Polypropylene-polyethylene copolymer buffer tubes for optical fi

Details Polypropylene-polyethylene copolymer buffer tubes for optical fi Polypropylene-polyethylene copolymer buffer tubes for optical fi

1996-09-19

1998-06-02

Lee, John D.

Optical waveguides

Optical transmission cable

Loose tube type

385113, 385141, G02B 644

Patent

active

057613621

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
The present invention relates to optical fiber cables and, more particularly, to a polypropylene-polyethylene copolymer core or buffer tube for an optical fiber cable, and a method for making the same.
2. Description of the Prior Art
Optical fiber cables have been used for many years to transmit information at high rates and very long distances. The transmission medium of the optical fiber cable are delicate hair-thin optical fibers. Ordinarily, these optical fibers reside in a buffer tube or in a core tube. Hereinafter, for the sake of simplicity, buffer tubes shall also include core tubes. The buffer tube provides the primary structure to protect the hair-thin optical fibers residing therein. The buffer tube may be fabricated from a single material or from a combination of two or more materials in a multi-layer form.
In virtually all optical fiber cables, it is desirable to have a buffer tube made from a material with a high Young's modulus. The use of a material with a high Young's modulus results in a cable with a buffer or core tube having relatively high tensile and compressive resistance capability, a trait useful in protecting the optical fibers in the event the cable is twisted, stretched or compressed. Also, it is important to select a material for the buffer tube that has a low thermal expansion coefficient. Too much shrinkage or expansion of the buffer tube caused by temperature changes could place tensile or compressive loads on the optical fibers residing therein. High tensile or compressive loads can result in damaged or broken optical fibers.
Prior art buffer tubes have been made from polybutylene terephthalate (PBT), polycarbonate (PC), a layered combination of PBT and PC, or a polyamide such as Nylon-b 12. The PBT, PC or Nylon are fairly good materials for making buffer or core tubes because they are materials that have high Young's modulus and low thermal expansion coefficients. However, such materials also have some disadvantages when compared to other materials such as polypropylene-polyethylene copolymers. These disadvantages include higher cost, lower flexibility, moisture sensitivity, and increased difficulty in handling and processing due to the mechanical properties of the materials.
While polypropylene-polyethylene copolymers are cheaper and easier to handle than PBT, PC or Nylon and could be used as buffer or core tubes for optical fiber cables, they generally have not been favored over PBT, PC or Nylon. The polypropylene-polyethylene copolymers generally have higher thermal expansion coefficients and lower Young's modulus than PBT, PC or Nylon such that a buffer tube made with the polypropylene-polyethylene copolymer would have greater shrinkage, and less compression-tension resistance. The disadvantages of greater shrinkage and lower compression-tension resistance of polypropylene-polyethylene copolymers have outweighed the material handling and cost advantages offered by this material.
To overcome some of the above noted disadvantages, U.S. Pat. No. 5,031,996 discloses an optical fiber cable having a dual-layer buffer tube. According to this patent the outside layer is formed from PBT and the inside layer from a modified polypropylene material. The patent further discloses that the modified polypropylene material may be a copolymer with polyethylene which is provided with an inorganic filler as a blend with cross-linked polypropylene to enhance its properties.
One disadvantage with the modified polypropylene material disclosed in U.S. Pat. No. 5,031,996 is that relatively large amounts of filler must be added to the polypropylene polymer to obtain the appropriate mechanical strengths required for optical fiber cables. When large amounts of filler are added, the buffer tube becomes too stiff and loses its flexibility at low temperatures. Also, the processibility of polymer with high filler content or loading (typically 20 to 40 percent by weight) becomes more difficult and causes extruder tooling abrasion. Furthermore, mod

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Plastic Additives Handbook, 3rd Edition by R. Gachter and H. Muller,

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