Optical fiber assemblies, and methods and apparatus for the manufacture thereof

Methods for manufacturing cables and cables assemblies include providing particulate matter within a tube extruded about optical fiber. The particles may be accelerated so that as they strike the tube they mechanically attach to the tube.

TECHNICAL FIELD

The present application relates generally to methods of manufacturing optical fiber assemblies used for transmitting optical signals. More particularly, the present application relates to the manufacture of optical fiber assemblies having relatively low-levels of mechanically attached water-swellable powder and methods of providing particulate materials for manufacturing processes.

BACKGROUND

Communications networks are used to transport a variety of signals such as voice, video, data and the like. As communications applications required greater bandwidth, communication networks switched to cables having optical fibers since they are capable of transmitting an extremely large amount of bandwidth compared with copper conductors. Fiber optic cables are also much smaller and lighter compared to copper cables having the same bandwidth capacity.

In certain applications, fiber optic cables are exposed to moisture that over time may enter the fiber optic cable. The moisture can migrate along the cable and enter cable splice enclosures, buildings, etc. To block water migration, fiber optic cables were provided with one or more components for blocking the migration of water along the fiber optic cable. By way of example, conventional fiber optic cables block water migration using a filling and/or a flooding material such as gel or grease within the fiber optic cable. Filling material refers to gel or grease that is inside a tube or cavity with the optical fibers, whereas flooding material refers to gel or grease within the cable that is outside of the cavity that houses the optical fibers. The gel or grease fills voids in the cable so that water does not have a path to follow in the fiber optic cable. Additionally, the gel or grease filling material provides cushioning and coupling of the optical fibers.

Gel or grease filling materials also have disadvantages. For example, the gel or grease may be messy and may drip from an end of the fiber optic cable. The filling material must also be cleaned from the optical fibers when being prepared for an optical connection, requiring the craft to carry cleaning materials into the field. Early fiber optic cable designs eliminated the flooding material by using cleaner, dry water-blocking components such as tapes or yarns outside the buffer tubes for inhibiting water migration. These dry water-blocking components typically include super absorbent polymers (SAPs) that absorb water and swell as a result, thereby blocking the water path for inhibiting the migration of water along the fiber optic cable. Generally speaking, the water-swellable components used a yarn or tape as a carrier for the SAP. Since the water-swellable yarns and tapes were first used outside the cavity housing the optical fibers, the other functions besides water-blocking such as coupling and optical attenuation did not need to be addressed.

Eventually, fiber optic cables incorporated water-swellable yarns, tapes, or super-absorbent polymers (SAPs) within the tubes that housed the optical fibers as a replacement for gel and grease filling materials. Generally speaking, the water-swellable yarns or tapes had sufficient water-blocking capabilities, but did not provide all of the functions of the gel or grease filling materials such as cushioning and coupling. For example, water-swellable tapes and yarns are bulky since they are relatively large compared with a typical optical fiber and/or can have a relatively rough surface. As a result, water-swellable yarns or tapes may cause problems if the optical fiber is pressed against the optical fibers. Likewise, the SAPs may cause problems if pressed against the optical fibers. In some cases, optical fibers pressed against conventional water-swellable yarns, tapes, and/or SAPs may experience microbending, which can cause undesirable levels of optical attenuation and/or cause other issues. Moreover, the desired level of coupling for the optical fibers with the tube may be an issue if the fiber optic cable is not a stranded design since the stranding provides coupling.

Other early fiber optic cable designs used tube assemblies that were highly-filled with SAPs in the form of loose powder for blocking the migration of water within the fiber optic cable. However, conventionally applied loose SAP powders within the fiber optic cable created problems since the superabsorbent powder particles, which were not attached to a carrier such as a yarn or tape, could migrate to and accumulate at positions within the fiber optic cable. When the cable was wound on a reel, such SAP powders could accumulate at the low points due to gravity and/or vibration, thereby causing inconsistent water blocking within the fiber optic cable. Also, loose SAP powders are free to fall out of the end of a tube.

FIGS. 15 and 16respectively depict a transverse cross-sectional view and a longitudinal cross-sectional view of a conventional dry fiber optic assembly10having a plurality of optical fibers1along with schematically represented loose water-swellable powder particles3disposed within a tube5. The conventional dry fiber optic assembly10uses a relatively large quantity of SAP powder3within tube5for blocking the migration of water therein. Other conventional fiber optic cable components have included embedded SAP powder in the outer circumferential surface of a tube, such as disclosed in U.S. Pat. No. 5,388,175. However, embedding the SAP in the outer surfaces of the tube greatly reduces the effectiveness of the powder since water may not reach the embedded particles.

SUMMARY

According to one embodiment, a mixture of motive gas and particulate matter is conveyed to an extrusion process. The mixture is conveyed to the extrusion process in a first phase, and launched or passed into an extrusion cone formed in the extrusion process in a second phase having an equivalent or higher velocity than the first phase. The particulate matter is conveyed so that at least a portion of the particulate matter becomes mechanically attached to the extrusion cone. The tube resulting from the extrusion process thereby receives an application of particulate matter to its inner or cavity surface. The first phase of conveyance can be, for example, strand phase flow, and the second phase can be dilute phase flow.

According to one embodiment, a mixture of motive gas and particulate matter is conveyed to the interior of an extrusion cone through a nozzle. The particulate matter is accelerated through the nozzle so that at least a portion of the particulate matter becomes mechanically attached to the extrusion cone.

According to another embodiment, a mixture of motive gas and particulate matter is conveyed to the interior of an extrusion cone through an annular passageway. The particulate matter is conveyed so that at least a portion of the particulate matter becomes mechanically attached to the extrusion cone.

According to one aspect of the present embodiments, the tube can be of the type used to form a fiber optic assembly, the fiber optic assembly having relatively low concentrations of particulate matter yet still retaining desired water-blocking properties.

Those skilled in the art will appreciate the above stated advantages and other advantages and benefits of various additional embodiments reading the following detailed description with reference to the below-listed drawing figures.

DETAILED DESCRIPTION

The exemplary methods for manufacturing fiber optic cables and fiber optic cable assemblies according to the present invention have several advantages compared with conventional methods. One advantage is that at least a portion of the particulate matter, which can be in the form of powder particles, are mechanically attached to a surface of the fiber optic assembly (e.g., the tube or cavity interior wall). The attachment may be carried out so that less than all of the surface area of the tube is covered while still effectively blocking the migration of water. The presence of water-swellable powder in the tube is nearly transparent to the craft since low levels of powder are sufficient. The water-swellable powder may be applied to the inside surface of the extruded tube, a cavity, a substrate, or the like to avoid an excess of loose powder in the tube or cavity. Migration of loose powder, such as occurs with conventional injection and fog methods, is thereby mitigated. Tubes or cavities of fiber optic assemblies constructed according to the methods disclosed herein can also have smaller cross-sectional areas than conventional dry cable assemblies that use tape or yarns for waterblocking. The relatively small quantities of powder are less likely to form large agglomerations, which may results in attenuation in optical fibers.

