Patent Description:
<CIT> discloses optical fiber cables with a polymeric protective component comprising a polymeric matrix material and a plurality of microcapillaries defining longitudinal channels. <CIT> discloses a crosslinked olefin elastomer foam. <CIT> and <CIT> show additional prior art.

Additional features and advantages will be set forth in the detailed description that follows, and in part will be readily apparent to those skilled in the art from the description or recognized by practicing the embodiments as described in the written description and claims hereof, as well as the appended drawings.

The accompanying drawings are included to provide a further understanding and are incorporated in and constitute a part of this specification. The drawings illustrate one or more embodiment(s), and together with the description serve to explain principles and the operation of the various embodiments.

Referring generally to the figures, the present disclosure relates to a thermoplastic foam that can be extruded around one or more ribbon stacks and/or buffer tubes of an optical fiber cable. The foam is formed from either a polymer blend of polyolefin elastomer (POE) and low density polyethylene (LDPE) or a thermoplastic elastomer (TPE) with LDPE. The foam provides cushion for the optical fibers within the buffer tube(s). That is, the foam helps prevent attenuation of the optical fibers when the cable is bent, crushed, twisted, flexed, etc. In particular, the foam, which has a low modulus, diminishes the transmission of outside stress forces to the ribbon stack, which could otherwise cause fiber attenuation.

Further, in armored cable designs, the foam prevents attenuation issues caused by armor contact during cable bending, flexing, or coiling. Additionally, the foam allows for the reduction of the wall thickness of the buffer tubes and of the cable jacket so as to allow for increased fiber density within a given cable outside diameter. In this way, the foam also allows for significantly improved cable designs along with cost reduction through elimination of free space in the tubes, reduction of cable outer diameter, and use of smaller strength members (such as glass-reinforced fiber rods). As will be discussed more fully below, the polymer blend is extruded around a central buffer tube or stranded buffer tubes in a manner that causes it to foam. These and other advantages and aspects of the foam will be discussed in relation to the embodiments disclosed and depicted herein, especially as they relate to an optical fiber cable. However, these embodiments are exemplary in nature, not limiting.

<FIG> depicts a longitudinal, cross-sectional view of an optical fiber cable <NUM>. The optical fiber cable <NUM> includes at least one buffer tube <NUM>, shown as a central tube. The buffer tube <NUM> has an inner surface <NUM> and an outer surface <NUM> that define an average buffer tube thickness T<NUM>. In embodiments, the thickness T<NUM> of the buffer tube <NUM> is from <NUM> to <NUM>. The inner surface <NUM> defines a central bore <NUM> that extends along the longitudinal axis of the optical fiber cable <NUM> for at a portion of the length of the optical fiber cable <NUM>. Disposed within the central bore <NUM> of the buffer tube <NUM> is a stack <NUM> of optical fiber ribbons <NUM>. The optical fiber ribbons <NUM> include a plurality of optical fibers <NUM> arranged in a substantially planar array. In embodiments, the optical fibers <NUM> may be held in the array via a binding matrix and at least one coating of a curable resin.

Surrounding the buffer tube <NUM> along the longitudinal axis is a foam layer <NUM>. As used herein, each element inside the foam layer <NUM> will be collectively referred to as an "optical fiber cable core" <NUM>. Thus, in the embodiment of <FIG>, the optical fiber cable core <NUM> includes the stack <NUM> of optical fiber ribbons <NUM> and the buffer tube <NUM>. In embodiments, the foam layer <NUM> is extruded and drawn over the outer surface <NUM> of the buffer tube <NUM>. Further, in embodiments, the foam layer <NUM> has a thickness T<NUM> of from <NUM> to <NUM>. In other embodiments, the foam layer <NUM> has an average thickness T<NUM> of from <NUM> to <NUM>, and in still other embodiments, the foam layer <NUM> has a thickness T<NUM> of from <NUM> to <NUM>.

