Coating processes – Electrical product produced – Wire conductor
Reexamination Certificate
2000-05-24
2001-05-15
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Wire conductor
C427S117000, C427S118000
Reexamination Certificate
active
06231919
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates broadly to flame retardant communication cables and more particularly to blowing agents for use in producing insulation for flame retardant communications cable.
BACKGROUND OF THE INVENTION
Insulated wires such as those used in communications cable often include flame retardant insulating materials. These flame retardant insulating materials allow these cables to be located in plenum air spaces of buildings or in other locations where flame retardancy and low smoke generation are important properties for the cable. In communications cables, these insulated wires are often provided as twisted pairs consisting of two insulated conductors twisted about each other to form a two conductor group.
The flame retardant insulating materials conventionally used include fluorinated polymers such as fluorinated ethylene-propylene (FEP), ethylenetrifluoroethylene (ETFE), and ethylenechlorotrifluoroethylene (ECTFE). Although these fluorinated polymers impart the necessary flame retardant properties to the plenum cable, these polymers are generally quite expensive. Therefore, it is desirable to minimize the amount of fluorinated polymer material used to insulate conductors for communications cables.
One method for minimizing the amount of insulating material used to insulate conductors is to foam the polymer insulating material. Foaming the insulating material also has the benefit of improving the electrical transmission characteristics of the resulting cable. Typically, the insulating materials are foamed using a gas blowing agent such as nitrogen or carbon dioxide. The conventional method of using gas blowing agents for insulation is to feed the polymer insulating material to an extruder and inject the gas blowing agent into the polymer melt. The polymer insulating material and blowing agent are then blended and a layer of the insulating material is applied around the conductor. Preferably, the insulating material is applied as a thin layer to further reduce the amount of insulating material used in the cable.
Although this is a common method for applying insulating material to conductors, there are problems associated with foaming polymer insulating material with gas blowing agents. In particular, it is difficult to control the amount of gas blowing agent fed to the extruder. Therefore, if a thin insulating layer of the foamed polymer is to be applied to the conductor, small variations in the process conditions often occur which result in disproportionately large changes in the characteristics of the foamed polymer. For this reason, it is difficult to maintain close manufacturing tolerances for density, thickness, dielectric constant, etc. This is particularly a problem at the high temperatures used to melt fluorinated polymers. As a result, it is difficult to produce a layer of foamed fluorinated polymer insulating material having uniform or consistent properties along the length of a wire. Therefore, the electrical properties of the insulated conductor and the cable suffer.
An additional problem that is encountered in using gas blowing agents and in particular, nitrogen, for foaming fluorinated polymers is that the cell size of the resulting polymer insulation is too large for thin insulating layers such as 30 mils or less. As a result, there are breaks in the insulation thereby affecting the insulative properties of the fluorinated polymer layer.
Another problem associated with using gas blowing agents is that the small port used to inject the gas blowing agent into the extruder often becomes blocked by the polymer material or by airborne dust and dirt. As a result, the extruder must be taken off-line and the port cleaned thereby preventing the operation of the insulating process. Therefore, there is a need to find an alternative to using gas blowing agents to produce foamed fluorinated polymer insulation for insulated conductors.
SUMMARY OF THE INVENTION
In accordance with the present invention, a communications cable is provided having at least one elongate electrical conductor surrounded by a layer of insulating material comprising a foamed fluorinated polymer such as fluorinated ethylene-propylene (FEP). The foamed fluorinated polymer is formed using a liquid or solid blowing agent having a vaporization temperature at ambient temperature of between about 140° F. and about 700° F. and the resulting foamed polymer insulation includes a small amount of this blowing agent. Preferably, the blowing agent is a nonflammable fluorinated hydrocarbon optionally substituted with an alkoxy group. Examples of these nonflammable fluorinated hydrocarbons include perfluorododecane (C
12
F
26
), methoxynonafluorobutane (C
4
F
9
OCH
3
), and mixtures thereof. Alternatively, the blowing agent can be a hydrocarbon selected from the group consisting of C8 to C22 straight chained or branched aliphatic hydrocarbons, C8 to C22 substituted or unsubstituted monocyclic or polycyclic aromatic hydrocarbons, and low molecular weight petroleum-based oils. For example, the blowing agent can be tetradecane, nonadecane, hexamethylbenzene, or a mixture thereof. The blowing agent is generally present in the insulation in an amount of less than about 1% by weight of the insulating material. The insulating material can also include a nucleating agent such as boron nitride. The insulating material is provided as a thin layer preferably having a thickness of less than about 30 mil, and more preferably of less than about 15 mil.
In the communications cables of the invention, the elongate electrical conductors are generally provided as at least one pair of twisted wires, each wire thereof surrounded by a layer of the foamed fluorinated polymer insulating material. In addition to the at lease one twisted pair of fluorinated polymer-insulated wires, the communications cable can further include at least one additional pair of twisted wires, each wire thereof surrounded by a layer of a non-fluorinated polymer insulation. For example, the non-fluorinated polymer insulation can be a foamed polyolefin such as a foamed polyethylene. The twisted pairs of insulated wire can be provided in a jacket which surrounds and protects the wires from the environment.
The present invention also provides a method of making an insulated conductor comprising the steps of feeding a fluorinated polymer (e.g. FEP) and a blowing agent having a vaporization temperature at ambient pressure of between about 140° F. and about 700° F. into an extruder apparatus, heating the fluorinated polymer and the blowing agent to a predetermined temperature above the melting point of the fluorinated polymer and above the ambient pressure vaporization temperature of the blowing agent, blending the melted fluorinated polymer and the blowing agent, applying a layer of the blend around an advancing electrical conductor, and vaporizing the blowing agent to foam and expand the fluorinated polymer to produce an insulated conductor with a foamed fluorinated polymer insulation. The blowing agent is preferably a blowing agent comprising a nonflammable fluorinated hydrocarbon optionally substituted with an alkoxy group. Exemplary nonflammable fluorinated hydrocarbons include perfluorododecane (C
12
F
26
), methoxynonafluorobutane (C
4
F
9
OCH
3
), and mixtures thereof. Alternatively, the blowing agent can be a hydrocarbon selected from the group consisting of C8 to C22 straight chained or branched aliphatic hydrocarbons, C8 to C22 substituted or unsubstituted monocyclic or polycyclic aromatic hydrocarbons, low molecular weight petroleum-based oils, and mixtures thereof (e.g. tetradecane, nonadecane, hexamethylbenzene, or a mixture thereof).
In operation, the blowing agent is preferably fed to the extruder at a rate of between about 0.05% and 1% by weight of the blend. The blowing agent can be fed into the extruder apparatus through the same port as the fluorinated polymer, especially when the blowing agent is solid at ambient temperature and pressure. The blowing agent can also be fed into a different port, generally downstrea
Alston & Bird LLP
CommScope Properties LLC
Talbot Brian K.
LandOfFree
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