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
1999-09-27
2002-07-09
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...
C524S502000, C524S524000
Reexamination Certificate
active
06417265
ABSTRACT:
FIELD OF INVENTION
The present invention relates to conducting polymer composite materials that are crosslinked and the method of making such composite materials. Specifically, the present invention concerns co-continuous polymer blends through the percolation-in-percolation approach or multiple percolation comprising conducting material wherein the polymers are crosslinked through various means.
BACKGROUND OF THE INVENTION
The ability of polymers to act as electrical insulators is the basis for their widespread use in the electrical and electronic fields. However, material designers have sought to combine the fabrication versatility of polymers with many of the electrical properties of metals. There are instances when an increased conductivity or relative permitivity of the polymer is warranted, such as in applications which require antistatic materials, low-temperature heaters, electromagnetic radiation shielding and electric field grading. A few select polymers, such as polyacetylene, polyaniline, polypyrrole and others, can be induced to exhibit intrinsic electronic conductivity through doping, though these systems tend to be cost prohibitive and difficult to fabricate into articles.
Conductivity may be imparted to a polymer through the creation of conducting polymer composite materials. Electronic conducting polymer composite materials require a random distribution of a conducting filler to be dispersed throughout an insulating polymer which results in a infinite network capable of supporting electron flow. A material is considered conducting when its volume resistivity drops below about 10
8
&OHgr;·cm to about 10
6
&OHgr;·cm. When conducting filler in a polymer is distributed at a volume level sufficient to support electron flow in a polymer/conducting filler blend, a continuous conducting network exists in the polymer.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite materials by predicting the convergence of conducting particles to distances at which the transfer of charge carriers between them becomes probable. The percolation threshold (p
c
) which is defined as the lowest concentration of conducting particles at which continuous conducting chains are formed, e.g., when a continuous conducting network is generated, is easily determined from the experimentally determined dependence of conductivity of the conducting polymer composite material on the filler concentration. For a general discussion on percolation theory, see the October 1973 Vol. 45, No. 4, Review of Modern Physics article, entitled, Percolation and Conduction, the contents of which are herein incorporated by reference. Much work has been done on determining the parameters influencing the percolation threshold with regard to the conducting filler material. See for example, Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conducting and Insulating Materials, 1993 Journal of Materials Science, Vol. 28; Resistivity of Filled Electrically Conducting Crosslinked Polyethylene, 1984 Journal of Applied Polymer Science, Vol. 29; and Electron Transport Processes in Conductor-Filled Polymers, 1983 Polymer Engineering and Science Vol. 23, No. 1; the contents of each of which are herein incorporated by reference.
Conducting composite polymer materials may be created through a “percolation-within-percolation” approach. For example, two immiscible polymers can be identified; the &agr;-polymer which is selectively filled with a conducting filler, and the &bgr;-polymer which is to be filled with the &agr;-polymer conducting filler blend. Denoting the critical weight fraction, or percolation threshold, of the conducting filler required to insure conductivity in the &agr;-polymer as p
&agr;
and the critical weight fraction or percolation threshold of the &agr;-phase required to insure conductivity of the &agr; polymer/conducting filler blend in &bgr; as p
&bgr;
, the critical weight fraction or percolation threshold of the conducting filler in the total ternary blend can be calculated based on the following mathematical equation:
p
c
=p
&agr;
p
&bgr;
(1)
Extending the approach to higher levels of percolation, the critical weight fraction or threshold of the conducting filler in the blend can be calculated based on the following mathematical equation.
p
c
=p
&agr;
p
&bgr;
. . . p
n−1
p
n
(2)
where p
n
is the percolation threshold of co-continuity of the (n−1)-polymer blend in the n-polymer, and allows, at least theoretically, for the feasibility to obtain a conducting composite with as low a level of conducting filler as desired via multiple percolation. This “multiple percolation” approach to forming conducting polymer composites has been reported in the scientific literature, see for example Multiple Percolation in Conducting Polymer Blends, 1993 Macromolecules Vol. 26, the contents of which is herein incorporated by reference. Polymer composites have been fabricated with low carbon black content which still exhibit favorable conductivity using a double percolation approach. However, these composites also exhibit extreme immiscibility between minor and major phases. Such materials have inherently poor mechanical properties, and as engineering materials, are not useful for most applications.
Applications of the heretofor described alternatives for reduction of conducting filler content in conducting polymer composite materials have been reported for polyethylene/polystyrene immiscible blends and for polypropylene/polyamide immiscible blends, both employing carbon black as the conducting filler. See for example, Design of Electrical Conducting Composites: Key Role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends, 1995 Macromolecules, Vol. 28 No. 5 and Conducting Polymer Blends with Low Carbon Black Loading: Polypropylene/Polyamide, 1996 Polymer Engineering and Science, Vol. 36, No. 10, the contents of both of which are herein incorporated by reference. Additionally, U.S. Pat. No. 4,929,388 to Wessling discloses a semiconductive polymer blend with a reduced amount of conductive filler comprised of two partially compatible thermoplastic polymers having a solubility parameter difference and a conductive additive. Wessling also discloses methods of making such materials and methods of crosslinking the materials by adding a crosslinking agent or by irradiation. Unlike the present invention, however, Wessling does not disclose quaternary conductive polymer composites of a percolation-within-percolation approach. Furthermore, Wessling does not teach silane crosslinking of ternary or quaternary composites or of using a major phase polymer with silane pregrafted onto the backbone of the polymer chain to effect crosslinking.
SUMMARY OF THE INVENTION
The present invention is a crosslinked conducting polymer blend or crosslinked conducting polymer composite as the two terms can be used interchangeably to generally describe the invention, and specifically a blend generated through percolation theory. The percolation results in conducting polymer blends that take advantage of the full potential of reduction of conducting filler content. Conducting polymer blends heretofor known in the art fail to take advantage of multiple percolation to generate crosslinked blends with reduced conducting filler requirements.
In particular, conducting polymer composites that are crosslinked are advantageous when the composite will be placed in environments where exposure to chemicals could solvate and deteriorate a thermoplastic version of the composite, or in high temperature environments, where the composite may be exposed to temperatures exceeding the melting temperature of the composite constituents. For example, insulation shields and conductor shields in power cables are typically crosslinked conducting polymer composites.
Crosslinking thermoplastics conventionally used for insulations shields generally results in losses in the conducting properties. In s
Cain Edward J.
Lee Katarzyna W.
Norris & McLaughlin & Marcus
Pirelli Cables and Systems LLC
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