Optionally, in embodiments, the optical fiber cable <NUM> includes an armor layer <NUM> disposed around the foam layer <NUM>. The armor layer <NUM> may be formed from a metal tape that is wrapped around the cable core <NUM>, or may be comprised of a dielectric material or any other suitable material for protecting the core elements of the optical fiber cable <NUM>. In certain embodiments, the armor layer <NUM> is corrugated. A cable jacket <NUM> surrounds the armor layer <NUM> (if provided) or the foam layer <NUM> (if no armor layer <NUM> is provided). The cable jacket <NUM> has an inner surface <NUM> and an outer surface <NUM> that define an average jacket thickness T<NUM>. In embodiments, the cable jacket <NUM> has a thickness T<NUM> of from <NUM> to <NUM>. In embodiments, the cable jacket <NUM> has a thickness T<NUM> of about <NUM>. In embodiments, the outer surface <NUM> of the cable jacket <NUM> defines the outermost surface of the optical fiber cable <NUM>. As depicted in <FIG>, the optical fiber cable <NUM> may include strength elements <NUM> embedded in the cable jacket <NUM> between the inner surface <NUM> and the outer surface <NUM>. Exemplary strength elements <NUM> include glass-reinforced plastic (GRP) rods and metal wire. In embodiments, the thickness T<NUM> is limited on the low end of the thickness T<NUM> range by the size of the strength elements <NUM>.

In another embodiment shown in <FIG>, the optical fiber cable <NUM>' includes a plurality of buffer tubes <NUM> stranded around a central strength member <NUM> having an upjacket <NUM>. The buffer tubes <NUM> contain a plurality of optical fibers <NUM> in a loose tube configuration. The buffer tubes <NUM> are stranded around the upjacketed central strength member <NUM>, e.g., in a helical or SZ-stranded manner. Thus, in this embodiment of the optical fiber cable <NUM>', the cable core <NUM> includes the plurality of buffer tubes <NUM>, the optical fibers <NUM>, the central strength member <NUM>, and the upjacket <NUM>. Surrounding the cable core <NUM> is the foam layer <NUM>, and disposed around the foam layer <NUM> is the cable jacket <NUM>.

In a further embodiment of an optical fiber cable <NUM>" depicted in <FIG>, the foam layer <NUM> surrounds the ribbon stack <NUM>. In the embodiment depicted, the buffer tube <NUM> is excluded (in which case the cable core <NUM> of the embodiment depicted includes just the ribbon stack <NUM>). However, in other embodiments, a buffer tube <NUM> may surround the foam layer <NUM>, and another foam layer <NUM> could surround the buffer tube <NUM> (in which case the cable core <NUM> includes the ribbons tack <NUM>, the first foam layer <NUM>, and the buffer tube <NUM>). In the embodiment depicted, the foam layer <NUM> is surrounded by an optional armor layer <NUM>, which is surrounded by the cable jacket <NUM>. Thus, in the embodiment of <FIG>, the foam layer <NUM> is in contact with the outer surface of the ribbon stack <NUM> and with the inner surface of the armor layer <NUM>. However, in other embodiments, the foam layer <NUM> can be in contact with the outer surface of the ribbon stack <NUM> and with the inner surface <NUM> of a buffer tube <NUM> or with the inner surface <NUM> of the cable jacket <NUM>.

Having described three embodiments of the optical fiber cables <NUM>, <NUM>', <NUM>" in which the foam layer <NUM> may be incorporated, the foam layer <NUM> will now be described in greater detail. As mentioned above, the foam layer <NUM> comprises either (<NUM>) a blend of polyolefin elastomer (POE) and LDPE or (<NUM>) a thermoplastic elastomer (TPE) in a blend with LDPE.

