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

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.

TECHNICAL FIELD 
This invention relates to an optical fiber cable having a dripless, 
non-bleeding and optical fiber coating-compatible waterblocking material 
in a core thereof. More particularly, this invention relates to an optical 
fiber cable having a core in which a composition of matter which is 
grease-like fills interstices in the core. 
BACKGROUND OF THE INVENTION 
In the cable industry, it is well known that changes in ambient conditions 
lead to differences in water vapor pressure between the inside and the 
outside of a plastic cable jacket. This generally operates to diffuse 
moisture in a unidirectional manner from the outside of the cable to the 
inside of the cable. Eventually, this will lead to an undesirably high 
moisture level inside the cable, especially if a plastic jacket is the 
only barrier to the ingress of the moisture. High levels of condensed 
moisture inside a cable sheath system may have a detrimental effect on the 
transmission characteristics of a metallic conductor cable. 
Furthermore, water may enter the cable because of damage to the cable which 
compromises its integrity. For example, rodent attacks or mechanical 
impacts may cause openings in the sheath system of the cable to occur, 
allowing water to enter, and, if not controlled, to move longitudinally 
along the cable into splice closures. 
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 terminals or 
associated equipment inside closures, for example, may cause problems 
especially in freezing environments and should be prevented. 
Consequently, it should be no surprise that cables for transmitting 
communications signals must meet industry standards with respect to 
waterblocking provisions. For example, one industry standard requires that 
there be no transmission of water under a pressure head of one meter in 
one hour through a one meter length of cable. 
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 longitudinal seam. However, the forming of a shield about a cable 
core requires the use of relatively low manufacturing line speeds. Also, 
the use of a metallic shield is destructive of the otherwise 
all-dielectric property of an optical fiber cable. Further, lightning 
strikes may cause holes in a metallic shield. 
It is not uncommon to include provisions in addition to or as an 
alternative to a metallic shield for preventing the ingress of water into 
the core. 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 filling 
material and which typically is in the form of a grease-like composition 
of matter, causes housekeeping problems for field personnel during 
splicing operations, for example, it continues 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. 
A grease-like composition of matter typically is a semisolid or semiliquid 
substance comprising a thickening or gelling agent in a liquid carrier. 
The gelling agents used in greases frequently are fatty acid soaps, but 
high melting point materials, such as clays, silica, organic dyes, 
aromatic amides, and urea derivatives also are used. Nonsoap thickeners 
are typically present as relatively isometric colloidal particles. All 
types of gelling agents form a network structure in which the carrier is 
held by capillary forces. 
When a low stress is applied to a grease-like material, the material acts 
substantially as a solid. If the stress is above a critical value, then 
the material flows and the viscosity decreases rapidly. The decrease in 
viscosity is largely reversible because it is typically caused by the 
rupture of network junctions between the filler particles, and these 
junctions can reform following the release of the critical stress. 
A cable filling material, especially an optical fiber cable filling 
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.degree. to about 76.degree. C. It is desirable that the composition of 
matter of the filling material be substantially free of syneresis, i.e. 
have an ability to retain uniform consistency, over the temperature range. 
Generally, syneresis is controlled by assuring dispersion of an adequate 
amount of collodial particles or other gelling agent. Other desirable 
properties of grease-like compositions include thermal oxidation 
resistance. 
Further complicating the optical fiber cable situation is the introduction 
of a waterblocking filling material into the cable core in order 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 fibers within the cable would be 
prevented and the fibers would buckle because they contact, with a 
relative small periodicity, a surface of the unyielding filling material. 
The smaller the periodicity of the fibers 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 difficult to 
control at long wavelengths than at short ones. Thus the requirements on 
the mechanical properties of a fiber cable filling material are typically 
substantially more severe for cable that is to be used at 1.55 .mu.m, for 
example, than they are if the cable is to be used at shorter operating 
wavelengths of 1.3 .mu.m, for example. Although, it has been found that 
some prior art filling materials perform quite satisfactorily at 
wavelengths up to about 1.3 .mu.m, it has also been found that this is 
often not the case at longer wavelengths. 
Because silica-based optical fibers typically have their lowest losses at 
or near the 1.55 .mu.m wavelength, there is great interest in operating 
optical fiber telecommunication systems at approximately that wavelength. 
