Wire and a process for its production

Details Wire and a process for its production Wire and a process for its production

2000-12-04

2003-03-18

Wu, David W. (Department: 1713)

Coating processes

Electrical product produced

Wire conductor

C427S118000, C427S120000, C526S348000, C526S348400, C526S348600

Reexamination Certificate

active

06534119

ABSTRACT:

FIELD OF THE INVENTION
The invention relates to a wire coated with an insulating material comprising an ethylene/&agr;-olefin copolymer and a process for producing it. More specifically, the invention relates to a wire coated with an insulating material having excellent physical properties such as mechanical properties, wear resistance, heat stability and crosslinking properties and an excellent appearance, and to a manufacturing process therefor showing excellent extrusion-coating characteristics.
BACKGROUND OF THE INVENTION
In the past, polyethylene has been used for an insulating material for many of the power cables and other electrical wires, and its excellent electrical insulating properties have been evaluated highly. However, it has been said that when such polyethylene is used in an environment in which there is much moisture, a water tree occurs in the insulating material layer, causing a decline in electrical insulating properties. Because of this, among other reasons, an improvement in its physical properties has been called for so that the polyethylene may be used as a satisfactory insulating material.
In recent years, a new polyethylene manufactured by use of a metallocene catalyst has come on the scene, and a study is under way to apply such new polyethylene to insulating material use. Japanese Laid-Open Patent Publication No. 509905/1994 and Japanese Laid-Open Patent Publication No. 312118/1995 disclose that such new polyethylene is capable of preventing from the occurrence of a water tree to a significant extent, thus improving its electrical insulating properties. However, since the new polyethylene has a narrow molecular weight distribution, it does not show satisfactory extrudability. Because of this, when the polyethylene is actually used in forming a thick-walled coating layer, the surface of the coating layer thus formed roughens, sometimes damaging the appearance. This has called for a further improvement in its extrudability.
DESCRIPTION OF THE INVENTION
Accordingly it is an object of the invention to solve the problems experienced by the conventional technology as mentioned above and to provide a wire which has a coating layer having a satisfactory appearance, while retaining the electrical insulating properties, mechanical strength, wear resistance and crosslinking properties inherently possessed by polyethylene.
Another object of the invention is to provide a process for producing a wire having such coating layer.
The invention relates to a wire whose conductor or conductor shielding layer is coated with an insulating material comprising a copolymer of an ethylene and an &agr;-olefin having 3 to 20 carbon atoms, wherein the ethylene/&agr;-olefin copolymer (a) meets following conditions (A) to (D):
(A) a density (d) is 0.880 to 0.950 (g/cm
3
);
(B) a melt flow rate (MFR) is 0.01 to 20 (g/10 minutes);
(C) a relationship between a decane soluble matter content (W (wt %)) at 23° C. and the density is:
when MFR≦10 (g/10 minutes),
W
<80×exp{−100(
d
−0.88)}+0.1,
when MFR>10 (g/10 minutes),
W
<80×(
MFR
−9)0.26exp{−100(
d
−0.88)}+0.1; and
(D) a relationship between a temperature (Tm (° C.)) at a position of the highest peak of an endothermic curve as measured by a differential scanning calorimeter (DSC) and the density (d) is:
Tm
<400
×d
−244.
Further, the insulating material may be a composition comprising the ethylene/&agr;-olefin copolymer (a) and another ethylene-based polymer. For example, if a composition obtained by compounding a high-pressure low-density polyethylene to the ethylene/&agr;-olefin copolymer (a) is used as such composition, the composition comprising 60 to 97 wt % of the ethylene/&agr;-olefin copolymer (a) and 3 to 40 wt % of the high-pressure low-density polyethylene is preferable.
