Patent Publication Number: US-2020301087-A1

Title: Flame retardant compound on cable central member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation application of International Application Number PCT/US2018/061537, filed Nov. 16, 2018, which claims priority to U.S. Patent Application No. 62/592,919, filed Nov. 30, 2017, the disclosure of each of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     Referring to  FIG. 1 , an optical communication cable, shown as cable  10 , includes a cable body, shown as cable jacket  12 , having an inner surface that defines an internal area or region within which the various cable components discussed below are located. Generally, a plurality of optical fibers  14  is included among the cable components, and the cable  10  provides structure and protection to a plurality of optical fibers  14  during and after installation (e.g., protection during handling, protection from elements, protection from vermin, etc.). 
     As shown in  FIG. 1 , the cable  10  includes a plurality of buffer tubes  16  wound in a pattern or arrangement (e.g., a spiral pattern, a helical pattern, SZ pattern, etc.) around a central support member, shown as central strength member  20 . The buffer tubes  16  house the optical fibers  14 , which may be loose fibers, intermittently connected groups of fibers, or optical fiber ribbons, for example. The central strength member may be formed from a material such as glass fiber reinforced plastic (GRP) or metal (e.g., steel). The buffer tubes  16  surround the central member  20  and may be up jacketed with an up jacket layer  22  to achieve a certain thickness for stabilization against shrink forces 
     New regulations, such as the Construction Product Regulation (CPR) in Europe, require improved flame retardant properties for optical fiber cables. To improve burn characteristics of the cables, such as a measurement of total heat release rate and smoke density during burning, all components of the cable have to be looked at for optimization. For example, the jacket  12  may be a halogen free flame retardant sheath. In addition, as also shown in  FIG. 1 , a high flame retardant bedding compound  30  may be used to fill an outer core area  32  comprising the gaps and interstices between the buffer tubes  16  and the jacket  12 . The usage of a bedding compound improves significantly the flame retardant properties of a cable. Bedding compounds are typically a relatively soft material with very low or nearly no mechanical strength. The flame retardant additive content (active or passive mineral filler) goes up to 80% with 20% of very soft polymer that works as a binder to ensure the shape of the produced geometry. 
     However, the up jacket layer  22  is typically comprised of Polyethylene (PE), which is a combustible material. Moreover, as shown in  FIG. 1 , there is typically free space in the interstices  32  of an inner core area between the up jacketed central member  20  and the buffer tubes  20 . As such, a flame can travel unhindered through the interstices  32 , which work as a chimney to potentially accelerate the heat and flame distribution within the cable  10 . To improve the burn characteristics of a cable such that shown in  FIG. 1 , there is a need to remove the up jacketed layer  22  on the central member  20  and fill the interstices  32  of the inner core area with a highly flame retardant compound. 
     SUMMARY 
     An optical fiber cable includes a central strength member, a bedding compound surrounding the central strength member, and a plurality of buffer tubes stranded around the central strength member and the bedding compound such that the bedding compound forms to the buffer tubes and occupies substantially the entirety of an inner core area between the buffer tubes and the central strength member. A jacket may be provided to surround the plurality of stranded buffer tubes. 
     A method of manufacturing an optical fiber cable includes extruding a bedding compound onto a central strength member and stranding buffer tubes around the central strength member such that the buffer tubes at least partially embed into the bedding compound while the bedding compound is in a molten state. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of a fiber optic cable in accordance with aspects of the present disclosure. 
         FIG. 2  is a cross-sectional view of a fiber optic cable in accordance with yet other aspects of the present disclosure. 
