CA2066948A1

CA2066948A1 - Optical fiber cable having dripless, non-bleeding and optical fiber coating-compatible waterblocking material in core thereof

Info

Publication number: CA2066948A1
Application number: CA002066948A
Authority: CA
Inventors: Ching-Fang Tu
Original assignee: Ching-Fang Tu; American Telephone And Telegraph Company
Current assignee: AT&T Corp
Priority date: 1991-04-26
Filing date: 1992-04-23
Publication date: 1992-10-27
Also published as: JPH05157952A; TW209299B; EP0510967A3; US5187763A; EP0510967A2

Abstract

Abstract An optical fiber cable (20) includes a core which (22) is filled with a filling material (26) comprising a composition of matter which passes industry wide drip tests, which substantially has no oil separation and which has excellent thermal oxidative stability. The filling composition includes a relatively high molecular weight aliphatic hydrocarbon which may be a polyalphaolefin, for example, or a relatively high molecular weight mineral oil. Also included in the composition of the filling material is a fumed silica, a block copolymer and a relatively high amount of an antioxidant system. Swelling of coating materials for optical fibers (25,25) which are relatively soft and which are in contact with the filling material is substantially less than that experienced with prior art filling materials.
Because of the inclusion of a low pour point aliphatic hydrocarbon in the filling material, the cable provides excellent optical performance at low temperatures.

Description

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Technical Field This invention relates to an optical ~lber cable having a dripless, non-bleeding and optical fîber coatillg-compatible waterblocking material in a core thereof.
5 Backgro~nd of the Inv~n~ion Optical fiber cables have made great inroads into the communications cable market. Although the presence of water itself within an optical fiber cable is not necessarily detrimental to its performance, passage of the water along the cable interior to connection points or 10 terminals or associated ecluipment inside closures, for example, may cause problems especially in freezing environments and should be preve1~ted.
Consequent!y, it should be no surprise that cables for transmitting communications signals must meet industry standards with respect to waterblocking provisions.
In the prior art, various techniques have been used to prevent the ingress of water through the sheath system of a cable and along the core. For example, a metallic shield which often times is used to protect a metallic conductor cable against lightning and rodent attacks is provided with a sealed longituclinal seam. However, the forming of a shield about a 20 cable core requires the use of relatively low manufacturing line speeds.
~lso, the use of a metallic shield is destructive of the otherwise all-dielectric property of an optical i~lber cable. Further, lightning strikes may cause holes in a metallic shield.
It is not uncommon to include provisions in addition to or as an 2S alternative to a metallic shield for preventing the ingress of water into thecore. Waterblocking Materials have been used to fill cable cores and to coat portions of cable sheath systems to prevent the movement longitudinally thereof of any water which enters the cable. Although the use of such a material, which typically is referred to as a fllling material and which 30 typically is in the form of a grease-like composition of matter, causes housekeeping problems for field personnel during splic;ng operations, for example, it contillues to be used to prevent entry of the water into the core.
In optical fiber cables, a further important function of a filling material is the maintenance of the optical fibers in a low stress state.

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A cable filling material, especially an optical ~lber cable rllling material, should meet a variety of requirements. Among them is the requirement that the physical properties of the cable remain within acceptable limits over a rather wide temperature range e.g., from about -40 S to about 76 (~. It is desirable that the composition of matter of the fillingmaterial be substantially free of syneresis, i.e. have an ability to retain uniform consistency, over the temperature range.
Further complicating the optical rlber cable situation is the introduction of a waterblocking rllling material into the cable core in order 10 to prevent the incursion of water. Suitable waterblocking materials in use must yield under strains experienced when the cable is made or handled.
Otherwise, movement of the optical ~lbers within the cable ~Ivould be prevented and the rlbers would buc~ile because they contact, with a relative small periodicity, a surface of the unyielding rllling material. The smaller 15 the periodicity of the ~lbers when contacting such an unyielding surface, the greater a loss which is referred to as microbending loss. Typically~
microbending loss in optical fiber cables is more difrlcult to control at long wavelengths than at short ones.
Filling compositions for use in optical fiber cables should have a 20 relatively low shear modulus, ~:e. However, it has been determined that, at least for some applications, a low value of Ge of the filling material is not suf~lcient to assure low cabling loss, and that a further parameter, the critical yield stress, ~c, needs to be controlled because it also affects the optical performance of fibers in a cable filled with a grease-like composition 25 of matter. A grease-lil;e filling composition of matter having a relatively low critical yield stress is disclosed in U.S. Pat. No. ~,701,016.
Waterproofing filling materials for use in cables also must pass industry standard drip tests. To pass these tests, filling materials in cable cores must be retained as cable samples, suspended vertically, are subjected 30 to specif~led elevated temperatures. Some prior art materials, which have been used, perfol m satisfactorily with respect to microbending and associated losses, but they bleed out excessively and have problems in meeting current drip tests. Also, it is desired that the low mean added losses exhibited by some prior art filling materials at least be met by filling 35 materials which pass the drip test and have suitable low temperature properties.

