Outdoor, dry-core loose-tube cable eliminates flooding compounds
Based on test measurements, nonflooded, water-blocked loose-tube fiber-optic cables perform equivalently to their flooded counterparts in aerial, direct buried and duct installations
clinton E. Clyburn III,
anne g. bringuier and
john c. williams
An outdoor, dry-core loose-tube fiber-optic cable design eliminates the need for a flooding compound outside the buffer tubes, but it still prevents water penetration. The flooding compound is replaced with water-swellable materials that permit fast removal for cable splicing, shorten installation time and delete the use of cleaning chemicals. These attributes, therefore, aid in lowering fiber-optic cable and installation costs.
The use of greaseless water-swellable materials for cable construction was initially developed for slotted core designs. In recent years, however, the development of superabsorbent polymers has improved the quality and performance of these materials. Different swellable materials for the outdoor, dry-core cable design include tape and yarn combinations and superabsorbent coatings attached by different binders to various cable components.
The design of a nonflooded core loose-tube cable presents mechanical and ergonomic performance challenges. Design goals opted to meet or surpass existing stranded loose-tube cable characteristics. These characteristics include robust protection of the fibers during installation, ease of access and quick identification of buffer tubes.
With the goal of eliminating flooding compound, the initial effort concentrated on preventing water from flowing down interstitial spaces in the cable. In single-layer tube cables, these spaces consist of areas above and below the buffer tubes. In high-density cables, another water path exists between buffer-tube layers. The size of these interstitial areas depends on the central-member and buffer-tube diameters, tube count and the thicknesses of components placed over the tube`s outer layer.
To overcome water absorption, superabsorbent polymers are incorporated into this cable design. These polymers, such as salts of polyacrylic acids, are well-characterized and effective in water absorption. They can be used in different forms and come with a variety of substrates.
Superabsorbent polymers are analyzed according to swell rate, swell capacity and gel strength. Gel strength of the polymers retains the cable`s blocking ability under expected levels of water pressure. After a careful study of polymer grades and performance levels, the materials investigated included superabsorbent polymer powder and other polymers bound in forms of tensile and non-tensile yarns, binders and tapes. Combinations of these forms and methods of application were studied to determine the polymer materials that met the cable`s design goals.
Based on initial studies, water-swellable tapes and yarns were chosen to replace the flooding compounds. Because armored and non-armored (duct) loose-tube cables share a common core construction, the duct design was first completed for a nonflooded core conversion. This work was then used as the foundation for various armored designs.
Design analyses yielded the desired nonflooded duct construction, including swell tape and yarn size, application method, tape type, yarn count and location of materials. Single-layer cables of 5- through 16-tube positions were studied followed by 16- through 24-position dual-layer cables, all using 3.0-millimeter buffer tubes.
Samples of the different core constructions were tested for the degree of water ingress in accordance with Telecommunications Industry Association/Electronic Industries Association Fiber Optic Test Procedure-82, or FOTP, Water Penetration. A 1-meter static head of tap water was applied at one end of a 1-meter length of cable for 24 hours. After testing, the cable samples were dissected. Then, the distance of water ingress was measured along different components in the cable--central member, swell yarns, outer core, swell tape, tensile yarns and ripcord. These six variables were identified to quantify the cable location and length the water was able to penetrate along the test sample. Based on this investigation, the single-layer duct cable construction was established.
A nonflooded core armored cable followed the development of the duct version. It included the same core as that designed for the duct cable, so design concentration was focused on components outside the tensile yarn wall. Performance levels were expected to be the same or better than that of a standard flooded loose-tube armored cable.
To take advantage of the nonflooded cable`s weight savings and assembly simplicity, one design version eliminated an inner jacket between the tensile yarn wall and the armor wrap. This version would allow the swell tape to provide water blockage in areas between the tensile yarns and the armor. Eliminating the inner jacket helped to decrease the outside diameter of the cable by 1 to 2 mm. This dimension equated to a 12% drop in the armored cable`s outside diameter for fiber counts to 36.
Another cable design version included an inner jacket between the core and armor. Unfortunately, an additional water path was created in this interface. To solve this problem, a water swell tape--which facilitated removal of the armor from the inner jacket--was wrapped over the inner jacket. Armor tape was then applied, and a final outer jacket was extruded over the cable.
Water penetration samples
The duct and armored cable designs were tested to FOTP-82 for water penetration. Unaged 5- and 24-position duct and armored cable samples were subjected to a three-week water penetration test with a 1-meter head. These tests revealed no statistical difference between 24-hour ingress values and 21-day measurements. All measurements involved less than 1 meter.
In addition, the water-swell materials employed in the nonflooded core cables were analyzed in terms of swell performance after aging. Long-term reliability of cable components is crucial because cables are expected to have lifetimes of 20 or more years when they are exposed to combinations of temperature and humidity.
Five-position single-layer and 24-position dual-layer duct and armored cables were subjected to 168 hours of cable aging at 85C. Twenty samples were randomly chosen from a 500-meter reel of each cable type and exposed to a 1-meter head of tap water for 48 hours. Measurements in different ingress areas were compared to unaged results for determining changes in water penetration performance. Statistical comparisons did not show any differences between aged and unaged water penetration.
