Polymeric Materials

This invention relates to materials possessing useful electrical properties and to electrical devices incorporating such materials.

The present materials are in general thermoplastic polymers and may be suitably shaped by conventional plastics processing techniques.

According to the present invention a polymeric material comprises an amorphous solid solution formed between a metallic compound and a polymer by virtue of ether, thioether or urethane linkages contained therein, the metal of the metallic compound being selected from the group consisting of Be, Mg, Sr, Ba, Sc, Y, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, Zr, Nb, Mo, Tc, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, Al, La and the other lanthanides. When the metal of the metallic compound is zinc, the non metallic moiety is preferably other than chlorine.

The ether, thioether or urethane linkages which may be present either in the polymer backbone or in pendant groups, or in both, usually amount to at least 5 mole % and preferably at least 10 mole % of the polymer and may represent up to 50 mole % thereof. The polymer may be a homo-polymer or a copolymer and may be a polyether, polythioether, a copolymer thereof or may be a different type of polymer, e.g. a polyurethane which may contain ether or thioether linkages. The polymer may for example comprise a homo- or copolymer of an alkylene oxide, e.g. poly (propylene oxide), poly (ethylene oxide), poly (tetramethylene oxide), of a vinylether, of phenylene oxide, styrene oxide, of formaldehyde, of epichlorohydrin, e.g. bisphenol-A-epichlorohydrin copolymer, a polyurethane containing one or more blocks of a poly (alkylene oxide) polymer or any of the above polymers in which ethereal oxygen is wholly or partly substituted by sulphur.

The metallic compound which is normally a salt, i.e. exists in ionic form, is preferably a compound of one of the metals Z, Co, Sn, Ni, Fe, Hg, Zr, Al, Mg, Cu or Ag and it is, in general, desirable that the metal is present in an atmospherically stable oxidation state. Salts may comprise a wide range of anions, of which the following are preferred: F.sup.-, Cl.sup.-, Br.sup.-, I.sup.-, NO.sub.3.sup.-, CNS.sup.-, NCS.sup.-, SO.sub.4.sup.2-, ClO.sub.4.sup.-, CO.sub.3.sup.2-, phosphates, acetate and other organic anions. The concentration of the metallic compound in the polymer is usually at least 2 mole % of the total ether, thioether or urethane linkages by virtue of which the solid solution is formed, and normally does not exceed 30 mole % though it may reach 50 mole %.

The molecular weight of the polymer from which the solid solution is prepared is dependent on the application which the solid solution finds. High polymers with molecular weight greater than 20,000 are of interest for incorporation in electrets useful for example in piezoelectric transducers, capacitors and charge storage devices whereas low polymers with molecular weight less than 20,000 are of interest as solid electrolytes and semiconductors.

The solid solutions of the present invention may be prepared by removing solvent from a solution of the polymer and the metallic compound in the solvent at a rate such that the product is predominantly, or preferably wholly, single phase. Should the rate of removal be too great, or the temperature of the solution at which solvent is removed not be properly chosen, precipitation or unreacted metallic compound or polymer is liable to result. In practice, the compound and polymer are generally diss olved separately in the chosen solvent, which may comprise a plurality of solvent components, typically maintained at a temperature above ambient in order to promote dissolution, and the solutions are then mixed. The mixture is then usually held at a temperature above ambient, typically 100.degree. C. or less and the bulk of the solvent is evaporated therefrom for example by passage of a stream of gas such as nitrogen over the surface of the solution. Residual solvent may be removed under vacuum if necessary at elevated temperature. In general the solvent for the mixture of metallic compound in the polymer comprises polar organic compounds such as the alcohols, tetrahydrofuran or acetone and may also comprise a less polar cosolvent, such as toluene. In particular, solid solutions of cobalt halides e.g. CoCl.sub.2 in high molecular weight polyalkylene oxides such as polypropylene oxide may require a carefully chosen mixture of a polar solvent and less polar cosolvent, for example an alcohol such as ethanol and a hydrocarbon such as toluene. About 30% toluene in ethanol is preferred.

It is usually highly desirable to ensure that the mixture in the solvent is kept anhydrous by for example ensuring that any gas stream employed for evaporation is dry.

