Patent Publication Number: US-11650387-B2

Title: Foam for optical fiber cable, composition, and method of manufacturing

Description:
CROSS-REFERENCED TO RELATED APPLICATIONS 
     This application is a Divisional of U.S. patent application Ser. No. 16/918,466 filed Jul. 1, 2020, which claims the benefit of priority under 35 U.S.C. § 119 of U.S. Provisional Application Ser. No. 62/869,929 filed on Jul. 2, 2019, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention is related to an optical fiber cable having a foam layer disposed between a ribbon stack and a cable jacket and, in particular, to polymer blend and method of forming the foam layer. Optical fiber cables incorporate a variety of materials with function-specific properties in multiple layers to achieve desired performance. For examples, the cable jacket and buffer tubes are often made of polyolefin materials. The optical fiber cable may also include a metal armor layer and one or more glass-reinforced plastic strength members. Though the polyolefins often provide good flexibility, the armor layer and/or strength members may create signal attenuation when the cable is bent, coiled, crushed, or twisted. 
     SUMMARY 
     In one aspect, embodiments of the present disclosure relate to an optical fiber cable having at least one optical fiber, a cable jacket and a foam layer. The cable jacket includes an inner surface and an outer surface in which the outer surface is an outermost surface of the optical fiber cable. The inner surface is disposed around the at least one optical fiber. The foam layer is disposed between the at least one optical fiber and the cable jacket. The foam layer is made of an extruded product of at least one thermoplastic elastomer (TPE), a chemical foaming agent, and a crosslinking agent. The foam layer has a closed-cell morphology having pores with an average effective circle diameter of less than 100 μm. Further, the foam layer has a compression modulus of less than 1 MPa when measured at 50% strain. 
     In another aspect, embodiments of the present disclosure relate to a method of preparing an optical fiber cable. In the method, a thermoplastic elastomer (TPE) blend is prepared that includes 100 parts of a polymer component, 0.1 to 3 parts of a chemical foaming agent, and 0.1 to 2 parts of a crosslinking agent. The TPE has a tensile modulus of at most 10 MPa at 100% secant. In the method, the TPE blend is extruded around an optical fiber cable core in a manner that produces a foam layer surrounding the optical fiber cable core along a longitudinal axis of the optical fiber cable core. 
     In yet another aspect, embodiments of the present disclosure relate to a thermoplastic elastomer (TPE) foam. The TPE foam includes an extruded product of 100 parts by weight of a polymer component including at least one TPE, 0.1 to 3 parts by weight of a chemical foaming agent, and 0.1 to 2 parts by weight of a crosslinking agent. The TPE foam has a closed-cell morphology having pores with an average effective circle diameter of less than 100 μm. The TPE foam has a compression modulus of less than 1 MPa at 50% strain, and the TPE foam has a compression set of no more than 5% as measured after compression to a strain of 60% for ten hours and after four hours of recovery time. 
     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. 
     It is to be understood that both the foregoing general description and the following detailed description are merely exemplary, and are intended to provide an overview or framework to understand the nature and character of the claims. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
         FIG.  1    is longitudinal, cross-sectional view of an optical fiber cable having a foam layer, according to an exemplary embodiment; 
         FIG.  2    is a perspective view of a portion of an optical fiber cable having a foam layer, according to an exemplary embodiment; 
         FIG.  3    is longitudinal, cross-sectional view of a portion of an optical fiber cable having a foam layer, according to an exemplary embodiment. 
         FIG.  4    is a micrograph of a TPE foam, according to an exemplary embodiment; 
         FIG.  5    depicts a histogram of pore size distribution of the TPE foam of  FIG.  4   ; 
         FIG.  6    is a hysteresis graph of compressive stress and strain on a TPE foam according to an embodiment of the present invention and a physically foamed polymer blend according to a comparative example; and 
         FIG.  7    depicts a graph of stress and strain over time for calculating compression set, according to an exemplary embodiment. 