Cold-temperature performance of loose-tube cables is analytically determined by comparing a geometrically-derived contraction "window" and the amount of expected cable contraction at a given temperature. This shrinkage is estimated from an effective cable coefficient of thermal expansion formula. For this equation, the assumption is made that the composite cable model employs full coupling between cable components. Nonflooded core cables correlate well with this model. It has been shown that coupling forces between buffer tubes and central member in nonflooded core loose-tube cables are five times higher than in flooded cables.
The tensile behavior of the cable can also be deduced from the composite cable model. Adding contributions from the different tensile members in the cable permits estimation of the amount of cable strain at various loads.
From the presence of higher coupling forces, it is surmised that nonflooded core cables exhibited improved tensile performance. Increased coupling forces lead to lower slippage between components under tensile loads. In effect, allowances made for construction stretch in stress-strain plots can be reduced. Given equivalent cabled excess fiber lengths, higher installation loads are realized before fiber strain is observed in the nonflooded cable.
Developmental armored cables in 6-position single-layer and 24-position dual-layer constructions were produced and evaluated for optical, mechanical and environmental performance according to various fiber-optic test procedure criteria for outdoor cables. All cables contained standard singlemode and 50- and 62.5-micron multimode fibers. Tests were chosen to subject the cable to rigors beyond what would be expected under typical use. These tests included temperature cycling and aging, tensile loading, crush, impact and cyclic flex. Test results disclosed that the environmental and mechanical performances of the nonflooded core loose-tube cable in duct and armored versions are equivalent to or better than their flooded counterparts.
FOTP-82 testing for water penetration employs tap water. However, in field installations where cables are submerged in ground water, charged ions are present in various concentrations. Ionic solutions are known to impose negative effects on the swellability of superabsorbent polymer materials. Testing was conducted to evaluate the interaction between water swell materials and typical ground waters.
The superabsorbent polymer material used in the tape and yarn is a sodium, salt-based polyacrylic copolymer. Each polymer chain is lined with carboxyl groups. In water, the carboxyl groups solvate and dissociate into negatively charged carboxylate groups. The repulsion between these negatively charged ions loosens the polymer coil, enabling absorption of water around the polymer chains.
Water swell performance
Water swell performance depends on ionic density and varies in proportion to the osmotic pressure. Hydrogen bonding drives the water absorption mechanism. When the water contains positive ions, these ions position themselves next to the carboxylate ions, thus limiting the swelling and absorbency capacity of the superabsorbent polymer. This positioning explains why distilled deionized water swells the superabsorbent polymer more than tap water does, which is already slightly ionized. This action also explains why swell performance decreases with increasingly ionic water.
The effect of ionic solutions on the swellability and, therefore, on the cable water penetration performance, was examined in the case of various monovalent ions, such as sodium and potassium, and polyvalent ions, such as calcium, magnesium, aluminum and iron. Polyvalent ions were expected to have the greatest effect on water absorbency. For example, calcium ions generally come in contact with twice as many carboxylate groups as a monovalent ion such as potassium. Concentrations of these ions vary greatly, depending on the geographical location of the cable. Typical ground water analyses were gathered from different reference books on water chemistry.
Several properties were investigated, including swell height, swell rate and mass uptake of the bare swell tape and yarn, and water penetration performance of the finished cable. Good correlation between the swell performance of yarn and tape was observed, which confirms that the superabsorbent polymers chemically react the same whether in tape or yarn form.
Measurements showed that a mixture of solutions found in ground water had less of an effect on swell performance than would be expected from a model in which the effects of individual ions alone are added. Interactions between ion groups in ground water and other phenomena serve to alleviate ionic affinity for the swell tape`s carboxyl groups. Solutions were also used to perform FOTP-82 water penetration testing on duct and armored versions of the dry core cable. All samples exhibited less than 1-meter water ingress after 24 hours.
Loose-tube optical fiber cables consist of a plurality of buffer tubes stranded about a rigid central member. These buffer tubes house and protect the optical fibers from mechanical degradations. The helical lay imparted by stranding the tubes protects the fibers against tensile loading of the cable and cold temperature cable contraction.
Stranding the tubes provides a protection window, whereby the fibers endure no exterior forces when the cable is subjected to tensile forces up to its rated load. Likewise, a window is established against compressive forces when the cable is exposed to its lowest rated cold temperature. Therefore, the optical fibers are not subjected to micro- and macro-bending, which could lead to increases in attenuation. Outside of the buffer tube layer, the cable is wrapped with a complement of tensile yarns, ripcords, armoring and outer jackets.
Loose-tube cables typically employ a grease-based flooding compound around the buffer tubes to block longitudinal water penetration. Greases can be effective water-blocking medium, but they require tedious cleaning with solvents during cable preparation for termination or splicing in the field.
Dry cables can be prepared faster because cleaning the flooding compound from the cable core is eliminated. In addition, fewer chemicals are required to prepare the cable, and only the filling compound on the fibers needs to be removed. Qualitatively, the lack of grease and the use of fewer chemicals make the processes of preparing and terminating dry cable much easier and cleaner. The choice of using nonflooded or flooded fiber-optic cables, therefore, hinges on the need for efficiency and timeliness in installation rather than on the particular application. u
Clinton E. Clyburn III is product development engineer, Anne G. Bringuier is senior materials engineer and John C. Williams is senior product specialist at Siecor Corp., Hickory, NC.