Patent Publication Number: US-2002004138-A1

Title: Novel optical fiber gel fluid

Description:
CROSS REFERENCE TO RELATED APPLICATION  
     [0001] This application claims priority from U.S. Provisional Application Ser. No. 60/187,459, filed Mar. 6, 2000, currently pending. 
    
    
     
       FIELD OF THE INVENTION  
       [0002] The present invention generally relates to a novel optical fiber gel fluid. More particularly the present invention relates to an optical fiber gel fluid comprising ethylene-alpha-olefin copolymers.  
       BACKGROUND OF THE PRESENT INVENTION  
       [0003] In providing cushioning of the fiber optic elements which are carried in a jacket or sheath, care must be taken to assure that the optical qualities of the fiber optic elements are not diminished. It is desirable for buffer tube as well as cable jacket polymer and waterblocking gels to have as little interaction as possible. Compatibility of a polymer gel system in flooded cable designs can have a serious effect on cable lifetime and reliability. Since 1995, polypropylene and polyethylene buffer tubes have become increasingly attractive from an application and installation standpoint.  
       [0004] Although polyolefins can lead to improvements in cable performance and reduced cost, they may not be compatible with many traditional, low cost polyolefin and hydrocarbon-based cable gel compounds. In systems where strong swelling interactions exist, lifetimes can be reduced to a small fraction of that which would be observed for the virgin polymer. This incompatibility is due to favorable solvent-polymer interactions between polyolefins and polyolefin-based gels. The commercially available cable waterblocking gels have very significant compound to compound variation in compatibility with polyolefins.  
       [0005] Currently, polyalphaolefins (PAOs) are the dominant hydrocarbon synthetic base oil on which the gels are based. However, costs are high compared to conventional mineral base oils. Generally, the PAO&#39;s are manufactured in two steps. First, ethylene is oligomerized with aluminum alkyl to produce a range of linear alpha-olefins having even numbers carbon atoms from C 4  to C 30+ . About fifteen linear alpha-olefins are produced that serve distinct markets growing at different rates. About 25% of the linear alpha-olefins are 1-Decene, the major component of PAO.  
       [0006] PAO is generally prepared by reaction of 1-Decene with a Lewis Acid catalyst such as BF 3  to produce primarily trimers and tetramers. 1-Decene is expensive and costs about three times more than ethylene making PAO very expensive. Other examples of Lewis Acid catalysts used to produce higher molecular weight PAOs are alkyl aluminum halides such as those disclosed in U.S. Pat. Nos. 4,469,910 and 4,594,469, both of which are incorporated herein by reference.  
       [0007] As an alternative, polyol-based and high molecular weight polyalphaolefin waterblocking gels have been utilized, especially for buffer tube filling applications, which offer better compatibility due to less favorable polymer-gel interactions. Compared to waterblocking gels based on mineral oils, however, these gels are even more expensive, which decreases the cost savings associated with use of polyolefins such as polypropylene for buffer tube applications.  
       [0008] Exemplary conventional gels are described in Brauer et al., U.S. Pat. No. 5,433,872; Brauer et al., U.S. Pat. No. 5,348,669; Randisi, U.S. Pat. No. 5,050,959; Kaufman et al., U.S. Pat. No. 5,285,513; and Tu, U.S. Pat. No. 5,187,763, all of which are incorporated herein by reference in their entirety.  
       [0009] While the known water resistant optical cable gel fluids possess some unique features, they are expensive and not completely suitable for many potential applications. Thus, there is a need in the art for an improved, lower cost cable gel fluid which is water resistant, has a wide service temperature range, high shear and oxidation stability, low toxicity and has reduced influence on the swelling interaction with polyolefin buffer tubes. 
     
    
    
