Note: Descriptions are shown in the official language in which they were submitted.
3~;3~
In our Canadian Patent No. 1,189,262, entitled "Melt
Extruded Elongated Member Suitable for Improved Service as a
Stiffening Support in an Optical Fiber Cable" is claimed a melt
extruded elongated member which is suitable for use in the
optical fiber cable of the present invention.
The present invention relates to fiber-optic cable and,
in particular, to the reinforcement of such cable.
Fiber-optic cable is rapidly gaining in preference over
metallic wire electric cables for a variety of reasons,
especially the wide band width and low attenuation which are
characteristic of optical fibers. Optical fibers, however, are
generally very thin and unable to withstand appreciable mechani-
cal loading, and thus do not aid in strengthening the cable as
do the metallic wires of electric wire cables. As a result,
special measures must be taken to reinforce fiber-optic cable
because the transmission capability of optical fibers
deteriorates as the fibers are subjected to strain. Below a
certain level of strain, the deterioration is reversible and is
thus permissible during cable installation, but not during cable
operation. Above that level of strain, however, the
deterioration is permanent and is thus not permissible at any
stage of handling or operation. Thus, it is critical that
stresses incurred during handling and operation be prevented
from excessively straining the optical fiber.
-- 1 --
B
3~1
The provision of strength members in the cable
represents one possible means of reinforcing the cable.
The use of metal reinforcement has been proposed in U.S.
Patent 3,865,466 to Slaughter and U.S. Patent 4,110,001 to
olszewski et al. Other materials, such as , polyester,
lyotropic liquid crystalline polymer (e.g., Kevlar~ poly
(p-phenylene terephthalamide)) , polyethylene, polyethylene
terephthalate, cotton, E and S glass/epoxy rods, etc.,
have also been proposed, e.g., see U.S. Patent 4,037,922
to Claypoole, U.S. Patent 4,093,342 to Foord et al, and
U.S. Patent 4,226,504 to Bellino.
However, the use of metal strengtheners is not
compatible with all applications of fiber-optic cable,
some of which may specify that the cable be free of
electrically conductive components. Conversely, in some
applications metallic elements may be tolerable, but may be
advantageously eliminated from the point of view of reducing
cable weight or increasing the useful temperature range of the
cable.
The use of poly ~p-phenylene terephthalamide) as the
reinforcement has first necessitatPd the dissolution of the
polymer in an appropriate solvent fcr the same, and the solution
spinning of a large number of relatively fine denier fibers
(e.g., thousands of filaments) which may optionally be embedded
in an appropriate resin (e.g., an epoxy resin) to form the
stiffening member. Such poly(p-phenylene terephthalamide) is
incapable of melt extrusion and the procedures required to form
the reinforcing member are time consuming, and involve considerable
k
ED
2;3 ~32~L
expense. Also, the resulting stiEfening member because oE the
fabrication techniques inherently required ls only with
difficulty amenable to formation into complex cross-sectional
configurations.
Reinforcing members available in the prior art which
are composed of E and S glass/epoxy rods are commonly formed
by pultrusion and have been found to present shortcomings during
service within the resulting cable assembly. For instance,
such rods may be susceptible to undesirable thermal expansion
and contraction and have tended to be unduly inflexible and
relatively brittle which may result in cable failure if the
cable assembly is sharply bent.
Some reinforcement arrangements while general]y serving
to prevent excessive deformation of the optical fiber may, under
certain conditions, such as temperature change for example,
actually contribute to such excessive deformations. That is,
the particular linear thermal expansion characteristics of the
reinforcement may render the overall coefficient of linear ther-
mal expansion of the cable significantly different from that
of the optical fiber. As a result, the optical fiber may be
subject to excessive deformation under extreme temperature
conditions.
Reference is made to the accompanying drawings showing,
by way of example, preferred embodiments of the invention, in
which:
Figure 1 is a cross-sectional view of a flber-optic
cable unit in which an optical fiber is disposed within a tube;
Figure 2 is a schematic longitudinal sectional view
through the unit depicted in Figure l;
Figure 3 is a view similar to Figure 2 depicting one
technique for resisting thermally-induced straining of the
optical fiber;
~2363Zl
Figure ls a view oE the unit depic-ted in Figure 3 in
response to the unit being subjected to cold temperatures;
Figure 5 is a schematic view of a plurality of cable
units being bundled together;
Figure 6 is a cross-sectional view of another type of
fiber-optic cable;
Figure 7 is a cross-sectional view through still another
type of fiber-optic cable;
Figure 8 is a cross-sectional view through a fiber-optic
cable employing a channel member;
Figure 9 is a schematic view of one technique for
producing a channel member; and
Figure 10 is a cross-sectional view of a fiber-optic
cable formed of rows of optical fibers separated by layers of
thermotropic liquid crystalline polymer and surrounded by a
jacket of that material.
Attention is directed to Figures 1 and 2 which depict a
fiber-optic cable unit 8 wherein an optical fiber 10 is encased
within a buffer tube 12 formed of a thermoplastic material.
