Telecommunications cable having good adhesion between a protective jacket and strength members

A telecommunications cable comprising a communications element, such as an optical fiber, and a jacket surrounding the communications element having at least one elongated strength member embedded therein is disclosed. The jacket of the telecommunications cable is formed by extruding a blend of a polyolefin material and a copolymer adhesion promoting material, such as graft copolymer of polyethylene and ethylene acrylic acid or a graft copolymer of polyethylene and maleic anhydride. The copolymer adhesion promoting material promotes adhesion between the strength member and the jacket. The resulting increase in adhesion between the strength member and the jacket improves the cable's resistance to water penetration, low temperature buckling and shrinkage as well as excessive high temperature expansion. The blending of an adhesion promoting material in the jacketing material also reduces the risk of armor cracking during cyclic flexing and strength member pistoning within the jacket.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to telecommunications cables. More
 particularly, the present invention relates to a telecommunication cable
 having strength members adjacent to or embedded in a protective jacket
 made from a polymeric material having an adhesion promoter blended
 therein.
 2. Description of the Prior Art
 Telecommunications cables containing optical fiber cables have been used by
 the telecommunications industry for a number of years to transmit
 information at very high rates over long distances. Because the optical
 fiber transmission elements are delicate, the telecommunications cables
 are provided with members which are designed to protect the optical
 fibers. For example, in a typical optical fiber telecommunications cable,
 one or more optical fibers are disposed in a buffer tube which provides
 some protection against abrasion, as well as outside tensile and
 compressive forces. In a basic central tube design, a single buffer tube
 may be surrounded by an armor layer made from aramid yarns, water blocking
 tapes, metallic sheaths or some combination of such materials. The armor
 layer is typically surrounded by an outer protective sheath made from a
 polyolfin material such as medium density polyethylene (MDPE) or high
 density polyethylene (HDPE). To provide additional compressive and tensile
 strength to the cable, the outer protective jacket may be extruded over
 strength members made from composite materials containing glass reinforced
 fibers or steel so that such members become embedded in the outer
 protective jacket.
 In some cables, copolymer adhesion promoters, such as ethylene-acrylic acid
 (EAA) are applied as a coating on composite strength members to promote
 adhesion between the strength members and the jacket extruded thereover.
 Alternatively, steel armor may be precoated with a copolymer material
 which promotes adhesion. Such strength members are marketed by t he Dow
 Chemical company under the ZETABON trademark. The jacket material is then
 extruded over this copolymer coated steel armor. Copolymer coatings,
 however, have the disadvantage that adhesion cannot be controlled during
 cable manufacturing.
 Control of bonding is desirable in fiber optic cables to allow a
 combination of mechanical integrity and ease of cable access. If adhesion
 between the jacket and strength members or jacket is too low, debonding
 may occur during handling or installation. If jacket debonding occurs, the
 cable may show numerous mechanical or other problems. For example, if the
 strength members become debonded from the jacket pistoning of the strength
 members may be seen with temperature variation due to the different
 coefficient of thermal expansion (CTE) values for the different materials.
 If t he jacket and strength members do not remain coupled, low temperature
 cable contraction may be too high and attenuation may result.
 Additionally, debonding can result in water penetration failures in the
 cable. If the level of adhesion is too high, especially between steel
 armor ad the jacket, cable access may be difficult for splicing operations
 thereby increasing the effective time for cable access and splicing.
 To control bonding, some have applied hot melt or other types of adhesives
 to the strength members or armor prior to the extrusion of the jacket
 material over the strength member or armor. These materials have become
 necessary to obtain the desired level of mechanical coupling or bonding
 between the jacket and the strength members or armor. However, the
 application of these materials to the strength member or armor during the
 cable manufacturing process increases the cost of manufacturing by adding
 another step to the manufacturing process.
 Accordingly, what is needed is a mechanism to provide and control the
 adhesion between embedded elongated strength members and the protective
 jacket material during the jacketing process without significantly adding
 to the manufacturing cost of the cable. The present invention is intended
 to provide such a mechanism.
 SUMMARY OF THE INVENTION
 It is an object of the present invention to provide a telecommunications
 cable having a jacket which is reinforced by elongated strength members
 embedded therein.
 It is another object of the present invention to provide a
 telecommunications cable having a jacket which has controlled adhesion to
 the elongated strength members embedded therein.
 It is yet another object of the present invention to provide a
 telecommunications cable having a jacket which has controlled adhesion to
 an armor layer adjacent thereto and in contact therewith.
