GB1604735A

GB1604735A - Ptc compositions and devices comprising them

Info

Publication number: GB1604735A
Application number: GB1475178A
Authority: GB
Original assignee: Raychem Corp
Current assignee: Raychem Corp
Priority date: 1978-04-14
Filing date: 1978-04-14
Publication date: 1981-12-16
Also published as: FR2423037B2; DE2915094A1; FR2423037A2; BE875504R; JPS5514690A

Description

(544 PTC COMPOSITIONS AND DEVTCES COMPRISING THEM (71) We, RAYCHEM CORPORATION, a Corporation organised under the laws of the State of California, United States of America, of 300 Constitution Drive, Menlo Park, California 94025, United States of America, do hereby declare the invention, for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- This invention relates to conductive polymer compositions which exhibit positive temperature coefficient (PTC) behavior, to devices containing such compositions and to methods of using such compositions.

Conductive polymer compositions comprise a conductive filler, usually carbon black, dispersed in a polymer. When the polymer is a thermoplastic crystalline polymer and contains a suitable amount of a suitable conductive filler, the composition exhibits PTC behavior, i.e. a sharp mcrease in resistivity over a particular temperature range, the increase beginning around the softening point of the polymer. Such compositions have been used for example in self-limiting strip heaters, and have been cross-linked by irradiation at room temperature to improve their form stability at higher temperatures.Although there have been reports of PTC behavior in conductive polymers in which the polymer is not a thermoplastic crystalline polymer, it has been accepted by those skilled in the art that in order to obtain a practically useful PCT composition the conductive filler must be dispersed in a thermoplastic crystalline polymer and that the sharp increase in resistivity always takes place at or near the crystalline melting point of the polymer (see for example the article by J. Meyer in Polymer Engineering and Science, November 1973, 13 No. 6, pages 462-468).

As examples of the prior art relating to PTC compositions, reference may be made to U.S. Patents Nos. 2,978,665; 3,243,753; 3,412,358; 3,591,526; 3,793,716; 3,823,217; and 3,914,363; British Patent No. 1,409,695; Brit. J. Appl. Phys, Series 2, 2, 567-576 (1969, Carley Read and Stow); and Kautschuk und Gummi II WT 138-148 (1958, de Meij); as well as the Meyer article referred to above, the disclosures of which are hereby incorporated by reference. For details of recent developments in this field, reference may be made to German Offenlegungschriften Nos.

2,543,314.1, 2,543,338.9, 2,543,346.9, 2,634,999.5, 2,635,000.5, 2,643,931.5, 2,643,932.6 and 2,655,543.1 and German Gebrauchsmuster No. 7,527,288.

We have now discovered that, provided a sufficient degree of cross-linking is introduced, the cross-linking of any conductive polymer will have a substantial effect on the resistance/temperature characteristics of the composition at a temperature which is related to the cross-linking temperature (which is frequently referred to herein as Tc), and/or at a temperature which is related to the temperature at which the polymer has already been cross-linked if the polymer had previously been cross-linked. Thus such cross-linking frequently will cause the composition to exhibit useful PTC behavior at a temperature related to Tc or will enhance previously existing useful PTC behavior around Tc.We have also found that the relation between Tc and the temperature range over which the effect (on resistance/temperature characteristics) is chiefly observed is, in the case of amorphous polymers, often at least in part dependent on the arrangement of the polymer chains at the time at which they are cross-linked. For example, if a polymer having high green strength is annealed at one temperature and is then heated (or cooled) to another temperature, both temperatures being above the melting point, and immediately cross-linked to an adequate extent, a PTC effect will be observed ccmmencing at a temperature between the annealing temperature and Tc.On the other hand if the polymer has low green strength which in this specification means one whose gum stock exhibits at 25 G a tensile stress of below 5 psi at 20% elongation, the effect will commence near Tc. For crystalline polymers which are cross-linked at temperatures within the melting range, and which have preferably been allowed to equilibrate at Tc prior to cross-linking, different effects will be observed depending upon whether Tc is below or above about the peak of the crystalline melting range, thus when Tc is above the peak of the crystalline melting temperature, and the composition is sufficiently cross-linked, it will exhibit PTC behavior in two temperature ranges, the lower related to the crystalline melting point and the higher related to Tc.