The inner diameter of the tube may be of greater diameter than
the outer diameter of the fiber, the space therebetween filled
with a water-repelling medium. Additional reinforcement (not
shown) would typically be provided
- 3a -
B
3.23G3~
(e.g., a central high-strength elongated number around
which the optical fiber is helically wound, or high-
strength wires helically wound around the tube 12), since
a conventional thermoplastic tube is too weak to constitute a
strength member. This can be expected to result in a condition
where the net linear thermal expansion coefficient of the
overall cable varies considerably from that of the optical
fiber itself. Accordingly, as the temperature increases, the
cable tends to expand to a greater extent than the optical fiber,
whereby the fiber is strained. One manner of minimizing
this problem is to preslacken the fiber, as depicted in
Figure 3, whereby the overall cable can expand to a greater
extent than the optical fiber itsalf, without straining the
fiber (i.e., the slack is "taken-up" during cable expansion).
However, the amount of pre~slack which can be
"built-into" the cable is limited, due to the fact that during
colder temperatures the overall cable contracts to a greater
extent than the optical fiber due to the sign ficant difference
in the coefficient of linear thermal expansion. Thus, as the
temperature decreases, the amount of slack increases due to
the lesser extent of contraction of the optical fiber. If this
results in the fiber bearing against the wall of the tube (Fig.
4), the "microbending losses" in the fiber are significantly
increased, thereby increasing attenuation losses of the fiber.
Therefore, it will be appreciated that a mismatch between
the linear thermal expansion coefficient of the optical fiber
and the net linear thermal expansion coefficient of the overall
cable places limits on the upper and lower temperatures in which
the cable may be effectively utilized; the greater the mismatch,
the smaller the range of effective utilization.
~23~
Another problem occurring in connection with fiber-
optic cable relates to the difficulty in repairing a broken
cable. When a break occurs, it is presently necessary to
locate and identify the damaged optical fiber(s) in order to
perform a splicing operation. This procedure is difficult
enough due to the small size of the fiber, but is made even
more difficult in conventional cables which are cluttered with
numerous reinforcing strands. Although it has been previously
proposed to position each fiber within its own individual tube,
e.g., an extruded polyethylene terephthalate tube, in order to
facilitate fiber identification, such an arrangement would add
to the size, weight, and internal clutter of the cable.
A further problem relates to the fact that conven-
tional techniques for reinforcing the cable must be adapted
to the particular type of cable being produced, i.e., the
cable must be redesigned for the particular end uses. One
reason for this relatively expensive requirement is that in
conventional cables the reinforcement is common to all of the
optical fibers. In an effort to deal with this problem, it
has been proposed to individually encase each fiber see U.S.
Patent No. 4,188,088 issued to Andersen et al on February 12,
1980). This is achieved by encasing each fiber within a
dumbbell-shaped sheath of flexible polymer material. A
separate strengthener strand is embedded within another por-
tion of the sheath. This arrangement, however, does not
minimize the bulk and weight problems, nor the thermall-
induced strain problems discussed earlier.
-- .D
~31E;32~
It is, therefore, an object of the present invention
to minimize or obviate problems of the type discussed above.
A further object of the invention is to minimize
the size, weight, and bulk of a fiber-optic cable while
maintaining ample strength of the cable.
Another object is to increase the temperature range
in which fiber-optic cable may be effectively utilized.
An additional object of the invention is to provide
a reinforced fiber-optic cable in which the coefficient of
linear expansion of the overall cable closely approximates
that of the optical fiber.
A further object of the invention is to facilitate
the splicing of fiber-optic cables.
Yet another object of the invention is to provide
a cable having multiple optical fibers in which the individual
fibers can be easily found and identified.
An additional object of the invention is to enable
a given component of fiber-optic cable to perform a multiplicity
of functions.
A further object of the invention is to facilitate
the manufacture of stength members for use in fiber-optic cable.
SUMMARY OF THE INVENTION
These objects are achieved in accordance with the
present invention which relates to fiber-optic cable and
methods for producing same.
In accordance with one aspect of the present invention,
a reinforced fiber-optic cable comprises at least one optical
._
:~%3~3Z~L
fiber, and at least one strength member comprising a
hollow tube formed-in-place around the at least one optical
fiber. The tube is formed of a thermotropic liquid
crystalline polymer which resists deformation and breakage
of the at least one optical fiber.
The at least one optical fiber may comprise a single
fiber, and the at least one strength member may comprise a
single tube surrounding the fiber.
The at least one optical fiber may comprise a plurality
of fibers, and the at least one stiffening member may comprise
a plurality of tubes individually surrounding the fibers,
with a jacket surrounding all of the tubes. The jacket may
comprise a thermotropic liquid crystalline polymer.
In another aspect of the present invention, a
reinforced optic-fiber cable may comprise a strength member
in the form of a channel member which includes a core and a
plurality of longitudinal channels. The channel member is
formed of a thermotropic liquid crystalline polymer which
resists deformation of the cable. A plurality of optical
fibers would be disposed in respective ones of the channels.
Preferably, the channel member includes a plurality
of ribs projecting radially from the core to define the channels.
Preferably, the channels are outwardly open, and a
jacket surrounds the channel member and encloses the channels.
The jacket may be formed of a thermotropic liquid c-rystalline
polymer.
. .
The optical fiber may be formed of glass, with the
thermotropic liquid crystalline polymer having a coefficient
of linear thermal expansion of from -10x10 6/oF to -4x10 6/oF.
