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
1999-07-22
2001-09-18
Dawson, Robert (Department: 1712)
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...
C252S06290R, C252S518100, C252S520210, C524S431000, C524S435000, C524S493000, C524S588000
Reexamination Certificate
active
06291568
ABSTRACT:
TECHNICAL FIELD
This invention relates to a polymer composition, and more particularly to an elastomeric conductive polymer composition which displays a large dynamic resistance range and isotropic electrical properties when subjected to distortion forces such as compression or extension forces or alignments created by mechanical energy, thermal energy, electric fields or magnetic fields.
BACKGROUND ART
Devices for switching electric current are conventionally of a mechanical nature and as such embody a number of disadvantages, for example the generation of significant transients such as sparks on actuation of the switch.
It has been proposed to provide polymer compositions capable of providing a variable electrical conductivity effect.
DE 195 10 100 A1 discloses elastically moldable resistors which are either non-conductive in the quiescent state, becoming conductive on the application of pressure, or are conductive in the quiescent state, becoming less conductive under tension. Significantly high pressures of the order of 100 Kgf/cm
2
are required to decrease the resistance to 2-3 ohm/cm, such pressures being two orders of magnitude above human ‘touch’ pressure which is typically in the range of 300 to 600 gf/cm
2
. Thus such devices are unsuitable for electrical switches relying on finger contact to change electrical resistance from open circuit to low resistance values.
DE 27 16 742 discloses pressure sensitive elastomer compositions that change electrical resistance from a semi-conducting state of typical resistivity 10
3
ohms/cm to a low resistance state of typical resistivity 10 ohms/cm by the application of high pressures, typically 5-15 Kgf/cm
2
. Again such materials are unsuitable for practical finger switching devices.
SUMMARY OF THE INVENTION
It would be desirable to be able to provide an improved polymer composition capable of directly carrying electric current, capable of operation with zero or minimal generation of transients, and capable of providing isotropic electrical properties when subjected to substantially lower forces than heretofore.
According to the present invention there is provided a polymer composition which is elastically deformable from a quiescent state and comprises at least one electrically conductive filler mixed with a non-conductive elastomer, characterised in that the volumetric ratio of filler to elastomer is at least 0.1:1 within the composition, the filler being mixed with the elastomer in a controlled manner whereby the filler is dispersed within and encapsulated by the elastomer and remains structurally intact, the nature and concentration of the filler being such that the electrical resistivity of the composition is variable in response to compression or extension forces and decreases from a given value in the quiescent state towards a value substantially equal to that of the conductor bridges of the filler when subjected to either compression or extension forces, the composition further comprising a modifier which, oa release of said forces, accelerates the elastic return of the composition to its quiescent state.
Such a composition, as well as being capable of carrying high currents and displaying a large dynamic electrical resistance range with electrical properties which are changed when the composition is subjected to either compression or extension forces or alignments, is capable of full recovery to the quiescent state when the forces are removed. The cycle may be repeated many times without deterioration of the property. It may also display piezo-charge properties when forces are applied and is capable of holding a charge when unstressed or lightly stressed prior to the commencement or completion of conduction. The polymer composition is produced by combining powdered forms of the metallic elements or their electrically conductive reduced oxides, either on their own or together, within an elastomer encapsulant under a controlled mixing regime.
Such an electrically conductive material is more specifically selected from the group consisting 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 lathanide and actinide series and at least one electroconductive agent. Alternatively, the conducting filler can be the basic element in the unoxidised state. An alternative conductive medium can be a layer of conducting element or oxide on a carrier core of powder, grains, fibres or other shaped forms. The oxides can be mixtures comprising sintered powders of an oxycompound.
The encapsulant elastomer will have the general properties:
i) low surface energy typically in the range 15-50 dyne/cm but especially 22-30 dyne/cm,
ii) a surface energy of wetting for hardened elastomer higher than its uncured liquid,
iii) a low energy of rotation (close to zero) giving extreme flexibility,
iv) excellent pressure sensitive tack both to the filler particles and electrical contacts to which the composite may be attached—that is possess a high ratio of viscous to elastic properties at time spans comparable to bonding times (fraction of a second),
v) high on the triboelectric series as a positive charge carrier (conversely will not carry negative charge on its surface),
vi) chemically inert, fire extinguishing and effective as a barrier to oxygen and air ingress.
The silicone elastomers typically but not exclusively based on polydimethylsiloxane, with leaving groups, cross-linkers and cure systems based on:
Leaving Group
Cross-Linker
Cure System
HOC(O)CH
3
CH
3
Si[OC(O)CH
3
]
3
ACETIC ACID
HOCH
3
CH
3
Si(OCH
3
)
3
ALCOHOL
HONC(CH
3
) (C
2
H
5)
CH
3
Si [ONC(CH
3
)C
2
H
5]
3
OXIME
CH
3
C(O)CH
3
CH
3
Si [OC(CH
2
)CH
3
]
3
ACETONE
HN(CH
3
)C(O)C
6
H
5
CH
3
Si [N(CH
3
)C(O)C
6
H
5
]
3
BENZAMIDE
meet all of the above mentioned property criteria. The elastomer can be mixtures comprising cured elastomers selected from the group comprising one, two or more component silicones, one, two or more component polygermanes and polyphosphazines and at least one silicone agent. The preferred embodiment of the invention employs a product with useful strength, pressure sensitive tack and useful life and is manufactured from high strength room temperature cured fumed silica loaded (RTV) silicone polymer.
Other additives are included with the silicone for the purpose of modifying the physical and electrical properties of the uncured or cured polymer composition. Such additives can include at least one property modifier from the group comprising: alkyl and hydroxyalkycellulose, carboxymethylcellulose, hydroxyethylcellulose, hydroxypropylcellulose, polyacrylamide, polyethylene glycol, poly(ethylene oxide), polyvinyl alcohol, polyvinylpyrrolidone, starch and its modifications, calcium carbonate, fumed silica, silica gel and silicone analogues and at least one silica analogue or silicone analogue modifier. Fumed silica is an example of a modifier as commonly used in elastomer technology. For this invention, in proportions of between 0.01-20% by weight of the final polymer composition, it increases the resilience of the polymer composition to accelerate the return of the composition to its quiescent state after any applied force is released.
The ratio of conductive medium to encapsulated elastomer is in the order of 7:4 by volume. Small changes of this ratio will be required to account for the difference in relative surface tensions of different types and grades of elastomer and the various surface energies of the different conductive oxides and modifiers. Changes of this ratio also have an effect on the piezo-charge properties, the overall resistance range, the recovery hysteresis and the pressure sensitivity of the polymer composition. The limits of the described effects range from approximately 1:1 to 3:1 conductive medium to elastome
Dawson Robert
Larson & Taylor PLC
Peng Kuo-Liang
Peratech Limited of a Company of Great Britain and Northern Irel
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