In this specification, “mechanical attachment” of a particle within a substrate, such as an SAP powder particle mechanically attached in an extruded tube, means at least a part of an attached particle extends into or is partially embedded or deposited within the substrate, below the surface of the substrate, as opposed to being solely adhered to the surface of the substrate by means such as adhesives. The mechanical attachment can be caused by a moving particle impinging upon and disrupting the surface of the substrate. Adhesives or binders may be used in addition to mechanical attachment so the two methods are not mutually exclusive. In this specification, the term “powder” is understood to include mixtures of powders of differing type and/or particle size as well as single composition powders. While the present specification describes the supply of particulate matter in the form of powder to various extrusion processes, the disclosed methods of providing powder to manufacturing processes can be used to convey any type of relatively fine particulate matter.

The movement of gases and particulate matter in this specification is often described in terms of “velocity” through a passage, from a nozzle exit, or in an interior area, etc. It is understood that individual gas molecules and individual particles may not travel with a constant velocity or direction, may swirl, etc. Therefore, in this specification, the “velocity” of a gas, gases, particulate matter, or mixtures thereof, refers to average velocity for the plurality of particles, gas molecules, etc., along a process or downstream direction.

This specification describes “motive gas” as conveying particulate matter through a manufacturing line. “Motive gas” is used for brevity of description and is intended to encompass a mixture of individual gases as well as individual gases.

FIG. 1is a schematic representation of an exemplary manufacturing line10for making fiber optic assemblies. The manufacturing line10may include a plurality of rotating reels20for paying out a plurality of respective optical fibers22along a process direction12. Other means, such as flying off, may also be used to pay off one or more optical fibers. The illustrated manufacturing line10illustrates a process utilizing twelve optical fibers22, but other numbers, including 1, 2, 6, etc., of optical fibers can be used. In some applications, the optical fibers22may leave their respective reels20with a static charge that can assist in the deposition of powder particles. The optical fibers22may, for example, pass from the reels20through a set of guide rollers28and a guiding die30. The bundle of fibers22then passes into a powder application/extrusion apparatus40. The powder application/extrusion apparatus40is schematically illustrated as a powder/motive gas supply50connected to an extrusion apparatus60by a powder/motive gas supply passage55, which may have the general form of a tube. While the apparatuses50,60are schematically illustrated as separate devices, they could, for example, be integrated as a single device and/or station of the manufacturing line10.

In general, the powder application/extrusion apparatus40extrudes a tube around the fibers22and applies particulate matter (e.g., powder) to the interior of the tube, which draws down around the fibers to form an uncooled fiber optic assembly65. The powder/motive gas supply50provides the powder and motive gas to convey the powder particles to the extrusion operation, and the extrusion apparatus60extrudes the tube around the fibers22. The uncooled fiber optic assembly65may then be cooled in a cooling device70, which may be, for example, a longitudinally extending trough filled with cooling fluid, such as liquid water. The cooling device70cools the recently extruded tube as the fiber optic assembly65moves along the process direction12. The resultant fiber optic assembly100is then collected on a take-up device90, such as, for example, a take-reel or take-up disc.

The Powder/Motive Gas Supply

The operation of the powder/motive supply50in supplying powder to the extrusion apparatus60is discussed below with reference toFIGS. 2-3C. In the following discussion, the exemplary powder is SAP powder used for waterblocking in fiber optic cables. Table 1 below includes a particle size distribution for SAP powder suitable for use in the present embodiment. The principles discussed, however, are applicable to other relatively fine particulate matter, such as, for example, flame-retardant particles (e.g., aluminum trihydrate, magnesium hydroxide, etc.), dry lubricants such as talc, graphite, boron, and/or the like, and/or mixtures thereof.

FIG. 2is a partially schematic cross-sectional perspective view of a powder/gas mixing section108of the powder/motive gas supply50shown schematically inFIG. 1. The venturi section of the section108is shown in more detail inFIGS. 3A and 3B. The powder/gas mixing section108may be supplied with powder particles by, for example, vibratory and/or gravimetric particle supply means (not illustrated). One commercially available powder feeder is the Model6C Vibratory Feeder available from Eriez Manufacturing Co. Vibratory feeders are particularly well suited for use with the present embodiments because of the relatively low powder flow rates used in the extrusion processes.

The powder/gas mixing section108is part of a venturi system having a transition funnel110with an open, funnel-shaped downwardly-tapered powder receiving portion112that receives falling powder or other particulate matter from the supply means. The lower end of the transition funnel110is operatively connected to a venturi funnel114having a vertically tapered passage116that terminates at a metering or powder feed aperture120located at the upper end of a venturi block124. InFIG. 2, the right or intake side of the venturi block124includes a threaded intake end128that receives a conveying passageway of a motive gas supply, which is discussed in further detail below with reference toFIG. 3A. The left or downstream exiting side of the venturi block124includes a stepped passage132having a threaded exit end134that receives the upstream or intake end of the powder/motive gas supply tube55(shown inFIG. 1). The powder/motive gas supply tube55may supply the SAP powder consumed by, for example, extrusion processes, as discussed in further detail below.

FIG. 3Ais a cross-sectional view of a portion of the powder/gas mixing section108, with a motive gas supply passage or tube138threaded into the intake end128of the venturi block124and the powder/motive gas supply tube55threaded into the exit end134.FIG. 3Bis a section view taken along line3B-3B inFIG. 3A. Referring toFIGS. 3A and 3B, in operation, the open top of the receiving portion112of the transition funnel110receives a mixture of atmospheric air and/or other gases and powder particles135, which inFIGS. 3A and 3Bare symbolically illustrated as particulate matter and as entering the transition funnel110with a conical shape. The ambient air and/or other gas or gases at the funnel110entrance may be at partial vacuum (i.e., at pressures less than ambient). At the same time, a stream of motive gas142, its direction indicated by an arrow inFIG. 3A, is provided to the motive air supply tube138by, for example, a blower, a compressed gas source, etc. (not shown). The motive gas142can be, for example, pressurized atmospheric air, or one or more “dry” gases, such as nitrogen, and mixtures of air, dry gases, etc. Dry gases may also be used to convey particles at the receiving portion112.

Still referring toFIGS. 3A and 3B, the motive gas142travels down the supply passage138, which is formed in an upstream or intake piece144of the venturi block124, through an intake chamber146, and is then forced through a relatively small motive gas orifice148(shown in isolation inFIG. 3C). The motive gas142then passes below the metering aperture120at the upper end of the venturi block124, where the motive gas142mixes with and accelerates the downwardly metered powder particles135. The movement of the motive gas142through the gas intake orifice148creates a partial vacuum in a venturi receiving chamber150in a downstream piece152of the venturi block124, which assists in drawing the powder particles135downwardly through the aperture120and into the receiving chamber150. The partial vacuum in the chamber150is caused by the acceleration of and the accompanying pressure drop in the motive gas142as it passes through the small motive gas orifice148, which acts as a venturi orifice. The direction of the accelerated mixture156of motive gas and powder particles is indicated by the arrow to the left in the figures. For simplicity of description, the portion of partial vacuum air drawn in with the powder particles135through the metering aperture120is considered to form a part of the “motive gas” as it mixes with the motive gas142. Referring specifically toFIG. 3B, the intake chamber146is in fluid communication with the motive air orifice148via a passage160in the upstream piece144.