In another aspect of the present invention, a rein-
forced fiber-optic cable comprises at least one optical fiber
and at least one strength member comprising an elongated member
formed of thermotropic liquid crystalline polymer extending
generally in the direction of the fiber. A jacket encloses
the fiber and the elongated member. The jacket may be formed
of a thermotropic liquid crystalline polymer.
Preferably, the elongated member is helically wound
around the fiber or extends parallel thereto.
There may be provided a plurality of fibers, with the
elongated member disposed centrally of the fibers.
In another aspect of the present invention, a
reinforced fiber-optic cakle comprises a plurality of optical
flbers and a strength member extending along the fibers. A
jacket formed of a thermotropic liquid crystalline poIymer
surrounds the fibers and strength member.
The fibers may be arranged in spaced rows, with a
layer of thermotropic liquid crystalline polymer separating
adjacent rows.
The present invention includes methods o making rein
forced fiber-optic cable. One method aspect comprises the
steps of forming a strength member by extruding a ~hermotropic
liquid crystalline polymer in the shape of a hollow tube sur-
rounding an optical fiber such that deformation and breakage
of the fiber is resisted by the tube. The tube may be formed,
-- 8
a
~3~3~l
for-example, by extrusion or by coating the optical fiber
with thermotropic liquid crystalline polymer.
Another method aspect of the invention comprises the
steps of forming a strength member by extruding a thermotropic
liquid crystalline polymer in the shape of a channel member
having a core and a plurality of longitudinal channels.
Optical fibers are positioned within respective ones of
the channels, with deformation and breakage of the fibers
being resisted by the strength member.
A further method aspect of the invention involves
the steps of forming a strength member by extruding a
thermotropic liquid crystalline polymer in the shape of
an elongated member. The elongated member is positioned
adjacent an optical fiber and extends generally in the
longitudinal direction thereof. A jacket is disposed in
surrounding relationship to the optical fiber and strand.
The jacket may be formed of a thermotropic liquid crystalline
polymer.
The thermotropic liquid crystalline polymer contains
unique characteristics in the area of tensile modulus, coeffi-
cient of linear thermal expansion, and ease of manufacture which
~Z363~:~
render i-t ideally suited to the manufacture of Eiber-optic
cable and enable a s:ingle component to perform mult:iple
functions whereby the size, weight, and bulk of the cable can
be greatly enhanced. In cases where the optical fiber is formed
of glass, and wherein the thermotropic liquid crystalline
polymer has a coefficient of linear thermal expansion of from
-10x10 6/oF to -~x10 6/oF, the temperature range in which the
fiber-optic cable may be effectively utilized, is significantly
increased.
DETAILED DESCRIPTION OF THE INVENTION
In accordance with the present invention, it has been
found that thermotropic liquid crystal polymers are uniquely
suited to form improved reinforcement for fiber-optic cables.
The thermotropic liquid crystalline polymer from which
the elongated members of the present invention is formed is of
the requisite molecular weight to be capable of undergoing melt
extrusion. Such thermotropic liquid crystalline polymers have
been known in the art but have notprior to the invention been
recognized to be suitable for forming the presently claimed
elongated article which has been found to be capable of improved
service as a stiffening support in an optical fiber cable.
- 10 -
~3~3~
As is known in polymer technology a thermotropic
liquid crystalline polymer exhibits optical anisotropy in
the melt. The anisotropic character of the polymer melt
may be confirmed by conventional polarized light techniques
whereby crossed-polarizers are utilized. More specifically,
the anisotropic nature of the melt phase may conveniently
be confirmed by the use of a Leitz polarizing microscope
at a magnification of 40X with the sample on a Leitz hot
stage and under a nitrogen atmosphere. The amount of light
transmitted changes when the sample is forced to flow; however,
the sample is optically anisotropic even in the static state.
On the contrary,typical melt processable polymers do not
transmit light to any substantial degree when examined under
identical conditions.
Representative classes of polymers from which the
thermotropic liquid crystalline polymer suitable for use in
the present invention may be selec-ted include wholly aromatic
polyester, aromatic-aliphatic polyesters, wholly aromatic poly
(ester-amides), aromatic-aliphatic poly(ester-amides), aromatic
polyazomethines, aromatic polyester-carbonates, and mixtures of
the same. In preferred embodiments the thermotropic liquid
crystalline polymer is a wholly aromatic polyester, a wholly
aromatic poly(ester-amide), or an aromatic-aliphatic poly(ester-
amide). In such wholly aromatic polyester and wholly aromatic
poly(ester-amide) each moiety present within the polymer chain
contribures at least one aromatic ring. Also, it is preferred
that naphthalene moieties be included in the thermotropic liquid
crysta]line polymer, e.g., 6-oxy-2-naphthoyl moiety, 2,6-
dioxynaphthalene moiety, or 2,6-dicarboxynaphthalene moiety,in a
2 -
~;~3~
concentration of not less than about 10 mole percent. The
particularly preferred naphthalene moiety for inclusion in the
thermotropic liquid crystalline polymer is the 6-oxy-2-naphthoyl
moiety in a concentration of not less thanabout 10 mole percent.