     
    
    
     While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
     DETAILED DESCRIPTION 
     Referring generally to the figures, embodiments of the present disclosure relate 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 the extruded product of a blend of thermoplastic elastomer (TPE), a chemical foaming agent, and a crosslinking agent. By using a chemical foaming process, the chemical foaming agent advantageously can be compounded with the TPE and extruded within or around a cable core without the need for any special processing equipment. That is, because the foaming action takes place chemically, no equipment modifications are necessary to physically create the foam. Further, as compared to physical foams, the foams prepared according to embodiments of the present disclosure have similar density reductions (e.g., at least 65%) with better compression set performance, at least in part because the chemical foaming agent produces smaller pores than physical foaming agents. In an optical fiber cable, the foam provides cushioning 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 plastic rods). As will be discussed more fully below, the TPE foam is extruded around a central buffer tube, a ribbon stack, 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.  1    depicts a longitudinal, cross-sectional view of an optical fiber cable  10 . The optical fiber cable  10  includes at least one buffer tube  12 , shown as a central tube. The buffer tube  12  has an inner surface  14  and an outer surface  16  that define an average buffer tube thickness T 1 . In embodiments, the thickness T 1  of the buffer tube  12  is from 0.25 mm to 0.30 mm. The inner surface  14  defines a central bore  18  that extends along the longitudinal axis of the optical fiber cable  10  for at least a portion of the length of the optical fiber cable  10 . Disposed within the central bore  18  of the buffer tube  12  is a stack  20  of optical fiber ribbons  22 . The optical fiber ribbons  22  include a plurality of optical fibers  24  arranged in substantially planar arrays. In embodiments, the optical fibers  24  may be held in the array via a binding matrix and at least one coating of a curable resin. 
     Surrounding the buffer tube  12  along the longitudinal axis is a foam layer  26 . As used herein, each element inside the foam layer  26  will be collectively referred to as an “optical fiber cable core”  27 . Thus, in the embodiment of  FIG.  1   , the optical fiber cable core  27  includes the stack  20  of optical fiber ribbons  22  and the buffer tube  12 . In embodiments, the foam layer  26  is extruded and drawn over the outer surface  16  of the buffer tube  12 . Further, in embodiments, the foam layer  26  has a thickness T 2  of from 0.5 mm to 3 mm. In other embodiments, the foam layer  26  has an average thickness T 2  of from 0.5 mm to 2 mm, and in still other embodiments, the foam layer  26  has a thickness T 2  of from 1 mm to 2 mm. 
     Optionally, in embodiments, the optical fiber cable  10  includes an armor layer  28  disposed around the foam layer  26 . In embodiments, the armor layer  28  is formed from a metal tape that is wrapped around the cable core  27 . In certain embodiments, the armor layer  28  is corrugated. A cable jacket  30  surrounds the armor layer  28  (if provided) or the foam layer  26  (if no armor layer  28  is provided). The cable jacket  30  has an inner surface  32  and an outer surface  34  that define an average jacket thickness T 3 . In embodiments, the cable jacket  30  has a thickness T 3  of from 1.25 mm to 1.75 mm. In embodiments, the cable jacket  30  has a thickness T 3  of about 1.50 mm. In embodiments, the outer surface  34  of the cable jacket  30  defines the outermost surface of the optical fiber cable  10 . As depicted in  FIG.  1   , the optical fiber cable  10  may include strength elements  36  embedded in the cable jacket  30  between the inner surface  32  and the outer surface  34 . Exemplary strength elements  36  include glass-reinforced plastic (GRP) rods and metal wire. In embodiments, the thickness T 3  is limited on the low end of the thickness T 3  range by the size of the strength elements  36 . 
     In another embodiment shown in  FIG.  2   , the optical fiber cable  10 ′ includes a plurality of buffer tubes  12  stranded around a central strength member  38  having a coating  40 . The buffer tubes  12  contain a plurality of optical fibers  24  in a loose tube configuration. The buffer tubes  12  are stranded around the coated central strength member  38 , e.g., in a helical or SZ-stranded manner. Thus, in this embodiment of the optical fiber cable  10 ′, the cable core  27  includes the plurality of buffer tubes  12 , the optical fibers  24 , the central strength member  38 , and the coating  40 . Surrounding the cable core  27  is the foam layer  26 , and disposed around the foam layer  26  is the cable jacket  30 . 
     In a further embodiment of an optical fiber cable  10 ″ depicted in  FIG.  3   , the foam layer  26  surrounds the ribbon stack  20 . In the embodiment depicted, the buffer tube  12  is excluded (in which case the cable core  27  of the embodiment depicted includes just the ribbon stack  20 ). However, in other embodiments, a buffer tube  12  may surround the foam layer  26 , and another foam layer  26  could surround the buffer tube  12  (in which case the cable core  27  includes the ribbon stack  20 , the first foam layer  26 , and the buffer tube  12 ). In the embodiment depicted, the foam layer  26  is surrounded by an optional armor layer  28 , which is surrounded by the cable jacket  30 . Thus, in the embodiment of  FIG.  3   , the foam layer  26  is in contact with the outer surface of the ribbon stack  24  and with the inner surface of the armor layer  28 . However, in other embodiments, the foam layer  26  can be in contact with the outer surface of the ribbon stack  24  and with the inner surface  14  of a buffer tube  12  or with the inner surface  32  of the cable jacket  30 . 
     Having described three embodiments of the optical fiber cables  10 ,  10 ′,  10 ″ in which the foam layer  26  may be incorporated, the foam layer  26  will now be described in greater detail. As mentioned above, the foam layer  26  is a TPE foam formed through a chemical foaming process. 