FIG. 3Cis an isolated view taken on line3C inFIG. 3A, and illustrates the operation of the venturi block124in greater detail. Referring toFIGS. 3B and 3C, the upstream piece144of the venturi block124can be connected to the downstream piece152by threaded attachment members166, and an o-ring seal168can be seated between the upstream piece144and the downstream piece152to provide a gas-tight seal between the pieces144,152in the vicinity of the motive gas intake orifice148. The powder/motive gas supply tube55has an outer sleeve170that may be threaded into the exit end134of the venturi block124, and a concentrically mounted inner passage174with an inner bore178having, for example, a circular cross-sectional area. As shown inFIGS. 3B and 3C, the stepped bore132includes a relatively small venturi passage180that conveys the accelerated mixture of motive gas and powder particles156downstream through the venturi block124and into the powder/motive gas supply tube55. A transverse monitor port184can be formed in the venturi passage180for monitoring conditions such as temperature, humidity, particle concentration, pressure, flow velocity, etc., within the venturi passage180. The venturi passage180may have, for example, a circular cross-section. Still referring toFIGS. 3B and 3C, the mixture of motive gas and powder particles advances through the venturi passage180and enters the inner bore178of the powder/motive gas supply tube55. The mixture may then be provided to a manufacturing process such as, for example, extrusion processes performed in the extrusion apparatus60(illustrated inFIG. 1). Examples of the uses of the mixture of motive gas and powder particles156from the powder/motive gas supply50in exemplary extrusion processes are discussed in detail below.

According to one aspect of the invention, the venturi powder supply50utilizes Bernoulli's principle in that the high velocity of motive gas through the motive gas supply orifice148creates a partial vacuum in the chamber150immediately downstream of the orifice148. The powder135is thereby drawn through the feed aperture120and into the venturi block124where it mixes with the relatively low pressure motive gas exiting the orifice148. The use of a dry motive gas142having a low dew point (e.g., 32° F., or 0° C., or lower) reduces the chance of powder agglomeration before the powder particles are applied to a substrate, such as, for example, an extruded tube or cavity. The velocity of the motive gas142may be regulated by controlling the pressure upstream of the motive gas orifice148to ensure consistent flow through the motive gas supply orifice148. The velocity of the motive gas142as it passes through the orifice148may also be controlled by controlling the cross-sectional area of the orifice148. A comparatively small orifice148, for example, can be used to create desirable flow velocities and powder densities for supply to manufacturing processes. For example, in some applications, motive gas supply orifice148diameters of 0.3 mm (cross-sectional area of about 0.08 mm2) or less provides high velocity flows through the orifice. In another application, orifice diameters of 0.2 mm (cross-sectional area of about 0.03 mm2) or less may be used.

In the venturi block124, the sum of the motive gas142and vacuum gas/powder138equals the total flow of gas and powder particles entering the powder/motive gas supply tube55. The powder/motive gas supply tube55receives this combined flow from the relatively small bore venturi passage180in the venturi block124. According to one embodiment of the invention, the powder/motive gas supply tube's inner diameter178may be sized such that the velocity of the gas/powder mixture is in the range of 5-20 m/s (meters/second).

For flow velocities below 5 m/s, the flow of a gas/powder mixture may be generally referred to as “dense phase conveyance.” In dense phase conveyance, the gas/powder mixture may lack uniformity across the cross section of the supply tube55, and much of the powder may move along the bottom (as opposed to being supported and conveyed by motive gas) of the powder/motive gas supply tube55or in slugs. Further, dense phase conveyance may lead to high pressure drops in the conveying passage. Dense phase conveyance is also undesirable because it can lead to a plugged conveyance tube and/or surging of the gas/powder mixture at the conveyance tube exit.

For flow velocities above 20 m/s, the flow of the gas/powder mixture may be generally referred to as “dilute phase conveyance.” In dilute phase conveyance, the powder particles generally occupy less than about 5% of the volume of the powder/gas mixture, although concentrations may vary for differing particle sizes, motive gases, etc. Dilute phase conveyance is desirable because the gas/powder mixture is generally uniform across the cross section of the supply tube55, which leads to better dispersion of powder when applied to the inner circumference of an extruded buffer tube, for example. However, increasing the flow velocity in the powder/motive gas supply tube55may result in high back pressures (e.g., above atmospheric pressure) in the venturi block124. High back pressures in the block124can affect the operation of the venturi in that high pressures at the venturi receiving chamber150may inhibit the flow of powder135and clog the powder metering aperture120.

Flow velocities falling generally within the range of 5-20 m/s, between the ranges for dense phase nor dilute phase conveyance, is referred to herein as “strand phase conveyance.” Although strand phase conveyance may not provide the degree of uniformity of powder particle size distribution through the entire cross section of the tube55that is possible in dilute phase flow, the length of the powder/motive gas supply tube55can be selected to minimize surge of powder within the supply tube55. Strand phase flows can be used in selected portions of the powder application/extrusion apparatus40to avoid or mitigate the high back pressures at the venturi associated with dilute phase flows. The use of particles conveyed in strand phase and in dilute phase flows is discussed below in the context of two exemplary extrusion processes, although the methods and apparatuses for providing such flows discussed above can be used in other applications requiring a flow of particulate matter conveyed by a motive fluids such as gas or mixtures of gases and other fluids.

Several parameters can be used to control the velocity within the powder/motive gas supply tube55. For example, flow velocity of the gas/powder mixture within the supply tube55increases with decreasing cross sectional area of the supply tube55. However, resistance in the tube may result in high back pressures as discussed above. The device illustrated inFIGS. 3A-3Ccan operate at, for example, a back pressure of up to 172.3 mBar, measured at the monitor port184. Higher back pressures may result in a positive gauge pressure at the chamber150. Back pressures experienced in generating dilute phase flows may, for example, be relieved by setting the gas/powder velocity through the powder/motive gas supply tube55to fall within the range of strand phase conveyance.

A powder/gas mixing section108as shown inFIGS. 2-3Chas a motive gas orifice148of 0.25 mm diameter (cross-sectional area of about 0.05 mm2). The incoming flow volume of the motive gas142at the motive gas orifice148is about 2.5 l/min. (liters/minute). The velocity of the motive gas flow as it exits the orifice148is at least Mach 1. The flow volume of the vacuum air drawn in with the powder135is about 1.7 l/min. The mixture of motive gas and powder in the venturi passage180is in dilute phase conveyance at a velocity of about 80 m/s. Powder135is conveyed through the venturi block124at a rate of about 2 grams/min. The powder is then conveyed through a 3.05 mm diameter passageway for a distance of 1.5 m in strand flow conveyance to an extrusion apparatus.

Extrusion with Powder Supply Through an Annular Passage

An exemplary method of extrusion of a tube around optical fibers and application of powder in the tube is discussed below with reference toFIGS. 1 and 4A-4C.