The polymer blend of POE or TPE and LDPE comprises from <NUM>% by weight to <NUM>% by weight of LDPE. In embodiments, the polymer blend comprises from <NUM>% by weight to <NUM>% by weight of LDPE, and in still other embodiments, the polymer blend comprises from <NUM>% by weight to <NUM>% by weight of LDPE. The polymer blend comprises from <NUM>% by weight to <NUM>% by weight of POE or TPE. In embodiments, the polymer blend comprises from <NUM>% by weight to <NUM>% by weight of POE or TPE, and in still other embodiments, the polymer blend comprises from <NUM>% by weight to <NUM>% by weight of POE or TPE.

A variety of POE are suitable for use in the polymer blend that forms the foam layer <NUM>. In an exemplary embodiment, the POE is selected to have an unfoamed tensile modulus of at most <NUM> MPa at <NUM>% secant according to ASTM D638. In other embodiments, the POE is selected to have an unfoamed tensile modulus of at most <NUM> MPa at <NUM>% secant according ASTM D638. In exemplary embodiments, the POE is a copolymer of ethylene and a C<NUM>-C<NUM> alpha-olefin, such as an ethylene-octene copolymer. Such copolymers offer a low modulus at low temperature and high recovery from mechanical deformations. Two commercially available ethylene-octene copolymers include the Engage™ copolymer family and Infuse™ Olefin Block Copolymers (OBCs). The Engage™ copolymers are random copolymers with melt temperatures up to <NUM>, and the Infuse™ OBC have alternating blocks of hard (highly rigid) and soft (highly elastomeric) segments and melt temperatures ranging from <NUM> to <NUM>. Exemplary commercial embodiments of LDPE include Agility™ <NUM> or AXELERON™ <NUM> (both available from Dow Chemical Company, Midland, MI).

In embodiments in which the foam layer <NUM> is made from a blend of TPE and LDPE, the TPE is selected to have an unfoamed tensile modulus of at most <NUM> MPa at <NUM>% secant according to ASTM D638. In other embodiments, the thermoplastic elastomer is selected to have an unfoamed tensile modulus of at most <NUM> MPa at <NUM>% secant according ASTM D638. Examples of thermoplastic elastomers suitable for use in forming the foam layer include thermoplastic polyolefins (TPO) and thermoplastic vulcanizates (TPV). Commercially available examples include Catalloy TPOs of Softell grades (LyondellBasell Industries, Houston, TX), Santoprene™ (Exxon Mobil Corporation, Irving, TX), and Sarlink® <NUM> (Teknor Apex, Pawtucket, RI).

In embodiments, the foam layer <NUM> is formed by extruding the polymer blend at a sufficiently high pressure so that a gaseous blowing agent, or agents, remain dissolved in the melt until the polymer-gas saturation pressure is reached near or just beyond the exit of the extrusion die. At this saturation pressure, dissolved gas demixes as many, well-distributed bubble nuclei expand and serve as nucleation sites for additional bubble formation and growth.

In the polymer blend, POE or TPE provides the elastomeric property to the foam while the LDPE provides a high expansion ratio as a result of high melt strength that results from long chain branching. POE copolymers are based on a polyethylene or polypropylene backbone with high flexibility, i.e., low modulus. During foam extrusion, if the melt strength of the blend is too low, the bubbles will rupture and coalesce before the foam is cooled and a poor quality foam with large bubbles will result. The LDPE with its long chain branching exhibits strain hardening. Strain hardening is measured in the melt and represents the increase in elongational viscosity while the melt is being strained. This allows for maximum bubble expansion without excessive rupturing and coalescing.