     In embodiments, the foam layer  26  primarily comprises a polymer component, a chemical foaming agent, and a crosslinking agent. In embodiments, the foam layer  26  comprises 0.1 to 3 parts of active chemical foaming agent and 0.1 to 2 parts of the active crosslinking agent per 100 parts of the polymer component. 
     In embodiments, the polymer component is primarily comprised of one or more TPE. In embodiments, the polymer component comprises at least 90% by weight of the one or more TPE. In embodiments, the polymer component may also comprise up to 10% by weight of low density polyethylene (LDPE). 
     A variety of TPE are suitable for use in the polymer component of the foam layer  26 . In embodiments, the TPE comprises at least one of a polyolefin elastomer (POE), a thermoplastic polyolefin (TPO), or a thermoplastic vulcanizate (TPV). In an exemplary embodiment, the TPE is selected to have an unfoamed tensile modulus of at most 10 MPa at 100% secant according to ASTM D638. In other embodiments, the TPE is selected to have an unfoamed tensile modulus of at most 5 MPa at 100% secant according ASTM D638. 
     In exemplary embodiments, suitable POE for the foam layer  26  include copolymers of ethylene and octene or butene, such as an ethylene-octene copolymer or an ethylene-butene 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). Commercially available examples of TPOs include Catalloy TPOs of Softell grades (LyondellBasell Industries, Houston, Tex.), and commercially available examples of TPVs include Santoprene™ (Exxon Mobil Corporation, Irving, Tex.), and Sarlink® 8145 (Teknor Apex, Pawtucket, R.I.). 
     In embodiments, the chemical foaming agent comprises at least one of azodicarbonamide, azodiisobutyronitrile, benzenesulfohydrazide, 4,4-oxybenzenesulfonyl semicarbazide, para-toluene sulfonyl semicarbazide, barium azodicarboxylate, N,N′-dimethyl-N,N′-dinitrosoterephthalamide, trihydrazino triazine, or sodium bicarbonate. In embodiments, the chemical foaming agent is introduced to the foam layer  26  via a masterbatch, which provides for ease of handling. Commercially available examples of chemical foaming agents include Foamazol™ (Bergen International, LLC, East Rutherford, N.J.), Hydrocerol® (Clariant, Muttenz, Switzerland), Safoam® (Reedy Chemical Foam &amp; Specialty Additives, Charlotte, N.C.), or similar chemical foaming agents. 
     In embodiments, the crosslinking agent comprises a peroxide. In particular embodiments, the peroxide comprises at least one of dicumyl peroxide, di-tert-butyl peroxide, ditertiary amyl peroxide, tert-butyl peroxide, tert-butyl cumyl peroxide, dibenzoyl peroxide, or tert-butyl hydroperoxide. Masterbatch of crosslinking agent is also preferred for the ease of handling. Commercially available examples include Luperox® (Arkema S.A., Colombes, France) and PCL (Polyvel Inc., Hammonton, N.J.). The crosslinking agent is used to produce free radicals during melt extrusion and induce partial crosslinks in the TPE. The partially crosslinked TPE has an increased melt strength so that the foam cell coalescence is minimized during foaming and density reduction is increased. 
     In embodiments, the foam layer  26  is formed by extruding the TPE blend around the cable core  27 . Advantageously, the TPE blend can be prepared by simply mixing the polymer component, the chemical foaming agent, and the crosslinking agent in an extruder. In particular embodiments, the polymer component, the chemical foaming agent, and the crosslinking agent are dry-mixed prior to adding them into the extruder hopper. Other additives may also be added to the TPE blend in the extruder, including nucleating agents, processing aids, UV stabilizers, and/or antioxidants, among others. Successful extrusion of the TPE blend as a foam is achieved by adjusting the temperature and pressure profiles within the extruder to efficiently use the chemical foaming agent. During extrusion, the temperature at the feed zone is kept low enough to prevent premature decomposition of chemical foaming agents in the barrel while still allowing a melt seal to form (otherwise gas loss may occur back through the hopper). The melt zone temperature should then increase rapidly to above the decomposition temperature of the chemical foaming agent(s) and at the same time initiate the peroxide decomposition. Sufficient pressure is maintained on the melt to prevent foaming in the extruder. In embodiments, the pressure is maintained by use of a high compression screw or temperature reduction after the melting zone of the extruder. The pressure is maintained until the TPE blend exits the die at which point the rapid pressure drop initiates nucleation and foaming of the TPE blend. The TPE blend melt temperature at this point is kept as low as possible so that cooling can take place quickly to control expansion and limit escape of the gas. In embodiments, the temperature is kept lower than that for unfoamed plastics to enhance surface appearance. 