Representative wholly aromatic polyesters which exhibit
thermotropic liquid crystalline properties include those
disclosed in the following United States Patents: 3,991,013;
3,991,014; 4,066,620; 4,067,852; 4,075,262; 4,083,829;
4,093,595; 4,118,372; 4,130,545; 4,146,702; 4,153,779;
4,156,070; 4,159,365; 4,161,470; 4,169,933; 4,181,792;
4,183,895; 4,184,996; 4,188~476; 4,201,856; 4,219,461;
4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144;
4,238,~98; 4,238,599; 4,238,600; 4,242,496; 4,245,082;
4,245,084; 4,247,514; 4,256,624; 4,265,802; 4,267,304;
4,269,965; 4,279,803; 4,299,756; 4,337,191; 4,337,190;
4,318,841 and 4,355,134. As discussed hereafter the wholly
aromatic polyester of United States Patent No. 4,161,470 is
particularly preferred for use in the present invention.
Representative aromatic-aliphatic polyesters which
exhibit thermotropic liquid crystalline properties are copolymers
of polyethylene terephthalate and hydroxybenzoic acid as
disclosed in Polyester X-7G-A Self Reinforced Thermoplastic, by
J. Jackson, Jr., H.F. Kuhfuss, and T.F. Gray, Jr., 30th
Anniversary Technical Conference, 1975 Reinforced Plastics/
Composites Institute, The Society of the Plastics Industry,
Inc., Sec'ion 17-D, Pages 1-4. A further disclosure of such
copolymers can be found in "Liquid Crystal Polymers:
Preparation and Properties of p-Hydroxybenzoic Acid Copolymers",
Journal of Polymer Science, Polymer Chemistry Edition, Vol. 14,
pages 2043 to 2058 (1976), by W.J. Jackson, Jr. and H.F. Kuhfuss.
See also our United States Patents Nos. 4,318,842 and 4,355,133.
Representative wholly aromatic and aromatic-aliphatic
- 12 -
~3632~
poly(ester-amides) which exhibit thermotropic llquid crys-talline
propertles are dlsclosed ln Unlted States Patents Nos.
4,272,625; 4,330,457; 4,351,917; 4,351,918; 4,341,688;
4,355,132 and 4,339,375. As dlscussed hereafter -the poly(es-ter-
amlde) of Unlted States Patent No. 4,330,457 ls partlcularly
preferred for use ln the presen-t lnventlon.
Representatlve aromatlc polyazomethlnes whlch exhlblt a
thermotroplc liquid crystalline propertles are dlsclosed ln
Unlted States Patents Nos. 3,493,522; 3,493,524; 3,503,739;
3,516,970; 3,516,971; 3,526,611; 4~048,148 and 4,122,070.
Speclfic examples of such polymers include poly(nitrilo-2-
methyl-1,4-phenylenenitrlloethylidyne-1,4-phenylenneethylidyne);
poly(nltrilo-2-methyl-1,4-phenylenenitrilomethyliddyne-1,4-
phenylene-methylidyne); and poly(nitrilo-2-chloro-1,4-
phenylenenitrilomethylidyne-1,4-phenylene-methyliddyne).
Representative aromatic polyester-carbonates which
exhibit thermotropic llquld crystalllne propertles are dlsclosed
ln United States Patents Nos. 4,107,143; 4,284,757 and
4,371,660. Examples of such polymers include those consisting
essentially of p-oxybenzoyl unlts, p-dioxyphenyl units,
dioxycarbonyl units, and terephthoyl units.
A thermo-tropic llquid crystalline polymer commonly ls
selected for use ln the formatlon of the elongated member of
the present lnventlon whlch possesses a meltlng temperature
within the range that ls amenable to melt extruslon while
employing commerclally avallable equipment. For lnstance,
thermotroplc llquld crystalllne polymers commonly are selected
which exhibit a melting temperature somewhere within the range
of approximately 250 to 400~C.
The thermotropic liquid crystalline polymer selected
preferably also exhibits an inherent viscoslty of at least 2.0
dl./g. when dlssolved in a concentration of 0.1 percent by
~23~
weight in pentafluorophenal at 60C. (e.g. an inherent
viscosity of approxima-tely 2.0 to 15.0 dl./g.).
The particularly preferred wholly aromatic polyester for
use in the present invention is that disclosed in United S-tates
Patent No. 4,161,470 which is capable of forming an anisotropic
melt phase at a temperature below approximately 350C. This
polyester consists essentially of the recurring moieties I and
II wherein:
- 14 -
~2~3%.~
I is -o c- , and
II is _O
The polyester comprises approximately 10 Jo 90 mole percent of
moiety I, and approximately 10 to 90 mole percent of moiety II.
In one embodiment, moiety II is present in a concentration of
approximately 65 to 85 mole percent, and preferably in a
concentration oE approximately 70 to B0 mole percent, e.g.,
approximately 73 mole percent. In another embodiment, moiety II
is present in a lesser proportion of approximately 15 to 35 mole
percent, and preferably in a concentration of approximately 20 to
30 mole percent. In addition, at least some of the hydrogen
atoms present upon tbe rings optionally may be replaced by
substitution selected from the group consisting of an alkyl group
of 1 to 4 carbon atoms, an alkoxy group of l to 4 carbon atoms,
halogen phenyl, substituted phenyl, and mixtures thereof. Such
polymer preferably has an inherent viscosity of approximately 3.5
to 10 dl./g. when dissolved in a concentration of 0.1 percent by
weight in pentafluorophenol at 60~C.