Each of the materials according to the present invention are of interest for one or more of the following applications: a. as a dielectric in a capacitor, b. as a solid electrolyte in a cell. c. as an electret or d. as a dielectric absorber. The suitability of a material for particular application is dependent on the nature of the metallic compound and of the polymer and on the concentration of the compound in the polymer.

a. As a dielectric in a capacitor: it will be appreciated that a capacitor comprises two conducting elements, e.g. metal foils, between which is sandwiched an insulating dielectric. A large contact area may be obtained by combining a plurality of conducting and dielectric elements in alternating sequence, e.g. by arranging metals foils and dielectric elements in a stack or in a roll. High electrical capacitance of the device (i.e. ability thereof to store electrical charge) is obtained by providing an insulating dielectric of high dielectric constant. The dielectric is usually in thin film form and able to withstand high electric voltages applied across the thickness thereof.

The present materials when at the optimum composition and temperature ranges are capable of giving improvements of more than 100 fold in capacitance over existing capacitors filled with thermoplastic materials. Materials which have a high dielectric constant, e.g. at least 1000 at low and radio frequencies, at temperatures above their Tg can accept very large electrical charges which, on cooling below the Tg, are stored in the polymeric material, being releasable on reheating. Materials containing zinc or tin salts in polymers such as poly (propylene oxide) are of particular interest for this application.

b. As a solid electrolyte in a cell: a cell comprises electrodes of different chemical potential in contact with a common electrolyte. A liquid electrolyte is usually employed but may be replaced by a material according to the present invention in solid form provided that ions of metals used as the electrodes are soluble in the material in the presence of a common anion. By using metal foil electrodes and thin films of the present materials as solid electrolytes very high surface areas can be achieved to give normal current densities. Such systems are lightweight and non-spillable. Salt concentrations in the polymer between 10% and 50% by weight are preferred for solid electrolyte applications, the following salts being of particular interest: Mg Cl.sub.2, Zn Cl.sub.2, Sn Cl.sub.2, Fe Cl.sub.3, Cu Cl.sub.2, Mg Cl.sub.2 and Co Cl.sub.2. Polyethers such as poly (ethylene oxide) are generally preferred as polymers.

c. As an electret: it will be appreciated that an electret is a material (usually in film or plate form) which can maintain an electric polarization in the absence of an external field. In the present materials, when of suitable composition, this may be achieved by polarizing the film at high temperature and cooling to below the glass transition temperature (Tg), with the field still applied. If Tg is sufficiently greater than room temperature a permanent electret can be produced.

Electret films can be used in conjunction with an adjacent counter electrode as a piezoelectric transducer e.g. a condenser type microphone or a loudspeaker. In the former case impinging sound waves cause movement of the electret film towards and away from the counter electrode with resulting potential changes therein. These voltage changes can be utilized in conventional ways, e.g. in telephones and tape recording systems. Zinc salts and in particular zinc bromide especially in polyethers such as polypropylene oxide, are of special interest for this application.

d. As a dielectric absorber: materials according to the present invention may be employed as absorbers of (i) radio frequency and low micro waves frequency radiation or (ii) high micro wave frequency radiation. In case (i) the materials find application for example as seals for use with microwave ovens and in welding operations which use radio frequency heating. Materials with high dielectric loss at or above room temperature may act as good dielectric heating materials for use in the dielectric welding of plastic components one to another. Materials of particular interest for this application include polyethers and polyurethanes containing salts of certain of the metals hereinbefore disclosed and in particular salts of Fe, Co, Cu, Zn and Sn. In case (ii) the materials find application in anechoic test chambers and as coatings for objects located near radar transmitters, which coatings minimise reflection from the objects. Examples of such objects are: ship's masts and airport structures. In effective materials the ratio of dielectric constant/magnetic permittivity approaches unity and the dielectric loss is preferably high. It is envisaged that these requirements can be met by selection of metal for the metallic compound with optimum paramagnetism, salts of iron cobalt and nickel being of particular interest, especially when dispersed in polyethers.

The present invention is illustrated by the following Examples.

High molecular weight, amorphous poly (propylene oxide), containing 5% co-units of allyglycidylether (Parel 58, M.sub.n =3.5.times.10.sup.5, Tg of -64.degree. C. supplied by Hercules Inc.) is purified in the following manner.

Parel 58 (50 g) is dissolved in acetone (1000 cm.sup.3) (dried over 3 A molecular sieve) and centrifuged at 10000 rev/min for 1 hour. This treatment removes traces of insoluble initiator. The polymer solution is then slowly poured, with stirring, into a large volume of distilled water. The polymer is precipitated from solution and the wet polymer is dried under vacuum at 60.degree. C. After drying, the polymer is dissolved in dry A.R. benzene (800 ml) (dried over 3 A molecular sieve) and the solution is frozen using liquid nitrogen or solid CO.sub.2. The frozen solution is placed in a vacuum desiccator at room temperature and the benzene is removed by pumping for 7 days. The resultant purified polymer (45 g) contains no water and is completely amorphous, as evidenced respectively by infrared analysis and wide angle X-ray scattering.