Yet different effects will be observed for crystalline polymers which are cross-linked below the peak of the crystalline melting range. We have also found that if a PTC composition is produced by a process which comprises two cross-linking steps carried out at different temperatures, the resistance/temperature characteristics can reflect either one or both of the cross-linking steps.

From the foregoing it will be clear that the present invention provides PTC compositions which have physical properties and/or resistance/temperature characteristics which have not previously been available. For example the invention makes it possible to prepare PTC compositions which (a) exhibit PTC behavior which is not associated with a first or second order thermodynamic transition of the polymer, e.g. the glass transition or the crystalline melting point, or with chemical instability (this term being used to include, for example, a lessening of the rigidity of the crcss-linked network above a certain temperature due to the lability of the cross-links above that temperature, e.g. the lability of disulfide links above about 700C);; or (b) have more than one useful Ts, as hereinafter defined, preferably separated by at least 250C, often by at least 50 C; or (c) exhibit PTC behavior which is associated with a thermodynamic transition of the polymer (as is known) but show sharper increases in resistivity than have hitherto been attained with the same polymer and same conductive filler.

In further describing and defining the present invention, the following terms and abbreviaticns are used in the ways defined below.

The term "switching temperature" (usually abbreviated to Ts) (which is in fact a temperature range over which a PTC composition shows a rapid increase in resistivity) is defined herein as the temperature at which extensions of the substantially straight portions of the plot of the log of the resistance against the temperature above the top and below the bottom the range cross.In order to be practically useful, a PTC composition must have at least one Ts which is between --100"C and 250"C, and which is associated with a temperature range in which the composition has an R14 value of at least 2.5 or an R100 value of at least 10 (and preferably both), where R14 is the ratio of the resistivities at the end and beginning of a 14"C range and R,, is the ratio of the resistivities at the end and beginning of a 1000C range. It is also preferred in practice that the resistivity is below about 105 ohm. cm. at the beginning of that temperature range.The term "critical range" is used herein to denote a temperature range in which the R14 value is at least 2.5 or the R,,, value is at least 10, and at the beginning of which the resistivity is below about 105 ohm. cm. A PTC composition is described herein as "having a useful T," if a plot of its resistivity against temperature shows a critical range (as defined above) which gives rise to a between --1000C and +250 C. The term "Peak Resistivity" is used herein to denote the maximum resistivity of the composition as it is heated to temperatures beyond any useful Ts, and the temperature at which the composition shows its peak resistivity is called the "Peak Temperature".The ratio of the Peak Resistivity to the resistivity at the useful Ts( or at the hlghest useful Ta if there is more than one) is called the Peak Ratio.

The gel fraction of a cross-linked polymer is the fraction by weight thereof that is not soluble in a solvent for the uncross-linked polymer.

In one aspect, the present invention provides a composition which comprises: (1) a cross-linked elastomeric polymer; and (2) conductive particles enclosed within the cross-linked network of the polymer, the particles having a size of at least 18 millimicrons; the polymer component of which composition has a gel fraction of at least 0.6 and has at least one useful Ts, wherein the polymer is derived from a gum exhibiting at 250C a tensile stress of at least 5 psi at 20% elongation, and the composition has a T.

not more than 450C below Tc (as hereinbefore defined), and advantageously not more than 35"C below Tc. The composition preferably has at least one useful Ts between 25"C and 2000 C. The resistivity of the composition will for many applications preferably be at least 25 ohm. cm. at the higher of 25"C and a temperature 500C below the useful Ts (or the lowest useful Ts if there is more than one). Especially when the composition is used at low voltages, the resistivity at 250C can be low, for example 1 ohm.

cm. or lower, though it is preferably at least 3 ohm. cm., especially at least 5 ohm. cm.

