Synthetic resins or natural rubbers -- part of the class 520 ser – Synthetic resins – Cellular products or processes of preparing a cellular...
Reexamination Certificate
2000-10-24
2002-09-24
Seidleck, James J. (Department: 1711)
Synthetic resins or natural rubbers -- part of the class 520 ser
Synthetic resins
Cellular products or processes of preparing a cellular...
C521S140000, C521S142000, C521S144000, C525S191000, C525S232000, C525S240000, C174S1100FC, C174S1100PM, C174S1100SR
Reexamination Certificate
active
06455602
ABSTRACT:
TECHNICAL FIELD
This invention relates to a polyolefin composition for cellular insulation that has improved capability to be processed at high line speeds with enhanced foamability.
BACKGROUND INFORMATION
Thin wall insulated wires incorporating foamed polyolefin insulating materials are commonly used in local area network (LAN), wiring and outdoor “telephone” cables. These insulated wires are typically produced using a wire coating extrusion process operating at production speeds ranging from about 500 to about 3000 meters per minute. Increased production speeds are desired for increased productivity resulting in improved production economics, but can be limited by the capability of the insulation material to be extruded as a very uniform foamed insulation layer within physical and electrical production tolerances. Very uniform cellular insulation is especially important to data-grade transmission characteristics, which are commercially evolving toward increasingly stringent requirements. Productivity can also be improved by increasing the foaming capability of the insulation to provide higher expansion rates, allowing reduced insulation thickness and material use.
Foaming can be accomplished by chemical foaming agents or by physical gas injection into the extruder during the high speed insulated wire production process. The insulated wire can be a single foamed insulation layer, or can be a multi-layer design such as the insulation widely used in telephone cables, which has a foamed inner skin and a solid outer skin. The foamed polyethylene insulation provides some improved electrical properties, for example, higher velocity of propagation and/or lower attenuation, versus solid (not foamed) polyolefin insulation. Foaming also improves cable economics via reduced insulation weight and decreased insulation thickness yielding decreased cable dimensions.
Thin wall cellular insulated wire for LAN or telephone cable use is typically twisted in a helical manner with another similar insulated wire to form a “twisted pair” transmission circuit. The insulated wires are usually colored to provide color coded transmission pairs that facilitate installation into communication networks. Multiple transmission pairs are usually grouped together, often by additional helical twisting, to form a transmission core. The transmission core is then protected by a sheathing system which incorporates a polymeric jacketing to provide mechanical and environmental protection. Often metallic armor and/or multiple jacketing layers are included in the sheathing system for added environmental protection, especially for outdoor cables. With outdoor cables, hydrocarbon greases are often used to fill air spaces in the cable, such as the interstices between the twisted pairs in the transmission core, to minimize the possibility of deterioration in electrical transmission performance by water/moisture ingress. Various other components such as polymeric wraps or metallized shields for electromagnetic interference (EMI) protection and flame retardancy are often incorporated into the cable design, especially for LAN cable applications. For indoor applications such as LAN cables, the cable usually must meet certain flame retardancy standards, which can affect material selection and cable design.
The selection of the polyolefin insulating material(s) is a critical factor in determining the production speeds and end-use performance capabilities for high speed thin wall cellular insulation applications. The best available commercial materials have limited production capabilities such that improved materials are very much desired. For example, existing commercial materials used in a physical foaming process for production of thin wall foamed insulated wires for LAN data-grade applications typically lack good high speed extrusion characteristics. Therefore, production speeds are typically limited to about 1500 meters per minute or less. Attempted production above such speeds typically results in insulation roughness with increased capacitance and diameter variations, yielding unacceptable electrical transmission characteristics. Despite production speed limitations, these resins have been used commercially for LAN applications due to good foamability that allows for expansion rates up to 65 percent. Until the present invention, other resins having better high speed extrusion capability lacked the desired foaming capability. Another example is the multilayer foam/skin telephone wire application in which chemically expanded foams are typically used. Commercial materials traditionally used in this application have good high speed extrusion characteristics but lack optimal foaming capability. Such materials allow for foam/skin production at speeds up to about 3000 meters per minute or more but their limited foaming capability restricts use to about a 45 percent maximum expansion level. Attempts to foam these materials above about a 45 percent expansion yields unacceptable foam quality and insulation variations that exceed allowable tolerances for acceptable electrical transmission characteristics. Due to the ability of processing at high production rates, existing commercial materials have been used despite their limited foaming capability.
Thus a base resin system providing a combination of improved high speed extrusion capability and improved foaming capability will provide substantial product improvement. Such an improved composition for cellular insulation would enable increased production speeds for, for example, gas injection foamed LAN insulation, and increased foaming capability for, for example, chemically foamed telephone wire insulation. This would improve productivity via increased output rates and increased expansion rates providing reduced material consumption.
The production of a chemically foamed insulation typically comprises the following steps. A chemical blowing agent is melt mixed, or compounded, with the base resin system at a temperature below the decomposition temperature of the selected blowing agent. The melt is then pumped through a pelleting dieplate and a pelleter/cutter to produce small pellets (solid beads) which are then cooled to solidification temperature in water. This pelleted composition is then used as the raw material for the foamed insulation extrusion process. It is also possible to obtain a chemical foam insulation composition by combining several constituents in the proper ratios at the fabrication extruder. For example, a pellet mixture comprising the proper ratio of base resin components and a chemical blowing agent masterbatch could be dry blended and then fed directly to the fabrication extruder.
To fabricate the foamed insulation, the specified composition is typically processed in a fabricating extruder for coating onto a conductor. In the fabricating extruder, the pellets are fluxed and mixed and the melt is then brought to a processing temperature exceeding the decomposition temperature of the blowing agent, thereby producing gas for the foaming process. As the melt passes through the coating die, forming a coating around the wire to be insulated, the dissolved gas nucleates and forms tiny cells in the plastic coating. This foaming mainly occurs in an air gap between the die exit and a water cooling trough, in which the insulation is solidified. Chemically foamed insulation is widely used because the required equipment investment is lower and because the operation has competitive processing capabilities relative to the physical foaming process, at lower expansion rates, especially for foam/skin telephone wire insulation production.
For high speed foam/skin telephone wire applications, commercially available chemically expanded foam materials have good cellular insulation processing capability to about 45 percent expansion. As noted above, typical foam/skin production line speeds range from 1500 to 3000 meters per minute. For other chemically foamed insulation processes, notably coaxial cables which are produced to larger diameters at considerably slower production speeds (l
Brown Geoffrey David
Frankowski David John
He Vicky Yafen
Maki Sandra
Wasserman Scott Hanley
Bissett Melanie
Seidleck James J.
Union Carbide Chemicals & Plastics Technology Corporation
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