As shown inFIG. 1, the powder/motive gas supply50supplies powder to the extrusion apparatus60via the powder/motive gas supply passage55. The powder and motive gas are used in an extrusion process used to form a tube.FIG. 4Ais a partial section view of selected portions of the extrusion apparatus60that utilizes the powder and motive gas from the supply passage55in the extrusion process.FIG. 4Aillustrates a crosshead extrusion die200, a powder/motive gas supply assembly210, and an extrusion tip212concentrically mounted on the downstream end of the assembly210. The components200,212illustrate a portion of the extrusion apparatus60, and can be connected to the assembly210, and incorporated into a surrounding crosshead extrusion apparatus of conventional design, the details of which are omitted for the sake of brevity.

Referring toFIG. 4A, the powder/motive gas supply assembly210includes a central fiber guide passage214through which the plurality of optical fibers22pass during the extrusion process. The fiber guide passage214may have, for example, a circular cross section. The powder/motive gas supply assembly210comprises an elongate tubular outer sleeve216and an elongate tubular inner sleeve220mounted concentrically within the outer sleeve216. An annular passage226is defined between the inner and outer sleeves and is arranged to convey powder and motive gas through the powder/motive gas supply assembly210. A powder/motive gas intake stem230is formed in the powder application/extrusion piece210and is in communication with the annular passage226via an intake port234. The powder/motive gas intake stem230is adapted to receive the downstream end of the powder/motive gas supply tube55, or an intermediate conveyance, such as a hose, that is connected to the downstream end of the supply passage55. The mixture of motive gas and powder particles supplied by the powder/motive gas supply50is thereby conveyed into the annular passage226of the powder/motive gas supply assembly210.

The extrusion tip212is mounted concentrically with the downstream end of the outer sleeve216, and a portion of the extrusion crosshead (not shown) abuts a stop edge244on the outer sleeve216. The downstream end of the extrusion tip212and the downstream end of the powder/motive gas supply assembly210are in turn mounted concentrically within the extrusion die200. The region between the extrusion die200and the inner guide212defines an annular channel248through which molten liquid extrudate250is provided for the extrusion process. The source (not illustrated) of molten extrudate may be any conventional means.

FIG. 4Bis an isolated section view of the extrusion apparatus60illustrating the extrusion/powder application process in the vicinity of the extrusion die200. As shown inFIG. 4B, the extrusion tip212has a tapered portion254that terminates at a cylindrical tip portion258. The inner sleeve220of the powder/motive gas supply assembly210extends concentrically through the cylindrical portion258, and may terminate at a location adjacent to the downstream end face of the cylindrical tip portion258. The downstream end of the annular channel226is defined as the annular space between the downstream end of the inner sleeve220and the extrusion tip212.

Still referring toFIG. 4B, in operation, the optical fibers22advance through the passage214in the inner sleeve220in the process direction12. As the fibers22advance through the inner sleeve220, molten extrudate250, which is in a soft, energy absorbing state, is advanced through the extrudate channel248generally in the direction of the heavy black arrows. The extrudate250forms an extrusion cone262around the fibers22that eventually shrinks radially or “draws down” and forms the tube around the fibers22(the drawn down tube is indicated by reference sign264inFIG. 4A). In the exemplary embodiment, the tube is a loose-fitting tube, although other tube forms are possible. An “extrusion cone” can be generally defined as the zone of molten or partially molten extrudate250between the downstream end of the extrudate flow channel258and the point where the extrudate is fully drawn down. The extrusion cone can, for example, extend into the cooling trough (illustrated inFIG. 1). It is understood that the term “extrusion cone” encompasses extrudate forms that deviate from a perfect conical shape, and includes, for example, tapered extrudate forms having concave and/or convex exterior surface contours, as appreciated by those of skill in the art. During the extrusion process, the mixture of motive gas and powder particles supplied by the powder/motive gas supply tube55travels through the annular channel226in a direction indicated by the arrows266, which may be generally parallel to the process direction12, and is introduced into the interior272of the extrusion cone262.

As the gas/powder mixture is launched into the hot extrusion cone262, the powder particles have sufficient momentum so that they collide with the interior272of the extrusion cone262. The gas/powder mixture may be conveyed through the annular channel226with sufficient velocity so that the particles' momentum causes all or a significant portion of the particles to become lodged in, embedded, or otherwise mechanically attached to the interior of the hot extrudate, particularly within the extrusion cone262. In certain applications, a portion of the powder may be adhered to exterior surfaces of the fibers22. During extrusion, the introduction of a gas flow is required to keep the tube from collapsing under atmospheric pressure while the extrudate tube is still molten. The motive gas from the channel226may be of a relatively low flow rate so as not to excessively distort the shape of the extrusion cone262during extrusion, while also conveying the powder particles into the extrusion cone262. As the extrusion cone262closes around the fibers22, the motive gas and excess powder not consumed by the process are returned through the fiber guide passage214in the direction of the arrows278, counter to the process direction12. Powder is therefore introduced into extrusion cone without adversely affecting the dimensional stability of extrusion process.

The particulate matter in the gas/powder mixture266, if having sufficient momentum, can partially embed at various locations in the extrudate. In general, extrudate having a temperature of at least 160° C. provides an application area that allows at least partial embedment of particulate matter. For more effective attachment, the extrudate can be at a temperature of at least 200° C. in the attachment area.

A portion of the particulate powder launched through the annular opening284may not become mechanically attached to the interior surface of the extruded tube through partial embedment. For example, up to 30% by weight of the particulate matter supplied to the extrusion process, and in some embodiments, up to 40%, or more particularly 60% by weight of the supplied particulate matter may mechanically attach to the tube, with the remainder either remaining loose in the tube or being discarded. For example, a portion of the unattached powder, along with the motive gas, can be exhausted through the opposite end of the passage214, or, if desired, recirculated for reuse in the extrusion process. About 10-25% by weight of the total powder, for example, may be exhausted out of the tube. A portion of the unused powder may also be conveyed forward into the tube by the optical fibers22and remain loose in the tube. About 25-45% by weight of the total supplied particular matter, for example, may remain loose in the tube. The passage214can be in communication with a filter, such as a HEPA filter, a collection vessel, or other means for collecting the exhausted unused powder and/or the unused motive gas.

For the quantity of particulate matter that remains in the tube, including the powder either loose, mechanically attached, and a small amount adhered to the fibers, about 45-80% by weight of that quantity may be mechanically attached to the interior of the tube. High levels of mechanically attached powder can be reliably obtained by varying process parameters such as particulate launch momentum. For example, up 60% by weight, or in some embodiments, up to 80% or even 90% by weight, of the total powder quantity in the tube can be mechanically attached to the tube interior. The total percentage of powder by weight mechanically attached can be determined by averaging the measured or calculated weight of the mechanically attached powder per meter length divided by the total weight of the powder per meter length disposed within the tube or cavity. Conversely, the total percentage of powder by weight loosely disposed can be determined by averaging the measured or calculated weight of the loosely disposed powder per meter length divided by the total weight of the powder disposed within the tube or cavity.

