The present invention is generally related to fiber optic cables and, more particularly, is related to fiber optic buffer tubes.
Optical fiber cables have been in use in communications industries for a number of years to transmit information at very high rates over long distances. In an optical fiber cable the information is carried in the form of light signals through glass fibers. These fibers are protected from the environment and external stresses by the cable structure.
Optical fiber cables may be classified into three general classifications based on cable structure: loose tube, slotted core, and monotube. In loose tube optical fiber cables, the optical fibers lie in a plurality of optical fiber buffer tubes which are generally filled with some type of water blocking compound such as a gel. The loose tube buffer tubes are stranded around a central strength member. In the loose tube design, in addition to the buffer tubes, filler rods may be stranded around the central member in order to provide symmetry in design for fiber counts lower than that of a full fiber count cable. The filler rods may be made of a solid or a cellular polymer.
In a slotted core cable the optical fiber reside in channels or slots that are generally filled with a water-blocking gel. The channel or slots are symmetrically placed round the central core and form helical or reverse-helical grooves extending along the longitudinal axis of the cable for receiving one or more optical fibers. In order to ensure that the optical fibers are not subject to destructive tensile and compressive stresses when the cable is bent, each slot may be made so as to follow a helical path. Thus, at a curved part of a cable, an optical fiber experiences compression and tension, and over the length of the curve the stresses at least partially cancel out. In some cables, the direction of lay of the helices may reverse at periodic intervals.
In a monotube cable the optical fibers reside in a central tube which is generally filled with some type of water-blocking compound. In all of these structures, the buffer tube or core provides the primary structure to protect the thin optical fibers contained within. Typically the buffer tubes or core are jacketed with additional protective layers. Additionally, reinforcing yarns or fibers as well as water-blocking materials in the form of gels or hot melts, water swellable powders, yarns, tapes, and/or corrugated armor may be placed between the jacket and the inner cable layers to strengthen and protect the optical fibers.
For each buffer tube design, it is important to choose material combinations which are compatible in terms of basic material properties and processability and exhibit desirable engineering thermoplastic characteristics. Key properties relevant in choosing the material and processing conditions include a low sensitivity to moisture, heat resistance, dimensional stability, chemical resistance, low density, and recyclability. More specifically, the choice of materials and processing conditions should produce a tube that is kink- and crush-resistant as well as having the ability to be score-snapped. Additional parameters that are relevant to choosing the material and processing conditions are tensile strength, flexural strength, and flexural modulus.
Materials. and processing conditions must be chosen which result in a cable which has high compression resistance and tensile strength, combined with a low amount of residual stress. It is also important to choose a combination of materials and processing conditions which has minimal changes in dimensions as a function of time and temperature. It is desirable for a material to have a low coefficient of thermal expansion to ensure that the fibers are not placed under stress as the cable endures the high and low temperature extremes encountered within its environment. Favorable material and processing conditions, which minimize process-induced orientation, are also desired since these will minimize the post-extrusion relaxation and shrinkage of cable components. Post-extrusion shrinkage of buffer tubes can lead to an increase in excess fiber length (a ratio of fiber length to actual tube length) which can, in turn, cause increases in fiber attenuation.
In designing the cable structure it is important to ensure that process- or construction-induced stresses related to cable production do not interfere with optical fiber performance. The general trend in the industry is to increase rates of production to meet demand and increase profitability by increasing line speeds on production equipment. For extruded components such as optical fiber buffer tubes, filler rods, cores, or jackets, higher line speeds may result in greater shear rates and higher orientation and residual stress in the finished product especially if an optimal material is not used.
Polycarbonates, fluoropolymer, polybutylene terephthalate, Nylon-12, polypropylene-polyethylene copolymer, polyester elastomer, acetal resins and the like have been used as buffer tube materials. However, these materials have material, product, performance, and economic drawbacks.
For example, polybutylene terephthalate (PBT) is the most commonly used polymeric material for making loose tube fiber optic buffer tubes. However, it has some inherent disadvantages, one of which is that PBT is susceptible to hydrolysis which leads to a loss in strength. Additionally, PBT is a stiff material (flexural modulus of at least 330,000 psi) and any attempt to improve its flexibility (make, it less stiff) results in a more expensive product.
Polyamides are reputed for their outstanding mechanical properties and chemical resistance. However, their resistance to hydrolysis is limited. Further, polyamides are hygroscopic and tend to absorb water which in turn affects their mechanical and electrical properties as well as their dimensional stability.
The copolymer polypropylene-polyethylene (PP) has been used as a buffer tube material as a substitute for PBT. This polymer has a flexural modulus of about 200,000 psi and therefore is more flexible than PBT. However, there are other undesired properties associated with the resin. PP copolymer shrinks during and after processing, which consequently has a heavy bearing on excess fiber length, an important parameter that influences the attenuation of optical fibers. In addition, PP has a lower tensile, flexural and compression strength than PBT. Also the thermal resistance is inferior to an engineering thermoplastic. All of these properties are required for some of the more demanding applications of fiber optic cables.
Polyethylene (PE) though it is flexible, has poor thermal and mechanical properties. It exhibits very high dimensional shrinkage resulting in higher xe2x80x9cexcess fiber lengthxe2x80x9d numbers. Nylon-6 is hygroscopic and thus, tends to absorb moisture in a humid environment. Further, the processing of nylon needs special screw/barrel design, which is less shear sensitive. It also exhibits shrinkage both during and after extrusion that in turn affects the fiber-to-tube ratio and causes excess fiber length. These are some of the reasons why neither Nylon-6 nor PE has ever been established as an xe2x80x9cindustry-standardxe2x80x9d buffer tube material in the fiber optic industry.
Accordingly, there is a need in the industry for a buffer tube material that has the advantages of thermo-mechanical performance of PBT, chemical resistance of nylon and flexibility and attractive pricing of a polyolefin.
The present invention provides a fiber optic buffer tube made of a polyamide/polyolefin blend, where the polyamide/polyolefin blend comprises a blend of Nylon-6 and polyethylene. The Nylon-6 and polyethylene blend is about 50-90% Nylon-6 and about 10-50% polyethylene. Alternatively, the Nylon-6 and polyethylene blend is about 60-80% Nylon-6 and about 20-40% polyethylene. The polyamide/polyolefin blend is made of a recyclable material and is heat resistant, kink-resistant, and score-snappable.
The present invention can also be viewed as a fiber optic cable. The fiber optic cable is constructed of at least one buffer tube and at least one transmission medium positioned within the buffer tube. The buffer tube is made of a polyamide/polyolefin blend, as described hereinabove, wherein the polyamide/polyolefin blend comprises a blend of a Nylon-6 and polyethylene. The Nylon-6 and polyethylene blend is about 50-90% Nylon-6 and about 10-50% polyethylene. Alternatively, the Nylon-6 and polyethylene blend is about 60-80% Nylon-6 and about 20-40% polyethylene. The polyamide/polyolefin blend is made of a recyclable material and is heat resistant, kink-resistant, and score-snappable.
Other features and advantages of the present invention will become apparent to one with skill in the art upon examination of the following drawings and detailed description. It is intended that all such additional features and advantages be included herein within the scope of the present invention.
USPTO Backgrounds
USPTO Backgrounds