Compositions – Electrically conductive or emissive compositions – Free metal containing
Reexamination Certificate
2000-07-24
2002-12-17
Kopec, Mark (Department: 1751)
Compositions
Electrically conductive or emissive compositions
Free metal containing
C252S06290R, C428S403000, C428S407000, C428S901000, C427S096400
Reexamination Certificate
active
06495069
ABSTRACT:
TECHNICAL FIELD
This invention relates to a polymer composition including a finely divided electrical conductor, particularly comprising such a composition in an advantageous physical form.
BACKGROUND ART
The use of such a composition has been proposed for devices for controlling or switching electric current, to avoid or limit disadvantages such as the generation of transients and sparks which are associated with the actuation of conventional mechanical switches. In particular, co-pending application PCT/GB98/00206 dated Jan. 23, 1998 and subsequently published under serial number WO98/33193, discloses such a composition and switches based on it. That application discloses for the first time a granular composition containing polymer and conductive filler. The present application relates to a polymer composition in that form and to further developments thereof.
DE-A-4315382, to which corresponds U.S. Pat. No. 5,589,222, discloses hydrophobic flowable granules comprising hydrophilic inorganic powder and 0.03 to 15% w/w of hydrophobic polyorganosiloxan and a process of producing them by mixing the powder in a granulator with an aqueous emulsion of the polyorganosiloxan and drying the resulting product at elevated temperature. Among the powders listed are metals and alloys. The granules are, however, for use in enamelling; there is no disclosure that they might form part of an electrical circuit or, further, that they could be electrically insulating when quiescent but conducting when subject to mechanical stress or electrostatic field.
SUMMARY OF THE INVENTION
According to the present invention there is provided an electrical conductor composite providing conduction when subjected to mechanical stress or electrostatic charge but electrically insulating when quiescent comprising a granular composition each granule of which comprises at least one substantially non-conductive polymer and at least one electrically conductive filler and is electrically insulating when quiescent but conductive when subjected to mechanical stress or electrostatic charge.
The granules are typically in the size range up to 1 mm, especially 0.04 to 0.2 mm. Thus the smaller granules behave as powders. These ranges are based on measuring the greater diameter of the granules if not regularly spherical. To suit user's requirements, the granules may be for example in an approximate Poisson size distribution, or sieved to a skewed distribution or a narrow spread (for example, largest granules no greater than 2×smallest) or classified so that small granules fill the spaces between larger granules.
Within the granules the conductor:polymer volumetric ratio (tapped bulk:voidless solid) is suitably at least 3:1. For the ratio of conductive medium to polymer small changes will be required to account for differences in relative surface tensions of types and grades of polymer and the various surface energies of the different conductive oxides and other solids present. Changes of this ratio have an effect on the piezo-charge properties, the overall resistance range, the recovery hysteresis and the pressure sensitivity of the granules.
In general the conductive material can be one or more metals, other conductive or semiconductive elements and oxides or intrinsically conductive or semiconductive organic or inorganic polymers. Thus it is suitably selected from powdered forms of the metallic elements or their electrically conductive alloys or reduced oxides either on their own or together. More specifically it is one or more of titanium, tantalum, zirconium, vanadium, niobium, hafnium, aluminium, silicon, tin, chromium, molybdenum, tungsten, lead, manganese, beryllium, iron, cobalt, nickel, platinum, palladium, osmium, iridium, rhenium, technetium, rhodium, ruthenium, gold, silver, cadmium, copper, zinc, germanium, arsenic, antimony, bismuth, boron, scandium and metals of the lanthanide and actinide series and, if appropriate, at least one electroconductive agent. The conducting filler can be the basic element in the unoxidised state; or can be a layer on a carrier core of powder, grains, fibres or other shaped forms. The oxides can be mixtures comprising sintered powders of an oxycompound. The alloy may be for example titanium diboride.
The micro-structure of the conductor particles is of substantial importance. Thus dendritic, filamentous, and spiked forms of the conductive materials have been shown to be capable of producing particularly sensitive granules when coated with a polymer such as silicone. Generally the conductor particles are rough-surfaced with smaller and spikier powders producing more sensitive granules. Preferably the particles comprise metal having at least one of these characteristics:
(i) spiky and/or dendritic surface texture;
(ii) filamentary structure, with a three-dimensional chain-like network of spiky beads, the chains being on average 2.5-3.5 microns in cross-section and possibly more than 15-20 microns in length.
Generally these characteristics are present in the conductor particles before mixing with polymer, and mixing is controlled to substantially preserve them.
As described further below, preferred conductor particles comprise carbonyl-derived metallic nickel. Other examples include dendritic copper.
The polymer constituent of the granules can be chosen from a wide range of materials, the only limitation being that the polymer or a precursor thereof should be available in a form sufficiently mobile to permit incorporation of conductor particles. In an extreme case it can be a fully or partly cured resin, such as a formaldehyde condensate, epoxy resin, maleimide resin or 3-dimensional olefin resin. Polymers having flexibility, such as linear thermoplasts, are of more general application. Very suitably the polymer constituent is an elastomer. Since elastomers are preferred in certain composites including the granules, they will be described further below.
The invention provides a method of making the granules by mixing conductor particles with liquid-form polymer in granule-forming conditions. The liquid-form polymer may be, for example, a precursor subject to polymerization or to cross-linking during the granule-forming step or later. Liquid-form means sufficiently flowable to undergo mixing with the conductor particles. The polymer may be very viscous. A liquid may be present to modify the viscosity of the polymer as an aid to mixing. It may be added, for example, by pre-mixing with the polymer or with the conductor powder. The liquid should of course be chemically inert with respect to the conductor and polymer. Preferably it is volatile, that is, has an atmospheric pressure boiling point under 120° C., to assist removal during and after mixing. Hydrocarbons, such as petroleum distillates, are very suitable. Before or during mixing there may be added a hydrophobising agent. This is believed to act by displacing adsorbed water from the surfaces of the components of the mix, for example the conductor particles, solid additives such as those described below, especially fumed silica, and possibly of newly exposed polymer and newly formed granules. The agent may also act as a lubricant limiting friction-at mixer surfaces. Since it can act by formation of very thin, even unimolecular layers, the quantity to be used is very small, for example 10-1000 ppm w/w of the mix. Examples of the agent are liquid hydrocarbons carrying groups favouring chemisorption on metals, and fluorocarbons.
The granules are made by coating conductive particles with a layer of polymer in a controlled mixing regime that imparts only sufficient force to the components of the mix to achieve the coating process and avoids additional force which has been found to have a degrading effect on the electrical properties of the final polymer composition. The relationship between filler, binder, mixing energy, time, rate of shear, temperature and pressure determine the particle size-distribution and electromechanical properties of the resulting granule. It appears likely that the conductor particles act as nuclei for
King Andrew Brian
Lussey Christopher John
Lussey David
Kopec Mark
Larson & Taylor PLC
Peratech Limited of a Company of Great Britain and Northern Irel
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