Furthermore, the insulating material may further comprise a crosslinking agent such as a peroxide or a silane compound. If the insulating material crosslinked with such crosslinking agent has following physical properties, it is preferable as the wire coating layer:
(1) a tensile strength of not less than 15 MPa;
(2) a tensile elongation of not less than 400%; and
(3) a wear amount of not more than 15 mg as determined by a taper wear test method.
Moreover, the invention relates to a process for producing the wire, wherein a relationship between a resin pressure P (kg/cm
2
) and a resin temperature T (° C.), an extrusion rate K (kg/hour), a lip cross-section area of extruder A (cm
2
) and a melt flow rate of resin MFR (g/10 minutes) is under following conditions when the insulating material is extrusion-coated onto a conductor or a conductor shielding layer:
100
×{K
/(
MFR
)
1.5
/(
T
−120)/
A}
0.2
≦P
≦1000
×{K
/(
MFR
)
1.5
/(
T
−120)/
A}
0.2
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The invention relates to a wire coated with an insulating material comprising an ethylene/&agr;-olefin copolymer (a) by extrusion-coating the insulating material onto a conductor or a conductor shielding layer such as a semiconductor layer, and a process for producing it. The followings are given specific explanations of each constitution of such wire and process.
Ethylene/&agr;-olefin Copolymer (a)
This copolymer is a polymer comprised primarily of an ethylene and an &agr;-olefin having 3 to 20 carbon atoms. Specific examples of such &agr;-olefin include propylene, 1-butene, 1-pentene, 1-hexene, 4-methyl-1-pentene, 1-octene, 1-decene and 1-dodecene.
The ethylene content of the copolymer is normally 94 to 99 mol %, preferably 96 to 98 mol %, and the comonomer &agr;-olefin content is normally 1 to 6 mol %, preferably 2 to 4 mol %. This copolymer is formed by random-copolymerizing the ethylene and &agr;-olefin whose contents are in the ranges shown above.
Here, the ethylene and &agr;-olefin contents can be measured by use of
13
C-NMR. Specifically, the composition can be determined normally by measuring
13
C-NMR spectrum of a sample obtained by dissolving approximately 200 mg of the copolymer uniformly in 1 ml of hexachlorobutadiene in a test tube 10 mm&phgr; under the conditions of the test temperature of 120° C., test frequency of 25.05 MHz, spectrum width of 1500 Hz, pulse repeating time of 4.2 sec. and pulse width of 6 &mgr;sec.
The ethylene/&agr;-olefin copolymer (a) constituting the insulating material allows a satisfactory coating layer to be formed when (A) its density, (B) its melt flow rate (MFR), (C) the relationship between the decane soluble matter content and density, and (D) the relationship between the temperature at the position of the highest peak of the endothermic curve as measured by use of a differential scanning calorimeter (DSC) and density are in the certain ranges explained below.
(A) The density of the copolymer is in the range of 0.880 to 0.950 (g/cm
3
), preferably 0.880 to 0.940 (g/cm
3
), more preferably 0.890 to 0.930 (g/cm
3
). It is convenient to impart flexibility to the wire coating layer if the density is in this range. Here, the density is measured at 23±0.1° C. by use of a density gradient tube in accordance with ASTM D-1505 after subjecting the strand, which is obtained by the operation of measurement of the melt flow rate, to heat treatment at 120° C. for one hour and then slowly cooling it to the room temperature for one hour.
(B) Moreover, the value of the MFR of the copolymer as measured at 190° C. and under a load of 2.16 kg in accordance with ASTM D-1238 is 0.01 to 20 (g/10 minutes), preferably 0.1 to 15 (g/10 minutes), more preferably 0.2 to 11 (g/10 minutes). It is preferable that the MFR should be in this range, because the coating layer having a satisfactory appearance may be obtained at high extrusion molding speed in the range of MFR.
(C) Furthermore, the copolymer shows the relationship between the decane soluble matter content (W (wt %)) at 23° C. and density (d) which meets the following conditions:
when MFR≦10 (g/10 minutes),
W
<80×exp{−100(

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