Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
1998-05-15
2001-12-11
Wilson, Donald R. (Department: 1713)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S379000, C428S375000, C428S383000, C174S1100PM, C174S11000P, C174S1100SR, C174S12000C, C385S100000, C385S106000, C525S240000
Reexamination Certificate
active
06329054
ABSTRACT:
The present invention relates to a cable-sheathing composition, as well as the use thereof as outer sheathing for a power cable or a communication cable.
Cables, by which is meant power cables for high voltage, medium voltage or low voltage, and communication cables, such as optical cables, coaxial cables and pair cables, generally comprise a core surrounded by a sheath consisting of one or more layers. The outermost layer is referred to as outer sheath or sheathing layer and is nowadays made of polymer material, preferably ethylene plastic. The highly diverse fields of application for various sorts of cables, such as telecommunication cables, including conventional copper cables and fibre-optical cables, as well as power cables, entail that the sheathing material has to meet a number of property requirements which in some respects are contradictory. Thus, important properties of cable-sheathing materials are good processability, i.e. it should be easy to process the material within a broad temperature range, low shrinkage, high mechanical strength, high surface finish as well as high environmental stress cracking resistance (ESCR). Since it has hitherto been difficult or even impossible to meet all these property requirements, prior-art sheathing materials have been the result of compromise, such that that good properties in one respect have been obtained at the cost of poorer properties in some other respect.
Thus, it would be highly advantageous if this compromise as regards the properties of cable-sheathing materials could be reduced or even eliminated. In particular, it would be advantageous if one were able to improve the ESCR of the material and reduce the shrinkage at a given processability.
The present invention achieves this goal by a cable-sheathing composition which, instead of the unimodal polyethylene plastic used in conventional cable-sheathing compositions, consists of a multimodal olefin polymer mixture having certain given values of density and melt flow rate, both as regards the polymer mixture and as regards the polymers forming part thereof.
The present invention thus provides a cable-sheathing composition, which is characterised in that it consists of a multimodal olefin polymer mixture having a density of about 0.915-0.955 g/cm
3
and a melt flow rate of about 0.1-0.3 g/10 min, said olefin polymer mixture comprising at least a first and a second olefin polymer, of which the first has a density and a melt flow rate selected from (a) about 0.930-0.975 g/cm
3
and about 50-2000 g/10 min and (b) about 0.88-0.93 g/cm
3
and about 0.1-0.8 g/10 min.
The invention further concerns the use of this cable-sheathing composition as outer sheath for a power cable or a communication cable.
Further distinctive features and advantages of the invention will appear from the following description and the appended claims.
However, before the invention is described in more detail, a few key expressions will be defined.
By the “modality” of a polymer is meant the structure of the molecular-weight distribution of the polymer, i.e. the appearance of the curve indicating the number of molecules as a function of the molecular weight. If the curve exhibits one maximum, the polymer is referred to as “unimodal”, whereas if the curve exhibits a very broad maximum or two or more maxima and the polymer consists of two or more fractions, the polymer is referred to as “bimodal”, “multimodal” etc. In the following, all polymers whose molecular-weight-distribution curve is very broad or has more than one maximum are jointly referred to as “multimodal”.
The “melt flow rate” (MRF) of a polymer is determined in accordance with ISO 1133, condition 4, at a temperature of 190° C. and a nominal load of 2,160 kg and is equivalent to the term “melt index” previously used. The melt flow rate, which is indicated in g/10 min, is an indication of the flowability, and hence the processability, of the polymer. The higher the melt flow rate, the lower the viscosity of the polymer.
By the term “environmental stress cracking resistance” (ESCR) is meant the resistance of the polymer to crack formation under the action of mechanical stress and a reagent in the form of a surfactant. The ESCR is determined in accordance with ASTM D 1693 A, the reagent employed being Igepal CO-630.
By the term “ethylene plastic” is meant a plastic based on polyethylene or on copolymers of ethylene, the ethylene monomer making up most of the mass.
As indicated in the foregoing, the cable-sheathing composition according to the invention is distinguished by the fact that it consists of a multimodal olefin polymer mixture of specified density and melt flow rate.
It is previously known to produce multimodal, in particular bimodal, olefin polymers, preferably multimodal ethylene plastics, in two or more reactors connected in series. As instances of this prior art, mention may be made of EP 040 992, EP 041 796, EP 022 376 and WO 92/12182, which are hereby incorporated by way of reference as regards the production of multimodal polymers. According to these references, each and every one of the polymerisation stages can be carried out in liquid phase, slurry or gas phase.
According to the present invention, the main polymerisation stages are preferably carried out as a combination of slurry polymerisation/gas-phase polymerisation or gas-phase polymerisation/gas-phase polymerisation. The slurry polymerisation is preferably performed in a so-called loop reactor. The use of slurry polymerisation in a stirred-tank reactor is not preferred in the present invention, since such a method is not sufficiently flexible for the production of the inventive composition and involves solubility problems. In order to produce the inventive composition of improved properties, a flexible method is required. For this reason, it is preferred that the composition is produced in two main polymerisation stages in a combination of loop reactor/gas-phase reactor or gas-phase reactor/gas-phase reactor. It is especially preferred that the composition is produced in two main polymerisation stages, in which case the first stage is performed as slurry polymerisation in a loop reactor and the second stage is performed as gas-phase polymerisation in a gas-phase reactor. Optionally, the main polymerisation stages may be preceded by a prepolymerisation, in which case up to 20% by weight, preferably 1-10% by weight, of the total amount of polymers is produced. Generally, this technique results in a multimodal polymer mixture through polymerisation with the aid of a chromium, metallocene or Ziegler-Natta catalyst in several successive polymerisation reactors. In the production of, say, a bimodal ethylene plastic, which according to the invention is the preferred polymer, a first ethylene polymer is produced in a first reactor under certain conditions with respect to monomer composition, hydrogen-gas pressure, temperature, pressure, and so forth. After the polymerisation in the first reactor, the reaction mixture including the polymer produced is fed to a second reactor, where further polymerisation takes place under other conditions. Usually, a first polymer of high melt flow rate (low molecular weight) and with a moderate or small addition of comonomer, or no such addition at all, is produced in the first reactor, whereas a second polymer of low melt flow rate (high molecular weight) and with a greater addition of comonomer is produced in the second reactor. As comonomer, use is commonly made of other olefins having up to 12 carbon atoms, such as &agr;-olefins having 3-12 carbon atoms, e.g. propene, butene, 4-methyl-1-pentene, hexene, octene, decene, etc., in the copolymerisation of ethylene. The resulting end product consists of an intimate mixture of the polymers from the two reactors, the different molecular-weight-distribution curves of these polymers together forming a molecular-weight-distribution curve having a broad maximum or two maxima, i.e. the end product is a bimodal polymer mixture. Since multimodal, and especially bimodal, polymers, preferably ethylene polyme
Asumalahti Markku
Martinsson Hans-Bertil
Rogestedt Laila
{umlaut over (Aa)}rilä Jari
Borealis Polymers OY
Merchant & Gould P.C.
Wilson Donald R.
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