Thus, it is important to have available optical fiber cable that has no 
significant cabling-induced losses at long wavelengths, including about 
1.55 .mu.m. 
Filling compositions for use in optical fiber cables should have a 
relatively low shear modulus, Ge. However, it has been determined that, at 
least for some applications, a low value of G.sub.e of the filling 
material is not sufficient to assure low cabling loss, and that a further 
parameter, the critical yield stress, .sigma..sub.c, needs to be 
controlled because it also affects the optical performance of fibers in a 
cable filled with a grease-like composition of matter. 
A grease-like filling composition of matter having a relatively low 
critical yield stress is disclosed in U.S. Pat. No. 4,701,016 which issued 
on Oct. 20, 1987 in the names of C. H. Gartside, III, et al. and which is 
incorporated by reference hereinto. The composition comprises oil, a 
gelling agent such as colloidal particles, and, optionally, a bleed 
inhibitor. It includes 93% by weight mineral oil and 7% by weight of 
hydrophobic formed silica. Among oils useful in the practice of the 
invention are ASTM type (ASTM D-226 test) 103, 104A, or 104B (or mixtures 
thereof) naphthenic oils having a minimum specific gravity of about 0.860 
and a maximum pour point (ASTM D97) of less than approximately -4.degree. 
C., and polybutene oils of minimum specific gravity of about 0.83 and a 
maximum pour point (ASTM D97) of less than about 18.degree. C. The 
colloidal particle filler material preferably comprises silica particles. 
Preferred bleed inhibitors are styrene-rubber or styrene-rubber-styrene 
block copolymers, and/or semiliquid rubbers, such as a high viscosity 
polyisobutylene. Other ingredients, such as, for example, a thermal 
oxidative stabilizer, may be present. The critical yield stress of the 
filling material of U.S. Pat. No. 4,701,016 is not greater than about 70 
Pa, measured at 20.degree. C. whereas the shear modulus is less than about 
13 kPa at 20.degree. C. 
Incorporating a block copolymer into the grease-like composition of matter 
allows a reduction of the amount of colloidal particles that has been to 
added to the mixture to prevent syneresis of the gel. This reduction can 
result in cost savings. Furthermore, it makes possible the formulation of 
less bleeding compositions having a very low critical yield stress. 
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 to 
specified elevated temperatures. Some prior art materials, which have been 
used, perform 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 means added losses 
exhibited by some prior art filling materials at least be met by filling 
materials which pass the drip test and have suitable low temperature 
properties. 
Oil separation is a property of a grease-like material which describes the 
tendency to bleed oil during its lifetime. What is desired is a filling 
material which has an oil separation no greater than 30% 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 viscosity of the sought after filling material also is 
important with respect to processing. These constraints usually are 
antagonistic to each other. For example, a reduction of oil separation and 
an increase in 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 disposed about drawn optical fiber to protect the optical fiber. If 
compatibility is lacking, the performance 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. 
Although some prior art compositions of matter are suitable for filling 
cable cores comprising optical fibers each having layers of particular 
coating materials thereon, the prior art does not appear to include a 
cable filling material which is suitable for filling cable cores which 
include optical fiber coated with some of the softer coating materials 
used today. What is 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 
specified elevated temperatures and one which does not exacerbate optical 
loss. 
SUMMARY OF THE INVENTION 
The foregoing problems of the prior art have been solved by a cable of this 
invention having a filling composition of matter disposed in a core 
thereof. A cable of this invention includes a core comprising a plurality 
of coated optical fibers and a filling composition of matter which is 
disposed about the fibers. Typically, the fibers and the filling material 
are disposed within a tubular member which is disposed within a sheath 
system. The sheath system includes longitudinally extending strength 
members and a plastic jacket. 
The filling composition of matter comprises at least about 85 percent by 
weight of an oil constituent which is a relatively high molecular weight 
aliphatic hydrocarbon, the molecular weight of which is at least about 
600. The aliphatic hydrocarbon constituent may be a synthetic oil such as 
polyalphaolefin, for example, or a relatively high molecular weight 
mineral oil. Relatively low pour point oils are used in order to improve 
optical loss at low temperatures. A thickening system which includes an 
inorganic constituent and a block copolymer is used to reduce the 
viscosity of the filling material as well as to reduce oil separation. 
Also a relatively large percent by weight of an antioxidant system is used 
to prevent thermal oxidative degradation of the filling material as well 
as of materials in contact with the filling material.

