Conductive polymer composite materials and methods of making...

Details Conductive polymer composite materials and methods of making... Conductive polymer composite materials and methods of making...

1999-05-07

2001-03-06

Gupta, Yogendra (Department: 1751)

Compositions

Electrically conductive or emissive compositions

C252S510000, C252S511000, C252S518100, C252S519320, C174S02500C, C174S02500C, C174S036000, C174S068100, C174S1020SP

Reexamination Certificate

active

06197219

ABSTRACT:

FIELD OF INVENTION
The present invention relates generally to conductive polymer composite materials and the method of making such composite materials.
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 permittivity 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 conductivity through doping, though these systems tend to be cost prohibitive and difficult to fabricate into articles. Furthermore, polymer composites have been fabricated which exhibit favorable conductivity characteristics. However, those composites which exhibit extreme immiscibility between their minor and major phase materials have inherently poor mechanical properties and, as engineering materials, are not useful for most applications.
Percolation theory is relatively successful in modeling the general conductivity characteristics of conducting polymer composite (CPC) 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 the level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material, can be determined from the experimentally determined dependence of conductivity of the CPC 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 conductive filler material. See for example,
Models Proposed to Explain the Electrical Conductivity of Mixtures Made of Conductive and Insulating Materials,
1993 Journal of Materials Science, Vol. 28;
Resistivity of Filled Electrically Conductive 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. See also,
Multiple Percolation in Conducting Polymer Blends,
1993 Macromolecules Vol. 26, which discusses “double percolation”, the contents of which are also herein incorporated by reference.
Attempts for the reduction of conductive filler content in CPC materials have been reported for polyethylene/polystyrene and for polypropylene/polyamide, both employing carbon black as the conductive filler. See for example,
Design of Electrical Conductive Composites: Key role of the Morphology on the Electrical Properties of Carbon Black Filled Polymer Blends,
1995 Macromolecules, Vol. 28 No. 5 and
Conductive 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.
SUMMARY OF THE INVENTION
However, none of the prior art has recognized the possibility of multi-phase (more than two polymer components) blends and taken advantage of the full potential of reduction of conductive filler content which can be realized by requiring a highly crystalline material as the minor phase of the immiscible polymer blend, nor have investigated processing approaches to improve the conductive network, nor have employed intrinsically conductive polymers as the conductive filler in the minor phase, as claimed herein.
It is therefore an object of the present invention to provide a conductive polymer composite (CPC) material having reduced conductive filler content while maintaining good conductivity by decreasing the percolation threshold required to generate a continuous conductive network in the composite material by the hereinafter described embodiments.
The present invention improves on the prior art by providing a CPC material and method of making same which results in an improved conductive network with a reduction of conductive filler content by reducing the percolation threshold. The present invention is based on immiscible polymer blends wherein the immiscibility is exploited to create semiconductive compounds with low content conductive filler through a multiple percolation approach to network formation. The conductive filler material content can be reduced to about 10% by weight of the total composite or less, depending, for example, on the final application or conductivity requirements for such application and the conductive filler material itself, without a corresponding loss in the conductivity performance of the compound. Correspondingly, the rheology of the melt phase of the inventive material will more closely follow an unfilled system due to the reduction of the reinforcing conductive filler content thereby increasing the ease of processing the material.
The physics of network formation of a minor second phase material in a differing major phase is effectively described by percolation theory as discussed heretofore. The level at which a minor phase material is just sufficiently incorporated volumetrically into a major phase material resulting in both phases being co-continuous, that is, the lowest concentration of conducting particles needed to form continuous conducting chains when incorporated into another material, is the “percolation threshold” (p
c
). A minor second phase material in the form of nonassociating spheres, when dispersed in a major phase material, must often be in excess of approximately 16% by volume to generate an infinite network. This 16 volume % threshold which is exemplary for spheres, is dependent on the geometry of the conductive filler particles, (i.e. the surface area to volume ratio of the particle) and may vary with the type of filler. The addition of a single dispersion of conductor filler particles to a single major phase is termed “single percolation”. It has been found that by altering the morphology of the minor/major phase a significant reduction in percolation threshold can be realized. The present invention exploits these aspects of percolation theory in developing very low conductive filler content CPC materials.
In accordance with the present invention, a method requiring an immiscible blend of at least two polymers that phase separate into two continuous morphologies is employed. By requiring the conductive filler to reside in the minor polymer phase, the concentration of conductive filler can be concentrated above the percolation threshold required to generate a continuous network in the minor polymer phase while the total concentration of conductive filler in the volume of the combined polymers is far below the threshold if the filler was dispersed uniformly throughout both phases. In addition, since the minor polymer phase is co-continuous within the major polymer phase, the total composite is conductive. This approach employs multiple percolation due to the two or more levels of percolation that are required: percolation of conductive dispersion in a minor phase and percolation of a minor phase in a major phase.
In a binary mixture of a semicrystalline polymer and a conductive filler, the filler particles are rejected from the crystalline regions into the amorphous regions upon recrystallization, which accordingly decreases the percolation threshold. Similarly, using a polymer b

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