According to the present embodiments, fiber optic assemblies can also have relatively small average concentrations of powder per meter of tube. Low average concentrations render the powder in the tube nearly transparent to the craft. The “average concentration” of powder or other particulate matter in a tube is the total weight of particulate matter per unit length of tube, and may be expressed as grams of particulate matter per meter of tube (g/m), or equivalently, milligrams per millimeter (mg/mm). The average concentration can then be used to calculate a “normalized concentration” in order to scale the concentration by tube inner or cavity cross-sectional area. The normalized concentration per square millimeter of cavity cross-sectional area is calculated by dividing the average concentration by the cavity cross-sectional area. The term “normalized concentration” is used in lieu of “volume concentration” because the particulate matter will not be evenly distributed throughout the tube's interior volume. Tubular fiber optic assemblies according to the present embodiments can be formed with low average and normalized powder concentrations yet still have sufficient water-blocking so as to block a one-meter pressure head of tap water within a one meter length for twenty-four hours. In one example, a tube has an average concentration of about 0.02 grams of powder per meter length for a tube having a 2.0 millimeter inner diameter. The cavity cross-sectional of about 3.14 square millimeters yields a normalized concentration value of about 0.01 grams of water-swellable powder per meter length of the tube assembly when rounded up. In another example, a tube has an inner diameter of 1.6 mm and an average concentration of about 0.0085 grams per meter of tube. The normalized concentration is about 0.004 gram of powder per meter length of tube per square millimeter of cavity cross-sectional areas. According to the present embodiments, low powder normalized concentrations of 0.01 or less, and even as low as 0.005 or less, provide desirable waterblocking characteristics, such as the ability to block a one-meter pressure head of tap water within a one meter length for twenty-four hours. Generally speaking, as the cross-sectional area of the cavity of the tube or the like increases, the amount of water-swellable powder needed for effectively blocking the migration of water along the same may increase generally proportionately for effective water-blocking.

According to one aspect of the invention, the powder particles can be passed or launched into the interior of the extrusion cone through the annular opening284formed between the concentrically arranged inner sleeve220and cylindrical tip portion258of the extrusion tip212, which define the terminal end of the annular channel226. The point of exit or launch of powder from the annular opening284is therefore immediately adjacent to the extrusion cone262that is in part defined by the extrusion tip212. The proximity of the annular opening284to the extrusion cone262ensures that minimal kinetic energy of the powder particles is dissipated before the powder particles strike the interior272of the extrusion cone. At the point of exit or launch of the gas/powder mixture from the annular opening284, the extrusion cone262interior cross sectional area is much greater than the cross sectional area of the annular opening284. The increase in cross sectional area that the gas/powder mixture encounters may cause the gas velocity to drop below the saltation velocity, which could cause the powder to fall out of the conveying gas. However, the momentum of the powder particles permits the particles to travel for a short distance at a velocity greater than the gas and to adhere, embed or otherwise mechanically attach to the inside of the extrusion cone262. The nearness of the annular opening284to the interior272of the extrusion cone262ensures a significant portion of the powder particles retain sufficient momentum to mechanically attach to the cone.

Referring toFIG. 4B, according to one embodiment of the invention, the downstream face (i.e., the face to the right inFIG. 4B) of the extrusion die200can be a distance D1in the range of +/−3 mm from the downstream face of the cylindrical tip portion258of the extrusion tip212. The end of the cylindrical tip portion258can be tapered so that the gas/powder mixture exiting the cylindrical channel226impinge essentially directly on the extrudate250. The downstream face of the cylindrical tip portion258can be a distance D2in the range of +/−3 mm from the downstream face of the inner sleeve220. Further, the downstream face of the inner sleeve220, as well as the downstream face of the cylindrical tip portion258, can be downstream of the downstream face of the extrusion die200. In this configuration, the ends of the tip212and the sleeve220lie inside of the extrusion cone262so that the powder particles are launched from the channel226at a launch area that lies inside of the extrusion cone262. In another embodiment, the downstream face of the extrusion die200may be 0-5 mm upstream of the downstream face of the cylindrical tip portion258.

In order to provide a desired degree of attachment of the powder particles to the extruded tube, the velocity of the gas/powder mix at the annular opening284may be in the range of 2 m/s to 50 m/s. The velocity at the annular opening may be, more particularly, at least 5 m/s. For example, for powder particles having an average particle size of about 60 microns or less, a flow velocity of at least 5 m/s (i.e., strand phase flow) at the annular opening284imparts sufficient momentum to the particles so that particles mechanically attach to the molten extrudate. According to this embodiment, the mixture of powder and motive gas may be conveyed in strand phase conveyance through the powder/motive gas supply passage55, and introduced into the extrusion process at strand flow velocities. At least 40% by weight of powder supplied to the process, or, in the range of 45-80% by weight of the total amount of powder that remains in the tube may be adhered to the tube interior using this process.

The powder/motive gas supply assembly210may, for example, be constructed so that the relative axial location of one or more of the inner sleeve220, the extrusion tip212, and the extrusion die200can be varied. For example, the downstream end of the extrusion tip212can be axially adjustable with respect to the extrusion die200. Referring toFIG. 4A, the extrusion tip212can be threadably mounted in the extrusion crosshead (not illustrated) so that it can be axially translated with respect to the extrusion die200. Referring back toFIG. 4B, the distance D1can therefore be varied to obtain desired flow properties at the annular opening284. Similarly, the axial location of the inner sleeve220can be varied with respect to the extrusion tip212and with respect to the extrusion die200. The inner sleeve220can, for example, be threadably mounted in the outer sleeve216to that the distance D2can be varied.

An extrusion process utilizes the powder/motive gas supply50to provide powder and motive gas to the extrusion apparatus illustrated inFIGS. 4A and 4B. The powder/motive gas supply50provides an SAP powder mixture to the powder/motive gas supply passage55in strand phase conveyance, having a motive gas velocity of about 10 m/s. The diameter of the motive gas orifice148is between 0.1 and 0.3 mm (cross-sectional areas of about 0.008 mm2and 0.07 mm2). The flow rate of the motive gas142is between 0.5 l/min and 5 l/min. The proportion of powder to air by volume is between 0.01-2.0%. The powder flow rate is between 0.05-10 gram/min. The powder/motive gas mixture is subsequently conveyed to the annular passage226, where it is conveyed in strand flow at a velocity of about 5 m/s. The average particle size for the powder is in the range of about 30 micrometers, with a size distribution in the range of about 0-63 micrometers as generally described in Table 1. The motive gas/powder mix is launched from the annular passage226at a velocity in the range of 5-8 m/s. The tube is polypropylene with an inner diameter of 1.6 mm and is at a temperature of about 230° C. in the powder application zone. At least 40% by weight of the powder supplied to the extrusion process becomes mechanically attached to the extruded tube. About 45-80% by weight of the powder in the tube is mechanically attached. The normalized concentration is less than 0.01 grams of powder per meter of tube per square millimeter of tube inner cross-sectional area.

Extrusion Utilizing Powder Supply Through a Nozzle

Another exemplary method of extrusion of a tube around optical fibers and application of powder thereto are discussed below with reference toFIGS. 1 and 5A-5B. In the embodiment ofFIGS. 5A and 5B, particulate matter is introduced into the extrusion process through a nozzle rather than an annular opening.

