New Ultradur grade for sheathing of fiber-optic cables

Presentation by Dr. Simon Kniesel

Business Development Universal

Engineering Plastics Europe

BASF SE, Ludwigshafen

Broadband fiber-optic cables for a fast Internet connection are a hallmark of modern economies today. With 82 percent of households connected to the Internet, Germany ranks among the top countries in Europe according to BITKOM, the telecommunications industry trade association.

While worldwide demand for fiber-optic cables is growing, on the one hand, the space available for overhead cross-country lines and in cable ducts as well as in cable conduits in buildings and optoelectronic assemblies is limited. Cables are required to be ever thinner, while the information density remains the same. To meet these increasingly challenging requirement, BASF has developed a new grade of Ultradur^® specifically for thin fiber-optic sheathing. Ultradur is the brand name of the semi-crystalline, thermoplastic, saturated polyester based on PBT (polybutylene terephthalate) from BASF. The new grade has the designation Ultradur B6550 LNX and offers considerably improved mechanical properties, above all at low thicknesses. With this new grade, BASF is expanding its existing line of Ultradur products for extrusion of protective sheathing for optical fibers. In contrast to the PBT grades optimized for injection molding these materials are characterized by a high molecular weight and correspondingly high viscosity.

Increasing Internet traffic – Protection for thin fiber-optic cables

Internet traffic is increasing along with the demand for telephone, television and industrial communication connections such as communication equipment for control of electricity grids. The operators of the central Internet node in Frankfurt/Main have estimated how great the growth in Internet traffic will be: According to these estimates, data traffic on the Internet will increase twentyfold from 2011 to 2015. The primary reason for this is the continued growth in transmission of multimedia content such as videos and television shows. This calls for thinner but more powerful fiber-optic cables with thinner sheathing. At the same time, the optical fibers must still be protected adequately under all circumstances. This can be assured only by plastic sheathing that has exceptionally good mechanical properties such as stiffness, for instance. 

Multilayer sheathing and extrusion

The cables used for optical data transmission have a complex structure. Up to 12 light-conducting glass fibers are combined into so-called multifiber loose buffer tubes and encased in a highly stiff and stabilizing polybutylene terephthalate (PBT) sheath in an extrusion process. Sheathing takes place in a completely automated extrusion process during which the individual fibers are fed from separate reels at speeds of 100 to 500 meters per minute and pulled into the simultaneously extruded tube. A distinction is drawn between filled and unfilled loose tubes, depending on whether or not the space within the sheath is filled with a gel-like mass. The engineering plastic PBT is characterized in this application by especially fast solidification of the melt during the extrusion process and high stiffness of the finished loose tubes.

The tubes are subsequently combined and typically wound around a glass fiber-reinforced aramid or epoxy core. Depending on the cable application, the entire bundle is then covered with different protective layers such as aramid fibers or glass rovings and finally sheathed with polyethylene or polyamide in another extrusion process.

Very stiff, but not brittle and easy to extrude

With the aim of assisting users in production of thin cable sheathing, BASF developed Ultradur B6550 LNX, which provides the same protection against kinking with a considerably thinner sheath around the loose tubes. The material has a high yield stress and an increased modulus of elasticity. The result is a material with high stiffness that is normally accompanied by brittleness. Here, however, the development engineers at BASF succeeded in combining low brittleness with the high elastic modulus – this means a high elongation at break.

In addition, because of its semi-crystalline morphology Ultradur B6550 LNX exhibits even faster crystallization than standard PBT grades and has a very high melt viscosity, especially at low shear rates. Both properties are beneficial in the extrusion process. 

Mechanical strength doubled

A key indicator for the user when evaluating the sheathing quality is the so-called crush resistance. It describes the uniformity and compressive strength of the sheath around the loose tubes. To measure the crush resistance, a cable with a defined length is clamped in a fixture, a signal is sent through the cable while pressure is simultaneously applied to the cable until the signal attenuation reaches a value of 0.05 dB. The quantity measured is the maximum force in newtons per decimeter (N/dm) that the cable can withstand until this attenuation is reached in the optical fiber. The PBT-based sheathing materials currently on the market offer a crush resistance of about 600 N/dm for cables with a 2 mm O.D. and approximately 1.5 mm I.D. In contrast, the new Ultradur B6550 LNX provides a crush resistance value that is more than twice as high (1,350 N/dm). Thus, the new material offers greater resistance to crushing at the same thickness – as desired. The crush resistance was determined in the laboratories of one of the world's leading manufacturers of fiber-optic cables.

The market for especially thin sheathing for fiber-optic cables is a specialty market. It presents demanding challenges for material developers and suppliers in terms of consistent quality, reliability of supply and technical support for customers during processing. With its worldwide supply network and its extensive PBT expertise, BASF can easily meet these challenges.

www.ultradur.de

Additional information on Ultradur^® materials from BASF is available by sending a message to the e-mail address Ultraplaste.infopoint@basf.com or by calling the telephone number +49 (0) 621 60-78780.

P-13-303