It is preferred that the critical range of the composition (or at least one of the critical ranges when the composition has more than one useful T) should have an Rao value of at least 6, where R,0 is the ratio of the resistivities at the end and beginning of a 30"C range. Another preferred feature is that the (or each) useful Ts of the composition should remain substantially unchanged when the composition is repeatedly subjected to thermal cycling which comprises heating the composition from a temperature below the useful Ts to a temperature above the useful Ts but below the Peak Temperature, followed by cooling to a temperature below the useful T. It is also preferred that the Peak Ratio is at least 20:1, especially at least 100:1.It is further preferred that when the useful Ts is less than 1500C, the ratio of the resistivity at 200"C to the resistivity at Ts is at least 20:1, and that when the useful Ts is greater than 150 C, the ratio of the resistivity at 250"C to the resistivity at Ts should be at least 20:1.

In the compositions of the invention the cross-linked polymer can be a mixture of cross-linked polymers and the composition may include other components such as fillers, flame retardants and antioxidants, as well as other cross-linked polymers not having conductive particles enclosed within the cross-linked network thereof, and thermoplastic amorphous or crystalline polymers which may or may not have conductive particles mixed therewith.

The invention also includes a process for making a composition as defined above, which process comprises: (1) dispersing conductive particles having a size of at least 18 millimicrons in a polymer (this term being again used to include a mixture of polymers) and annealing the dispersion; and (2) cross-linking the dispersion from step (1) to obtain a composition the poly mer component of which has a gel fraction of at least 0.6 and which com prises conductive particles enclosed within the cross-linked network of the polymer; the particles, the polymer, and the cross-linking conditions being such that the crosslinked composition has at least one useful Ts. The annealing time may be as little as 1 minute in some cases but times of at least 2 minutes, e.g., 3 to 15 or 3 to 10 minutes are preferred.When cross-linking is effected by a chemical cross-linking agent, the elevated temperature should be below the cross-linking temperature (or other measures taken) to prevent excessive premature cross-linking.

In one embodiment of the process, the cross-linking is effected in two stages carried out at substantially different temperatures. It is generally preferable that the first cross-linking should be effected at a higher temperature than the second.

The invention also provides an electrical device comprising a pair of electrodes separated by an electrically conductive element comprising a composition of the invention.

The invention further provides a device, e.g. a strip heater, comprising an element comprising a composition of the invention and at least one electrode, generally two electrodes which can be connected to a source of electrical power (e.g. D.C. from one or more 12 volt batteries or AC from a 110 or 220 volt source) to cause current to flow through the element. As discussed in detail in German Offenlegungsschrift No.

2,543,314.1 and in German Gebrauchsmuster No. 7,527,288, a problem which has frequently arisen in the operation of PTC devices is the formation of so-called "hotlines", that is the formation of a narrow band in the PTC material which is at a higher temperature than the surrounding PTC material. In the devices of the invention, the current preferably flows through the PTC element in a way which substantially prevents the formation of hotlines.Thus one preferred device is in the form of a strip heater comprising two parallel spaced-apart electrodes which are separated by a distance d (e.g. of 0.15 to 1 cm.) and are enclosed within a web of the PTC composition, the portion of the web between the electrodes having a minimum thickness t which is greater than the maximum dimension of the electrodes in the same direction and is such that d/t is less than 9, preferably less than 7, especially less than 5, particularly 3 to 4. In another type of preferred device, the PTC composition is in the form of a layer and the electrical connections thereto are such that, at least when the layer is at or near Ts, the current flow is predominantly through the thickness nf the laver. Such layers can be relatively thick, for example more than 0.05 inch (0.125 cm.) thick.In another type of device the electrical connection to the PTC composition is made by an element composed of a conductive polymer which does not exhibit a useful Ts at any temperature below a useful Ts of the PTC composition. For further details of ways in which hotlines can be prevented reference should be made to the publications referred to above.

The invention also includes a heat-recoverable device which comprises a device as defined above in thermal contact with a heat-recoverable member of an organic polymer which will recover at a temperature below, preferably not more than 50"C.

below, the useful Ts of the PTC composition (or the lowest useful Ts if there is more than one).