Referring toFIG. 1, the powder/motive gas supply50supplies powder to the extrusion apparatus60via the powder/motive gas supply passage55.FIG. 5Ais a partial section view of the extrusion apparatus60, which includes a crosshead extrusion die500, a powder/motive gas supply assembly510, and an extrusion tip512mounted on the downstream end of the powder/motive gas supply assembly510. The components500,510,512can be incorporated into a crosshead extrusion apparatus of conventional design, the details of which are omitted for the sake of brevity.

Referring toFIG. 5A, the powder/motive gas supply assembly510includes a central fiber guide passage514through which the plurality of optical fibers22pass during the extrusion process. The powder/motive gas supply assembly510comprises an injection head portion516joined to an entry head518by a collar520and a connecting nut522. An exhaust piece526having an exhaust port528can be attached to the entry head518by, for example, a detent mechanism (not shown) disposed in the transverse opening530. Alternatively, the entry head518and the exhaust piece526can be a unitary piece. A powder/gas intake port536is defined in the entry head518and is in communication with a powder/gas passage540in the injection head516. A powder/gas intake stem or other device (not illustrated) may be formed in or attached to the entry head518in communication with the powder/gas intake port536. The intake stem is adapted to receive the downstream end of the powder/motive gas supply passage55, or an intermediate conveyance, such as a hose, connected to the passage55. The mixture of motive gas and powder particles supplied from the powder/motive gas supply passage55is thereby conveyed into the powder/gas passage540.

The powder/motive gas supply assembly510can be mounted to a crosshead extrusion apparatus by inserting the injection head516into the extrusion apparatus, from left to right inFIG. 5A, until the tapered portion548abuts the extrusion apparatus. The connecting nut522is slid over the injection head516along with the collar520. The entry head518is then threaded into the collar520by rotating the collar. The port536and the short longitudinal passage in the entry head518can be aligned with the powder/gas passage540in the injection head516by, for example, a dowel (not illustrated) that prevents relative rotation between the injection head516and the entry head518. The extrusion tip512is mounted concentrically with the downstream end of the injection head516and forms a channel550with the extrusion die500through which molten extrudate554flows during extrusion.

FIG. 5Bis an isolated section view of the extrusion apparatus60illustrating the extrusion/powder application process in the vicinity of the extrusion die500. As shown inFIG. 5B, the extrusion tip512has a tapered portion560that terminates at a terminal downstream (i.e., to the left inFIG. 5B) cylindrical portion564. The end of the injection head516extends concentrically through and is seated within (e.g., may abut) the interior of the cylindrical portion564, and may terminate at a location adjacent to the end of the cylindrical portion564.

In operation, the optical fibers22advance through the powder/motive gas supply assembly510along the process direction12. As the fibers22advance, molten extrudate554, which is in a soft, energy-absorbing state, is advanced through the channel550in the direction of the heavy black arrows, and forms an extrusion cone570around the fibers22that eventually draws down and forms the tube576around the fibers22. The tube576may be, for example, a loose fitting tube, although tight fitting tube forms such as tight buffer tubes are also possible. During the extrusion process, the mixture of motive gas and powder particles supplied by the powder/motive gas supply tube55travels through the powder/gas passage540and is introduced or launched into the interior572of the extrusion cone570.

As the gas/powder mixture enters the extrusion cone570, powder particles impinge the interior572of the hot extrusion cone570. The momentum of the particles causes a significant portion of the particles become adhered, embedded, or otherwise mechanically attached to the interior of the hot extrudate, particularly within the extrusion cone570. In certain applications, a portion of the powder may be adhered to the fibers22. During extrusion, the motive gas may be used to maintain the shape of the extrusion cone570during extrusion, and also serves to convey the powder into the extrusion cone570.

As discussed above, a portion of the total particulate powder launched through the nozzle580may not become mechanically attached to the interior surface of the extruded tube. As the extrusion cone570closes around and adheres to the fibers22to form the tube576, the powder and motive gas not consumed by the process are returned through the fiber guide passage514in the direction of the arrow578. The motive gas and a portion of the unattached powder is exhausted through the exhaust port528in the exhaust piece526(FIG. 5A). A portion of the unattached powder may also be conveyed forward into the tube by the optical fibers22and remain loose in the tube. The exhaust port528can be in communication with a filter, such as a HEPA filter, a collection vessel, or other means for collecting the unused powder and/or the unused motive gas. A portion of the unused powder may also be conveyed forward by the optical fibers22.

According to one aspect of the invention, a nozzle580of smaller cross-sectional area (e.g., a smaller diameter) than the powder/gas passage540can be included at the end of the passage540to accelerate the flow of gas and powder. The nozzle580may be constructed, for example, to accelerate the flow to dilute phase velocities as it is discharged or launched from the nozzle exit. At the exit or launch point of the gas/powder mixture from the nozzle580, the extruded cone570interior cross sectional area is much greater than the cross sectional area of the powder/gas passage540. The increase in cross sectional area that the gas/powder mixture encounters may cause the gas velocity to drop below the saltation velocity, which may cause the powder to fall out of the conveying gas. However, the momentum of the powder particles can be selected to permit the particles to travel for a short distance at a velocity greater than the gas and to adhere, embed or otherwise mechanically attach to the inside of the extrusion cone570.

In order to provide adequate attachment of the powder particles to the extruded tube, the velocity of the gas/powder mix exiting the nozzle580may be in the range of 2 m/s to 100 m/s. For example, for powder having a particle size distribution as shown in Table 1, a flow velocity of at least 20 m/s at the nozzle580imparts sufficient momentum to the particles so that at least 40% of the supplied particles mechanically attach to the molten extrudate. The exit of the nozzle580may be relatively close to the interior surface of the extrusion cone. For example, the nozzle exit may be within +/−7 mm of the downstream face of the extrusion die500.

Referring toFIG. 5B, the exit point of the nozzle580can be positioned so that it flush or nearly flush (e.g., within =/−0.5 mm) with the end of the extrusion tip512. In this embodiment, the particulate matter exiting the nozzle580retains much of its launch velocity as it strikes the extrusion cone570. Locating the nozzle580at extremely close vicinities to the extrusion cone570accordingly ensures high embedment rates for the particles.

The powder/motive gas supply assembly510and the surrounding extrusion apparatus may, for example, be constructed so that the relative axial location of the injection head516with respect to the extrusion tip512can be varied. The injection head516may be, for example, threadably mounted to vary the distance with respect to the downstream end face of the extrusion tip512. The distance D3from the nozzle580to the downstream end of the extrusion tip512, and accordingly the distance of the nozzle580to the extrudate, can then be varied. The extrusion tip512can be also threadably mounted so that the extrusion tip512and/or the injection head516can be axially translated with respect to the downstream end of the extrusion die500. The axial position of the nozzle580with respect to the extrusion die500and with respect to the extrusion tip512can therefore be varied to obtain desired launch properties for the mix of gas and particulate matter.

