Polyethylene crosslinkable composition

Details Polyethylene crosslinkable composition Polyethylene crosslinkable composition

1999-07-22

2001-02-13

Lipman, Bernard (Department: 1713)

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

Synthetic resins

Processes of preparing a desired or intentional composition...

C524S581000, C525S333800, C525S375000

Reexamination Certificate

active

06187847

ABSTRACT:

TECHNICAL FIELD
This invention relates to polyethylene compositions useful in the preparation of cable insulation, semiconducting shields, and jackets.
BACKGROUND OF THE INVENTION
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.
In many cases, crosslinking of the polymeric materials is essential to the particular cable application, and, in order to accomplish this, useful compositions generally include a polymer; a crosslinking agent, usually an organic peroxide; and antioxidants, and, optionally, various other additives such as a scorch inhibitor or retardant and a crosslinking booster. Crosslinking assists the polymer in meeting mechanical and physical requirements such as improved high temperature properties.
The crosslinking of polymers with free radical initiators such as organic peroxides is well known. Generally, the organic peroxide is incorporated into the polymer by melt blending in a roll mill, a biaxial screw kneading extruder, or a Banbury™ or Brabender™ mixer at a temperature lower than the onset temperature for significant decomposition of the peroxide. Peroxides are judged for decomposition based on their half life temperatures as described in Plastic Additives Handbook, Gachter et al, 1985, pages 646 to 649. An alternative method for organic peroxide incorporation into a polymeric compound is to mix liquid peroxide and pellets of the polymer in a blending device, such as a Henschel™ mixer or a soaking device such as a simple drum tumbler, which are maintained at temperatures above the freeze point of the organic peroxide and below the decomposition temperature of the organic peroxide and the melt temperature of the polymer. Following the organic peroxide incorporation, the polymer/organic peroxide blend is then, for example, introduced into an extruder where it is extruded around an electrical conductor at a temperature lower than the decomposition temperature of the organic peroxide to form a cable. The cable is then exposed to higher temperatures at which the organic peroxide decomposes to provide free radicals, which crosslink the polymer.
Polymers containing peroxides are vulnerable to scorch (premature crosslinking occurring during the extrusion process). Scorch causes the formation of discolored gel-like particles in the resin and leads to undesired build up of extruder pressure during extrusion. Further, to achieve a high crosslink density, high levels of organic peroxide have often been used. This leads to a problem known as sweat-out, which has a negative effect on the extrusion process and the cable product. Sweat-out dust is an explosion hazard, may foul filters, and can cause slippage and instability in the extrusion process. The cable product exposed to sweat-out may have surface irregularities such as lumps and pimples and voids may form in the insulation layer.
It is known that phenolic compounds can reduce scorch during extrusion of peroxide-containing insulation materials. B. Gustafsson, J. -O. Bostrom, and R. C. Dammert, Die Angewandte Makromolekulare Chemie 261/262, 1998, pages 93 to 99 studied the effect of degree of steric hindrance of phenolic compounds on scorch inhibition and antioxidant capability in peroxide crosslinked polyethylene. Gustafsson et al teach that the less hindered the phenol is, the more effective it is as a scorch inhibitor. In addition, they teach that those phenols that provide the highest level of scorch inhibition are least effective as stabilizers. Furthermore, they teach that the less hindered the phenol is, the higher is the non-productive consumption of peroxide, leading to a higher peroxide requirement to achieve a desired level of cure. In U.S. patent application Ser. No. 09/098,179, filed on Jun. 16, 1998, the inventor, Keogh, describes a scorch retarding semi-hindered phenol. Although the additive imparts scorch resistance, it does so at the expense of crosslinking density, requiring either excess peroxide or use of a cure booster in order to achieve adequate crosslinking. While higher peroxide levels will result in higher peroxide sweat out, use of a cure booster is not always desirable, since added formulation complexity complicates the manufacturing process.
Industry is constantly seeking to find polyethylene crosslinking compositions which can be extruded at high temperatures (although limited by the decomposition temperature of the organic peroxide) and rates with a minimum of scorch and yet be crosslinked at a fast cure rate to a high crosslink density, all without the requirement of excess peroxide or cure boosters, and without sacrificing long-term heat aging stability.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a polyethylene composition with a scorch inhibitor, which minimizes scorch, and maximizes crosslink density without requiring excess peroxide or a cure booster and without sacrificing long term stability. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The composition comprises:
(a) polyethylene;
(b) as a scorch inhibitor, [1,3,5-tris(4-tert-butyl-3-hydroxy-2,6-dimethylbenzyl)-1,3,5-triazine-2,4,6-(1H,3H,5H)-trione]; and
(c) an organic peroxide.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
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 can be produced in the gas phase, or 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.

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