The particularly preferred wholly aromatic poly(ester-
amide) or ~romatic-aliphatic poly(e~ter-amide) for use in the
/S
~3~
present invention is disclosed in commonly assigned United States
Serial No. 214,557, fi.led December 9, 1980, which is capable of
forming an anisotropic melt phase at a temperature below
approximately 400C. The poly(ester-amide)s there disclosed
consist essentially of recurring moieties I, II, III, and,
optionally, IV wherein:
I is
O O
n n
II is - C _ A _ C - , where A is a divalent
radical comprising at least one aromatic ring or a
divalent trans-1,4-cyclohexylene radical;
III is - Y - Ar - Z - .~ where Ar is a divalent
radical comprising at least one aromatic ring, Y is
O, No, or NR, and Z i8 NH or NR, where R is an
alkyl group of 1 to 6 carbon atoms or an aryl
group; and
IV is - O - Ar' - O - , where Ar' is a divalent
radical comprising at least one aromatic ring;
~3
@~
~3~3
wherein at least some of the hydrogen atoms present upon the
rings optionally may be replaced by substitution selected from
the group consisting of an alkyl group of 1 to 4 carbon atoms, an
alkoxy group of 1 to 4 carbon atoms, halogen, phenyl, substituted
phenyl, and mixtures thereof, and wherein said poly(ester-amide)
comprises approximately 10 to 90 mole percent of moiety I,
approximately 5 to 45 mole percent of moiety II~ approximately 5
to 45 mole percent of moiety III, and approximately O to 40 mole
percent of moiety IV. The preferred dicarboxy aryl moiety II
is:
lo7 1ol
go
the preferred moiety III is:
_ NH O _ or _ NH NH -
and the preferred dioxy aryl moiety rv is:
--o~o-
Such polymer preferably has an inherent viscosity of
approximately 2.0 to ld dig when dissolved in a concentration
of 0.1 percent by weight in pentafluorophenol at ~0C.
/7
~;~36312~
When forming the melt extruded elongated member of the
present invention conventional melt extrusion apparatus can be
used wherein an ex-trusion clie is selectecl having a shape which
corresponds to the cross-sectional configura-tion of the
elongated member to be formed with the exception that the
orifice dimensions will be larger than the dimensions of the
resulting elongated member in view of drawdown of the molten
polymer which occurs immediately following extrusion. Polymers
other than thermotropic liquid crystalline polymers are
recognized to be incapable of melt extrusion to form articles
of the cross-sectional area herein discussed wherein the
profile will accurately correspond to the die shape. Accord-
ingly,.the thermotropic liquid crystalline polymers do not
exhibit any substantial elastic recoil upon exiting from the
extrusion die as do conventional polymers which are melt
extruded. Suitable extrusion apparatus are described, for
example, in the "Plastics Engineering Handbook" of the Society
of the Plastics Industry, Pages 156 to 203, 4th Edition, edited
by Joel Frados, Van Nostrand Reinhold Company, 1976. The
elongated members of the present invention optionally may be
formed in accordance with the teachings of United States Patent
No. 4,332,759, filed July 15, 1980 of Yoshiaki Ide, entitled
"Process for Extruding Liquid Crystal Polymer".
The temperature and pressure conditions selected for
extruding the molten thermotropic liquid crystalline polymer
will be influenced by the melting temperature of the polymer
and its viscosi-ty as will be apparent to those skilled in the
art.
- 18 -
~63~2~
Typically extrusion temperatures approxima-tely 0 to 30C.
above the polymer melting temperature and pressures of
approximately 100 to 5,000 psi are selected. In order to
induce relatively high molecular orientation coextensive
with the length of the elongated member, the extrudate is
drawn while in the melt phase immediately adjacent -the extru-
sion orifice and prior to complete solidification. The extent
of such drawdown is influenced by the takeup speed under which
the elongated member is wound or otherwise collected on an
appropriate support or collection device. The resulting draw
ratio is defined as the ratio of the die cross-sectional area
to that of the cross-sectional area of the fully solidified
extrudate. Such draw ratios commonly range between 4 and 100,
and preferably between approximately 10 and 50 while utilizing
the equipment described in the Examples.
In addition to the drawdown appropriate cooling must
be applied to the extrudate of thermotropic liquid crystalline
polymer intermediate the extrusion orifice and the point of
collection. Appropriate fluid media, e.g., a gas or a liquid,
may be selected to impart the desired cooling. For instance,
the extrudate may be simply contacted by a stream of air or
other gas or preferably immersed in a circulating bath of
water or other liquid which is maintained at an appropriate
temperature to impart the cooling required for solidification.
One preferred type of fiber-optic cable in accordance
with the present invention is depicted in Figures 1 and 2,
wherein a conventional optical fiber 10 is arranged within a
buffer tube 12. Thus, the arrangement of the fiber and tube
~23~32~
in accordance with the present invention can be similar to
that of the prior art. In accordance with the present inven-
tion, however, the buffer tube is formed from a thermotropic
liquid crystalline polymer.
A buffer tube formed of thermotropic liquid crystalline
polymer constitutes more than a mere envelope for the optical
fiber. Due to its high modulus and strength in tension, this
tube constitutes a strengthening member which, in many instances
satisfies all of the reinforcement requirements of the fiber.
