Optical waveguides – Optical transmission cable
Reexamination Certificate
1999-03-03
2001-02-06
Healy, Brian (Department: 2874)
Optical waveguides
Optical transmission cable
C385S113000, C385S141000, C525S240000
Reexamination Certificate
active
06185349
ABSTRACT:
FILED OF THE INVENTION
The present invention relates to a multimodal polymer composition for fibre optic cables and to a fibre optic cable.
TECHNICAL BACKGROUND
Fibre optical cables have been used for many years to transmit information. The fibre optical cables includes hair-thin optical fibres as the transmission medium. These optical fibres are protected by core or buffer tube structures from external forces and elements. Depending on the construction, the fibre optic cables may be divided in four different categories. These are the slotted core type, the tight-coated tube type, the stranded-loose tube type, and the central tube type. Some of these categories can be subdivided further depending on how the fibres are inserted into the core. Thus, the fibres may be inserted as individual fibres, fibre ribbons, or fibre bundles. The most frequently used type of fibre optic cable today is the stranded-loose tube type, but an increasing interest has been shown in the other types, especially the slotted core type which is of particular significance in connection with the present invention.
As an example of fibre optic cables reference may be made to U.S. Pat. No. 5,574,816 which discloses buffer tubes for optical fibre cables.
In the stranded-loose tube type cable the optical fibres reside in protecting and supporting elements in the form of gel filled buffer tubes. These buffer tubes are stranded around a central strengthening member.
In a fibre optical cable of the slotted core type, the optical fibres reside in gel filled channels or slots in the surface of a so-called slotted core supporting element. The core has the form of a central strengthening member of steel or fibre glass reinforced plastic around which is extruded a plastic rod with circumferential slots or groves in the surface. These slots of the slotted core are symmetrical and form a winding helical or SZ like path along the longitudinal axis of the cable. The optical fibres inserted into the slots may be loose fibres or fibre ribbons. The slotted core with its optical fibre and gel filled slots is secured with a binding tape and covered by an outer sheathing. For further information regarding fibre optical cables of the slotted core type reference may be made to an article by Mikko Saikkonen, “Extrusion of slotted core elements”, Wire Technology International, November 1995.
The supporting elements, such as the slotted core elements for slotted core type cables and the buffer tubes of the stranded loose tube type cables are made to very exacting requirements. This puts very severe requirements on the materials used for these elements. Normally, the elements are made of polymer materials by melt extrusion. For easy production the polymer material should have a good melt processability; to cope with the mechanical stresses it is exposed to in use it should have good mechanical properties such as a sufficient strength in the solid state; and, very important particularly to the slotted core type cable, the polymer material should have a good dimensional stability to keep the profile shape stable, i.e. so that the ridges of the slotted core are not deformed or collapses. Another important property is the melt strength of the material, and further, the polymer material should have a low shrinkage. Many of these requirements are opposed to each other, i.e. if one property of the polymer material is optimized another property is likely to be deleteriously affected. The polymer materials used hitherto which are mostly unimodal polyethylene polymers therefore tend to be compromises with regard to their properties.
There is an ongoing need in the art for improved polymer materials for fibre optical cables, and more particularly for supporting elements such as slotted cores and buffer tubes. Specifically there is a need for a polymer material with improved dimensional stability and the need for this is particularly strong with regard to slotted cores.
SUMMARY OF THE INVENTION
It has now been discovered that the above deficiencies of the prior art may be alleviated or eliminated with an improved supporting element, such as a slotted core, provided for a fibre optical cable if a certain defined type of multimodal polymer composition is used as the material for the supporting element. The multimodal polymer composition of the present invention may be used for supporting elements of all types of fibre optical cables, such as buffer tubes and slotted cores, but in view of its outstanding dimensional stability it is of particular advantage as a material for the slotted core of fibre optical cables of the slotted core type. The superior properties of the multimodal polymer composition of the present invention are achieved by carefully selecting certain parameters of the polymer composition. Thus, the polymer composition should comprise a multimodal polyethylene having a low to high density, a high viscosity at low shear stress, and a selected ratio between its low molecular weight fraction and high molecular weight fraction.
Thus, the present invention provides a multimodal polymer composition for fibre optical cables, characterized in that it comprises a multimodal polyethylene with a density of 0.920-0.965 g/cm
3
and a viscosity at a shear stress of 2.7 kPa (&eegr;
2.7 kPa
) of at least 150 kPa.s, said multimodal polyethylene comprising a low molecular weight (LMW) ethylene homo- or copolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction, said multimodal polyethylene composition having a weight ratio of the LMW fraction to the HMW fraction of (35-55): (65-45).
Further, the present invention provides a fibre optical cable, characterized in that the cable has a fibre supporting element selected from slotted cores and buffer tubes, and that the fibre supporting element con- sists of a multimodal polymer composition according to any one of claims
1
-
13
.
Other distinguishing features and advantages of the invention will appear from the following specification and the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
As stated above the multimodal polymer composition of the present invention is a multimodal polyethylene.
The “modality” of a polymer refers to the form of its molecular weight distribution curve, i.e. the appearance of the graph of the polymer weight fraction as function of its molecular weight. If the polymer is produced in a sequential step process, utilizing reactors coupled in series and using different conditions in each reactor, the different fractions produced in the different reactors will each have their own molecular weight distribution. When the molecular weight distribution curves from these fractions are superimposed into the molecular weight distribution curve for the total resulting polymer product, that curve will show two or more maxima or at least be distinctly broadened in comparison with the curves for the individual fractions. Such a polymer product, produced in two or more serial steps, is called bimodal or multimodal depending on the number of steps. In the following all polymers thus produced in two or more sequential steps are called “multimodal”. It is to be noted here that also the chemical compositions of the different fractions may be different. Thus one or more fractions may consist of an ethylene copolymer, while one or more others may consist of ethylene homopolymer.
The multimodal polyethylene of the present invention is a low to high density polyethylene, as mentioned above, and has a density of 0.920-0.965 g/cm
3
, preferably 0.940-0.960 g/cm
3
.
The multimodal polyethylene of the present invention comprises a low molecular weight (LMW) ethylene homo- or copolymer fraction and a high molecular weight (HMW) ethylene copolymer fraction. Depending on whether the multimodal polyethylene is bimodal or has a higher modality the LMW and HMW fractions may comprise only one fraction each or include sub-fractions, i.e. the LMW fraction may comprise two or more LMW sub-fractions and similarly the HMW fraction may comprise two or more HMW sub-fractions. As a matter
Dammert Ruth
Heino Eeva-Leena
Korvenoja Tarja
Martinsson Hans-Bertil
Borealis Polymers OY
Healy Brian
Merchant & Gould P.C.
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