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
1999-07-22
2001-02-20
Lipman, Bernard (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...
C525S265000, C525S281000, C525S305000, C428S374000
Reexamination Certificate
active
06191230
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 thermal aging and lower deformation under pressure.
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 lead to the crosslinking of 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. Further, to achieve a high crosslink density, high levels of organic peroxide have 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.
Industry is constantly seeking to find polyethylene crosslinkable 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 with essentially no sweat out., i.e., diffusion of the organic peroxide to the surface of the extrudate.
Further, with regard to very low density polyethylenes (VLDPEs), improvement in processing, reduction in stiffness, and better water tree growth resistance (WTGR) are sought after.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a polyethylene composition with a scorch inhibitor, which minimizes scorch, maximizes crosslink density, and does not form crystals in the matrix which fail to melt blend on extrusion. Such a composition will be in the form of a masterbatch composition, which can be used with a wide range of VLDPEs. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The masterbatch composition comprises:
(a) a copolymer of ethylene and 1-octene prepared with a metallocene catalyst;
(b) as a scorch inhibitor, a substituted hydroquinone; 4,4′-thiobis(2-methyl-6-t-butylphenol); 4,4′-thiobis(2-t-butyl-5-methylphenol); or mixtures thereof;
(c) as a cure booster, triallyl trimellitate (TATM); 3,9-divinyl-2,4,8, 10-tetra-oxaspiro[5.5]undecane (DVS); triallylcyanurate; triallyl isocyanurate; or mixtures thereof; and
(d) an organic peroxide.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The copolymer of ethylene and 1-octene used in the masterbatch composition is a linear polymer and is prepared with a metallocene catalyst. Examples of the catalyst, the process for making the polymer, and the polymer itself can be found in U.S. Pat. Nos. 5,272,236 and 5,278,272. Other examples of metallocene catalysts, processes, and polymers are referred to in patents mentioned below. The copolymer can have a density in the range of 0.860 to 0.900 gram per centimeter; a melt index in the range of about 2 to about 35 grams per 10 minutes; and a melt flow ratio in the range of about 18 to about 170. Melt index is determined under ASTM D-1238, Condition E. It is measured at 190 degrees C. and 2160 grams. Flow index is determined under ASTM D-1238, Condition F. It is measured at 190 degrees C. at 10 times the weight used in the melt index test. Melt flow ratio is the ratio of flow index to melt index.
The masterbatch composition (MB) can be mixed with a VLDPE in any proportions; however, a suggested ratio, by weight, for MB:VLDPE is about 0.1:1 to about 0.8:1, and is preferably in the range of about 0.15:1 to about 0.4:1.
The VLDPE with which the masterbatch composition can be mixed can be a copolymer of ethylene and one or more alpha-olefins having 3 to 12 carbon atoms and preferably 3 to 8 carbon atoms. Examples of the alpha-olefins are propylene, 1-butene, 1-hexene, 4-methyl-1-pentene, and 1-octene. It is noted that the masterbatch polymer can be a VLDPE. The density of the VLDPE can be in the range of 0.860 to 0.915 gram per cubic centimeter. The melt index of the VLDPE can be in the range of about 0.1 to about 20 grams per 10 minutes and is preferably in the range of about 0.3 to about 5 grams per 10 minutes. The portion of the VLDPE attributed to the comonomer(s), other than ethylene, can be in the range of about 1 to about 49 percent by weight based on the weight of the copolymer and is preferably in the range of about 15 to about 40 percent by weight. A third comonomer can be included, e.g., another alpha-olefin or a diene such as ethylidene norbornene, butadiene, 1,4-hexadiene, or a dicyclopentadiene. The third comonomer can be present in an amount of about 1 to 15 percent by weight based on the weight of the copolymer and is preferably present in an amount of about 1 to about 10 percent by weight. It is preferred that the copolymer contain two or three comonomers inclusive of ethylene.
The VLDPE 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 VLDPEs are produced by low 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
Jow Jinder
Keogh Michael John
Bresch Saul R.
Lipman Bernard
Union Carbide Chemicals & Plastics Technology Corporation
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