 These objects are accomplished, at least in part, by a telecommunications
 cable comprising a communications element, and a jacket surrounding the
 communications element having at least one elongated strength member
 embedded therein, wherein the jacket is formed by extruding a blend of a
 polyolefin material and a copolymer adhesion promoting material which
 promotes adhesion between the strength member and the jacket.
 Other objects and advantages of the present invention will become apparent
 to those skilled in the art from the following detailed description read
 in conjunction with the attached drawing and claims appended hereto.

DETAILED DESCRIPTION OF THE PRESENT INVENTION
 Referring to FIG. 1, an embodiment of a telecommunications cable 10 made
 according to the present invention is formed, in part, by one or more
 communications elements 12, which are preferably ordinary optical fibers.
 A centrally located buffer tube 14 is extruded over the communications
 elements 12 so as to envelope them. The buffer tube may be extruded from
 any extrudable thermoplastic material such as polyvinyl chloride,
 polyethylene, polycarbonate (PC), polybutylene terephalate (PBT) or a
 thermoplastic polyolefin elastomer compound, but preferably, the buffer
 tube 14 is extruded from a nucleated polypropylene-polyethylene copolymer.
 Such a material is described in U.S. Pat. No. 5,574,814. Techniques for
 extruding thermoplastic material to form the buffer tube 14 over the
 communications elements 12 are well known in the art and therefore not
 described herein. Water blocking gels, water swellable yarn or water
 swellable powders (not shown) may also be disposed in the buffer tube 14,
 if desired. The buffer tube 14 provides the primary structure to protect
 delicate communications elements 12, such as optical fibers, contained
 therein.
 To provide support and protection for the buffer tube 14 in the cable 10
 after the buffer tube 14 is formed over the communications elements 12,
 the buffer tube 14 may be surrounded by a layer 16 formed by radial
 strength yarns, filling or flooding compounds, swellable water blocking
 tapes, corrugated metallic sheathing, or a combination of these various
 armor materials. Methods for forming this layer 16 over the buffer tube 14
 are well known by those skilled in the art and a detailed description of a
 typical armor layer can be found in U.S. Pat. No. 5,029,974, which is
 incorporated herein by reference in its entirety.
 Finally, the telecommunications cable is completed by the extrusion of a
 polyolefin, such as a medium density polyethylene (MDPE) or high density
 polyethylene (HDPE), outer jacket 18 over the buffer tube 14, or the layer
 16 if a buffer tube or a layer has been provided. Typically, to provide
 additional strength to the telecommunications cable 10, elongated strength
 members 20a, 20b are positioned in a parallel spaced apart relationship
 relative to the buffer tube, or to the layer 16 if the cable 10 has such a
 layer, prior to the extrusion of the outer jacket 18. The elongated
 strength members 20a, 20b may be formed from a composite material such as
 a thermoplastic polymer reinforced with glass fibers, steel alloys or
 other suitable reinforcing materials. Provided that good adhesion is
 obtained between the outer jacket 18 and the elongated strength members
 20a, 20b so that such components are mechanically coupled, the elongated
 strength members 20a, 20b enable the cable 10 to resist ordinary
 compressive and tensile forces which are placed on the cable 10 during
 installation and use in the field. It is important that such forces are
 resisted so that changes in the length of the cable 10 during installation
 and use are minimized to prevent straining, breaking or substantial
 bending of the communications elements, which is undesirable.
 As illustrated in FIG. 2, alternatively, the jacketing material may be
 extruded directly around the communications elements 12 to form a
 surrounding jacket and waterblocking compound while encapsulating the
 strength members 20a, 20b. Such a process and cable design eliminates the
 need for a buffer tube.
 To promote good adhesion between the outer jacket 18, the elongated
 strength members 20a, 20b and between the outer jacket 18 and the layer
 16, if the cable 10 has such a layer and the layer is of the typical armor
 type, the MDPE or HDPE jacketing material is blended with a graft
 copolymerized adhesion promoter prior to extrusion. According to the
 present invention, the graft copolymer is configured to contain polymer
 segments of two dissimilar chemical species that can promote improved
 adhesion between two dissimilar substrates, i.e., the polyolefin jacket
 and the elongated metallic or polymer strength member. Adhesion is
 promoted by providing chemical units within the jacketing polymer that
 migrate to the first substrate with like chemistry, such as the elongated
 glass reinforced polymer strength member. The same graft copolymer can
 have other chemical units that are similar to the other substrate, such as
 the polyolfin jacket material.