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Oil separation is a property of a grease-like material which descr;bes the tendency to bleed oil during its lifetime. What is desired is a - filling material which has an oil separation no greater than 30~0 when centrifuged at 10,000 rpm for one hour.
Because cable drip is related to oil separation, constraints on the sought after filling material include oil separation, critical yield stress and viscosity. The vjscosity of the sougllt after filling material also is importantwith respect to processing. These constraints usually are antagonistic to each other. For example, a reduction of oil separation and an increase in 10 cable drip temperature require high viscosity and yield stress whereas to facilitate processing and to reduce optical loss requires low viscosity and yield stress.
Another problem relating to filled optical fiber cables is the compatibility of the filling material with some coating materials which are 15 disposed about drawn opt;cal fiber to protect the optical fiber. If compatibility is lacking, the per~orll1ance and/or the appearance of the optical fiber could be affected adversely. The compatibility of otherwise suitable prior art filling materials with some coating materials, particularly those which are relatively soft, is something less than desired.
~lthough some pr;or art compositions of matter are suitable for filling cable cores comprising optical fibers each having layers of particular coating materials thereon, the pr;or art does not appear to include a cable filling material which is suitable for filling cable cores which include opticalfiber coated with some of the softer coating materials used today. What is 25 sought after and what does not appear to be disclosed in the prior art is an optical fiber cable filling composition of matter which is compatible with a broad range of optical fiber coating materials, which does not bleed and which does not drip from the cable core at specif~led elevated temperatures and one which does not exacerbate optical loss.
30 _lmmary oî the Invention The foregoing problems of the prior art have been solved by a cable of this invention as set forth in claim 1.
Brief l)escription of the Drawin~
FIG. 1 is a perspective view of an optical fiber cable of this 35 invention which has a core in which is disposed a fïlling material;

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2~69~8 li`IG. 2 is an end view in section Or the cable of FIG. l;
FIGS. 3 and ~ are graphs wh;ch depict the effects of a fumed silica and a copolymer eonstituent on the viscosity of the filling material;
FIGS. 5 and 6 are graphs which depict the effect of a fumed 5 silica and a copolymer constituent on oil separation of the filling material;
FIG. 7 depicts a generalized stress-strain curve of a ~lling material;
FIGS. 8 and ~ are graphs wh;ch depict the effect of a fumed silica and a copolymer constituent on yield stress; and FIG. 10 is a graph which depicts the effect of varying amounts of a fumed silica and a copolymer constituent on a drip test.
Detailed Description Referr;ng now to FIGS. 1 and 2, there is shown a communications cable which is designated generally by the numeral 20 and 15 which has a longitudinal axis 21. It includes a core 22 comprising optical fibers 25-25 which may be arranged in one or more units 24-24. Each of the optical rlbers is provided with a protective coating system which typically includes an inner ~rimary coating layer and an outer secondary coating layer. Also, each of the coated ilbers may be buffered with an outer layer of 20 polyvinyl chloride (PVC), for example. Each of the units 24-24 may be wrapped with a binder ribbon 23. The core 22 includes a waterblocking material 26 which is disposed within a tubular member 28 of a sheath system 27. The tubular member 28 often is referred to as a core tube.
The tubular member 28 may be enclosed by a metallie shield 2 25 and an outer plastic jacket 32. The sheath system 27 also may include strength members 30-30. Also, a waterblocking tape 35 may be wrapped about an outer surface of the core tube 2~. The tape 35 may be a waterblocking tape which is disclosecl, for example, in U.S. patent 4,867,526.
Also, the filling material 26 may be used to iill the core of a cable which 30 inclucles optical fiber ribbons such as those disclosed in U.S. patent 4,~00,176. names of K. W. Jackson, et al.
Constraints on the sought after filling material which includes an oil constitutent include oil separation, and associated cable drip temperature, critical yield stress and viscosity of the filling material. As 35 mentioned hereinbefore, these constraints usually are antagonistic to each othel. Priorly, it has been demonstrated that low pour point oils produee , ~ ;