Purified poly (propylene oxide) (5.01 g) is dissolved in A.R. methanol (50 ml) (dried over 3 A molecular sieve).

Anhydrous zinc chloride (1.02 g, supplied by BDH Chemicals Ltd), dried for several weeks under vacuum, is also dissolved in A.R. methanol (50 ml, dried over 3 A molecular sieve) and the resultant solution is stirred under dry nitrogen to remove very small traces of zinc oxide (.about.0.01 g). The solutions of poly (propylene oxide) and zinc chloride are added together and thoroughly mixed for several hours. The solution is then poured into a circular polytetrafluoroethylene mould and the bulk of the methanol removed by blowing a stream of dry nitrogen over the solution at 50.degree. C. for 48 hours. Last traces of methanol are removed under vacuum initially at 50.degree. C. for 24 hours, and then at 100.degree. C. for several days. The resulting solid solution of zinc chloride (8.72 mole % with respect to moles of poly (propylene oxide) monomer) in poly (propylene oxide) is stored under vacuum at room temperature.

The procedure described in Example 1 is repeated, but the amount of zinc chloride used is altered so as to yield solid solutions containing respectively: 1.09, 13.3 and 27.1 mole % zinc chloride in poly (propylene oxide).

The glass transition temperature (Tg) is determined using a DuPont 900 Differential Thermal Analyser, equipped with the Differential Scanning Calorimetry Accessory. Samples (.about.5-10 mg) are scanned from -100.degree. to 100.degree. C. at a heating rate of 15.degree. C./min using mercury as a reference standard.

For dielectric and electret analysis the solid solutions are compression moulded into 5 cm diameter circular discs, approximately 1.0 mm thick.

Dielectric measurements are made in the frequency range 120-10.sup.5 Hz and in the temperature range -150.degree. to +100.degree. C. using a three-terminal cell (Wayne Kerr "Solid Dielectric Permittivity" Jig D321) under vacuum. The cell is sealed in a copper container and the temperature is monitored by a thermocouple adjacent to the sample. The measuring bridge used in the temperature range -150.degree. to 20.degree. C. is a Rohde and Schwarz "Dielectric Test Bridge" Type VKB BN 3520 with a VKB Extension Unit BN 35208 where required. The cell is cooled from room temperature to -78.degree. C. using solid carbon dioxide-methanol solutions, and below -78.degree. C. by liquid nitrogen passing through a copper coil surrounding the cell. Temperature control is achieved to within 0.5.degree. C.

The measuring bridge in the temperature range 20.degree. to 100.degree. C. is a Wayne-Kerr Universal Bridge B221. The three-electrode cell, still sealed in its copper canister, is immersed in an oil bath thermostatted to .+-.0.1.degree. C.

To ensure good electrical contact between the sample surface and the electrodes, tin foil is attached to the specimen with a thin film of silicone grease. Results with gold-plated samples are not significantly different.

At a given temperature, readings of dielectric constant and dielectric loss are obtained as a function of frequency by balancing the bridge. These readings are repeated at 5.degree. C. temperature intervals throughout the entire temperature range.

Solid solutions for electret thermal analysis are poled at a temperature above Tg using a field strength of 2000 volts/cm. The sample is then cooled to -100.degree. C. using liquid nitrogen with the field still applied. At -100.degree. C. the field is removed and the thermally stimulated discharge current is measured using an electrometer (Keighley Model 615 Digital Electrometer) as the sample is heated at 5.degree. C./min to 100.degree. C.

The solid solution produced as described in Example 2, containing 1.09 mole % ZnCl.sub.2 has a Tg of -62.degree. C. and a dielectric loss of 350 at 200 Hz and 50.degree. C. Because of the high value for the dielectric loss the material is of interest in application where dielectric absorption is required.

The solid solution produced as described in Example 3, containing 13.3 mole % ZnCl.sub.2 has a Tg of -42.degree. C. and a dielectric constant of 250 at 200 Hz and 50.degree. C. Because of the high value for the dielectric constant the material is of interest as a dielectric in capacitors.

The solid solution produced as described in Example 4, containing 27.1 mole % ZnCl.sub.2 has a Tg of 33.degree. C. and is of interest as an electret. The material may be poled at field strengths of .about.10.sup.4 volts/cm and, due to the very large dielectric constant, is capable of storing very high charge densities for several days.