In the optical fiber cables <NUM>, <NUM>', <NUM>", the recovery from impact, crush, or bending stresses is achieved with a closed cell foam. In embodiments, the foam is manufactured in a physical foam process. However, in embodiments, the physical foam process can also incorporate one or more chemical foaming agents to further improve foam structure. Physical nucleating agents include talc, calcium carbonate, PTFE particles, and other high surface area inorganic and polymeric materials. In embodiments, such physical nucleating agents are present in the polymer blend in an amount of <NUM> to <NUM> wt%. Chemical foam agents include endothermic foaming agents, such as Reedy Safoam FPE- <NUM> (available from Reedy Chemical Foam & Specialty Additives, Charlotte, NC) and exothermic foaming agents, such as azodicarbonamide and <NUM>,<NUM>'-oxybis(benzenesulfonyl hydrazide (commercially available as XO-<NUM> and XO-<NUM> from Bergen International, LLC, East Rutherford, NJ). Additionally, in embodiments, the process of physical foaming with carbon dioxide, nitrogen, or other gases (such as small chain hydrocarbons) is further enhanced with the addition of chemical foaming agent and/or physical nucleators. Additionally, the blend may include one or more additives that prevent bubbles from coalescing, improve stability, and reduce gas diffusion out of the bubble; such as glycerol monostearate (GMS).

In embodiments, the foaming process is configured to achieve a fine, narrowly distributed closed cell morphology with equivalent circle diameter (ECD) of <NUM> to <NUM>. In other embodiments, the ECD is in the range of <NUM> to <NUM>, and in still other embodiments, the ECD is in the range of <NUM> to <NUM>. In embodiments, the resulting foam has an expansion ratio of at least <NUM>%. In other embodiments, the resulting foam has an expansion ratio of at least <NUM>%. Further, in embodiments, the resulting foam has a density reduction (as compared to an unfoamed blend) of <NUM>% - <NUM>%, more particularly of <NUM>% - <NUM>%.

As mentioned, the strain hardening behavior of the polymer melt influences the properties of the foam produced. In <FIG>, the extensional viscosity vs. time at a contestant strain of <NUM> sec-<NUM> for example polymer blend compositions is provided. The polymer compositions contain, in weight percent, <NUM>% POE ("CE1"), <NUM>%/<NUM>% LDPE/POE ("CE2"), <NUM>%/<NUM>% LDPE/POE ("E1"), and <NUM>%/<NUM>% LDPE/POE ("E2"). The LDPE used in each of the examples was Agility™ <NUM>, and the POE used in each example was Infuse™ <NUM> (both available from Dow Chemical Company, Midland, MI). As can be seen from <FIG>, E1 and E2 demonstrate more strain hardening behavior (defined as extensional viscosity increases that deviate from the linear viscoelastic region) than CE1 and CE2. As compared to CE1 and CE2 where the POE is at or above <NUM>%, E1 and E2, which contain higher amounts of LDPE, demonstrate better straining hardening behavior for foaming while still retaining the elastomeric properties and low modulus of the POE.

<FIG> depict photomicrographs of foams having similar compositions (about <NUM>/<NUM> LDPE/POE by wt%, i.e., composition E1) made using different foaming agents. The foam E1a of <FIG> was created using a physical nucleator, and the foam E1b of <FIG> was created using a blowing agent. As can be seen in a comparison of the two foams, the foam E1a of <FIG> exhibits a greater density reduction of <NUM>% as compared to the foam E1b of <FIG>, which exhibits a density reduction of <NUM>%. The pores of the foam E1a of <FIG> are also larger. The average area of each pore in <FIG> is <NUM><NUM>, whereas the average pore size in <FIG> is <NUM><NUM>. Another way to describe the pore size is equivalent circle diameter (ECD). The average ECD of the foam E1a of <FIG> is <NUM>, and the average ECD of the foam E1b of <FIG> is <NUM>. The pore size distribution of the foam E1b of <FIG> was much narrower than the pore size distribution of the foam E1a of <FIG>. That is, the foam E1b of <FIG> had more pores of roughly the same size than did the foam E1a of <FIG>.

