastic rubber in the blend. Kraton.RTM. G1650 and Kraton G1652 have approximately the same styrene/rubber ratio (about 0.4) but differ in molecular weight. Shell Chemical Company uses the viscosity of a 20 weight percent solids solution in toluene at 25 degrees Celsius as a measure of molecular weight. The viscosity range for Kraton G1650 is 1000-2000 cps, and for Kraton G1652 is 350-1000 cps. The Kraton G1650 used for the tests herein had a 1190 cps viscosity according to the Shell test. The regions defined in FIGS. 1 and 2 were determined for this viscosity of Kraton G1650; other viscosities can also be used. I recommend a viscosity according to this test in the range of 500 to 1800 cps. Very high viscosity (high molecular weight) rubbers, for example Kraton G1651, may be used, but present problems in blending with the other components.

The acceptable regions in FIGS. 1 and 2 are shifted down on the block copolymer axis (i.e., less block copolymer) by approximately 3 parts by weight in going to high molecular weight Kraton G1650 (viscosity 1800 cps) and are shifted upward (i.e., more block copolymer) by approximately 6 parts of block copolymer in going to low molecular weight Kraton G1652 (viscosity 500 cps), with other viscosities producing corresponding shifts. Similarly, lower molecular weight coumarone-indene resins than Cumar LX 509 can be added at higher levels with less of a viscosity effect.

Substitution of the resin for the rubber, as referred to in the previous section, is governed by compatibility as well as viscosity limitations. Incompatibility is defined here as the resin level at which a 180-190 degrees Celsius solution first becomes hazy. This criterion is reflected as lines BC and B'C' in FIGS. 1 and 2, respectively. The end points C and C' are discussed in subsequent sections.

The Ring and Ball softening point (ASTM E-28) was run at a heating rate of 1-2 degrees Celsius/minute as a function of both rubber and resin content. The results indicate a 3 degrees Celsius increase for each part of the resin added and a 2 degrees Celsius increase for a 1 part increase in the rubber. Lower melting resins must be added at higher levels to obtain a similar effect on softening point.

The minimum rubber content for a 100 degrees Celsius softening point composition containing no end-block resin is 19.5 parts. This is the value at point D in FIG. 1. The data on Cumar LX 509 content versus softening point were used with the compatibility data to determine the composition at point C in FIG. 1.

The 30 minute tensile relaxation modulus at -40 degrees Celsius was determined from data taken on a Rheometrics Thermomechanical Spectrometer. The data for compositions both with and without the microcrystalline wax are shown in Table II for blends A and B. The value for the wax-containing composition at -40 degrees Celsius is 9.times.10.sup.7 dynes/cm.sup.2, which is below the 1.0.times.10.sup.8 dynes/cm.sup.2 maximum value set as the requirement. Removal of the wax reduces the modulus by approximately an order of magnitude. Since the wax is controlling in terms of modulus, all the compositions bounded by ABCD in FIG. 1 and A'B'C'D' in FIG. 2 meet the modulus requirement for compositions containing up to an estimated 12 parts wax.

Microcrystalline and polyethylene waxes were also evaluated with respect to blocking. Both plaque and fiber coating experiments were run. In the plaque tests, a load of 6 g/cm.sup.2 was placed on top of two layers of coating material, each layer about 1.20 mm thick. The coatings contained 10 parts each wax and coumarone-indene resin, with the balance being rubber and oil. The tests were run for one hour at a series of temperatures. The 91 degrees Celsius (195 degrees F.) melting point microcrystalline wax prevented blocking up to 50 degrees Celsius, with some blocking noted at 64 degrees Celsius. A 116 degrees Celsius melting polyethylene wax containing material did not block at 42 degrees Celsius, but did at 50 degrees Celsius.

In these coating experiments, the coatings were placed on 110 .mu.m OD fiber in-line with drawing. The coated fiber OD was approximately 225 .mu.m. Fiber coated with material containing no wax could not be removed from the wind-up reel without either damaging the coating or breaking the fiber. When the coating contained 10 parts of 116 degrees Celsius melting point polyethylene wax but no coumarone-indene resin, severe blocking still occurred. Including 10 parts of coumarone-indene resin with the polyethylene wax gave a much improved, but marginal performance. Use of the 195 degrees F. melting point microcrystalline wax and the coumarone-indene resin gave coated fiber which did not block.

