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
2000-04-11
2002-08-27
Cain, Edward J. (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...
C524S496000
Reexamination Certificate
active
06441084
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to semi-conductive compositions useful for extrusion coating wires and cables. More specifically, the semi-conductive compositions of the invention are linear low density polyethylenes containing two or more different carbon blacks which impart a superior balance of properties, including dispersibility, moisture vapor transmission, environmental stress crack resistance, thermo-oxidative stability and physical/mechanical properties.
2. Description of the Prior Art
Linear low density polyethylene (LLDPE) is widely used in the wire and cable industry as a coating in electrical and telephone applications where flexibility, strength, low brittleness temperature and high resistance to abrasion are required. LLDPE has been found to be particularly useful for overhead and underground medium and low voltage power cable constructions. Power cables are multi-layer constructions of specifically formulated compositions which impart the requisite characteristics to the final construction and LLDPE-based compositions have been widely used for the conductor shield layer, the insulation shield layer and the exterior jacketing layer for such constructions. Conductor shield and insulation shield layers are semi-conductive and typically have large amounts of conductive carbon black compounded with the LLDPE. LLDPE-based jacketing compositions are usually non-conductive and generally contain about 2.5 to 3 percent conventional carbon black to provide protection against ultraviolet radiation.
While the use of smaller particle size carbon blacks generally provides increased electrical conductivity, these small particle size, high surface area blacks are more difficult to disperse in plastic materials. This is particularly true when using LLDPEs which are widely recognized as some of the more difficult polyolefin resins to process due to their narrow molecular weight distribution. Dispersibility of small particle size conductive blacks in LLDPE is even more of a problem when incorporating the higher black loadings typically required, e.g., 10-35 weight percent, for the manufacture of conductive compounds.
It would be highly useful, therefore, to develop LLDPE semi-conductive formulations having improved processability and extrudability for wire and cable applications. It would be even more advantageous and desirable if this could be accomplished while retaining or improving conductivity and the other properties generally considered to be essential for wire and cable applications. These and other advantages are realized with the LLDPE compositions of the present invention wherein a mixture of conductive carbon blacks with specific characteristics is utilized.
U.S. Pat. No. 5,733,480 discloses polyolefin compositions containing a mixture of conductive blacks having different structures useful for the extrusion of mono- and multi-layer films and coatings. The '480 compositions comprise conventional low density polyethylene (LDPE) resin having a density of 0.910-0.935 g/cm
3
and 6 to 15 weight percent of the carbon black mixture. Conventional LDPE resins are produced in high pressure polymerization processes and are homopolymers with substantial long-chain branching. LDPEs are distinguished from LLDPEs which are comprised of linear molecules with no long-chain branching. LLDPEs are produced by low pressure copolymerization of ethylene and one or more C
3-8
&agr;-olefins and contain only short-chain branches as a result of the incorporated comonomer. While the reference does disclose that LLDPE and higher density PEs can be blended with the LDPE, the amount of these additive resins is preferably kept below 15 weight percent of the total polyolefin component. There is no suggestion in the reference to the use of mixed blacks with polyolefin compositions where LLDPE is the sole or predominant resin component or that LLDPE compositions useful for wire and cable applications exhibiting a superior balance of properties can be achieved by the use of a mixture of specific conductive carbon blacks.
SUMMARY OF THE INVENTION
The present invention relates to semi-conductive extrusion compositions having complex viscosities at 210° C. and 100 rad/sec from 6000 to 25000 poise and dispersion numbers from 50 to 700 psi comprising: (a) 75 to 95 weight percent, based on the total weight of the composition, of a base resin comprising linear low density polyethylene having a density from 0.890 to 0.925 g/cm
3
and melt index from 0.3 to 15 g/10 min and (b) 5 to 25 weight percent, based on the total weight of the composition, of a carbon black mixture containing a major portion of a higher structure conductive carbon black and a minor proportion of a lower structure conductive carbon black. Preferably, the higher structure black has a BET surface area greater than 500 m
2
/g and dibutyl phthalate absorption number from 200 to 600 ml/g and the lower structure black has a BET surface area of 125 to 500 m
2
/g and dibutyl phthalate absorption number of 80 to 250 ml/g.
In a highly useful embodiment of the invention the base resin is a mixture of linear low density polyethylene with another polyolefin, preferably low density polyethylene. It is particularly advantageous when the linear low density polyethylene is a copolymer of ethylene with 2 to 25 weight percent butene-1 or hexene-1. Another highly useful embodiment utilizes from 0.1 to 2.5 weight percent of a stabilizer which is a mixture of a hindered phenol and a mercaptobenzimidazole compound. Preferably, the weight ratio of hindered phenol to mercaptobenzimidazole compound is 1:1 to 1:4. Preferred semi-conductive extrusion compositions have complex viscosities from 8000 to 15000 poise, dispersion numbers from 50 to 350 psi.
DETAILED DESCRIPTION OF THE INVENTION
The present invention relates to semi-conductive LLDPE compositions for wire and cable applications. The LLDPE compositions of the invention contain a mixture of conductive carbon blacks which impart semi-conductivity and provide a balance of useful properties. The compositions are particularly useful as the semi-conductive layer in power cable constructions. In addition to having low resistivity, the semi-conductive formulations exhibit good processability and dispersion of the carbon black, low moisture vapor transmission, good low temperature properties and environmental stress crack resistance and good physical/ mechanical properties. Additionally, the compositions have good abrasion resistance and, when properly stabilized, good short and long-term oxidative stability.
To obtain compositions having the requisite properties, a base resin having LLDPE as the sole or predominant component is employed. LLDPE, which is typically produced by the copolymerization of ethylene with one or more C
3-8 &agr;
-olefins comonomers using transition metal catalysts in accordance with well-known processes, is characterized by linear molecules having no long-chain branching. Short-chain branching is instead present and is one of the primary determinants of resin density and physical properties.
LLDPE densities will range from 0.890 to 0.925 g/cm
3
and, more preferably, from 0.905 to 0.922 g/cm
3
. Comonomers typically copolymerized with ethylene to obtain LLDPEs useful for the invention include propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, 1-octene and mixtures thereof. By incorporating these comonomers, linear polymer molecules having short-chain branches along the polymer backbone are produced. The amount of comonomer will typically not exceed 35 weight percent and, most commonly, the comonomer is present in an amount from about 2 to 25 weight percent of the polymer composition. The specific comonomer or comonomer mixture used is primarily based on process compatibility and the desired resin specifications. LLDPE resins which are copolymers of ethylene and butene-1 and/or hexene-1 have been found to be particularly advantageous when formulating of the semi-conductive compositions of the invention. Also, for best processability and extrudab
Lee Chun D.
Pickering James J.
Baracka Gerald A.
Cain Edward J.
Equistar Chemicals LP
Heidrich William A.
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