The foams E1a, E1b of <FIG> were also subjected to compression set testing. Compression set measurement was assessed via a parallel plate compression fixture on a dynamic mechanical analyzer (DMA Q800, available from TA Instruments, New Castle, DE). During testing, each foam specimen was compressed at a constant strain of <NUM>%, <NUM>%, or <NUM>% for <NUM> hours. The compression load was removed from the foam specimens, and the specimens were monitored for strain relaxation over the next <NUM> hours. The results of the compression test are shown, below, in Table <NUM>. As can be seen in Table <NUM>, E1b had better compression set performance than E1a. That is, E1b recovered to closer to its original dimensions after compression than did E1a. The compression testing indicates that the pore morphology depicted in <FIG> provides better compression set performance for this particular application than the pore morphology depicted in <FIG>.

The foam specimens were also tested to determine other compression properties using a parallel plate compression fixture on an electromechanical tensile test machine (MTS Insight <NUM> kN) according to ASTM <NUM> - Standard Test Methods for Flexible Cellular Materials with the exception of part for test samples (specimens) preparation. Specimens size were significantly smaller than required by standard procedure. In particular, samples of E1a and E1b were loaded at a constant strain rate of <NUM>% per min until <NUM>% strain was reached. Thereafter, the samples were unloaded at a constant rate of <NUM>% per minute until the parallel plates returned to their original position. <FIG> show the stress-strain curve and the modulus-strain curves, respectively, for E1a and E1b. As can be seen in <FIG>, E1b is more stiff than E1a, requiring more compressive stress to reach a strain of <NUM>%. Additionally, E1b exhibited better recovery than E1a as the strain returned to <NUM> for E1b when the sample was unloaded. From <FIG>, it can be seen that E1b has a higher compression modulus than E1a (<NUM> MPa vs. <NUM> MPa).

The foams E1a, E1b were incorporated into optical fiber cable samples with the foam extruded over a subunit. Dimensional stability of the samples was measured according to ASTM D4565-<NUM> - Standard Test Methods for Physical and Environmental Performance Properties of Insulations and Jackets for Telecommunications Wire and Cable. The samples were tested along the direction of foam extrusion. The samples exhibited good dimensional stability over the temperature range of -<NUM> to <NUM>. In particular, as shown in Tables <NUM> and <NUM>, below, it can be seen that the samples each had a length change of less than <NUM>% after being held at <NUM> for <NUM> hours and after being held at <NUM> for <NUM> hours.

Foams having different compression modulus can be utilized in the same optical fiber cable <NUM>, <NUM>', <NUM>". For example, the optical fiber cable <NUM>, <NUM>', <NUM>" may consist of one or more layers of foams. In one embodiment, the optical fiber cable <NUM>, <NUM>', <NUM>" includes a relatively softer inner layer (i.e., lower modulus), such as foam E1a, which directly contacts the stranded core or central tube and a relatively stiffer outer layer, such as foam E1b, which may contact with cable sheath or armor layer. Such a foam structure allows for further improvement of the cable mechanical performance by absorbing the strain/stress transferred to the core. Specifically, a softer inner layer reduces the compression stress imposed on a stack of optical fiber ribbons during bending and cable coiling. A stiffer outer layer together with the softer inner layer can deform under crushing and impact loading and therefore functions as spacer to reduce the loads.