The prior discussion indicates that both the microcrystalline wax and the coumarone-indene resin are functional in allowing the coated fiber to be removed from the take-up reel without damaging the fiber. For this reason, the compositions to be used as single-layer coating (or the outer layer of a multiple-layer coating) desirably contain about 9 parts of microcrystalline wax and some level of the coumarone-indene resin. A level of microcrystalline wax up to about 12 parts appears to be feasible in the present formulation, with even higher levels possible for lower melting point (softer) waxes than the preferred wax used herein. In the case of dual-layer coatings, wherein the inner layer need not be optimized to prevent blocking, the wax used can alternately be paraffin, or polyethylene, among others, or may be omitted entirely.

A material which is adhered to a substrate is more effective in protecting that substrate from mechanical damage than an unadhered material having the same mechanical properties. The coumarone-indene resin improves adhesion of the coating to the glass fiber. In the case of a single-layer coating, the improved adhesion is highly desirable to protect the fiber.

As a measure of adhesion, T peel testing was conducted on composites consisting of a 0.25 mm thickness of coating material between two 0.25 mm thick sheets of oriented poly(ethylene terephthalate) film. The composites were prepared in a press at 180 degrees Celsius using a molding frame to maintain the coating thickness. T peel measurements were made at 5.0 cm/minute. Based on these data, a coumarone-indene content of approximately 3.5 percent by weight with a minimum of 24 percent by weight rubber is sufficient to give T peel bonds of at least 1.0 kg/cm of width. These data then define the boundary line A'D' in FIG. 2. In the case of dual-layer coatings, the protection afforded the fiber by improved adhesion is not necessary in all cases. Therefore, the amount of coumarone-indene resin can be as low as zero, as indicated by line AD in FIG. 1.

The toughness of the coating, as well as the coating-to-fiber adhesion which was previously discussed, reflects the ability of the coating to protect the fiber from mechanical damage. One source of potential damage is the proof-test procedure during manufacture, whereby the coated fiber is contacted by wheels or other members while under stress. Toughness is defined herein as the area under the stress-strain curve obtained on microtensile specimens (ASTM D-1708) run at 50 mm/minute (about 225 percent/minute). Toughness measurements indicate a strong dependence of toughness on both the SEBS rubber and coumarone-indene resin content. These data show that the formulation A'B'C'D' shown in FIG. 2 meets the 500 psi toughness requirement. The line C'D' defines formulations having the minimum toughness, about 500 psi. The blend of point B' gives a maximum toughness of approximately 1300 psi. (As a comparison, one commercial silicone material marketed for optical fiber coating has a toughness of about 550 psi and an elongation of about 150 percent in this test.) However, to protect the fiber from proof-test or other damage, including microblending loss, the thickness of one or more coated layers can be increased compared to the Examples herein, while relaxing the toughness requirement noted. A lower proof-test level also relaxes the toughness requirement. The coated fibers of the Examples herein using formula A have been found suitable for proof-testing at 70,000 psi, and even higher values appear feasible.

Formulation A in Table II was evaluated as a buffer (inner) coating on optical fiber drawn from germanium phosphosilicate preforms in Examples 1 and 2 below. A UV curable epoxy acrylate overcoat with a 30 minute tensile relaxation modulus of approximately 6.times.10.sup.9 dynes/cm.sup.2 at 23 degrees Celsius and approximately 3.times.10.sup.10 dynes/cm.sup.2 at -40 degrees Celsius was used as the outer coating. Both coatings were applied in-line with drawing. The formulation A was filtered prior to application through a stainless steel cartridge filter having a 5 micron rating. Filtering is desirable to reduce the occurrence of particles in the melt, which can degrade the tensile strength of the coated fiber. Both the inner and outer coatings were applied by passing the fibers through two successive open cup coating applicators having flexible tips. The temperature of formulation A in the cup was about 190 degrees Celsius. In all the tests, fiber coated only with the UV material was used as a control.