In this regard, a tube of thermotropic liquid crystalline
polymer material of 38 mils outer diameter and 9.5 mils
thickness possesses a tensile modulus of 4.5x106psi. The
tensile modulus of a tube of the same diameter formed of
polyethylene terephthalate is much lower by comparison.
Such an extruded hollow tubular configuration cannot be
formed by extrusion of poly(p-phenylene terephthalamide),
an organic material commonly employecl as strength material
in fiber optic cable, since such material cannot be melt
extruded. Such a tube could only be formed of poly~p-phenylene
terephthalamide) by the use of an adhesive matrix, at much
greater cost due to the technological difficulty and slow
speed of the pultrusion process and slow rate of cure of the
required matrix.
A fiber-optic cable 16 can be fabricated by bundling
together a desired plurality of the tube-and-fiber units 8.
For example, the bundling can be achieved either by conven-
tional cable jacketing techniques or by an outer jacket 18`
in the form of a helical wrap of thermotropic liquid
~.~ _ I* _
~23632~
crystalline polymer (Fig. 5). In the cable 16 thus produced,
the tubes 12 serve as the strength members for the individual
fibers, and thus for the cable as a whole. The need for
additional reinforcement may thus be eliminated or signifi-
cantly minimized. In addition, the use of thermotropic liquid
crystalline polymer tape as a cable wrap allows greater control
of the thermal expansion of the cable, by controlling pitch,
tape thickness, or the thermal expansion coefficient of the
tape itself via control of molecular orientation within the
tape.
Moreover, the fabrication of different cable designs
is simplified since each fiber is individually reinforced
rather than the fibers being commonly reinforced as a group.
Thus, cables of different capacity can be provided by selecting
a desired number of the individually reinforced fibers. The
individual fibers are easier to isolate and identify when
repair to the cable is necessary, since each fiber is
contained in its own reinforcing tube and since there is
no multitude of reinforcement strands or yarns which clutter
the inside of the cable.
Furthermore, the use of a strength membex formed
of thermotropic liquid crystalline polymer enables the
fiber to be used within a relatively wide temperature range.
This advantage results from the fact that the thermotropic
liquid crystalline polymer material possesses a negative
and controllable linear thermal expansion coefficient so
that the overall linear l:hermal expansion coefficient o the
I"
~3~32
finished cable closely approximates that of glass optical
fibers (i.e., the linear thermal expansion coefficient
of thermotropic liquid crystalline polymer can range from
approximately ~lOxlO 6/oF to -4xlO 6/oF, as compared with
a coefficient of approximately +0.25xlO 6/oF to +1.25xlO /F
for commercially available glass optical fibers. Accordingly,
the expansion (or contraction) of the thermotropic liquid
crystalline polymer counteracts the expansion (or contrac-
tion) of the other elements and structures within the cable
composed of conventional materials, so that the expansion
and contraction of the overall cable and fiber are so close
that excessive straining of the fibers is avoided over a
much wider range of temperatures than has heretofore been
possible.
The coefficient of linear thermal expansion can
be conveniently determined with a DuPont thermomechanical
analyzer while examining the elongated member at tempera-
tures below 100C.
The tube 12 is formed-in-place around the optical
fibers 10 by means of, for example, an extrusion or pultru-
sion process, or coating the optical fiber with the thermotropic
liquid crystalline polymer in a liquid state and later cured.
Conventional extrusion apparatus can be used to
extrude the thermotropic liquid crystalline polymers.
~2~
-- ~3
~;~363~
Such apparatus are described in, for example, Plastics
Engineering Handbook of the Society of the Plastics Industry,
pp. 156-203, 4th edition, edited by Joel Frados, Van Nostrand
Reinhold Company, 1976.
The conditions of temperature and pressure under
which the thermotropic liquid crystalline polymer can be
extruded are not critical and can easily be determined
by one of ordinary skill in the art. Typically, extrusion
temperature ranges from 0 to 30C above the melting point
(which ranges from 200C to 350C~ and the pressure ranges
from 100 psi to 5,000 psi depending on the temperature and
the viscosity of the polymer.
The thermotropic liquid crystalline polymers can be
extruded to form a variety of shaped articles. Appropriate
dies must be chosen for this purpose but can be determined
easily by one of ordinary skill in the art.
In order to achieve high molecular orientation throughout
the part, and thus high mechanical properties, the extrudate
must be drawn down in the melt phase. The draw ratio, defined
by the ratio of die cross-sectional area to extrudate cross-
sectional area, should range between 4 and 100, preferably
between 10 and 50. Appropriate cooling must be applied before
take-up or haul-off in order to keep the integrity of the
extrudate. For example, when the extrudate is small, blowing
of air or other gases may be sufficient, but largex extrudates
(for example, smallest dimension> 20 mils) require water cooling.
_ _
~23~ 2~
,
~"~
.
The extrudates as produced exhibit very high
mechanical properties. On the other hand, conventional
polymers extruded with the above method have an order of
magnitude lower properties. Even if these extru~ates
of conventional polymers are drawn in the solid state, the
properties are much lower than those of thermotropic liquid
crystalline polymers produced according to the above method.