The embodiments of the optical fiber cables <NUM>, <NUM>', <NUM>" disclosed herein are envisioned to pass relevant telecommunications standards for reliability. For example, the malleability and flexibility of the foam allows movement of the ribbon stack subunit during cable coiling at 15x the cable outer diameter (i.e., minimum bend radius) over the temperature range of -<NUM> to <NUM> and allows stress dispersion during impact, crush, and other mechanical tests. Further, by replacing free space in the optical fiber cables <NUM>, <NUM>', <NUM>" with foam, the attenuation issue experienced by some conventional cables during the cable crush testing at the corner fibers of the ribbon stack is addressed and attenuation remains below <NUM> dB at all the corner fibers during the <NUM> N/cm compression load of Telcordia GR-<NUM>. Embodiments of the foam disclosed herein have a low modulus of less than <NUM> MPa for cables of having <NUM> optical fibers and below, and a lower modulus of <NUM> MPa for larger optical fiber counts, when measured at <NUM>% strain. Additionally, cable designs incorporating the foam layer have improved mid-span coiling over traditional designs because the foam layer allows for much more robust cable twist performance without attenuation increase. Indeed, according to industry standard GR-<NUM> for twist requirements, a two-meter piece of cable must be able to be twisted <NUM> degree in both directions without having any attenuation greater than <NUM> dB. Embodiments of the disclosed foam allow superior performance in twist testing with two full twists (<NUM>°) in both directions with attenuation less than <NUM> dB.

Additionally, the foam stays flexible at low temperature. The foam has a brittleness temperature of below -<NUM>. Further, the foam is dimensionally stable over the temperature range of -<NUM> to <NUM>, and has a shrinkback less than <NUM>%, as required per GR-<NUM> industry standard for jacket components.

The closed cell morphology and the selection of a POE or TPE deliver a balance of foam properties, combining low modulus to provide stress dispersion (and, consequently, fiber strain reduction) with over <NUM>% thickness recovery after being compressed to <NUM>% of its original thickness. The foam delivers instantaneous high recovery from large deformations of <NUM>% strain and low compression set after <NUM> hours at <NUM>% strain. When the foam is compressed for <NUM> hours at <NUM>% strain, it recovers to <NUM>% strain at <NUM> minutes and to <NUM>% after <NUM> hour.

The foam has low tensile strength due to its high density reduction, low modulus, and the elongation of the cells in the longitudinal direction during the draw down extrusion process over the core. Additionally, this low tensile strength results in low tear strength which provides easy access into the cable core during mid-span access or end access. Also advantageously, cable components, such as the cable jacket and buffer tube, can be made thinner. For example, cable jacket thickness can be decreased from <NUM> to about <NUM> for a stranded design (e.g., as shown in <FIG>). Further, buffer tubes having <NUM> optical fibers conventionally have a wall thickness of about <NUM>, and buffer tubes having <NUM> optical fibers have a wall thickness of about <NUM>. The thicknesses of the buffer tube according to the present disclosure is decreased to <NUM> to <NUM>.

Unless otherwise expressly stated, it is in no way intended that any method set forth herein be construed as requiring that its steps be performed in a specific order. Accordingly, where a method claim does not actually recite an order to be followed by its steps or it is not otherwise specifically stated in the claims or descriptions that the steps are to be limited to a specific order, it is in no way intended that any particular order be inferred. In addition, as used herein, the article "a" is intended to include one or more than one component or element, and is not intended to be construed as meaning only one.

Claim 1:
An optical fiber cable (<NUM>) comprising:
at least one optical fiber (<NUM>);
a cable jacket (<NUM>) having an inner surface (<NUM>) and an outer surface (<NUM>), wherein the outer surface (<NUM>) is an outermost surface of the optical fiber cable (<NUM>) and wherein the inner surface (<NUM>) is disposed around the at least one optical fiber (<NUM>);
a foam layer (<NUM>) disposed between the at least one optical fiber (<NUM>) and the cable jacket (<NUM>);
wherein the foam layer (<NUM>) comprises a polymer blend of polyolefin elastomer, POE, and low density polyethylene, LDPE, or a polymer blend of a thermoplastic elastomer, TPE, and LDPE,
characterized in that
the polymer blend of the foam layer (<NUM>) comprises from <NUM>% to <NUM>% by weight of POE or TPE and from <NUM>% to <NUM>% by weight of LDPE;
wherein the foam layer (<NUM>) comprises a closed-cell morphology having pores with an average effective circle diameter of <NUM> to <NUM>; and
wherein the expansion ratio of the foam layer (<NUM>) is at least <NUM>%.