The invention also includes a heat-recoverable article which comprises a heatrecoverable member composed of a PTC composition as defined above and at least two electrodes which can be connected to a source of electrical power to cause current to flow through the PTC composition and thus heat the article to its recovery temperature.

The invention further includes a method of controlling the size of an electrical current flowing through an electrical circuit containing a PTC element by maintaining the temperature of the PTC element within a crtical range thereof or by changing the temperature of the PTC element from a value outside a critical range thereof to a value inside that critical range (or vice verso), wherein the PTC element comprises a composition of the invention. The temperature can be maintained or changed by the self-heating effect of the current passing through the element, or by extemal heating or cooling, or by a combination thereof.

Care is needed in the selection of the conductive filler, the polymer in which it is dispersed, and the cross-linking conditions, in order to ensure that a PTC composition having the desired characteristics is obtained. However, those skilled in the art will have no difficulty, having regard to the disclosure herein and their own knowledge, in making and using the invention and obtaining the advantages thereof.

Choice of Conductive Filler.

We have found that conductive filler must have a particle size of at least 18 millimicrons, in order for a useful PTC effect to be obtained. As the particle size increases, the PTC behavior tends to become more pronounced. However, this valuable effect is counter balanced by the need to include greater proportions by weight of the larger-sized fillers to obtain the same resistivity. This does not put any very serious upper limit on the size of highly conductive fillers, e.g. metal particles, which may have sizes up to or even more than 1 micron. However, when using conductive carbon blacks, as is preferred, it is very difficult to achieve satisfactory physical properties when using particle sizes greater than 100 millimicrons, because of the high proportions of carbon black needed to obtain the required resistivity.It is preferred that the carbon blacks used in this invention should have a maximum size of 80 millimicrons (average size when a mixture of carbon black is used).

As is well known in the art, carbon blacks are conventionally characterised by their particle size and by their nitrogen absorption and DBP (dibutyl phthalate) absorption values, which provide a measure of the porosity and aggregation of the primary particles. For details of these characteristics and their measurement, see for example "Analysis of Carbon Black", by Schubert, Ford and Lyon, in volume 8, at page 179, of Encyclopedia of Industrial Chemical Analysis (1969) published by John Wiley & Son, New York.

The preferred particle size of the carbon black is also dependent on the crosslinking temperature (Tc). As Tc increases, PTC behavior tends to become less pronounced, but this can be offset by an increase in the particle size of the carbon black.

Thus it is preferred that the particle size should be at least 20 millimicrons when Tc is above 20"C, at least 30 millimicrons when Tc is above 10000, and at least 40 millimicrons when T, is above 1500C.

Good results may be obtained through the use of carbon blacks having particle sizes above that minimum, and below 50 millimicrons, e.g. less than 42 millimicrons, including the case where the polymer is a butyl rubber, and/or a halogenated butyl rubber, and/or a ethylene/propylene rubber, crcss-linked with the aid of a chemical cross-linking agent.

A large variety of carbon blacks are commercially available, but only a small proportion of them are known as conductive blacks and recommended for use in conductive polymer compositions. The preferred types of black for this invention are furnace and acetylene blacks, but the less conductive thermal and channel process blacks can also be used.

Examples of other conductive fillers, in addition to carbon black, are graphite, metal powders, conductive metal salts and oxides, and boron- or phosphorus-doped silicon or germanium.

The choice of a conductive filler will in addition be influenced by the polymer to be used, in particular the compatibility of the filler and polymer.

Choice of Polymer.

As noted previously, the invention is useful with elastomers because of the variety of physical properties then available coupled with the ability to vary the Ts of the composition in a predictable way. The cross-linked polymer may be substantially free of carbon-carbon unsaturation, e.g. less than 5% molar concentration (C = C).

