Stabilized insulation composition

Details Stabilized insulation composition Stabilized insulation composition

1999-04-16

2001-02-13

Truong, Duc (Department: 1711)

Synthetic resins or natural rubbers -- part of the class 520 ser

Synthetic resins

At least one aryl ring which is part of a fused or bridged...

C524S609000, C524S611000, C528S086000, C428S379000, C428S377000

Reexamination Certificate

active

06187858

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a stabilized insulation composition used in cable construction.
BACKGROUND INFORMATION
A typical electric power cable generally comprises one or more conductors in a cable core that is surrounded by several layers of polymeric materials including a first semiconducting shield layer (conductor or strand shield), an insulating layer, a second semiconducting shield layer (insulation shield), a metallic tape or wire shield, and a protective jacket. Additional layers within this construction such as moisture impervious materials are often incorporated. Other cable constructions such as plenum and riser cable omit the shield.
It is well known in the cable art that it is desirable to have insulation, which is crosslinkable, and superior with respect to the following properties: blooming prevention, anti-scorch, process stability, water tree retardancy, thermal distortion resistance, and heat aging resistance.
Crosslinked cables are typically prepared by extruding around an electrical conductor a compounded insulation composition comprising a polyethylene, one or more antioxidants, and a crosslinking agent, and curing same. When the insulation is processed into a cable, the resulting properties, mentioned above, are affected significantly by the kind and amount of the antioxidants, which are compounded into the insulation. Conventionally, thiobisphenol has been most widely used as the antioxidant of choice for cable insulation; however, when used as the only antioxidant, it has been known to cause various problems such as unsatisfactory heat aging resistance. This leads to blooming, which results in the deterioration of water tree retardancy because of the micro-voids produced by the blooming. On the other hand, if, in order to avoid the blooming, the compounding amount of the antioxidant is lowered, problems such as anti-scorch and process stability arise.
In order to solve this problem, it has been proposed to compound polyethylene with 4,4′-thiobis-(3-methyl-6-t-butylphenol), which is one of the thiobisphenols, and one other antioxidant as described in Japanese Patent Publications Gazette No. 9173/1987 or 36061/1987, but these combinations do not satisfactorily achieve the following properties: blooming prevention, anti-scorch, process stability, water tree retardancy, thermal distortion resistance, and heat aging resistance.
DISCLOSURE OF THE INVENTION
An object of this invention is to provide an insulation composition having a combination of antioxidants in effective amounts to provide the aforementioned desirable properties at a superior level. Other objects and advantages will become apparent hereinafter.
According to the invention, such an insulation composition has been discovered. The composition comprises:
(a) polyethylene;
(b) as a first antioxidant, a thiobisphenol;
(c) as a second antioxidant, a compound containing 3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate in the molecule;
(d) as a third antioxidant, distearyl thiodipropionate; and
(e) an organic peroxide,
with the proviso that each antioxidant is present in an amount of about 0.01 to the about 0.2 part by weight and the organic peroxide is present in an amount of about 0.5 to about 3 parts by weight, all per 100 parts by weight of polyethylene.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
Polyethylene, as that term is used herein, is a homopolymer of ethylene or a copolymer of ethylene and a minor proportion of one or more alpha-olefins having 3 to 12 carbon atoms, and preferably 4 to 8 carbon atoms, and, optionally, a diene, or a mixture or blend of such homopolymers and copolymers. The mixture can be a mechanical blend or an in situ blend. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. The polyethylene can also be a copolymer of ethylene and an unsaturated ester such as a vinyl ester, e.g., vinyl acetate or an acrylic or methacrylic acid ester.
The polyethylene can be homogeneous or heterogeneous. The homogeneous polyethylenes usually have a polydispersity (Mw/Mn) in the range of about 1.5 to about 3.5 and an essentially uniform comonomer distribution, and are characterized by single and relatively low DSC melting points. The heterogeneous polyethylenes, on the other hand, have a polydispersity (Mw/Mn) greater than 3.5 and do not have a uniform comonomer distribution. Mw is defined as weight average molecular weight and Mn is defined as number average molecular weight. The polyethylenes can have a density in the range of 0.860 to 0.950 gram per cubic centimeter, and preferably have a density in the range of 0.870 to about 0.930 gram per cubic centimeter. They also can have a melt index in the range of about 0.1 to about 50 grams per 10 minutes.
The polyethylenes can be produced by low or high pressure processes. They are preferably produced in the gas phase, but they can also be produced in the liquid phase in solutions or slurries by conventional techniques. Low pressure processes are typically run at pressures below 1000 psi whereas high pressure processes are typically run at pressures above 15,000 psi.
Typical catalyst systems, which can be used to prepare these polyethylenes, are magnesium/titanium based catalyst systems, which can be exemplified by the catalyst system described in U.S. Pat. No. 4,302,565 (heterogeneous polyethylenes); vanadium based catalyst systems such as those described in U.S. Pat. Nos. 4,508,842 (heterogeneous polyethylenes) and 5,332,793; 5,342,907; and 5,410,003 (homogeneous polyethylenes); a chromium based catalyst system such as that described in U.S. Pat. No. 4,101,445; a metallocene catalyst system such as those described in U.S. Pat. Nos. 4,937,299, 5,272,236, 5,278,272, and 5,317,036 (homogeneous polyethylenes); or other transition metal catalyst systems. Many of these catalyst systems are often referred to as Ziegler-Natta catalyst systems or Phillips catalyst systems. Catalyst systems, which use chromium or molybdenum oxides on silica-alumina supports, can be included here. Typical processes for preparing the polyethylenes are also described in the aforementioned patents. Typical in situ polyethylene blends and processes and catalyst systems for providing same are described in U.S. Pat. Nos. 5,371,145 and 5,405,901. The various polyethylenes can include low density homopolymers of ethylene made by high pressure processes (HP-LDPEs), linear low density polyethylenes (LLDPEs), very low density polyethylenes (VLDPEs), medium density polyethylenes (MDPEs), high density polyethylene (HDPE) having a density greater than 0.940 gram per cubic centimeter and metallocene copolymers with densities less than 0.900 gram per cubic centimeter. The latter five polyethylenes are generally made by low pressure processes. A conventional high pressure process is described in Introduction to Polymer Chemistry, Stille, Wiley and Sons, New York, 1962, pages 149 to 151. The high pressure processes are typically free radical initiated polymerizations conducted in a tubular reactor or a stirred autoclave. In the stirred autoclave, the pressure is in the range of about 10,000 to 30,000 psi and the temperature is in the range of about 175 to about 250 degrees C., and in the tubular reactor, the pressure is in the range of about 25,000 to about 45,000 psi and the temperature is in the range of about 200 to about 350 degrees C. Blends of high pressure polyethylene and metallocene resins are particularly suited for use in the application, the former component for its excellent processability and the latter for its flexibility.
Copolymers comprised of ethylene and unsaturated esters are well known, and can be prepared by the conventional high pressure techniques described above. The unsaturated esters can be alkyl acrylates, alkyl methacrylates, and vinyl carboxylates. The alkyl group can have 1 to 8 carbon atoms and preferably has 1 to 4 carbon atoms. The carboxylate group can have 2 to 8 carbon atoms and preferably has 2 to 5 carbon atoms, The portion of the copolymer attributed to the ester comonomer c

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