         FIG. 3  is a chart illustrating the impact of lateral support on a buffer tube&#39;s stiffness. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 2  illustrates an optical fiber cable, shown as cable  110  that includes a cable body, shown as cable jacket  112 , having an inner surface that defines an internal area or core region within which the various cable components discussed below are located. Generally, a plurality of optical fibers  114  is included among the cable components, and the cable  110  provides structure and protection to a plurality of optical fibers  114  during and after installation (e.g., protection during handling, protection from elements, protection from vermin, etc.). The optical fibers are preferably silica-based, single mode fibers, but they can be any type of optical fiber including, for example, a multi-mode or dispersion shifted optical fibers. The optical fibers  114  may be intermittently or continuously connected with a UV curable matrix, for example, to be rollable or standard optical fiber ribbons. 
     As shown in  FIG. 2 , the cable  110  includes a plurality of buffer tubes  116  wound in a pattern or arrangement (e.g., a spiral pattern, a helical pattern, SZ pattern, etc.) around a central support member, shown as central strength member  120 . The buffer tubes  116  house the optical fibers  114 , which may be loose fibers, intermittently connected groups of fibers, or optical fiber ribbons, for example. The central strength member  120  may be formed from a material such as glass fiber reinforced plastic (GRP) or metal (e.g., steel). The buffer tubes  116  surround the central member  120 . The buffer tubes may be provided with at least one water-blocking substance, such as gel, grease, or a super-absorbent polymer powder. 
     The jacket  112  may be a halogen free flame retardant sheath. In addition, as also shown in  FIG. 2 , a high flame retardant bedding compound  130  may be used to fill an outer core region  132  comprising the gaps and interstices between the buffer tubes  116  and the jacket  112 . The usage of a bedding compound improves significantly the flame retardant properties of a cable. Bedding compounds are typically a relatively soft material with very low or nearly no mechanical strength. The flame retardant additive content (active or passive mineral filler) goes up to 80% with 20% of very soft polymer that works as a binder to ensure the shape of the produced geometry. 
     As shown in  FIG. 2  however, the up jacket layer  22  of  FIG. 2  is removed, as is the combustible presence of the PE material that typically comprises the up-jacket layer. As also shown in  FIG. 2 , a second bedding compound  134  may be applied during extrusion and used to fill the free space in the interstices of an inner core area  136  between the central member  120  and the buffer tubes  116 . The bedding compound  134  may be applied before or as the buffer tubes  120  are being stranded about the central strength member  120 . Accordingly, the bedding compound  134  will provide certain softness during the manufacturing process to allow the buffer tubes  116  to be pressed into the bedding compound  132  and fill all interstices of the inner core area  136 . 
     An active flame retardant effect is thus brought into an area where no flame retardancy conventionally exists and the flammable PE of the up jacket layer may be replaced. The chimney effect discussed with respect to  FIG. 1  is prevented and the additional flame retardant materials reduce heat transfer and flame spread. The buffer tubes  116  are effectively bedded on the central member  120  and stabilized (from the center of the cable and between each other), which improves crush resistance. In addition, the bedding works as a stabilizing filler along the cable by increasing the contact area between central member  120  and the buffer tubes  116  which increases friction in the case of thermal contraction. The bedding materials can be chosen to avoid sticking to the buffer tubes  116  for installation and a suitable production process (e.g., coilable types, hardness, filler amount). 
     Table 1 below illustrates the impact of the bedding compound on the flame retardant characteristics of the cable  110  shown in  FIG. 2 . Two samples of a conventional PE up-jacket layer are compared to two bedding compounds. As illustrated, Sample A and Sample B of the up-jacket layer have significantly lower limiting oxygen index (LOI) and significantly higher peak heat release rates. As shown, the two bedding compound samples, Sample C and Sample D, have an LOI of 65% or higher and peak heat release rate 144 kW/m 2  or lower. 
     