     In the TPE blend, the TPE provides the elastomeric property to the foam while the crosslinking agent provides a high expansion ratio as a result of high melt strength that results from the crosslinking. 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. Additionally, the blend may include one or more additives that prevent bubbles from coalescing and that improve stability, such as glycerol monostearate (GMS). 
       FIG.  4    provides a picture of the TPE foam. Using the picture of the foam, an analysis of the pore size distribution was performed using the equivalent circle diameter (ECD) methodology, which involves tracing pore outlines within a given area and measuring the diameter of the tracings.  FIG.  5    provides a histogram of the pore size distribution. As can be seen in  FIG.  5   , the pores have an ECD of 100 μm or less. In particular, the pores have an ECD of 50 μm or less. In particular, the pores have an ECD distribution with a peak between 20 μm and 30 μm. In embodiments, the TPE foam has a density reduction (as compared to an unfoamed blend) of at least 60%, more particularly at least 65%. 
     The TPE foam specimen of  FIG.  4    was tested to determine the compression modulus at 50% strain using a parallel plate compression fixture on an electromechanical tensile test machine (MTS Insight 5 kN) according to ASTM 3574—Standard Test Methods for Flexible Cellular Materials. In particular, the TPE foam specimen was loaded at a constant strain rate of 30% per min until 50% strain was reached. Thereafter, the specimen was unloaded at a constant rate of 30% per minute until the parallel plates returned to their original position.  FIG.  6    shows the modulus-strain curve, for the TPE foam. As can be seen in  FIG.  6   , the TPE foam had a compression modulus of less than 1 MPa at 50% strain. In particular, the TPE foam has a compression modulus of less than 0.25 MPa at 50% strain. Further, the TPE foam returned to a strain of less than 0.1 (e.g., about 0.05) when unloaded. 
     The foam of  FIG.  4    was 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, the TPE foam specimen was compressed at a constant strain of 60% for 10 hours. The compression load was removed from the foam specimen, and the specimen was monitored for strain relaxation over the next 4 hours. The results of the compression test is shown in  FIG.  7   . As can be seen in  FIG.  7   , the specimen only experienced a compression set of less than 5%. That is, after compression at a strain of 60% for 10 hours, the specimen recovered to 95% of its original thickness after strain was removed. 
     Foams having different compression modulus can be utilized in the same optical fiber cable  10 ,  10 ′,  10 ″. For example, the optical fiber cable  10 ,  10 ′,  10 ″ may consist of one or more layers of foams. In one embodiment, the optical fiber cable  10 ,  10 ′,  10 ″ includes a relatively softer inner layer (i.e., lower modulus), which directly contacts the stranded core or central tube, and a relatively stiffer outer layer, 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. In embodiments, increasing the amount of chemical foaming agent or decreasing the amount of crosslinking agent reduces the compression modulus (e.g., to create the lower modulus inner layer), and decreasing the amount of chemical foaming agent or increasing the amount of crosslinking agent increases the compression modulus (e.g., to create the higher modulus outer layer). 
     The embodiments of the optical fiber cables  10 ,  10 ′,  10 ″ 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 15× the cable outer diameter (i.e., minimum bend radius) over the temperature range of −30° C. to 70° C. and allows stress dispersion during impact, crush, and other mechanical tests. Further, by replacing free space in the optical fiber cables  10 ,  10 ′,  10 ″ 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 0.15 dB at all the corner fibers during the 110 N/cm compression load of Telcordia GR-20. 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-20 for twist requirements, a two meter piece of cable must be able to be twisted 180 degree in both directions without having any attenuation greater than 0.15 dB. Embodiments of the disclosed foam allow superior performance in twist testing with three full twists)(1080°) in both directions with attenuation less than 0.15 dB. 
     Additionally, the foam stays flexible at low temperature. The foam has a brittleness temperature below −50° C. Further, the foam is dimensionally stable over the temperature range of −40° C. to 85° C., and has a shrinkback less than 5%, as required per GR-20 industry standard for jacket components. 
     The combination of a crosslinking agent with a chemical foaming agent exhibits higher density reduction (&gt;60%) than standard foams made with chemical foaming agents only. The closed cell morphology and the selection of a TPE blend deliver a balance for the TPE foam properties that combines low modulus to provide stress dispersion (and subsequent fiber strain reduction) with &gt;90% thickness recovery after being compressed to 60% of its original thickness. The TPE foam also delivers instantaneous high recovery from large deformations of 50% strain. 
     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. 
     It will be apparent to those skilled in the art that various modifications and variations can be made without departing from the spirit or scope of the disclosed embodiments. Since modifications, combinations, sub-combinations and variations of the disclosed embodiments incorporating the spirit and substance of the embodiments may occur to persons skilled in the art, the disclosed embodiments should be construed to include everything within the scope of the appended claims and their equivalents.