The arrangement of the nozzle580with respect to the interior surface of the tube allows targeted application of the powder to the extrusion cone570. As shown inFIG. 5B, the powder particles will have their greatest momentum at the exit of the nozzle580, which is offset with respect to the axial centerlines of the extrusion cone570, the optical fibers22, and the end of the extrusion die500. A targeted section of the interior of the extrusion cone570therefore receives a majority of the particulate matter. An example of targeted application of powder particles is discussed below with reference toFIG. 11.

An extrusion process utilizes the powder/motive gas supply50to provide powder and motive gas to the extrusion apparatus illustrated inFIGS. 5A and 5B. The powder/motive gas supply50provides an SAP powder mixture to the powder/motive gas supply passage55in strand phase flow, having a motive gas velocity in the range of 3-20 m/s. The diameter of the motive gas orifice148is between 0.05-0.3 mm (cross-sectional area of about 0.002 mm2and 0.07 mm2). The powder/motive gas mixture is subsequently conveyed to the powder/gas passage540, where it is conveyed in strand phase flow at a velocity in the range of 3-20 m/s. The average size for the powder is in the range of about 0-63 micrometers, with a size distribution as generally described in Table 1. The motive gas/powder mix is launched from the nozzle580at a velocity of at least 20 m/s. At least 45% by weight of the powder supplied to the process becomes mechanically attached to the extruded tube. About 45-80% by weight of the powder in the tube is mechanically attached. The normalized powder concentration is less than 0.01 grams of powder per meter of tube per square millimeter of tube inner cross-sectional area.

The extrusion methods discussed above disclose methods for mechanically attaching particles to the interior of an extruded tube during the extrusion process.FIGS. 6A-6D, 7A-7D, and 8A-8Dare photographs illustrating varying degrees of mechanical attachment of SAP particles to a tube interior. The photographs were obtained using a scanning electron microscope. In order to obtain the photographs, a fiber optic assembly was cut lengthwise using a razor, carbon coated, and photographed in the scanning electron microscope at a 55 degree tilt. The particles700shown in the figures were applied using a nozzle assembly similar to the nozzle shown inFIGS. 5A and 5B.

FIGS. 6A-6Dare a set of scanning electron microscope photographs at varying magnifications of a tube for a cable assembly with a size distribution as generally described in Table 1. In the photograph, mechanically attached SAP particles700are visible as attached to the interior surface705of a tube708. Attachment marks, or “footprints”710are also visible. The footprints710in the interior surface705may indicate mechanically deformed portions of the interior surface where the photograph technician has removed attached particles to illustrate the depth of the surface deformations. The momentum of the particles700imparted by the nozzle_launch allowed the portions of the particles to deform the tube wall and at least partially embed in the tube wall.

FIGS. 7A-7Dare another set of scanning electron microscope photographs of a tube for a cable assembly at varying magnifications. The mechanically attached SAP particles700and footprints710are visible on the interior surface705of a tube708.

FIGS. 8A-8Dare yet another set of scanning electron microscope photographs of a tube for a cable assembly at varying magnifications. Mechanically attached SAP particles700and footprints710are visible on the interior surface705of a tube708.

Fiber Optic Assemblies Having Mechanically Attached Powders

FIGS. 9 and 10respectively schematically depict a cross-sectional and an enlarged longitudinal cross-sectional view of a fiber optic assembly800(i.e., a tube assembly) that can be manufactured according to the above-described methods. The fiber optic assembly800includes a water-swellable powder or powder blend804within a tube805, and a plurality of optical fibers822extending through the tube. The optical fibers822discussed in the present specification may be any suitable type of optical waveguide. Moreover, the optical fibers may be a portion of a fiber optic ribbon, a bundle of optical fiber or the like. In other words, the illustrated optical fibers822are non-tight buffered, but the methods of the present invention may be used with optical fibers having other configurations such as tight buffered, ribbonized, stranded, etc. Cables incorporating micromodules may also be constructed according to the principles of the present invention, with the micromodule tubes, for example, including mechanically attached powder. As shown, the water-swellable powder804is, generally speaking, represented as disposed about the inner surface of tube805with at least a portion thereof mechanically attached to the tube inner wall. Further, the water-swellable powder804is mechanically attached to a relatively small percentage of a surface area of the tube inner wall. The presence of the powder is nearly transparent to the craft while being surprisingly effective in its water-blocking performance.

The assembly800has a relatively high proportion of its water-swellable powder804mechanically attached while still being able to block a one-meter pressure head of tap water within a one meter length for twenty-four hours. As used herein, “tap water” is defined as water having a saline level of 1% or less by weight. Similarly, fiber optic tube assemblies disclosed herein may also block saline solutions up to 3% by weight within 3 meters for 24 hours, and the blocking performance may even stop the 3% saline solution within about 1 meter for 24 hours, depending on the design. Mechanical attachment of the powder allows a portion of an attached water-swellable particle to protrude beyond the surface so that if water enters the cavity it may contact the particle. It is theorized that after the water contacts the water-swellable particle and initiates swelling that some of the particles break free of the surface so they can fully swell and/or move to form a water-blocking plug with other particles.

The water-swellable powder804is disposed within an extruded tube having an inner wall with a given surface area per meter length. In one embodiment, about 30 percent or less of the surface area of the inner wall of the tube has water-swellable powder and/or powder blends mechanically attached thereto, but other percentages are possible, such as 25 percent or less. The mechanical attachment may be generally uniformly disposed on the surface area, such as 30 percent or less, of the entire surface as depicted.

Alternatively, mechanical attachment may be concentrated in longitudinal stripes, strips (either interrupted or continuous) in the tube inner wall. For example, 100 percent or less, or more specifically at least 70%, mechanical attachment can be achieved in one or more stripes that cover 30 percent or less of the surface area, with substantially no mechanical attachment at other locations, as shown schematically inFIG. 11. This configuration may be alternatively stated as applying powder within arc sector of the tube interior. For example, arcs of 90 degrees or less, or 60 degrees or less may have mechanically-applied powder particles with the remainder of the tube interior being free of mechanically-attached particles. This type of targeted mechanical attachment may be obtained by, for example, providing a directing extension from the nozzle580illustrated inFIG. 5Bso that the flow of powder impinges on a specific region of the extrudate cone570.

FIG. 12is a photograph showing a magnified view (about 50×) of the inner wall of an extruded tube having powder mechanically attached thereto by the methods discussed above, viewed using I-Solutions software after any loose water-swellable powder or powder blend has been removed.FIG. 13is the same photograph shown inFIG. 12with the powder within the region of interest850identified using the software to determine the percentage of surface area within region of interest850that is mechanically attached thereto. The software allows the measurement of the percentage of surface area having powder mechanically attached thereto since the gray scale differentiation reveals the surface area having powder mechanically attached thereto relative to the tube wall. When using the software to determine the percentage of surface area having mechanical attachment, the threshold lighting should be properly adjusted to view the contrast between areas. Region of interest850shown inFIG. 13has the powder mechanically attached to about 30 percent or less of the region of interest850as depicted. In other embodiments, the powder can be mechanically attached to 25 percent or less of the surface area. Further, from the image the size and shape of the powder are observable.

