Plastic and nonmetallic article shaping or treating: processes – Recycling of reclaimed or purified process material
Reexamination Certificate
1998-03-31
2003-05-20
Eashoo, Mark (Department: 1732)
Plastic and nonmetallic article shaping or treating: processes
Recycling of reclaimed or purified process material
C264S105000, C264S140000, C264S176100
Reexamination Certificate
active
06565779
ABSTRACT:
TECHNICAL FIELD
This invention relates to a process for the preparation of semiconducting compositions useful in the preparation of cable semiconducting shields.
BACKGROUND INFORMATION
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.
Polymeric semiconducting shields have been utilized in multilayered power cable construction for many decades. Generally, they are used to fabricate solid dielectric power cables rated for voltages greater than 1 kilo Volt (kV). These shields are used to provide layers of intermediate conductivity between the high potential conductor and the primary insulation, and between the primary insulation and the ground or neutral potential. The volume resistivity of these semiconducting materials is typically in the range of 10
−1
to 10
8
ohm-cm when measured on a completed power cable construction using the methods described in ICEA S-66-524, section 6.12, or IEC 60502-2 (1997), Annex. C. Typical shield compositions contain a polyolefin, such as ethylene/vinyl acetate copolymer with a high vinyl acetate content, conductive carbon black, an organic peroxide crosslinking agent, and other conventional additives such as a nitrile rubber, which functions as a strip force reduction aid, processing aids, and antioxidants. These compositions are usually prepared in granular or pellet form. Polyolefin formulations such as these are disclosed in U.S. Pat. No. 4,286,023 and European Patent Application 420 271. The shield composition is, typically, introduced into an extruder where it is co-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.
The process of compounding resin formulations for wire and cable applications is a well documented and understood practice. A variety of techniques and/or equipment may be utilized in order to achieve the required product quality specifications. As product demands become more stringent, formulations are being developed to meet the diverse physical (dispersion, surface appearance), mechanical (tensile strength, brittleness), electrical (conductivity, resistivity), and chemical (melt index, melt temperature) property requirements. As these compounds are introduced, the ability of available compounding techniques to yield competitive products at a profitable margin becomes limited.
The products are unique in that they offer a variety of performance characteristics in one package. This is due to a high loading of various additives in the polymer matrix. Each of these additives performs a specific function to improve the characteristics of the final product. These additives also have their own physical limits, such as flowability, temperature sensitivity, and viscosity in the polymer matrix to name just a few. For this reason, as the level of additives in the polymer increases and becomes more diversified, several limitations are encountered. Physical and/or mechanical capabilities of the equipment limit the ability of the unit to produce acceptable quality material at substantial rates. This may be due to one of several reasons, such as inability of raw materials to feed at a higher loading level, instability of the raw materials at elevated temperatures, and/or excessive pressure. These system constraints force the user to reduce the production rate of the equipment in order to either meet the existing market demands or maintain the mechanical integrity of the equipment. As these demands become more stringent, either the efficiency of the equipment is further reduced, or the user is forced to make a substantial re-investment in new technology and/or additional facilities.
The introduction of carbon black to form semiconducting compositions is such a limitation. The additive is generally fed into the feed hopper of a melt/mixer or an extruder. Carbon black, however, is a low bulk density material, which tends to bridge or flood feed hoppers when introduced at typical loadings of, for example, 30 percent by weight at elevated feed rates, i.e., at rates above 2000 pounds per hour (pph) in a 200 millimeter Buss™ co-kneader or 1000 pph in a 140 millimeter Buss™ co-kneader. In addition to bridging or flooding, the high loading can lead to excessive temperatures, which tend to decompose some of the standard additives, and cause degradation of the product. The surge of the feed due to the bridging or flooding can lead to variations in composition viscosity, which, in turn, can lead to excessive power draw fluctuations on the mixer motor, temperature variations at the die plate, and pressure fluctuation upstream of the die pack. The surge also causes a quick build-up of particulates resulting in plugged screen packs and increased pressure, and eventually mixer shut down. Finally, it is found that when the semiconductive shield composition is extruded around a wire or core of wires, the coating is rough rather than smooth.
DISCLOSURE OF THE INVENTION
An object of this invention, therefore, is to provide a process for the preparation of semiconducting shield compositions, which permits the elastomers to be mixed with carbon black at elevated rates without the disadvantageous results outlined above. Other objects and advantages will become apparent hereinafter.
According to the invention, a process has been discovered for the preparation of a semiconducting shield composition comprising:
(i) introducing an elastomer into a melt/mixer having a melting zone and a mixing zone;
(ii) introducing particulate conductive carbon black into the melt/mixer in an amount of about 10 to about 25 percent by weight based on the weight of the resin;
(iii) melting the elastomer in the melting zone;
(iv) mixing the carbon black and the molten elastomer in the mixing zone;
(v) optionally, pelletizing the mixture of carbon black and elastomer;
(vi) recycling the mixture of carbon black and elastomer from step (iv) or the pellets from step (v) to a melt/mixer;
(vii) introducing additional particulate semiconductive carbon black into the melt/mixer in an amount sufficient to provide a total amount of carbon black in the range of about 25 to about 50 percent by weight based on the weight of the resin;
(viii) melting and mixing the mixture from step (vii); and
(ix) pelletizing or extruding the mixture from step (viii).
DESCRIPTION OF THE PREFERRED EMBODIMENT(S)
The resins most commonly used in semiconducting shields are elastomers of varying degrees of crystallinity from amorphous through low and medium crystallinity, preferably copolymers of ethylene and unsaturated esters having an ester content of at least about 10 percent by weight based on the weight of the copolymer. The term “elastomer” as used in this specification is considered to include mixtures of elastomers. The ester content is often as high as 80 percent by weight, and, at these levels, the primary monomer is the ester. The preferred range of ester content is about 30 to about 45 percent by weight. The percent by weight is based on the total weight of the copolymer. Examples of the unsaturated esters are vinyl esters and acrylic and methacrylic acid esters. The ethylene/unsaturated ester copolymers are usually made by conventional high pressure processes. These high pressure processes are typically run at pressures above 15,000 psi (pounds per square inch). The copolymers can have a density in the range of 0.900 to 0.990 gram per cubic centimeter, and preferably have a density in the range of 0.920 to 0.970 gram per cubic centimeter. The copolymers can also
Eashoo Mark
Union Carbide Chemicals & Plastics Technology Corporation
LandOfFree
Cable semiconducting shield compositions does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Cable semiconducting shield compositions, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Cable semiconducting shield compositions will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3020212