Suitable polymers in which the filler can be dispersed prior to cross-linking, and which are converted to elastomers by the cross-linking, include rubbers, elastomeric gums and thermoplastic elastomers. The terms "elastomeric gum", "gum" and "gum stock" are used herein to denote a polymer which is non-crystalline and has a glass transition temperature below Tc and preferably below room temperature (20 C), and which exhibits rubbery or elastomeric characteristics after being cross-linked. The term "thermoplastic elastomer" is used herein to denote a material which, even if not crosslinked, yet exhibits, in a certain temperature range, at least some elastomeric properties; such materials generally contain thermoplastic and elastomeric moieties.The filler can also be dispersed in a polymer which has been dynamically cross-linked, i.e. crosslinked while milling or otherwise shearing the polymer so as to obtain a product which can be subjected to further compounding or forming processes.

We have found that the higher the green strength of the initial polymer, the more pronounced the PTC effect (other things being equal). On the other hand, as previously indicated, it may be desirable to anneal polymers having high green strength at or near Tc prior to cross-linking. Depending on the circumstances, therefore, it may be preferred to employ a polymer having the minimum green strength i.e. between 5 and 10 psi at 20% elongation, or a polymer having high green strength at 2500.

The term "polymer having high green strength" is well known in the art and denotes a polymer which exhibits a tensile stress of at least 10 psi at 20% elongation. Such polymers after being equilibrated at elevated temperature to the configuration favored at that temperature, will not change, or will change only very slowly, from this configuration, when cooled to room temperature. They also possess form stability at room temperature such that articles prepared from them do not distorr or flow to any significant extent, even though not cross-linked.

Suitable gums for use in the invention include polymers which contain carbon in the polymer backbone, as well as others, for example polyisoprene (both natural and synthetic), ethylene-propylene random copolymers, poly(isobutylene), styrene-butadiene random copolymer rubbers, styrene-acrylonitrile-butadiene terpolymer rubbers with and without added minor copolymerized amounts of a,p-unsaturated carboxylic acids, polyacrylate rubbers, polyurethane gums, random copolymers of vinylidene fluoride and, for example, hexafluoropropylene, polychloroprene, chlorinated polyethylene, chlorosulphonated polyethylene, polyethers, plasticized poly(vinyl chloride) containing more than 21% plasticizer, and substantially non crystalline random co- or ter-polymers of ethylene with vinyl esters or acids and esters of ass-unsaturated acids.

Thermoplastic elastomers suitable for use in the invention, include graft and block copolymers such as: (i) random copolymers of ethylene and propylene grafted with polyethylene or polypropylene side chains, (ii) Block copolymers of a-olefins such as polyethylene or polypropylene with ethylene/propylene or ethylene/propylene/diene rubbers, polystyrene with polybutadiene, polystyrene with polyisoprene, polystyrene with ethylene propylene rubber, poly(vinylcyclohexane) with ethylene-propylene rubber, polv ( a-methvlstvrene) with polysiloxanes, poly-carbonates with polysiloxanes, poly(tetramethylene terephthalate) with polyktetramethylene oxide) and thermoplastic polyurethane rubbers.

Choice of cross4inking conditions.

As noted previously, both Tc and the extent to which cross-linking is effected are extremely important in determining the resistance/temperature characteristics imparted to the composition. Tc not only plays a major role in determining Ts but in addition the higher Tc, the less the degree of PTC exhibited (i.e. its slope and magnitude), other things being equal. Thus a carbon black/polymer combination which shows good PTC behavior due to the cross-linking when cross-linked at room temperature may show little or no PTC behavior due to the cross-linking when cross-linked at, for example, 170"C. The inhibiting effect of increasing Tc can often be counteracted by using a carbon black having a larger particle size and/or by increasing the level of cross-linking.We have found that it is essential that the cross-linked polymer have a gel fraction of at least 0.6, and often a higher gel fraction is necessary, e.g. at least 0.75.

For most polymers the gel fraction should generally not exceed 0.96, but many unsaturated rubbers for example can be cross-linked to higher gel fractions without undesirably affecting the physical properties of the composition.