According to one aspect of the present embodiments, the relatively low levels of powder that can be used to provide adequate water reduce attenuation in the optical assemblies. For example, in conventional cables, excessive water-blocking powder may gather and agglomerate when moistened. The agglomerated powder can reach a size such that it presses against one or more optical fibers and induces delta attenuation.

One factor that can affect optical performance is the maximum particle size, average particle and/or particle size distribution of water-swellable powder, which can impact microbending if the optical fibers should contact (i.e., press against) the water-swellable particles. The average particle size for the water-swellable powder is preferably about 150 micron or less, but other suitable average particles sizes are possible such as 60 micron or less. The skilled artisan understands that since the powder is sieved using an appropriated sized mesh it has a distribution of particle sizes. For instance, individual particles may have an aspect ratio (i.e., longer than wide) that still fit through the sieving mesh in one direction and are larger than the average particle size. Using SAPs with a somewhat larger average maximum particle size may still provide acceptable performance, but using a larger maximum particle size increases the likelihood of experiencing increased levels of optical attenuation. One exemplary water-swellable powder is a crosslinked sodium polyacrylate available from Evonik, Inc. of Greensboro, N.C. under the tradename Cabloc GR-211. The particle distribution for this powder is given by Table 1:

Of course, particulate matter including other powders, powder blends, and/or other particle distributions is possible. Another suitable crosslinked sodium polyacrylate is available from Absorbent Technologies, Inc. under the tradename Aquakeep J550P, still other types of water-swellable materials are also possible. By way of example, another suitable water-swellable powder is a copolymer of acrylate and polyacrylamide, which is effective with saline solutions. Furthermore, powder blends of two or more materials and/or water-swellable powders are possible such as blend of a slow-swelling water swellable powder and a fast-swelling water swellable powder. Likewise, a blend of water-swellable powder can include a first water-swellable powder that is highly-effective for a saline solution and a second water-swellable powder effective for tap water. Powder blends may also include components that are not inherently water-swellable. By way of example, small amounts of silica, such as, for example, a fumed silica, up to 3% may be added to a water-swellable powder for improving flow properties and/or inhibiting anti-caking due to moisture absorption. Additionally, concepts of the invention allow the use of other types of particles with or without the water-swellable particles such as flame-retardant particles (e.g., aluminum trihydrate, magnesium hydroxide, etc.), a dry lubricant like talc, graphite, boron, and/or the like.

A further factor to consider when selecting a water-swellable material is its absorption capacity. Absorption capacity is the amount of water that a unit of water-swellable material can absorb and is typically measured in grams of water absorbed per gram of water-swellable material. In one embodiment, the water-swellable material used in the methods described herein preferably has an absorption capacity of at least about 100 grams per grams of water-swellable material, but other values lower or higher are possible. For instance, the water-swellable material can have an absorption capacity of about 200 grams or more per gram of material, 300 grams or more per gram of material, or 400 grams or more per gram of material.

According to another aspect of the invention, the powder or powder blends used in the processes described herein can inhibit the sticking between the optical fibers and the tube without using a separation layer or other material. Specifically, fiber optic assemblies can have issues with the optical fibers contacting and sticking to the tube while it is molten state when being extruded about the optical fibers. If the optical fiber sticks to the inside of the tube it can cause the path of the optical fibers to be distorted (i.e., the optical fiber is prevented from moving at that point), which may induce undesirable levels of optical attenuation. As depicted inFIGS. 9 and 10, tube805is disposed about optical fibers822of fiber optic assembly800without using a further material or component as a separation layer (e.g., no gel, grease, yarn, tape, etc.) for inhibiting contact between the optical fibers and the molten tube. Sticking is inhibited because the water-swellable powder is a cross-linked material so it does not promote sticking thereto at typical extrusion temperatures. Thus, water-swellable powder804tends to act as a separation layer since it inhibits optical fibers822from sticking to the molten tube during manufacture.

The water-swellable powder804also acts to reduce the friction between the optical fibers and the tube or cavity wall by acting as a slip layer. The particulate matters effectively acts as ball-bearings between the optical fibers822and the inner wall of the tube to reduce friction and to reduce tension in the optical fibers. The optical fibers822may also include an outer layer such as an ink having a suitable lubricant for inhibiting optical fibers822from sticking to the molten tube805during extrusion. The tube805may include one or more suitable fillers in the polymer, thereby inhibiting the adherence of the optical fibers with the tube. Additionally, the use of other polymer materials for the tube such as a highly-filled PVC can inhibit sticking of the optical fibers to the tube.

The extruded tubes discussed in this specification may be constructed using any suitable polymer material for housing and protecting the optical fibers822therein. For example, the tube805and the tubes discussed in the extrusion methods can be a polypropylene (PP), polyethylene (PE), or blends of materials such as a blend of PE and ethylene vinyl acetate (EVA). In other embodiments, the tube805is formed from a flame-retardant material such as flame-retardant polyethylene, flame-retardant polypropylene, polyvinyl chloride (PVC), or polyvinylidene fluoride PVDF, thereby forming a portion of a flame retardant fiber optic cable. However, the tube805need not necessarily be formed from a flame-retardant material for making a flame-retardant fiber optic cable. In still other embodiments, the tube805may comprise a thin sheath that is easily tearable by the craft without tools. For example, a tube formed from a highly filled material renders it easily tearable by the craftsman merely using their fingers to tear the same. By way of example, tubes that are easily tearable may include a filled materials such as polybutylene terephthalate (PBT), a polycarbonate and/or a polyethylene (PE) material and/or an ethylene vinyl acrylate (EVA) or other blends thereof having fillers like a chalk, talc, or the like; however, other suitable materials are possible such as a UV-curable acrylates. Generally speaking, all other things being equal, the tube805can have a smaller inner diameter ID compared with dry tube assemblies that include a water-swellable yarn, tape, or thread (i.e., a carrier for the SAP) with the optical fibers. This is because the tube805does not have to provide the space for both the optical fibers and the carrier of the SAP (i.e., the yarn(s) or tapes); consequently the inner diameter of the tube may be smaller. A smaller inner diameter also allows for a smaller outer diameter, a more flexible assembly having a smaller bend radius (which may reduce kinking), lighter weight per length, and longer lengths of cable on a reel.

FIG. 14is a cross-sectional view of a fiber optic cable900manufactured according to the present invention. The cable comprises a plurality of subunits902arranged around a central strength member908and enclosed within a jacket920. A fire-retardant layer924is arranged on the interior of the jacket920, and a water-swellable layer928is arranged on the interior of the layer924. Each subunit902has a tube940enclosing plurality of optical fibers944. The tubes940can be extruded using the methods described herein, such that water-swellable particles are embedded in the interior tube surfaces948. The subunits902can have relatively low levels of water-swellable powder with normalized concentrations of less than 0.01, for example.

Many modifications and other embodiments of the present invention, within the scope of the claims will be apparent to those skilled in the art. For instance, the concepts of the present invention can be used with any suitable fiber optic cable design and/or method of manufacture. For instance, the embodiments shown can include other suitable cable components such as an armor layer, coupling elements, different cross-sectional shapes, or the like. Thus, it is intended that this invention covers these modifications and embodiments as well those also apparent to those skilled in the art.