 Graft copolymerized adhesion promoters that will work in accordance with
 the present invention include polar molecules such as ethylene acrylic
 acid or maleic anhydride which can be graft copolymerized to a polyolefin
 backbone to form an adhesion promoting material. The adhesion promoting
 material is blended into the MDPE, HDPE or polyolefin jacket material.
 This blending can take place directly during jacketing or as a prior
 compounding step. The polarity of the polar acrylic acid or maleic
 anhydride molecule provides good adhesion to metallic, glass or epoxy
 substrates. The polyolefin backbone has good compatibility and miscibility
 with the polyolefin jacket material. As jacket material is extruded over
 the elongated strength member during cable manufacturing, the similar
 chemical components of the graft copolymer migrate to the surface of the
 elongated strength member. This lowers the interfacial surface energy
 between the elongated strength member and the jacket material, thereby
 increasing adhesion between the elongated strength member and the jacket
 material. Two illustrative examples are set forth below.
 EXAMPLE 1
 Two cables containing embedded fiberglass reinforced epoxy elongated
 strength elements and jacketed with Union Carbide DHDA-8864 MDPE were
 manufactured under identical processing conditions with and without the
 addition of adhesion promoter. The cable that did not include the adhesion
 promoter failed a water penetration test with a penetration result of over
 1M using a 1M head of water within one hour. A cable containing 5 weight
 percent of the adhesion promoter Polybond.RTM. 3009 in the polyolefin
 jacketing material blend had water penetration of less than 2 cm in 24
 hours. Polybond.RTM. 3009 is a polyethylene/maleic anhydride graft
 copolymer produced by Uniroyal Chemical. The glass reinforced polymer
 (GRP) pullout forces for the cable that included no adhesion modifier were
 as low as 20 pounds, whereas the GRP pullout force on cables made with the
 blend containing Polybond.RTM. 3009 surpassed 100 pounds. The blending of
 5 weight percent Polybond.RTM. 3009 adhesion promoter with the polyolefin
 jacketing material was sufficient to change the mode of pullout failure
 from interfacial failure between the reinforcing member to failure of the
 MDPE surrounding the reinforcing member.
 FIG. 3A schematically shows the debonded surface of an elongated fiberglass
 reinforced epoxy rod 20a after a pullout test from a jacket containing no
 adhesion promoter. No evidence of the MDPE jacket remains on the rod after
 the pullout test, leaving it with a substantially clean surface. FIG. 3B
 schematically shows the debonded surface of fiberglass reinforced epoxy
 rod 20a after a pullout test from a jacket containing 5% Polybond.RTM.
 3009 adhesion promoter. The debonded surface of the sample made with the
 blend of polyolefin and the adhesion promoter shows that the interfacial
 strength between the reinforcing member and the MDPE jacket was
 significantly increased to the point where jacket material 18 remains on
 the reinforcing member after the pullout test. The resulting increase in
 interfacial strength between the strength member and the jacket increases
 reinforcing member pullout force and also causes a failure mode that can
 dissipate more energy.
 EXAMPLE 2
 Two cables containing embedded steel strength elements and jacketed with
 Union Carbide DHDA-8864 MDPE were manufactured under identical processing
 conditions with and without the addition of adhesion promoter. The cable
 that did not include the adhesion promoter in the jacketing material
 failed a water penetration test with a penetration result of over 1M using
 a 1M head of water within one hour. A cable made from a blend of 95 weight
 percent polyolefin and 5 weight percent of the adhesion promoter
 Polybond.RTM. 1009 had water penetration of less than 2 cm in 24 hours.
 Polybond.RTM. 1009 is a polyethylene/acrylic acid graft copolymer produced
 by Uniroyal Chemical.
 The increased adhesion between strength member and jacket material obtained
 by blending the adhesion promoter into the jacketing material reduces
 pistoning and improves water tightness of the cable as shown by the
 improved water penetration test results. Because the adhesion promoting
 material can be blended into the MDPE or HDPE jacket material,
 manufacturing costs are potentially lower due to the elimination of a
 separate strength member coating step and coating equipment.
 It will thus be seen that the objects and advantages set forth above and
 those made apparent from the preceding descriptions, are efficiently
 attained and, since certain changes may be made in the above construction
 without departing from the scope of the invention, it is intended that the
 matter contained in the above description or shown in the accompanying
 drawings shall be interpreted as illustrative and not in a limiting sense.
 It is also to be understood that the following claims are intended to
 cover all of the generic and specific features of the invention herein
 described, and all statements of the scope of the invention which, as a
 matter of language, might be said to fall there between.