, 2 ~ 8 filling materials the cl~itical yield stress o~ which at low temperatures decreases with decrea~sing pour point. The pour point of a material is the lowest temperature at which a sample of the material may be poured.
Theoretically, the use of a low pour point oil is conducive to the re~luctlon 5 of optical loss at low temperatures. Cable construction and cable processing conditions also affect the optical performance of fibers and, therefore, the benefit of a low pour point oil may become obscured.
The critical yield stress of a ~ g rnaterial is considered to affect the optical performance of fibers in a cable rllled with the fllling 10 material. The prior art rllling material typically has a critical yield stress of 0.001¢ psi at room temperature and 0.0096 psi at -40 C. The critical yield stress of the ~llling materisl 26 should be such that it does not cause an increase in optical fiber loss over that of prior art ~llling materials at all temperatures.
lS The viscosity requirement is needed to accommodate processing, not cable performance. The viscosity of prior art filling material as measured by a helipath viscometer should be 15 to 45 units using spindle TB at room temperature. In order to assure the waterhead resistance of an optical rlber cable, it is preferred to have the helipath viscosity in excess o~20 20 units. It is desired that the viscosity of the rllling material be in the vicinity of that of prior art rllling materials so that presently available processing facilities can be used.
The composition of matter of the fllling material 26 which is used to rlll interstices in the core of the cable 20 and which meets the 25 foregoing requirements includes an oil constituent system in the range of about 85 to about 02 percent by weight. A suitable oil constituent is a relatively high molecular weight aliphatic hydrocarbon. By relatively high in this description is meant a molecular weight in excess of about 600.
The aliphatic hydrocarbon constituent may be a relatively high 30 molecular weight minel al oil such as Sunpar 2280 available from the Sun Rerlning and Marketing Co., or Tufflo 80 mineral oil available from the Shell Chemical Company, for example. In the alternative, the aliphatic hydrocarbon constituent may be a synthetic oil such as, polyalphaolerln oil, polypropene oil or polybutene oil for example. Mixtures of polyalphaolefln 35 with mineral oils and polybutene oils also may be used. In a preferred embodiment, the composition includes about 87% by weight of a , - . ~- ~ : . :

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polyalphaole~ln such as HITEC 17~1 oil available from the Ethyl Corporation or S~lF ~l01 Oil available from the I~obil Corporat;on. The synthetic oil of the prcferred embodiment is a hydrogenated oligomer of alpha-decene and has an average molecular weight of 1280. The viscosity of the oil at 100 C
5 is approximately '10 centistokes. It has a pour point of less than -34 C.
The polyalphaolefin aliphatic hydrocarbon also may be one which is characterized by a viscosity in the range of about 10 centistokes at 100 C . Suitable polybutene oils have a viscosity in the range of 1~0 to 300 centistokes whereas a suitable mineral oil has a viscosity greater than 150 10 SUS which equates to about 35 centistokes. If it has a viscosity substantially greater than 10 centistokes, such as, ~or example, ~0 cent;stokes, the filling material may become more compatible with the coated optical fiber. ~lso, if the viscosity is less than about l0, for example 8, the percent swelling of the primary coating material on the optical fiber 15 may increase to about 42~ which exceeds the presently allowable 40%.
The oil constituent neecls to be thickened so that it will not run out of a cable and so that oil separation is reduced. Oil separation or syneresis is a property of a grease-like fill;ng material which describes the tendency to bleed oil during the lifetime of the filling material. Qne prior 20 art filling material is known to separate oil if left undisturbed for a certain period of time. The syneresis is usually a slow process and, therefor, has to be determined by an accelerated method, centrifugation. As mentioned hereinbefore, it is desired that the rllling material 26 be characterized by a 30% maximum oil separation when centrifuged at 10,000 rpm 25 ~approximately 12000 G) for one hour~ In order to accomplish this, inorganic and organic thickening agents are included in the composition of the filling material.
Colloidal fillers are used as inorganic thickening agents to adjust the yield stress of the composition. Colloidal fîller particles in oil gel the oil 30 by bonding surface hydroxyl groups to form a network~ Such gels are capable of supporting a load below a critical value of stress. Above this stress level, the netwolk is disrupted, and the material assumes a liquid-like character and flows under stress. Such beha~rior often is referred to as thixotropic and is desirable to facilitate processing.

