Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Mixing of two or more solid polymers; mixing of solid...
Reexamination Certificate
2002-03-13
2004-06-29
Egwim, Kelechi C. (Department: 1713)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Mixing of two or more solid polymers; mixing of solid...
C525S240000
Reexamination Certificate
active
06756447
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to cables, in particular for power transmission, for telecommunications or for data transmission, or also combined power/telecommunications cables, wherein at least one coating layer consists of a recyclable material which is halogen-free and has superior mechanical, electrical, and flame-retardant properties.
2. Description of the Related Art
There is currently a great need for highly environmentally friendly products, consisting of materials which are not harmful to the environment either during their production or when in use, and which are readily recyclable at the end of their working life. However, the option of using ecological materials is, in all cases, subject to the need to keep costs within acceptable limits, while still guaranteeing performances which are at least equivalent to those of conventional materials and which are, in any case, satisfactory under the most common conditions of use.
In the cables sector, in particular power transmission cables, the various coatings surrounding the conductor commonly consist of crosslinked polymer materials, in particular polyethylene or ethylene copolymers suitably crosslinked during extrusion, so as to give satisfactory mechanical performances even under heating in continuous use and under conditions of current overload, while at the same time maintaining a high level of flexibility. These materials are crosslinked and therefore cannot be recycled since they are devoid of thermoplastic properties, hence they can only be disposed of at the end of their working life by means of incineration. Moreover, in certain cases the outer protective sheath consists of polyvinyl chloride (PVC) which is difficult to separate by conventional methods (for example in water by density differences) from the crosslinked polyolefins containing inorganic fillers (for example from ethylene/propylene rubbers containing inorganic fillers), and, on the other hand, PVC cannot be incinerated together with crosslinked polyolefins since this produces highly toxic chlorinated products by combustion.
In U.S. Pat. No. 4,948,669 cable-coating compositions are described comprising from 29 to 50% by weight of low-density polyethylene, containing as comonomer an alpha-olefin having from 4 to 12 carbon atoms, in particular 1-octene, in an amount such as to obtain a density of between 0.90 and 0.92 g/cm
3
, in admixture with: (a) a propylene homopolymer; (b) a non-elastomeric copolymer of propylene with ethylene; or (c) heterogeneous copolymers of propylene with ethylene, obtained in reactor. As polyethylene it is particularly suggested using product Dowlex® 4000E from Dow Chemical, containing about 17% of 1-octene and having a melt index equal to 3.3 and a density of 0.912 g/cm
3
. These are products obtained using titanium-based Ziegler-Natta catalysts, having a relatively high density and thus little flexibility.
In patent application WO 96/23311 a low-voltage, high-current cable is described, wherein the insulating coating, the inner sheath, and the outer sheath are made of the same non-crosslinked polymer-based material which is black coloured by addition of carbon black. Using the same base material would allow recycling without the need to separate different materials. As polymer material for the outer sheath, it is suggested using, in place of PVC, ultra-low-density polyethylene (ULD-PE), for example products Engage® from DuPont-Dow Elastomers and Exxpol® from Exxon. Inorganic fillers such as aluminium or magnesium hydroxide are added to these materials in order to give them flame-retardant properties.
In U.S. Pat. No. 5,246,783 cables are described, having as insulating and/or semiconductive coatings polymer materials based on copolymers of ethylene with at least one C
3
-C
20
alpha-olefin, with a density of from 0.86 to 0.96 g/cm
3
, known commercially under the tradename Exact® from Exxon, preparable using metallocene catalysts. These copolymers are used in crosslinked form, achieved by chemical means (for example with dicumyl peroxide) or by irradiation.
SUMMARY OF THE INVENTION
The Applicant has perceived that the technical problem of obtaining a cable with a coating made of a non-crosslinked, and thus recyclable, polymer material which also has mechanical and electrical properties suitable to the usual conditions of use is dependent on the use of a crystalline propylene homopolymer or copolymer mixed with a copolymer of ethylene with an alpha-olefin having a low density and a high structural uniformity, in particular having a highly homogeneous distribution of the alpha-olefin between the polymer molecules. This high structural uniformity is obtainable in particular by copolymerization of the corresponding monomers in the presence of a single-site catalyst, for example a metallocene catalyst.
In particular, the Applicant has found that excellent performances, both in terms of mechanical properties, in particular elongation at break, stress at break, and modulus, and in terms of electrical properties, may be obtained by using, as non-crosslinked base material for at least one of the coating layers of the cable, a mixture as defined hereinbelow, comprising polypropylene and a copolymer of ethylene with at least one C
4
-C
12
alpha-olefin and optionally with a diene comonomer, having a density of from 0.90 to 0.86 g/cm
3
and a Composition Distribution Index, defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin, of greater than 45%.
Therefore, according to a first aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers comprises, as non-crosslinked base polymer material, a mixture comprising: (a) a crystalline propylene homopolymer or copolymer; and (b) a copolymer of ethylene with at least one alpha-olefin having from 4 to 12 carbon atoms, and optionally with a diene; the said copolymer (b) being characterized by a density of from 0.90 to 0.86 g/cm
3
and a Composition Distribution Index, defined as the weight percentage of copolymer molecules having an alpha-olefin content within 50% of the average total molar content of alpha-olefin, of greater than 45%.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has electrical insulating properties and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers has semiconductive properties and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least one of the said coating layers is an outer protective sheath and comprises a mixture as defined above as non-crosslinked base polymer material.
According to a further aspect, the invention relates to a cable comprising a conductor and one or more coating layers, wherein at least 70%, preferably at least 90%, by weight relative to the total weight of the base polymer material of the said coating layers consists of the mixture as defined above.
The Composition Distribution Index provides a measure of the distribution of the alpha-olefin between the copolymer molecules (the higher the value of this index, the more homogeneous is the distribution of the comonomer between the copolymer molecules) and can be determined by techniques of Temperature Rising Elution Fractionation, as described, for example, in U.S. Pat. No. 5,008,204 or in Wild et al.,
J. Poly. Sci. Poly. Phys. Ed.
, Vol. 20, p. 441 (1982).
The copolymers (b) have a molecular weight distribution index, defined as the ratio between the weight-average molecular weight M
w
and the number
Albizzati Enrico
Castellani Luca
Grizante Redondo Eduardo
Zaopo Antonio
Egwim Kelechi C.
Finnegan Henderson Farabow Garrett & Dunner L.L.P.
Pierelli Cavi E Sistemi.S.p.A.
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