It will be appreciated that the physical properties of the above solid solutions depend upon the mole % of zinc chloride present. An increase in the mole % of zinc chloride present results in an increase in the glass transition temperature (Tg), although all the materials are thermoplastic, a low mole % of zinc chloride gives a rubbery material, whereas a high mole % of zinc chloride gives a glass at room temperature. Materials of intermediate composition are tough and leathery.

It will be seen from the Results that solid solutions of zinc chloride in poly (propylene oxide) are of interest as dielectrics in capacitors, dielectric absorbers, and as electrets. The application to which they are best suited depends upon the amount of zinc chloride present.

Poly (propylene oxide) (1.80 g), purified as described in Example 1, is dissolved in A.R. methanol (50 ml, dried over 3 A molecular sieve). Anhydrous zinc bromide (1.58 g, supplied by BDH Chemicals Ltd), dried under vacuum for several weeks, is also dissolved in A.R. methanol (50 ml, dried over 3 A molecular sieve). These solutions are thoroughly mixed and the solvent removed in a manner analogous to the poly (propylene oxide)-zinc chloride solid solutions.

The resulting solid solution contains 20.0 mole % zinc bromide in poly (propylene oxide) and is a clear, colourless glass at room temperature with a Tg of 45.degree. C. The material has a dielectric constant of 30 at 200 Hz and 60.degree. C., (65 at 200 Hz and 80.degree. C.) as measured by the techniques described previously. This solid solution is of interest for use in electrets since it is capable of maintaining high charge densities for up to 10 weeks at room temperature.

The procedure of Example 5 is repeated using poly (propylene oxide) (5.0 g) in A.R. methanol (100 ml) and Zinc iodide (4.4 g) in A.R. Methanol (100 ml).

The resulting solid solution contains 15 mole % zinc iodide in poly (propylene oxide has a Tg of -2.degree. C. and dielectric constant of 25 at 200 Hz and 80.degree. C.

Amorphous, low molecular weight poly (propylene glycol) (Trade name PPG 2000, Mn=2000, Tg=-64.degree. C., supplied by Shell Limited) is degassed at 50.degree. C. under vacuum and requires no subsequent purification. This is then used to prepare solid solutions with metal salts.

Poly (propylene glycol) (5.0 g) is dissolved in absolute ethanol (50 ml, azeotroped with benzene and dried over 3 A molecular sieve). Anhydrous cobalt (II) chloride (2.50 g, supplied by BDH Chemicals Ltd) is dissolved in similarly purified absolute ethanol (50 ml) and the two solutions are thoroughly mixed together. The solvent is then removed in the manner described previously to leave a solid solution of 18.3 mole % Co(II)Cl.sub.2 in poly (propylene glycol). This material is a dark blue, transparent, brittle glass at room temperature and may be compression moulded at elevated temperatures. Tg of this material is 38.degree. C.

The procedure of Example 7 is repeated using poly (propylene glycol) (1.5 g) in absolute ethanol (50 ml) and ferric chloride (1.4 g) in absolute ethanol (50 ml).

The resulting material is a brown transparent brittle glass at room temperature with Tg of 22.degree. C. and may be compression moulded at elevated temperatures.

A solid solution of 17.5 mole % Hg (II) Cl.sub.2 in poly (propylene glycol) is prepared by thoroughly mixing solutions of anhydrous mercuric chloride (1.00 g in 50 ml ethanol) and poly (propylene glycol), (1.01 g in 50 ml ethanol). The method of preparation is analogous to the preparation of the poly (propylene glycol)--cobalt (II) chloride solid solution. The mercuric chloride solid solution is a colourless transparent, extremely viscous liquid at room temperature with a Tg of -13.degree. C.

The procedure of Example 9 is repeated using poly (propylene glycol) (1.4 g) in A.R. methanol (50 ml) and Stannous Chloride (1.4 g) in A.R. methanol (50 ml).

The resultant solid solution of 22.2 mole % Stannous Chloride in poly (propylene glycol) is a colourless, transparent, extremely viscous liquid at room temperature with Tg of -5.degree. C.

A polyurethane is prepared using conventional polymer techniques from poly (propylene glycol) (10 g), hexamethylene diisocyanate (3.7 g) and butanediol (1.08 g). The resultant polyurethane (Tg -69.degree. C.) is dissolved in dry absolute ethanol, and a solution of anhydrous zinc chloride in ethanol is added with thorough mixing. The solvent is removed as previously described to give a solid solution of 16.6 mole % zinc chloride in polyurethane. This material is rubbery elastomer at room temperature with a Tg of -30.degree. C.