Compositions – Electrically conductive or emissive compositions
Reexamination Certificate
1998-10-14
2001-03-20
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Reexamination Certificate
active
06203727
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to electronically-conductive polymers, and to composite articles or polymer blends containing such polymers. More specifically, this invention relates to electronically-conductive polymers having both improved conductivity and processability, and to composite articles comprising these.
Intrinsically conductive polymer compositions (ICPs), such as conductive forms of polyaniline, which are relatively thermally stable may be used to produce a thermally stable processible composition containing the ICP and one or more thermoplastic, solution-processible or thermoset polymers. See e.g., U.S. Pat. No. 5,160,457. U.S. Pat. No. 5,232,631 describes a composition comprising polyaniline, a solvent or plasticizing liquid, and a protonic acid solute which has been functionalized so that the polyaniline:protonic acid combination is compatible with nonpolar or weakly-polar organic liquids or solutions, or molten or liquefied oligomers, polymers or polymer mixtures. However, while the combination of the doped polyanilines and polymers described in these patents are thermally and solution processible to some extent, a relatively high weight content of the doped polyaniline is required to achieve the required conductivity for certain applications, such as electrostatic painting applications. The physical properties of the blends may also be less than desirable for certain applications. For example, the high acid dopant content of such compositions is relatively corrosive to metal and is a safety hazard. Corrosivity is particularly detrimental in applications, wherein the electronically-conducting polymer composite is processed through metal equipment, or the composite contacts metal during use. EP Application 582,919 describes a method for lowering the polyaniline content of such a blend by adding a metal salt to the blend. However, the physical properties of such a blend, particularly its impact resistance at low temperatures, may be less than desirable for certain applications.
Complexes of polyaniline and dopants having a relatively high molecular weight are also known and described, for example, in U.S. Pat. No. 5,378,402. Such complexes have relatively high solubility in solvents and matrix polymers, but their conductivity may be less than desirable for certain applications. Conductive polymers doped with more than one dopant are known and are described, for example, in U.S. Pat. No. 5,378,404. Such polymers are prepared by partially un-doping an ICP solid particle doped with a first dopant, and then doping with the second dopant which is more compatible with a matrix polymer with which the conductive polymer is to be blended. However, the thermal stability and processing characteristics of the doped polymer may still be less than desirable.
SUMMARY OF THE INVENTION
The inventors have discovered that electronically-conductive polymers possessing excellent conductivity, thermal stability, and good compatibility with matrix polymers may be prepared by utilizing a combination of at least two dopants one having a relatively low molecular weight and one having a relatively high molecular weight. The polymers and composites of the invention may be used to prepare electrostatically-paintable plastic articles, electronically-conductive coatings, films, fibers, and other shaped articles for other applications wherein a semiconductive or conductive substrate is desirable, such as, for example, other electromotive coating processes, anti-static applications, electroactive applications such as corrosion prevention or sensors, and the production of conductive fillers, such as polyaniline-coated talc and other platelets. These and other advantages of the invention will be apparent from the description which follows.
In one aspect, this invention is a doped intrinsically-conductive polymer having a conductivity of at least about 10
−12
Siemens/cm (S/cm). This ICP is doped with at least two different dopants, based on molecular weight, including a first dopant (hereafter, “short-chain dopant”) having a molecular weight of less than about 1,000 and a second dopant (hereafter, “long-chain dopant”) which is polymeric and has a weight average molecular weight of greater than about 2,000. (Note that for oligomeric and polymeric compounds all molecular weights are weight average molecular weight unless otherwise specified.) The short-chain dopant is present in an amount sufficient to increase the conductivity of the composition, relative to the conductivity of a composition which is the same in all respects except that the intrinsically-conductive polymer has the same molar amount of available sites doped solely with the long-chain dopant.
In a second aspect, this invention is a compatible mixture of: (i) a thermoplastic or thermosetting polymer, and (ii) the doped intrinsically-conductive polymer of the first aspect of the invention.
DETAILED DESCRIPTION OF THE INVENTION
The term “intrinsically-conductive polymer” or “ICP” as used herein refers to a polymer with extended pi-conjugated groups which may be rendered conductive with an acid or a redox agent to form a charge transfer complex with a conductivity of at least about 10
−12
S/cm. The charge transfer may be full or partial, depending on the specific electron donor/electron acceptor pair. For example, partial charge transfer between certain lithium salts and polyaniline has been found to increase the conductivity of the polyaniline. Full charge transfer is believed to occur with polyaniline and protons, and polythiophene and protons or transition metals. The process of rendering the polymer electronically conductive is referred to herein as “doping”. ICPs which have been rendered conductive and have not been rendered conductive are referred to herein as “doped” ICPs and “undoped” ICPs, respectively. The compounds and polymers which may be used in such doping processes to render the ICPs conductive are referred to herein as “dopants”.
Examples of suitable ICPs include polyaniline, polyacetylene, poly-p-phenylene, polypyrrole, polythiophene, poly(phenylene sulfide), polyindole, derivatives thereof such as poly(3-alkylthiophene), polyethylene dioxythiophene and poly(o-methoxy aniline), copolymers, and mixtures thereof. When low cost and high temperature stability are important, the ICP is preferably a polyaniline, polypyrrole, or polythiophene, but is most preferably a polyaniline. However, if the ICP is used to prepare a composite with a thermoplastic or thermoset polymer, the choice of ICP may also depend on its compatibility with such polymer. For example, polypyrrole is especially compatible with polymers with which it can form hydrogen bonds along its backbone; polyalkyithiophenes are particularly compatible with polyolefins and polystyrene; and polyacetylene; are particularly compatible with polyolefins.
Polyaniline can occur in several different oxidative states such as leucoemeraldine, protoemeraldine, emeraldine, nigraniline, and pernigraniline, depending on the ratio of amine groups to imine groups present in the backbone of the polymer. In addition, each oxidative state may or may not be protonated. For example, the emeraldine salt form of polyaniline, in which about 50 percent of the nitrogen atoms are contained in imine groups, is a very conductive and stable form of a protonated polyaniline. The nonconductive base is blue in color, while the protonated form (emeraldine salt) is green.
The ICP may be doped by any suitable method. The effectiveness of the various doping methods and the conductivity of the doped ICP obtained thereby may vary depending on the doping method, the particular ICP, the particular dopant(s), (and the stage in a composite fabrication process at which the ICP is doped, if the ICP is used to prepare a composite). The ICP may be doped, for example, by mixing a solution, melt, or dispersion of the dopant(s) with the ICP either in solution or with the ICP in the solid state, contacting a solid ICP with solid dopant(s) (solid state doping), by contacting a solid
Babinec Susan J.
Drumright Ray E.
Sen Ashish
Kopec Mark
The Dow Chemical Company
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