Fabrication of the tube requires a tube die. Preferably,
the tube is melt extruded while simultaneously passing the
optical fiber therethrough. In order to provide melt drawdown
only to the tube, the melt should not touch the optical fiber
and the extrusion rate or take-off speed should be controlled
to accomplish this. Specifically, the extrusion speed of the-
annular melt should be slower than the take-off speed by factor
of the drawdown ratio, and the take-off speed of the tube and
the optical fiber should be nearly the same.
An alternate method of fabricating the tube comprises
passing a plurality of filaments of thermotropic liquid
crystalline polymer through a heating die wherein the fila-
ments are converged and fused tosether.
In cases where a space is provided between the fiber
and tube, such space may be filled with any desired medium
such as gas or a water-repellant agent for example, dependlng
upon the intended use of the cable. Alternatively the tube
can be provided with a tight-fit whereby such space is not
created. Such a tight-fit can be created, for example, by
the afore-mentioned coating process.
_ 2y_
~2363~
It will be appreciated that cables will be subject
to differing types and magnitudes of forces, depending upon
the manner of installing and operating the cable. For
example, during aerial use, where the cable may be suspended
from poles, greater tensile forces may be encountered from
wind, ice, etc. Depending upon the particular use to which
the cable is subjected, then, it Jay be desirable to provide
the cable with a different type of reinforcement, comprising
helically wound elongated members 22 of circular cross-
section which encompasses the jacket 18 (Fig. 6). In this
case the tubes 8 may be formed of conventional thennoplastic
materials, or may also be formed of thermotropic liquid
crystalline polymers. An outer jacket 25 can be positioned
around the elongated member 22 and the optic fiber bundle.
The elongated members 22 are formed of thermotropic liquid
crystalline polymer, and would preferably be extruded in a
manner similarly to that described earlier with regard to
tubes, except that a rod die must be used instead of a tube
die.
Another type of additional reinforcement is depicted
in Figure 7 wherein a large thermotropic liquid crystalline
polymer elongated member 28 is positioned centrally of the
cable and is surrounded by a plurality of tube-encased fiber
units 8. A jacket 30 surrounds the units 8. This jacket may
comprise a wrap of thermotropic liquid crystalline tape if
additional control of cable thermal expansion is desired.
Such a tape wrap may contribute significantly to the tensile
stiffness of the final cable.
~3
_ I,_
~L~36~%~
In thls regard, it will be appreciated that it is
possible, in accordance witll the present invention, to employ
extruded thermotropic liquid crystalline polymer in the form
of an elongated member as a replacement for the reinforcement
presently employed in fiber-optic cable, regardless of whether
tubes of thermotropic liquid crystalline polymer are employed.
Thus, in accordance with the present invention, a fiber-optic
cable having reinforcing elongated members, e.g., elongated
members of circular cross-section which are helically wrapped
or positioned linearly, can be improved by forming those
elongated members of a thermotropic liquid crystalline polymer
material (preferably formed by extrusion).
Such an extrusion process would be similar to that
discussed above with reference to the filament 22.
A single elongated member of thermotropic liquid
crystalline polymer of 25 mils diameter possesses a tensile
modulus of 4-lOx106 psi. A strand of Xevlar poly(p-phenylene
terephthalamide) of 26 mils diameter (which strand is formed
of poly(p-phenylene terephthalamide) fibers adhered together
by adhesive) possesses a tensile modulus of llx106 psi. However,
the elongated member of thermotropic liquid crystalline is highly
superior. That is, since poly(p-phenylene terephthalamide) is
lyotropic, it is not melt extrudable, and thus cannot be
extruded in the form of large or complex cross~sections as
can a thexmotropic liquid crystalline polymer in accordance
with the present invention.
Another advantage of employing elongated members of
thermotropic liquid crystalline polymer material occurs in
cases where glass optical fibers are employed, since the
- Xl.-
~%3~
coefficient of linear thermal expansion of thermotropicliquid crystalline polymer is negative, and controllable,
so that a fiber-optic cable utilizing such members can be
made to have a linear thermal expansion coefficient very
similar to that of glass optical fibers, as discussed earlier.
It may be desirable, then, to support an optical cable
with a melt extruded elongated member of substantially uniform
cross-sectional configuration, the elongated member being
composed of a thermotropic liquid crystalline polymer having a
tensile modulus of approximately 4,000,000 to 20,000,000 psi,
and a tensile strength of at least 40,000 psi. The cross-
sectional area of the elongated member is preferably at least
7.85x10 5 square inch wherein no substantial portion of the
cross-section measures less than approximately 0.01 inch or
more than approximately 0.2 inch. The elongated member possésses
a length of at least one mile, and an aspect ratio of at least
31~,800 computed on the basis of the minimum cross-section
measurement.
The melt-extrudable characteristic of thermotropic
liquid crystalline polymer material is particularly beneficial
in the formation of a channel-defining element 30, as depicted
in Figure 8. Such an element 30 comprises a central hub 32
and a plurality of radial ribs 34. Outwardly open channels
36 are formed between circumferentially adjacent ribs 36.
Optical fibers 38 are positioned within respective ones of
the channels.
v. _ _
_
~2~3~
Channel elements of such configuration have heretofore
been proposed for use in fiber-optic cable, primarily because
(i) the individual fibers are precisely located by the channels,
(ii) a void within the cable can be more readily filled with
a water-repelling agent or the like, as compared with the
arrangement of a tube-encased fiber as depicted in Figure 1,
(iii) microbending losses at long wave lengths are reduced,
and (iv) mass-splicing techniques can be used whereby an
entire grooved rod with all fibers is spliced in one operation
rather than having to break-out individual fibers for splicing.
