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...
Reexamination Certificate
1998-06-16
2001-01-30
Niland, Patrick D. (Department: 1714)
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...
C174S1130AS, C174S1100SR, C174S1100PM, C174S1200SR, C428S375000, C428S378000, C428S379000, C428S383000, C525S387000
Reexamination Certificate
active
06180706
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to 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 crosslink the polymer.
Polymers containing peroxides are vulnerable to scorch (premature crosslinking occurring during the extrusion process). High pressure, low density polyethylene (HP-LDPE) is particularly vulnerable to scorch. 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 crosslinkable HP-LDPE 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., crystallization of the organic peroxide on the surface of the extrudate.
A substituted hydroquinone is suggested as a scorch inhibitor in U.S. Pat. No. 5,292,791 in quantities of at least 0.1 percent by weight based on the polymer. It is found, however, that this results in low crosslink densities.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a HP-LDPE composition, which minimizes scorch and maximizes crosslink density. Other objects and advantages will become apparent hereinafter.
According to the invention, such a composition has been discovered. The composition comprises:
(a) a low density homopolymer of ethylene prepared by a high pressure process;
(b) a scorch inhibitor selected from the group consisting of a substituted hydroquinone; 4,4′-thiobis(2-methyl-6-t-butylphenol);
2,2′-thiobis(6-t-butyl-4-methylphenol); and 4,4′-thiobis(2-t-butyl-5-methylphenol) in an amount of about 0.02 to about 0.08 part by weight of scorch inhibitor per 100 parts by weight of homopolymer;
(c) a cure booster; and
(d) an organic peroxide.
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The low density homopolymer of ethylene prepared by a high pressure process is conventional. It is a free radical initiated polymerization 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. The density of the homopolymer is preferably in the range of 0.910 to 0.930 gram per cubic centimeter, and the melt index can be in the range of about 1 to about 5 grams per 10 minutes and is preferably in the range of about 0.75 to about 3 grams per 10 minutes. Melt index is determined under ASTM D-1238, Condition E, at 190 degrees C. and 2.16 kilograms.
The scorch inhibitor is selected from the group consisting of a substituted hydroquinone; 4,4′-thiobis(2-methyl-6-t-butylphenol); 2,2′-thiobis(6-t-butyl-4-methylphenol); and 4,4′-thiobis(2-t-butyl-5-methylphenol) with the proviso that the amount of scorch inhibitor is maintained within the range of about 0.02 to about 0.08, preferably about 0.04 to about 0.06, part by weight per 100 parts by weight of homopolymer. The most preferred amount of scorch inhibitor is about 0.05 part by weight. If another polymer is added to the composition, then, the amount of scorch inhibitor will be based on total polymer, but the same limitations will apply.
The substituted hydroquinone can be selected from the group consisting of a hydroquinone substituted at the 2 or the 2 and 5 positions with the same or different tertiary alkyl groups; a hydroquinone substituted at the 2 position with a tertiary alkyl group and a benzene ring sharing the double bond between the 5 and 6 positions; and two hydroquinones bridged at the 6 and 2 positions with an alkylene group having 1 to 18 carbon atoms, one hydroquinone having a tertiary alkyl group at the 2 position and the other hydroquinone having a tertiary alkyl group at the 5 position.
The cure (crosslinking) booster can be any one, or a mixture, of a broad selection of boosters. For example, it can be an ester, ether, or ketone containing at least 2, and preferably 3, unsaturated groups such as a cyanurate, an isocyanurate, a phosphate, an ortho formate, an aliphatic or aromatic ether, or an allyl ester of benzene tricarboxylic acid. The number of carbon atoms in the ester, ether, or ketone can be in the range of 9 to 40 or more, and is preferably 9 to 20. Preferred esters, ethers, and ketones are essentially non-volatile at storage temperatures, and the unsaturated groups are preferably allyl groups. Specific examples are triallyl cyanurate (TAC); triallyl-1,3,5-triazine-2,4,6(1H,3H,5H)-trione also known as triallyl isocyanurate (TAIC); triallyl phosphate; triallyl ortho formate; tetra-allyloxy-ethane; triallyl benzene- 1,3,5-tricarboxylate; diallyl phthalate; zinc dimethacrylate; ethoxylated bisphenol A dimethacrylate; methacrylate terminated monomer with average chain lenght of C
14
or C
15
; pentaerythritol tetraacrylate; dipentaerythritol pentaacrylate; pentaerythritol triacrylate; dimethylolpropane tetraacrylate; ethox
Bresch Saul R.
Niland Patrick D.
Union Carbide Chemicals & Plastics Technology Corporation
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