DETAILED DESCRIPTION 
Referring now to FIGS. 1 and 2, there is shown a communications cable which 
is designated generally by the numeral 20 and 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 fibers is 
provided with a protective coating system which typically includes an 
inner primary coating layer and an outer secondary coating layer. Also, 
each of the coated fibers may be buffered with an outer layer of 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 metallic shield 29 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 28. The tape 35 may be a waterblocking tape which 
is disclosed, for example, in U.S. Pat. No. 4,867,526 which issued on Sep. 
19, 1989 in the name of C. J. Arroyo. Also, the filling material 26 may be 
used to fill the core of a cable which includes optical fiber ribbons such 
as those disclosed in U.S. Pat. No. 4,900,176 which issued on Feb. 13, 
1990 in the 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 mentioned hereinbefore, these constraints usually are antagonistic to 
each other. Priorly, it has been demonstrated that low pour point oils 
produce filling materials the critical yield stress of which at low 
temperatures decreases with decreasing 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 conductive to 
the reduction 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 filling material is considered to affect the 
optical performance of fibers in a cable filled with the filling material. 
The prior art filling material typically has a critical yield stress of 
0.0016 psi at room temperature and 0.0096 psi at -40.degree. C. The 
critical yield stress of the filling materisl 26 should be such that it 
does not cause an increase in optical fiber loss over that of prior art 
filling materials at all temperatures. 
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 
fiber cable, it is preferred to have the helipath viscosity in excess of 
20 units. It is desired that the viscosity of the filling material be in 
the vicinity of that of prior art filling materials so that presently 
available processing facilities can be used. 
The composition of matter of the filling material 26 which is used to fill 
interstices in the core of the cable 20 and which meets the foregoing 
requirements includes an oil constituent system in the range of about 85 
to about 92 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 at least about 600. 
The aliphatic hydrocarbon constituent may be a relatively high molecular 
weight mineral oil such as Sunpar 2280 available from the Sun Refining 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, polyalphaolefin oil, 
polypropene oil or polybutene oil for example. Mixtures of polyalphaolefin 
with mineral oils and polybutene oils also may be used. In a preferred 
embodiment, the composition includes about 87% by weight of a 
polyalphaolefin such as HITEC 174 oil available from the Ethyl Corporation 
or SHF 401 oil available from the Mobil Corporation. The synthetic oil of 
the preferred embodiment is a hydrogenated oligomer of alpha-decene and 
has an average molecular weight of 1280. The viscosity of the oil at 
100.degree. C. is approximately 40 centistokes. It has a pour point of 
less than -34.degree. C. 
The polyalphaolefin aliphatic hydrocarbon also may be one which is 
characterized by a viscosity in the range of about 10 centistokes at 
100.degree. C. Suitable polybutene oils have a viscosity in the range of 
190 to 300 centistokes whereas a suitable mineral oil has a viscosity 
greater than 150 SUS which equates to about 35 centistokes. If it has a 
viscosity substantially greater than 10 centistokes, such as, for example, 
40 centistokes, the filling material may become more compatible with the 
coated optical fiber. Also, if the viscosity is less than about 10, for 
example 8, the percent swelling of the primary coating material on the 
optical fiber may increase to about 42% which exceeds the presently 
allowable 40%. 
The oil constituent needs 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 filling material which describes the 
tendency to bleed oil during the lifetime of the filling material. One 
prior 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 filling material 26 be 
characterized by a 30% maximum oil separation when centrifuged at 10,000 
rpm (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 filler particles in oil gel the 
oil 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 network is disrupted, and the material assumes a 
liquid-like character and flows under stress. Such behavior often is 
referred to as thixotropic and is desirable to facilitate processing. 
Colloidal fillers 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 400 m.sup.2 /gm. The higher 
the surface area, the lower the oil separation. An increase in the 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 80-120 m.sup.2 /gm, containing about 5% 
b.w. carbon, and being available from the Cabot Corporation of Tuscola, 
Ill. under the trade designation Cab-O-Sil TS720. An exemplary hydrophilic 
colloidal material is fumed silica with a BET surface area of about 
175-225 m.sup.2 /gm, nominal particle size of 0.012 .mu.m, and a specific 
gravity of 2.2, available form the Cabot Corporation under the designation 
Cab-O-Sil M-5. Other colloidal fillers that may be useful in the practice 
of the invention are 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 filling material 26 may be improved by the 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, does not contribute as much to increasing yield stress and 
viscosity. 