In accordance with the present invention, such a
channel-defining element is formed of thermotropic liquid
crystalline polymer material. Accordingly, the channel
element itself constitutes a strength member due to the
high tensile properties of thermotrop~c liquid crystalline
polymer material. Thus, the need for additional reinforcement
can be eliminated or significantly minimized. Also, the
negativity and magnitude of the coefficient of linear thermal
expansion of the thermotropic liquid-crystalline polymer
material relative to that of glass is of great significance
as explained earlier.
A further advantage relates to the simplicity of
fabrication, since a strength member of thermotropic liquid
crystalline polymer material can be easily extruded.
An alternate method of forming such a channel member
is to machine channels into rods of thermotropic liquid
crystalline polymer formed by methods described above.
~3
3~
A simple method of performing such a machinlng operation
would be to pass the rod through a tool or die consisting
of a multiplicity of circularly disposed cutting teeth,
which would continuously cut channels corresponding to the
shape of the teeth.
By comparison, the direct melt extrusion of such a
channel member of a strength material such as poly(p-phenylene
terephthalamide), for example, would not be possible. Such a
configuration could be extruded from poly(p-phenylene terephthala-
mide) by passing performed filaments of poly(p-phenylene
terephthalamide) through a hot sizing die, but only if an
adhesive substance is added tc bond the individual poly(p-
phenylene terephthalamide) filaments togehter. The use of
an adhesive is not required, however, if filaments of thermo-
tropic liquid crystalline polymer are employed, since the
thermotropic liquid crystalline polymer filaments are capable
of being fused together in the die. Such an extrusion process
is depicted in Figure 9, wherein a plurality of filaments 40 of
thermotropic liquid crystalline polymer are passed through a
hot sizing die 42 and, in so doing, are fused together in the
form of a unitary channel member 44.
As noted earlier, the thermotropic liquid crystalline
polymer is highly suited for use as a wrap 18 for the fiber-
optic cable, especially due to the relatively large negative
value of the coefficient of thermal expansionO Such an
expedient may also take the form of a ribbon array, as
~3~i3~
depicted in Figure 10. A series of tapes 50 are passed
through a die simultaneously with spaced rows 52 of optical
fibers. The fibers adhere to layers of ethylene vinyl
acetate adhesive on the tapes 50. Side tapes ~4 enclose the
sides of the ribbon, with the optical fibers sandwiched between
the layers 50 of thermotropic liquid crystalline polymer.
This type of cable can be employed as is, or can be helically
twisted and surrounded by reinforcement.
It will be appreciated that the present invention
enables fiber-optic cables of less size, bulk, and weight
to be prGduced, without sacrificing stiffness and the ability
to avoid straining of the optic fiber. The high tensile
modulus and ease of fabrication of a thermotropic liquid
crystalline polymer material as compared to a lyotropic
liquid crystalline polymer, such as poly(p-phenylene tere-
phthalamide), enables a cable component formed of a thermo-
tropic liquid crystalline polymer to perform multiple functions,
e.g., to act as a strength member as well as a sheathing member.
Thus, the quantity of elements which must be incorporated
within the cable can be reduced.
Since a thermotropic liquid crystalline polymer material
is dielectric, it can be employed in fiber-optic cable in cases
where metallic reinforcement it unacceptable due to its elec-
trical conductivity.
The high negativity of the coefficient of linear thermal
expansion of thermotropic liquid crystalline polymer material
and the ability to control the value of such a coefficient,
renders the use of a thermotropic liquid crystalline polymer
_ _
=
^~
~2;~6
highly beneficial in fiber-optic cable containing fibers
of glass, since the temperature range in which such cable
can be effectively employed is significantly increased.
The ability of thermotropic liquid crystalline
polymer material to be melt extruded enables members of
relatively large and complex cross-section to be formed
of a strength material. Previously, such members were in
some instances formed of non-strength materials such as poly-
ethylene terephthalate. Fabrication thereoE from strength
materials has required the gluing together of individual
elongated members, a relatively expensive procedure. Thus,
channel members of thermotropic liquid crystalline polymer
material may be extruded by direct melt extrusion or by fusing
together a plurality of elongated members of thermotropic
liquid crystalline polymer material without the need for an
adhesive.
Reinforcing members formed of thermotropic liquid
crystalline polymer material can be thinned to the point
where the permissible elongation reaches fairly low levels,
e.g., 1-3%, and closely resembles that of glass. This assures
that the glass and the reinforcement will both break when
elongated by similar amounts, thereby avoiding the situation
where some optical fibers break, but not the reinforcement.
That is, it is considered preferable that the fiber-optic
cable either function at full capacity or not at all. This
is achieved by having the glass fibers and the reinforcement
break at about the same time.
I/
_ I_
~363'~:~
lthough the invention has been described in
connection with preferred embodiments thereof, it will
be appreciated by those skilled in the art, that additions,
modifications, substitutions, and deletions not specifically
described, may be made without departing from the spirit
and scope of the invention as defined in the appended claims.
_.
