This invention relates to static dissipative polymers.
Most organic polymers are poor conductors of electricity. As such, they cannot be satisfactorily used without modification in applications which require a conductive or semi-conductive material, such as static dissipative materials.
Due to their beneficial properties such as low cost, easy processability, good strength and light weight, it is often desirable to substitute polymeric materials into applications which in the past required metals or other materials. Accordingly, it has been attempted to prepare semi-conductive or conductive polymers.
Conductivity has been imparted to polymers, for instance, by incorporating conductive fibers, particulates or powders into a polymer. Although good conductivity can be achieved in this manner, the high loadings of filler materials (generally 20% or more) needed to obtain such conductivity greatly alter the properties of the polymer, often making it unsuitable for its desired purpose. In addition, such highly filled polymers are often much more expensive than the unfilled polymer. Yet another problem encountered with certain such fillers, especially fibers, is they often break, oxidize or otherwise lose their effectiveness during processing or over time. Also, such fillers tend to slough from a polymer matrix and contaminate sensitive objects nearby, such as electronic equipment that may be packaged or stored in the conductive polymer.
It is also known to impart antistatic properties to polymers using amines, surfactants or quaternary ammonium compounds. When such a compound is incorporated into a polymer or used as a static dissipative surface treatment thereon, the compound generally exudes to the surface of the polymer, where it absorbs atmospheric moisture to form an electrolyte microlayer. The microlayer is generally sufficiently conductive to render the polymer static dissipative. However, such treatments are often removed from the polymer during its normal use, causing the polymer to lose its antistatic properties.
Another approach has been to incorporate ionic salts into a polymer to increase its conductivity. For example, in Dupon et al. J. Elec. Chem. Soc. 128:715 (1981) it is taught to incorporate salts such as sodium thiocyanate into a high molecular weight poly(ethylene oxide) to increase its conductivity. French Pat. No. 2,442,513-4 demonstate the dissolution of mixed alkali metal thiocyanate salts into poly(alkylene oxide) polymers for the same reason. In these references, the concentration of salt is generally at least about 5 weight percent, and is preferably as high as about 25 weight percent. Such a high level of salt often imparts undesirable properties to the polymer, such as sensitivity to water. When such levels of salts are employed in a flexible polyurethane foam, the foam often fails, prunes or collapses due to the formation of undesired closed cells. U.S. Pat. Nos. 4,617,325 and 4,618,680 teach the use of enhancers to increase the static dissipative effectiveness of certain ionizable salts so that lower concentrations of the salts are necessary for static dissipation.
Although the use of such enhancers improve the conductivity of polymer compositions containing relatively low concentrations of salts, it would be desirable to further improve static dissipative polymer compositions such that the compositions maintain relatively high levels of conductivity with relatively low levels of salts after the polymer compositions have been exposed to elevated temperatures. Salts used to maintain such levels of conductivity would desirably also be stable in the presence of other additives, such as flame retardants, used in polymer compositions. Elevated temperatures often occur in production or processing of polymer compositions. It can also be desirable to incorporate antistatic additives as aqueous solutions.