The bleed inhibitor may be a block copolymer, a relatively high 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 syneresis of the gel and can result in cost savings. Furthermore, 
it makes possible the formulation 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-rubber-styrene block copolymers having a styrene/rubber ratio 
between approximately 0.1 and 0.8 and a molecular weight, as indicated by 
viscosity in toluene at 25.degree. C., of from about 100 cps in a 20% 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 0.59, a 
specific gravity of about 0.93, a break strength per ASTM D-412 of 300 
psi, and being available from the Shell Chemical Company of Houston, Tex., 
under the trade designation Kraton G1701; (b) a styrene-ethylene propylene 
block copolymer having a sytrene to rubber ratio of about 0.39 and 
available from the Shell Chemical Company under the designation G1702; (c) 
styrene-ethylene butylene-styrene block copolymer (SEBS), unplasticized, 
and having a styrene/rubber ratio of about 0.16, a specific gravity of 
about 0.90, 750% elongation, 300% modulus per ASTM D-412 of 350 psi, and 
being available from the Shell Chemical Corporation under the trade 
designation Kraton G1657 and (d) a diblock copolymer of ethylene and 
propylene (EP) available from the Shell Chemical Company under the 
designation G1750. Another copolymer which may be used is Kraton 1726 
copolymer which is a mixture of 30% styrene-ethylene butylene-styrene 
triblock copolymer (SEBS) and 70% styrene-ethylene butylene diblock 
copolymer (SEB). The preferred embodiment includes Kraton G 1701 block 
copolymer. 
Also included in the composition of the filling material 26 is an 
antioxidant system in the amount of about 1-2% by weight. The antioxidant 
constituents are high molecular weight, hindered phenolic antioxidants 
which are relatively soluble in mineral oil. An acceptable antioxidant is 
one available from the Ciba-Geigy Company under the trade designation 
Irganox 1035. In a preferred embodiment, the filling composition includes 
0.3% 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 12 g/100 
ml at 22.degree. C. Another suitable non-precipitating antioxidant is 
Irganox 1520 high molecular weight liquid antioxidant, also available from 
the Ciba Geigy Company. 
Exemplary compositions of this invention are shown in TABLES I, II, III, 
IV, and V, with the constituents being given in percent by weight. A 
summary of properties also is presented in each TABLE. Included in the 
TABLES are measurements of the swell of the primary optical fiber coating 
material, viscosity, oil separation and yield stress at room temperature. 
Cable drip test results at 65.degree. 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 used 
filling material but is acceptable based on loss results in a cable having 
such a filling material. Cables filled with this composition of matter 
passed the drip test at 70.degree. C. 
TABLE I 
__________________________________________________________________________ 
FILLING MATERIAL BASED ON POLYALPHAOLEFIN OILS 
Composition (% by wt) Primary Drip 
Synthetic Oil 
Fumed Silica 
Copolymer Antioxidant 
Coat Hel. 
Oil 
Yield 
Test 
Hitec 
Hitec 
Aerosil 
Cab-O-Sil 
Kraton 
Kraton 
Kraton 
Irganox 
Irganox 
Swell 
Visc. 
Sep. 
Stress 
at 
Ex. 
174 170 R974 
TS720 G1701 
G1702 
G1750 
1035 1076 (Vol. %) 
(Units) 
(%) 
(psi) 
65.degree. C. 