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Colloidal flllers useful in the cable 20 include colloidal silica, either hydrophilic or hydrophobic, preferably a hydrophobic fumed silica having a BET surface area between about 50 and about ~100 m2 /gm. The higher the surface area, the lower the oil separation. An increase in the 5 fumed silica level decreases oil separation, but adversely increases the critical yield stress and the viscosity of the grease. An example of a hydrophobic fumed silica is a polydimethylsiloxane-coated fumed silica having a BET surface area of about 8~120 m2 / gm, containing about 5%
b.w. carbon, and being available from the Cabot Corporation of Tuscola, Ill.
10 under the trade designation Cab-O-Sil TS720. An exemplary hydrophilic colloidal material is fumed silica with a BET surface area of about 17~225 m2 /gm, nominal particle size of 0.012 ~m, and a specirlc gravity of 2.2, available form the Cabot Corporation under the designation Cab-O-Sil M-5.
~)ther colloidal fillers that may be useful in the practice of the invention arelS precipitated silicas and clays such as bentonites, with or without surface treatment. In the preferred embodiment, a hydrophobic fumed silica such as the Cab-O-Sil TS720 fumed silica in the amount of about 5 to 8 percent by weight is used as the inorganic thickening agent.
Oil retention of the ~llling material 26 may be improved by the 20 addition of one of more organic thickening agents or bleed inhibitors to the composition. Copolymers used as bleed inhibitors are known to reduce the oil separation of a grease-like filling material, and, unlike fumed silica, doesnot contribute as much to increasing yield stress and viscosity.
The bleed inhibitor may be a block copolymer, a relatively high 25 viscosity semiliquid, sometimes referred to as semisolid, rubber, or other appropriate rubber. Block copolymers and semiliquid rubbers may be referred to collectively as rubber polymers. Incorporating a rubber polymer into the grease-like composition of matter allows a reduction in the amount of colloidal particles that must be added to the mixture to prevent syneres;s 30 of the gel and can result in cost savings. Furthermore, it makes possible theformulation of nonbleeding compositions having a relatively low critical yield stress.
Among the block copolymers that can be used in waterblocking compositions for the cable of the invention are styrene-rubber and styrene-35 rubber-styrene block copolymers having a styrene/rubber ratio between approximately 0.1 and 0.8 and a molecular weight, as indicated by viscosity : . i ., ,,: -~

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in to}uene at 25 C, of from about 100 cps in a 20~o b.w. rubber solution to about 2000 cps in a 15~ b.w. rubber solution. Exemplary block copolymers are (a) a styrene-ethylene propylene block copolymer (SEP), unplasticized, having a styrene/rubber ratio of about 0.5~, a spec;fic gravity of about 0.~3, 5 a break strength per ASTM D-412 of 300 psi, and being available from the Shell Chemical Company of Houston, Texas, under the trade designation Kraton G1701; (b) a styrene-ethylene propylene block copolyrner having a sytrene to rubber ratio of about 0.39 and available from the Shell Chemical Company under the desi~nation G1702; lc) styrene-ethylene butylene-10 styrene block copolymer ~SEBS), unplasticized, and h~ving a styrene/rubberratio of about 0.16, a specific gravity of about 0.~0, 750% elongation, 30û%
modulus per ASTM D-412 of 350 psi, and being available frorn the Shell Chemical Corporation under the trade designation l~raton G1657 and (d) a diblock copolymer of ethylene and propylene (EP) available from the Shell 15 Chemical Company under the designation G1750. Another copolymer which may be used is Kraton 1726 copol~mer which is a mixture of 30%
styrene-ethylene butylene-styrene triblock copolymer (SEBS) and ~0%
styrene-ethylene butylene diblock copolymer (SEB). The preferred embodiment includes I~raton G 1701 block copolymer.
Also included in the composition of the filling material 26 is an antioxidant system in the arnount of about 1-2% by weight. The antioxidant constituents are high molecular weight7 hindered phenolic antioxidants which are relatively soluble in mineral oil. An acceptable antioxidant is one available from the Ciba-Geigy Company under the trade 25 designation Irganox 1035. In a preferred embodiment, the rllling composition includes 0.3$~o by weight of Irganox 1035 antioxidant and 1.7%
by weight of Irganox 1076 antioxidant, the latter constituent being used to prevent the antioxidant from settling out. The solubility of Irganox 1035 antioxidant in mineral oil is about 0.30 g/100 ml and that of Irganox 1076 is 30 12 g/100 ml at 22 c C. Another suitable non-precipitating antioxidant is Irganox 1520 high molecular weight liquid antioxidant, also available from the Ciba Geigy Cornpany.
Exemplary compositions oï this invention are shown in TABLES
I II, III, IV, and V, with the constituents being given in percent by weight.
35 A summary oï properties also is presented in each TABLE. Included in the TABLES are measurements of the swell of the primary optical fiber coating .

