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
2000-02-16
2001-12-04
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...
C525S192000, C525S240000
Reexamination Certificate
active
06326434
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to compositions useful for extrusion coating wire and cables. More specifically, the compositions of the invention which exhibit improved melt flow under high shear comprise a blend of a major proportion of a first polymeric component which is a mixture of crystalline polypropylene homopolymer and amorphous or semi-crystalline ethylene-propylene copolymer and a minor proportion of a second polymer component which is a fractional melt index ethylene polymer, said blend dynamically modified in the presence of an organic peroxide.
2. Description of the Prior Art
Polyolefin resins are widely utilized for the construction of insulated wire and cable products. For high-speed extrusion of wire and cable insulation, the insulation composition should have a melt flow rate of about 1 to 5 g/10 min and, more preferably, 1.5 to 3.5 g/10 min for best results. Whereas polypropylene (PP) resins generally have better dielectric properties and abrasion resistance than polyethylene (PE) resins, processability problems have limited its use for extrusion coating of wire and cable products.
Particularly troublesome is the phenomenon called melt fracture. This phenomenon can be observed with virtually all polyolefin resins, including polyethylenes such as linear low density polyethylene (LLDPE) and high density polyethylene (HDPE); however, it is more pronounced and less easily controlled with PP resins. Melt fracture results in the distortion observed with extrudates obtained when resins are extruded through annular dies and capillaries at flow rates above certain critical limits. At first this appears as surface waviness or roughness but, as flow rates continue to increase, it results in the formation of helical extrudates and, ultimately, gross melt fracture.
Severe melt fracture during extrusion coating of electrical conductors results in non-uniform thickness of the insulation around the conductor, i.e., eccentricity. Failure to have the conductor consistently positioned at the geometric center of the construction, i.e., concentric, can result in decreased signal performance and crosstalk. Furthermore, those areas where thickness of the insulation layer is inadequate are more prone to pinholes and cracking from bending or abrasion. As wire manufacturers continue to push for higher line speeds, eccentricity becomes one of the major limiting factors. Melt fracture and its effect on conductor eccentricity is described in an article by J. S. Borke entitled, “Oscillatory Flow of Polypropylene and Its Effect on Conductor Eccentricity,” Proceedings of the International Wire and Cable Symposium, 47, 294-298 (1998).
It is highly desirable to have PP compositions within the desired melt flow range suitable for insulating wires and cables which exhibit improved melt flow during extrusion coating, thus minimizing eccentricity within the finished insulated article. It is more advantageous if the compositions are capable of being utilized at high line speeds for the production of telecommunication singles.
U.S. Pat. No. 4,375,531 to Ross discloses high impact visbroken, either by chemical or thermal treatment, blends of propylene and ethylene polymer components. Even though in one embodiment Ross discloses the compositions can be reactor-made or physically blended mixtures of PP and random copolymer of propylene and ethylene with an ethylene polymer, such as HDPE, the reference does not disclose use of the visbroken blends for wire and cable extrusion coating. More importantly, Ross does not disclose or even remotely suggest that, by judicious selection of the polymer components, high melt flow rate compositions with significantly improved melt flow suitable for high-speed wire and cable extrusion can be obtained.
SUMMARY OF THE INVENTION
According to the present invention, there is provided dynamically modified composition suitable for high-speed wire extrusion coating and which exhibit improved melt flow under high shear comprising (a) about 80 to 97.5 weight percent, based on the total weight of the polymer components, of a first polymeric component selected from the group consisting of (i) reactor-made intimate mixtures of crystalline polypropylene homopolymer having an isotacticity index greater than 80 percent and melt flow rate from 0.1 to 10 g/10 min and amorphous or semi-crystalline ethylene-propylene copolymer having an ethylene content of 20 to 60 weight percent and log complex viscosity at 0.1 rad/sec and 210° C. of 5.7 or greater and (ii) post-reactor blends of crystalline polypropylene homopolymer having an isotacticity index greater than 80 percent and melt flow rate from 0.1 to 10 g/10 min and ethylene-propylene rubber having an ethylene content of 50 to 80 weight percent and log complex viscosity of 0.1 rad/sec and 210° C. of 5.3 or greater; (b) 2.5 to 20 weight percent, based on the total weight of the polymer components, of a second polymer component which is a fractional melt index ethylene homopolymer or copolymer having a density greater than 0.935 g/cm
3
; and (c) 100 to 1000 ppm, based on the total weight of the polymer components, of an organic peroxide having a decomposition temperature less than 230° C.
In one particularly useful embodiment of the invention, the extrusion composition is comprised of 85 to 95 weight percent of a first polymer component wherein the PP and EP are present at a weight ratio of 5:1 to 1:1 and the PP has an isotacticity of 90 to 98 percent; the second polymeric component is a PE homopolymer or copolymer having a density from 0.940 to 0.958 and melt index from 0.1 to 0.8 g/10 min; and the organic peroxide has a half-life greater than one minute at 160° C. and is present in an amount from 100 to 600 ppm, based on the total weight of the first and second polymeric components. In an even more preferred embodiment, the organic peroxide is 2,5-dimethyl-2, 5-di(t-butylperoxy)-hexane.
The invention also provides extrusion coated articles comprising a wire or cable which is substantially uniformly coated with a 10 to 50 mil thick layer of the above-defined extrusion coating composition. The coated wire and cable articles may be adapted for use as electrical conductors or for the transmission of light. Insulated wires and cables obtained by extrusion coating 20 to 26 AWG copper wire with a 7 to 10 mil thick layer of the extrusion composition are particularly useful.
DETAILED DESCRIPTION
The extrusion compositions of the present invention which exhibit improved melt flow under high shear suitable for use on high-speed extrusion lines are dynamically modified blends of crystalline, i.e., isotactic, polypropylene homopolymer with specific ethylene copolymers having a rubbery or amorphous phase crystalline ethylene polymer and peroxide. Isotacticity of the PP will be 80 percent or greater. More specifically, the improved compositions of the invention comprise (a) a major proportion of a first polymer component which is an in-reactor or post-reactor produced mixture of crystalline PP homopolymer and a high molecular weight amorphous or semi-crystalline ethylene-propylene (EP) copolymer, (b) a minor proportion of a second polymer component which is a medium to high density ethylene homopolymer or copolymer having a fractional melt index and (c) an organic peroxide. The first polymeric component may be directly produced in the polymerization reactor(s) or may be a post-reactor blend.
Dynamic modification entails heating the blend under shear conditions and at a temperature sufficient to melt the polymers and decompose the organic-peroxide. Since it is known that propylene polymers undergo visbreaking and ethylene polymers undergo chain extension reactions in the presence of peroxides under these conditions, it is believed that the blends of the invention have a unique, complex network structure as a result of associations between the various molecular species formed during processing. These associations are believed to impart the improved Theological behavior, specifically, melt fracture-free ex
Jones Jeffrey A.
Lee Chun D.
Shroff Ramesh N.
Baracka Gerald A.
Equistar Chemicals LP
Heidrich William A.
Lipman Bernard
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