The most common sheathing material for fiber optic cable, popular in many countries and preferred by both cable plants and installation groups is Polyethylene. Polyethylenes comes in HDPE, LDPE, LLDPE, and MDPE forms to mention the first few. Some countries like India, New zealand etc use Nylon 12 as the outer jacekting material for duct and direct buried cables. Nylon sheath protect the cables from termites and soil chemical corrosion. PVC is used for Indoor
fiber optic cables.
Our topic of discussion today is about special materials used for fiber optic cable sheathing. Some customers ask Fluorine sheathed cables. The fluorine plastic material FEP or
perfluoroethylene propylene is a viscoelastic material. FEP exhibits good electrical, mechanical, thermal and chemical properties. PFA is another fluorine plastic commonly used as sheathing or tubing material.
PFA and related fluoropolymers are almost universally inert to industrial chemicals and solvents. Often referred to as Teflon® tubing, PFA tubing can handle virtually any corrosive chemical in use today, with few exceptions. It is also recognized for its excellent resistance to heat — up to 500°F continuously. Lesser known is its usefulness in extremely cold temperatures — well below -200°F. It will remain reasonably flexible to -110°F
Fluoropolymers are one of the most versatile engineering materials known and, since their discovery over 60 years ago, have been used in nearly every field of modern industry, technology, and science.
PFA (Perfluoroalkoxy) belongs to a class of melt-processible fluoroplastics. PFA tubing is known for its gas and vapor permeability properties and excellent UV transmission ratings. While very similar in composition to FEP, it has better heat resistance and has a smoother surface. It is more clear and flexible than PTFE.
The original fluoropolymer resin, PTFE was discovered by Dr. Roy Plunkett, a chemist at DuPont, in 1938. The use of fluoropolymers became well known to the public in 1946 when E.I. DuPont began marketing “Teflon®” brand product for use on cookware, as a fiber treatment for stain resistance, and in other household applications. PFA was later introduced in 1972. These days, high quality fluoropolymers are manufactured by a number of respected companies for use in the manufacture of PFA tubing.
Both PFA and FEP are used for sheathing optical cables, when temperatures higher than 100 deg C is required. In those cases, attention must be paid to other cable elements including the optical fibers whether they can withstand the anticipated temperatures or not. FEP and PFA sheaths have a smooth surface and are resistant to weathering.
Another most common special sheathing material type used for optical fiber cables is Halogen free sheathing materials. These are sheaths made of materials containing ethylene vinylacetate - EVA. They hae a filling upto 50 percent by weight of Aluminium trioxyhydrate. Then who will dare to come near me with fire ! asks the fiber optic cable !
Halogen - Free Sheath ? Halogens are a group of chemical eleements in periodic table of elements. These elements include, Chlorine, Bromine, Iodine, Astatine etc. Presence of these elements in sheathing materials is considered to be the cause of smoke generating when the sheath catches fire in case of an accident in indoors buildings. Smoke from burning cables kill more people almost equally with other factors. PVC is the typical example for a sheathing material with Halogen group as the 'C' in PVC stands for Chlorine.
There are different terminologies used to denote these Halogen Free materials as Low smoke Zero Halogen, ZHLS, LSOH etc. These are the preferred choice of sheathing materials when both flame resistance and freedom from halogens are required by the customers. In such cases, PE, PVC and plastics with Fluorine are not suitable to use.
When the fire catches and under the influence of flame, the temperature goes higher than 250 deg C, the water separates out of the aluminium trioxyhydrate. With the release of water and evaporation, the flames lose the energy. The temperature falls down than it is required to maintain the fire. The concentration of inflammable gases and oxygen will get reduced by the addition of steam and this will extinguish fire. The material remaining will be incombustible aluminium oxide.
Fiber Optic cables with EVA sheaths have equal mechanical properties almost identical to those of other sheath cables. Low smoke zero halogen sheathed cables are getting popular as the indoor/outdoor application of fiber optic cables have increased recently.
Read more with http://fiberopticcableprocess.blogspot.com/
Published: July 19, 2008
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