(1) 
(2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) 
(13) 
(14) 
(15) 
__________________________________________________________________________ 
A 92 8 5.0 B24.3 
13.6 
-- Fail 
B 89 7 3 1 -- B53.3 
3.1 
0.0045 
Pass 
C 88 7 4 1 -- B64.3 
5.8 
-- Pass 
D 87 6 6 1 -- B39.4 
1.3 
0.0030 
Pass 
E 87 6 6 1 -- B43.3 
1.2 
0.0030 
Pass 
F 86 6 6 2 -- B39.4 
-- 0.0033 
Pass 
G 86.5 6 5.5 2 -- B34.1 
-- 0.0033 
Pass 
H 86.4 5.6 6.0 2 -- B30.9 
-- -- Pass 
I 89 6 4 1 -- B42.2 
1.58 
0.0030 
Pass 
J 88 6 5 1 -- B53.1 
0.00 
0.0040 
Pass 
K 87 6 6 1 -- B56.4 
2.28 
0.0030 
-- 
L 87 7.6 3.4 0.3 1.7 36 B29.5 
7.02 
0.0035 
-- 
M 84.8 7.6 5.6 0.3 1.7 -- B51.2 
0.00 
0.006 
Pass 
N 85.5 8.0 4.5 0.3 1.7 -- B48.7 
0.15 
0.005 
Pass 
O 85.5 6.5 6 0.3 1.7 -- B39.7 
0.00 
0.0032 
Pass 
P 87 6.5 4.5 0.3 1.7 -- B23.1 
0.00 
0.0033 
Pass 
__________________________________________________________________________ 
TABLE II 
__________________________________________________________________________ 
FILLING MATERIAL BASED ON POLYPROPENE OIL 
Composition (% by wt) 
Polypropene Oil 
Fumed Silica 
Primary 
Helipath 
Oil Drip 
Amoco 
Amoco 
Aerosil 
Coat Swell 
Viscosity 
Separation 
Test 
Example 
9013 9015 
R974 (Vol %) 
(Units) 
(%) at 65.degree. C. 
(1) (2) (3) (4) (5) (6) (7) (8) 
__________________________________________________________________________ 
Q 90 10 46.8 C55.1 
0.00 Pass 
R 92 8 34.3 C46.7 
0.00 Pass 
__________________________________________________________________________ 
TABLE III 
__________________________________________________________________________ 
FILLING MATERIAL BASED ON POLYBUTENE OIL 
Composition (% by wt) 
Polybutene Oil Fumed Silica 
Copolymer 
Primary 
Helipath 
Oil Drip 
Indopol 
Indopol 
Aerosil 
Kraton 
Coat Swell 
Viscosity 
Separation 
Test 
Example 
H100 H300 R974 G1701 (Vol %) 
(Units) 
(%) at 65.degree. C. 
(1) (2) (3) (4) (5) (6) (7) (8) (9) 
__________________________________________________________________________ 
S 94 6 -- B37.5 
0.00 -- 
T 90 10 40.4 C54.5 
0.00 Pass 
U 94 6 15.6 B70.3 
0.00 Fail 
V 92 8 -- C60.7 
0.00 Pass 
W 90 10 -- D43.0 
0.00 Pass 
X 90 5 5 -- C69.5 
0.00 -- 
__________________________________________________________________________ 
TABLE IV 
__________________________________________________________________________ 
FILLING MATERIAL BASED ON MINERAL OIL 
Composition (% by wt) Primary 
Mineral Oil Fumed Silica 
Copolymer 
Antioxidant 
Coat Hel. 
Oil 
Yield 
Drip 
Drakeol 
Sunpar 
Aerosil 
Cab-O-Sil 
Kraton 
Irganox 
Swell 
Visc. 
Sep. 
Stress 
Test 
Ex. 
35 2280 
R974 
TS720 G1701 1035 (Vol. %) 
(Units) 
(%) 
(psi) 
at 65.degree. C. 
(1) 
(2) (3) (4) (5) (6) (7) (8) (9) (10) 
(11) 
(12) 
__________________________________________________________________________ 
Y 93 7 0.1 76.2 B30.0 
62.5 
0.0016 
Fail 
Z 90 10 -- C60.0 
33.7 
-- Fail 
AA 87 7 5 1.0 -- B39.3 
0.0 
0.0010 
Pass 
BB 87 7 5 1.0 -- B69.0 
-- 0.0055 
Pass 
CC 90 10 -- -- -- 36.5 B45.3 
0.00 
-- -- 
DD 88 12 -- -- -- -- C70.7 
1.18 
-- Fail 
EE 90 6 3 1.0 -- B60.9 
0.31 
0.005 
-- 
FF 89 -- 4 3 1.0 B23.8 
-- 0.001 
-- 
__________________________________________________________________________ 
TABLE V 
__________________________________________________________________________ 
FILLING MATERIAL BASED ON MIXTURES OF SYNTHETIC OIL AND MINERAL OIL 
HITEC or 
Sunpar 
Indopol 
Cab-O-Sil 
Kraton 
Irganox 
Helipath 
Oil Yield 
Ethylflo 170 
2280 H100 TS720 G1701 1035 Viscosity 
Separation 
Stress 
Example 
Synthetic Oil 
Mineral Oil 
Mineral Oil 
Fumed Silica 
Copolymer 
Antioxidant 
(Units) 
(%) (psi) 
__________________________________________________________________________ 
GG 45 45 -- 6 3 1 B33.8 
2.34 0.003 
HH 44.5 44.5 6 4 1 B42.9 
0.00 0.003 
__________________________________________________________________________ 
The test results indicate that high molecular weight oils are required to 
prevent some presently used optical fiber coatings from swelling. The 
higher the molecular weight of the mineral oil, the higher the pour point. 