       
         
           
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Limited  
                 peak heat  
                   
               
               
                   
                 oxygen 
                 release rate 
                   
               
               
                   
                 index  
                 (kW/m 2 ) 
                   
               
               
                   
                 (the higher, 
                 (the lower  
                 Price 
               
               
                 Material 
                 the better) 
                 the better) 
                 €/kg 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
            
               
                 Sample A-Up-jacket 
                 40% 
                 234 
                 1.88 
               
               
                 Sample B-Up-jacket 
                 35% 
                 360 
                 1.98 
               
               
                 Sample C-Bedding compound 1 
                 65% 
                 42 
                 1.26 
               
               
                 (FM 474/5) 
                   
                   
                   
               
               
                 Sample D-Bedding compound 2 
                 80% 
                 144 
                 1.11 
               
               
                 (FM 361) 
                   
                   
                   
               
               
                   
               
            
           
         
       
     
     An additional bonus for the cable manufacturer is that the bedding compounds typically cost substantially less than comparative amounts of the PE up jacket material, as also shown in Table 1. 
     The percentage weight of a cable that is comprised by the up jacket layer is typically between 10-20% for many types of indoor optical fiber cables. Accordingly, the replacement of materials such as the up jacket layer can have a huge impact on price and burn performance of the cable. 
     The lower melting temperature of the bedding materials disclosed herein results in a level of softness in the material at relative low processing temperatures (50-100° C.), which allows for the buffer tubes  116  to be pressed into the bedding  134  to fill the inner interstices of the inner core area  136 . By cooling to room temperature, the modulus of the bedding  134  increases significantly and creates a supporting structure in the inside. As shown in the chart in  FIG. 3 , the lateral support created by the supporting structure in the core of the cable may double the tube stiffness and thus the crush resistance of the cable  110 . 
     The bedding compound  134  should be in a soft state during the stranding step of manufacturing the cable  110  (i.e., where the buffer tubes  116  are stranded around the central strength member  120 ). The bedding material should consist of polymeric materials with a relative low melting temperature to produce a soft, gel-like behavior during processing at temperatures of approx. 100° C., for example. 
     To apply the bedding into the construction of the cable, the processing methods can be divided into 2 types. First, the bedding compound  134  may be applied by direct extrusion shortly onto the central member  120  shortly before stranding. The buffer tubes  116  may then be stranded directly onto the central member while the bedding compound  134  is still in a substantially molten state. 
     A second method is to use a coilable bedding compound that is coiled on a drum and applied to the central strength member  120  in a separate processing step. During stranding, heat is subsequently applied to the central member  120  (e.g., hot air blower) to soften the bedding layer before the buffer tubes  120  are pressed into the bedding compound  134 . 
     To estimate the bedding materials mechanical resistance at certain temperature in the stranding step, compression tests were performed. A 100 mm piece of buffer tube with 2.5 mm diameter and 3.3 mm diameter were pressed into plates of bedding compound at different temperatures. Assuming a maximum penetration of ⅓ tube diameter into the bedding and a coupling length of 10 mm within the stranding process, the following forces were measured at each of the processing temperatures shown: 
     
       
         
           
               
               
               
             
               
                   
               
               
                 Temperature 
                 tube (3.3 mm) 
                 tube (2.5 mm) 
               
               
                   
               
             
            
               
                  23° C. 
                 17.8N 
                 — 
               
               
                  50° C. 
                 11.3N 
                 5.3N 
               
               
                  60° C. 
                  8.7N 
                 4.8N 
               
               
                  70° C. 
                  4.1N 
                 3.5N 
               
               
                  80° C. 
                  4.0N 
                 3.3N 
               
               
                  90° C. 
                  3.8N 
                 3.3N 
               
               
                 100° C. 
                  3.7N 
                 3.0N 
               
               
                   
               
            
           
         
       
     
     The relative low forces exhibited at processing temperatures above 60° C. do not damage the buffer tubes and may be realized by either a die the strand runs through, tension of the buffer tubes in the stranding process, or tension of yarns that run on top of the tubes. 
     It is to be understood that the foregoing description is exemplary only and is intended to provide an overview for the understanding of the nature and character of the features which are defined by the claims. The accompanying drawings are included to provide a further understanding of the embodiments and are incorporated and constitute part of this specification. The drawings illustrate various features and embodiments which, together with their description, serve to explain the principals and operation. It will become apparent to those skilled in the art that various modifications to the embodiments as described herein can be made without departing from the spirit or scope of the appended claims. 
     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.