2 ~ 8 material, viscosity, oil separation and yielcl stress at room temperature.
Cable drip test results at 65 C are also provided in some of the TABLES.
In TABLE I, the composition example designated (F) meets all the desired properties. The yield stress is higher than that of a presently S used filling material but is acceptable based on loss results in a cable having such a filling material. Cables ~Illed with this composition of matter passed the drip test at 70 C.

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The test results indicate that high molecular weight oils are required to prevent some presently used optical rlber coatings from swel}ing.
The higher the molecular weight of the mineral oil, the higher the pour point. Test results have shown that a low viscosity polyalphaolefin oil 5 swelled the primary coating 36% but that a high viscosity polyalphaolefin oil, such as HITEC 174, for example, only swelled the optical fiber primary coating material approximately S%. At approximately the same molecular weigh$, polyalphaolerln oil has a lower viscosity than other oils and thus ~Illing materials made from these oils have a lower viscosity than filling 10 materials made from other oils.
FIGS. 3 and ~ show the effect of Cab-O-Sil TS720 fumed silica and Kraton G1701 copolymer, respectively, on the viscosity of filling materials made with HITEC 174 oil. As can be seen, the effect of the fumed silica is pFonounced when it is more than 5% by weight, while the effect of 15 the copolymer becomes more pronounced if it contains more than 3%.
For a filling material which includes ~IITEC 17~1 polyalphaole~ln oil, fumed silica does not reduce tlle oil separation without an adverse increase in viscosity and critical yield stress. A block copolymer was added to reduce further the oil separat;on and also to mitigate the viscos;ty 20 increase. Unlike fumed silica, the block copolymer does not contribute as much as fumed silica in increasing yield stress and viscosity. The efïects oî
Cab-O-Sil TS720 fumed silica and Kraton G1701 copolymer on the oil separation of filling materials are shown in FIGS. 5 and 6 respectively.
Without the copolymer, the fumed silica is not effective in reducing o;l 25 separation. Also, without the fumed silica, the filling material even with high levels of the block copolymer tends to flow. Therefore, the fumed silica and the block copolymer should be used together and their ratio optimized.
Advantageously, the filling material 26 which is used to fill the core of a cable of this invention yields at a low enough stress so that the 30 optical fibers 2~-2~1 and units 22-22 are capable of moving within the core when the cable is loaded or bent. The yielding filling material allows the optical fibers to move within the tube 28 which reduces the stress therein and lengthens the life of the optical fibers.
FIG. 7 shows a generalized stress-strain curve 37 at constant 3S strain rate for a thixotropic material such as that used as the waterblockingmaterial 26, and identifies several irnportant parameters. Along a segment , ., :: -: :
: ,-; ' :, ~ , 2~fi~8 38 of the stress-strain curve 37, the material acts essentially as an elastic solid. The segment extends from zero stress to the critical yield stress c~c.
The strain corresponding to ~c is identified as Yc, the critical shear strain.
13y definition, the coordinates aC and ~c indicate the onset of yielding and 5 the quantity crCl~c (or dcT/dry for Y < Yc) is known as the shear modulus, Ge~ of the material.
A segment 3~ of the stress-strain curve of FIG. 7 represents increasing values of incremental strain for increasing stress. The stress ~y is the maximum value of stress sustainable by the material at a given strain 10 rate with ~y being the corresponding strain. For strains in excess of oy, thestress at first decreases as shown by a segment 40, becoming substantially independent of strain for still greater values of strain as shown by a segment 41. The waterblocking material thus exhibits a liquid-like behavior ror a>C~y~
FIGS. 8 and 9 show the effect of Cab-O-Sil TS720 fumed silica and I~raton G 1701l copolymer on the yield stress of filling materials. Frorn the slopes of the curves in FIGS. 8 and ~), it should be apparent that the effect of Cab-O-Sil fumed silica is greater than that of Kraton G1701 copolymer. For cables to pass a 65 C no drlp requirement, the yield stress 20 of the filling material may, in most instances, be at least about 0.003 psi.
The composition of the filling material 26 unexpectedly results in excellent properties. It would be expected that to increase the drip temperature, the yield stress and hence the viscosity would have to be increased, perhaps to unacceptable levels. Unexpectedly, the filling material 25 of cable of this invention provides excellent results notwithstanding its relatively low viscos;ty. The bleed inhibitor performs several functions; not only does it reduce oil separation, the bleed inhibitor also keeps the viscosity low and increases the yield stress but not as much as the fumed silica.
Also, it should be observed that the level of the antioxidant constituent is relatively high. This provides a reservoir Or antioxidant which increases the oxidative stability of the tubular member 28 and optical fiber coatings to prevent prernature degradation of the optical fiber cable.
The filling material 26 of this invention has enhanced 35 perforrnance at low temperature because of the use of a low pour pOil1t oil, has a relatively high cable drip temperature ancli very low oil separation.