Test results have shown that a low viscosity polyalphaolefin oil 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 5%. At approximately the same molecular 
weight, polyalphaolefin oil has a lower viscosity than other oils and thus 
filling materials made from these oils have a lower viscosity than filling 
materials made from other oils. 
FIGS. 3 and 4 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 
pronounced when it is more than 5% by weight, while the effect of the 
copolymer becomes more pronounced if it contains more than 3%. 
For a filling material which includes HITEC 174 polyalphaolefin oil, fumed 
silica does not reduce the oil separation without an adverse increase in 
viscosity and critical yield stress. A block copolymer was added to reduce 
further the oil separation and also to mitigate the viscosity increase. 
Unlike fumed silica, the block copolymer does not contribute as much as 
fumed silica in increasing yield stress and viscosity. The effects of 
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 oil 
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 optical 
fibers 25--25 and units 24--24 are capable of moving within the core when 
the cabale is loaded or bent. The yielding filling material allows the 
optical fibers to move within the tubular member 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 strain rate 
for a thixotropic material such as that used as the waterblocking material 
26, and identifies several important parameters. Along a segment 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 
.sigma..sub.c. The strain corresponding to .sigma..sub.c is identified as 
.gamma..sub.c, the critical shear strain. By definition, the coordinates 
.sigma..sub.c and .gamma..sub.c indicate the onset of yielding and the 
quantity .sigma..sub.c /.gamma..sub.c (or d.sigma./d.gamma. for 
.gamma.&lt;.gamma..sub.c) is known as the shear modulus, G.sub.e, of the 
material. 
A segment 39 of the stress-strain curve of FIG. 7 represents increasing 
values of incremental strain for increasing stress. The stress 
.sigma..sub.y is the maximum value of stress sustainable by the material 
at a given strain rate with .gamma..sub.y being the corresponding strain. 
For strains in excess of .sigma..sub.y, the stress 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 for .sigma.&gt;.sigma..sub.y. 
FIGS. 8 and 9 show the effect of Cab-O-Sil TS720 fumed silica and Kraton G 
1701, copolymer on the yield stress of filling materials. From the slopes 
of the curves in FIGS. 8 and 9, 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.degree. C. no drip requirement, the yield stress 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 of cable of this invention provides excellent results 
notwithstanding its relatively low viscosity. 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 of antioxidant which 
increases the oxidative stability of the tubular member 28 and optical 
fiber coatings to prevent premature degradation of the optical fiber 
cable. 
The filling material 26 of this invention has enhanced performance at low 
temperature because of the use of a low pour point oil, has a relatively 
high cable drip temperature and very low oil separation. The filling 
material 26 is compatible with presently used fiber coating materials and 
other cable materials which it contacts. There is no bleeding of oil and 
it is expected that the optical loss at -40.degree. C. will not exceed 
that of the prior art filling 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.degree. C. cable drip test. Apparently, fumed silica as the only 
thickening agent in a mineral oil-based filling composition of matter 
cannot enable a cable to pass the drip test without an adverse viscosity 
increase. To avoid this result, a thermoplastic rubber is used in 
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 Kraton G 
1701 copolymer on the drip test. The compositions on the left side of the 
curve passed the 65.degree. C. drip test whereas those on the right 
failed. 
As stated before, what had been sought after and what has been 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 processing characteristics of 
the filling material disclosed in previously mentioned U.S. Pat. No. 
4,701,016 were 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 satisfies 
this requirement. 
It is to be understood that the above-described arrangements are simply 
illustrative of the invention. Other arrangements may be devised by those 
skilled in the art which will embody the principles of the invention and 
fall within the spirit and scope thereof.