Fiber having a 110 .mu.m OD and with a 2/1 cladding/core ratio (i.e., ratio of the outer diameters of the cladding and core) was coated as described above. The dual-coated fiber had an inner coating OD of about 175 .mu.m and a total-coated OD of about 230 .mu.m. The control fiber had a coated OD of about 230 .mu.m.

Two ribbons approximately 1 km long were manufactured, one with each coated fiber type. Each ribbon consisted of 12 fibers sandwiched contiguously between two layers of a polyester pressure sensitive coated tape. (This "ribbon" configuration is further described in U.S. Pat. No. 4,096,010, coassigned with the present invention.) The average added loss of the fibers in each ribbon was determined at -10, -26, and -43 degrees Celsius. The "added loss" is compared to the same ribboned fiber at room temperature (24 degrees Celsius). These data, shown below, indicate a much lower sensitivity to microbending for the dual-coated fiber:

The fiber and ribbon manufacturing procedure in Example 1 was repeated with the exception that the fiber OD was 125 .mu.m and the cladding/core ratio was 2.5/1. The coated fiber diameter was approximately the same as in Example 1. Three filled cables were manufactured having ribbons of each coated fiber type. The average added loss of the control fiber in going from the ribbon to the cable was 1.67 dB/km, as compared to 0.11 dB/km for the dual-coated structure.

Based upon the above tests, the presently preferred of the above compositions for use as the inner layer of a dual-layer coating has the formula A in Table II above, with an acceptable variation of at least plus or minus 1 weight percent for the major components, and plus or minus 0.5 percent for the antioxidant. Formula A also meets the above criteria as an outer layer coating. This formulation can typically be coated onto an optical fiber at a speed of up to several meters per second when the formulation is at typically about 190-230 degrees Celsius in the coating applicator. The higher temperatures reduce the viscosity of the formulations and allow coating at higher speeds. Formula B in Table II is also acceptable as the inner layer of a dual-layer coated fiber.

Other coating formulations within the range of FIGS. 1 and 2 have also been evaluated. They provide for reduced viscosity at the application temperature as compared to formulation A above; see Table III below. Formulas C and E were measured using a SEBS block copolymer (Kraton G1650) having a viscosity of about 1400 cps according to the Shell test above. Formula C is considered to be within the boundaries of A'B'C'D' as adjusted for the block copolymer viscosity shift previously noted.

The reduced viscosity allows a reduction of the application temperature of from about 5 to 15 degrees Celsius as compared to formulation A. Thus, longer lifetime in the coating applicator is possible. Also, a reduced viscosity at a given temperature facilitates higher fiber coating speeds. Formulas C, D, and E all meet the criteria for an inner coating of a dual-layer coating fiber; formulation C is presently preferred, due to its higher toughness and adhesion. In addition, formula C meets the requirements of an outer (or single layer) coating. The major components of formula C can vary at least plus or minus 1 weight percent, and the antioxidant can vary at least plus or minus 0.5 weight percent for acceptable performance. Formula D, which omits the coumarone-indene resin, is preferred in situations wherein low adhesion to the fiber is desired, as when ease of stripping the coating of the fiber by mechanical means is important. The toughness of formula D is somewhat lower than the others, due to the lower molecular weight of Kraton G1652 as compared to Kraton G1650. This is reflected in the viscosity of the Kraton G1652 used, being 475 in the above-noted Shell tests.

A nitrogen atmosphere can be used to extend the lifetime of the material in the applicator and reservoir, and is desirable if the material is heated above about 200 degrees Celsius.

It is apparent that the value of the inventive coating resides largely in the optical fiber coated thereby, due to its improved performance in operation, in many cases. This in turn relates to economies of the entire optical fiber system. For example, low optical loss over a wide temperature range relaxes the system signal margin requirements. Further value resides in the ease and economies of the coating operation itself. While preferred components and formulation ranges have been disclosed herein, persons of skill in the art can extend these ranges using appropriate material, according to the principles discussed herein. Furthermore, other coating sequences, such as a relatively low modulus hot melt inner layer followed by a high modulus hot melt outer layer, are possible. All such variations and deviations which rely on the teachings through which the present invention has advanced the art are considered to be within the spirit and scope of the present invention.