     DESCRIPTION OF THE INVENTION  
     [0010] The present invention provides a novel optical fiber cable gel for use with fiber optic elements. The gels are made from fluids of high molecular weight ethylene-alpha-olefin polymers produced by polymerization of ethylene, an alpha-olefin different from ethylene, and optionally a third monomer different from the alpha-olefin and having 3 to 20 carbon atoms, in the presence of a combination catalyst comprising a compound of a transition metal of Group IVb of the Periodic Table and an aluminoxane. The copolymer or terpolymer may be further processed by thermal cracking to yield novel cracked polymers, and the cracked polymers may be hydrogenated. The copolymers or terpolymers may also be hydroisomerized.  
     [0011] The ethylene-alpha-olefin polymers useful in the practice of the present invention are those having a low pour point, less than about −30° C. and a relatively high molecular weight, such as greater than 700, preferably greater than 1000, more preferably greater than 1100 and most preferably up to about 2000 or more. These ethylene-alpha-olefin polymers and their method of production as fully set forth in International Application No. PCT/US98/12621 published under International Publication Number WO 98/58972 which is hereby incorporated by reference in its entirety.  
     [0012] The above-described fluids are generally present in an amount of 85% by weight or more based on the weight of the entire gel composition. The molecular weight distribution of these fluids has a dramatic influence on the swelling interaction. Gels with large molecular weight components swell a given polymer most severely. The polymerized fraction utilized in the present invention has a kinematic viscosity at 100° C. of greater than 10 cSt, a flash point of greater than 235° C. and a pour point of −30° C. or less. The fluids of the present invention create a swelling weight gain of less than 3% in three different fiber optic grades of impact modified polypropylene.  
     [0013] The above-described fluids are then gelled with an effective amount of gelling agent such as are known to those of ordinary skill in the art. Preferred gelling agents are fumed silica, colloidal silica, either hydrophilic or hydrophobic, precipitated silicas and clays such as bentonite with or without surface treatment. Other inorganic colloidal particles may also be used, if desired, although the silicas are preferred. Generally the gelling agent is present in an amount ranging from about 5 to 8 percent by weight based on the weight of the entire gel composition.  
     [0014] Optionally, an effective amount of a bleed resistant agent may be added to the gels. Any such bleed resistant agents known to those skilled in the art to prevent the gels from bleeding through the polymer sheath may be employed. Block copolymers are preferred bleed resistant agents, such as styrene-rubber and styrene-rubber-styrene block copolymers. Commercially available examples are a styrene-ethylene-propylene block copolymer (SEP) sold under the trade designation Kraton G1701 or G1702, a styrene-ethylene butylene-styrene block copolymer (SEBS) sold under the trade designation Kraton G1657, or a styrene block copolymer mixture sold under the trade designation Kraton 1726. Preferably the bleed resistant agent is employed in an amount ranging from about 3 to about 6 weight percent based on the weight of the total gel composition.  
     [0015] Optionally, an effective amount of thermal and/or oxidative stabilizer may also be added to the gels of the present invention as is known to those of ordinary skill in the art. Preferred are hindered phenolic stabilizers such as those available from Ciba-Geigy Company under the trade designation Irganox 1035. In preferred embodiments these are present in amounts ranging from about 1 to about 2 percent by weight based on the weight of the total gel composition.  
     [0016] It is also contemplated to add effective amounts of thickening agents where necessary. These especially useful in the practice of the present invention to increase the viscosity of the gel by increasing the interaction possibilities between the silica and the oil. A small amount of glycol, such as polypropylene glycol, may be added.  
     [0017] Swelling of coating materials for optical fibers in contact with the filling gels of the present invention is substantially reduced over that seen with prior art filling materials. The use of a low pour point aliphatic hydrocarbon in the filling gel allows the cable to provide superior optical performance at low temperatures.  
     [0018] The filling gel composition of the present invention encompasses the fibers. Typically, the fibers and the filling material are contained within a tubular member which is disposed within a sheath system. The sheath system includes longitudinally extending strength members and a plastic jacket. See for example the above-mentioned U.S. Pat. Nos. 5,285,513 and 5,187,763.  
     [0019] The filling composition of the present invention comprises at least about 85 percent by weight of the above-described oils having a molecular weight of at least about 700. Relatively low pour point oils are preferred in order to improve optical loss at low temperature.  
     [0020] A thickening system which includes an inorganic constituent and a block copolymer is preferred to form the gel as well as to reduce oil separation. An antioxidant is used to prevent thermal oxidative degeneration of the filling gels of the present invention.  
     [0021] A grease-like filling composition prepared in accordance with the present invention having a relative high critical yield stress (as it relates to the stiffness of the gel) allows the inside diameter of tubes to increase. A larger tube can incorporate a larger amount of optical fibers in the form of stacked ribbons. The result in an increase in bandwidth.  
     [0022] The compositions formed in accordance with the present invention are thixotropic and are operable over a wide temperature range. They are water resistand and remain soft at both ends of the relevant temperature spectrum. They are compatible with a variety of jacket materials, including polypropylene, polyethylene, polycarbonate, etc, at other cable materials which they contact.  
     [0023] The filling material of the present invention has enhanced performance at low temperature because of the use of a low pour point oil and very low oil separation. There is no bleeding of oil and it is expected that the optical loss at −40° C. will not exceed that of the prior art filling materials.