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The filling material ~(; is compati~le with presently used fiber coating materials and other cable materials which it contacts. There is no bleeding of o;l and it is expected that the optical loss at -40 C will not exceed that of the prior art rllling materials.
The test results show that a filling material made with an increase in fumed silica level in mineral oil, although reducing the oil separation and greatly increasing the viscosity, was still unable to pass the 65 C cable drip test. Apparently, fumed silica as the only thickening agent in a mineral oil based f~llling composition of matter cannot enable a cable to 10 pass the drip test without an adverse viscosity increase. To avoid this result, a thermoplastic rubber is used ;n combination with fumed silica.
Also interesting is that at the same fumed silica level, a higher viscosity mineral oil produced filling materials having a viscosity lower than those prepared by a lower viscosity mineral oil.
In FIG. 10 is shown the effect of Cab-O-Sil TS720 fumed silica and I~raton G 1701 copolymer on the drip test. The compositions on the left s;de of the curve passed the 65 C drip test whereas those on the right failed.
As stated before, what had been sought after and what has been 20 achieved is a filling material in which oil separation has been reduced, cable drip temperature has been increased, optical fiber coating swell has been reduced, in which low temperature optical loss has been reduced or maintained at current levels, and in which proc&ssing characteristics of the fllling material disclosed in previously mentioned U.S. 4,701,016 were 25 retained. The goal was to provide a filling material which has zero oil separation at 15,000 rpm for two hours using an IEC model centrifuge. The filling material of the preferred embodiment satisrles this requirement.

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Claims

1. An optical fiber cable, said optical fiber cable comprising a core comprising at least one optical fiber transmission medium, a sheath system which is disposed about said core, and a filling composition of matter which is disposed in said core, said filling composition of matter being characterized by at least about 85% by weight of an aliphatic hydrocarbon having a molecular weight of at least 600, a block copolymer and a relatively high amount of an antioxidant system.

2. The optical fiber cable of claim 1, wherein said relatively high molecular weight aliphatic hydrocarbon is selected from the group consisting of a synthetic oil, a relatively high molecular weight mineral oil and a mixture comprising a synthetic oil and a relatively high molecular weight mineral oil.

3. The optical fiber cable of claim 1, wherein said synthetic oil is selected from the group consisting of polyalphaolefin, a polybutene, and a polypropene.

4. The optical fiber cable of claim 1, wherein said composition of matter also includes an inorganic thickening agent and an organic thickening agent.

5. The optical fiber cable of claim 4, wherein said inorganic thickening agent comprises a hydrophobic fumed silica.

6. The optical fiber cable of claim 5, wherein said organic thickening agent comprises a mixture of 70% of a diblock copolymer comprising a polystyrene block and a poly (ethylene - propylene) block having a styrene-rubber ratio in the range of about 0.39 to 0.59 and 30% of a triblock copolymer.

7. The optical fiber cable of claim 5, wherein said organic thickening agent comprises a mixture of a 30% triblock copolymer and 70%
of a diblock copolymer.

8. The optical fiber cable of claim 5, wherein said organic thickening agent comprises a diblock copolymer of ethylene and propylene.

9. The optical fiber cable of claim 1, wherein said antioxidant system comprises a high molecular weight, hindered phenolic antioxidant which is relatively soluble in mineral oil.

10. The optical fiber cable of claim 1, wherein said antioxidant system comprises a high molecular weight, hindered phenolic antioxidant which is non-precipitating in the filling composition of matter.