The crosslinks formed in the cross-linking operation should be stable in the temperature range in which the PTC composition is required to operate. Suitable covalent crosslinks include simple covalent bonds and cross-links comprising one or more of the linking structures -0-, -C-N-, -0-0-, -0-S-C-, -0-SO2-, -Si-, --CC-CCO-, -C-CO-C-, --COO-NNR- and --COO--S-.

Ionic cross4inks are also suitable provided that the composition exhibits form stability at least in the range in which PTC behavior is desired. Thus carboxylated elastomers partially or completely neutralized by sodium ion may be useful in the range up to 10000. However, for most applications the more thermally form-stable compositions, which are partially or completely neutralized by di or polyvalent metal ions, are preferred.

In considering the above discussion, those skilled in the art will readily understand that it is possible for polymer molecules to become linked together through mutual attachment to a third body for example, by chemical or strong physical bonding to the surface of the conductive filler, especially carbon black. Thus, the term crosslinking as used in this specification connotes any means of forming bonds between polymer molecules both directly or through the mediation of another small or large molecule or solid body provided only that such bonds result in coherency of the article and a degree of form stability throughout the operating or service temperature range of the composition.

When covalent crosslinking is contemplated, there can be used any crosslinking process which will yield a product which has the required PTC characteristics and is form-stable in the range of utility. Thus crosslinking may be accomplished by irradiation or by chemical treatment. Suitable chemical crosslinking agents include, but are not limited to, organic peroxides and other precursor materials capable of yielding free radicals on the application of heat or other activation means, for example metal oxides and amines, or suitable reactive derivatives of amines; isocyanates and other compounds capable of reacting with groups containing active hydrogen to yield, for example ureas, urethanes, allophanates and the like; nitroso or oxide compounds such as p-quinone dioxine, and other difunctional chemicals containing at least one group capable of adding across double bonds such as organo-silane hydrides. Compounds containing sulfur which react with double bonds to yield mono sulfide bridged crosslinks such as thiuram disulphide are also suitable. In many instances it may also be desired to add other materials to enhance the crosslinking effect such as, in the use of ionizing radiation or other free radical initiators, crosslinking coagents including, but not limited to, polyunsaturated compounds.

Irradiation and peroxide crosslinking are preferred. Where ionizing radiation is used, doses between 5 and 50 mrads will generally achieve the desired crosslinking density without excessively compromising the physical properties of the product.

Where peroxides are used, in general between 1 and 10 percent based on the polymer may be used. Part of the peroxide may be replaced by coagents. We have also discovered that when the cross-linked polymer is an elastomer, the degree of similarity between T and the crosslinking temperature (T,-) will depend on the thermal history of the composition prior to crosslinking. As indicated above, the process requires the maintenance of the composition at an annealing temperature, and this may be the crosslinking temperature, for a brief period prior to subjecting said composition to the crosslinking process.For the majority of compositions the time required may be as short as one half minute, but for polymers possessing significant amounts of high molecular weight material (high Mooney viscosity gums), and which therefore have high green strength, a longer annealing period is preferred. Although we do not wish to be bound to any particular theory, the shortness of the annealing period indicates that the annealing serves to allow the polymer molecules to relax at their characteristic molecular relaxation rate into their preferred configurations at the crosslinking temperature. Thus Ts most nearly approaches Tc when the polymer molecules have been previously caused to assume an unstrained configuration at Te prior to crosslinking.

The aforesaid annealing is not related to the annealing of carbon black containing compositions which results in a diminution in the resistance of the composition, for example as described in Smith-Johannsen U.S. Patent No. 3,861,029, which process is believed to result in the "structuring" of the carbon black dispersion, i.e. the formation of preferred conductive pathways. Thus annealing for structuring takes much longer than the relaxation annealing referred to hereinabove. We do find, however, that it is also advantageous to facilitate the structuring of the carbon black by annealing.When the desired crosslinking temperature is above about 1700C, it is often convenient to combine the two annealing steps and hold the composition in its final shaped form at Tc for a sufficient period of time to cause the desired structuring to occur prior to crosslinking the composition. Thus, a ten to twenty minute anneal at 2000C prior to crosslinking at that temperaure is suitable.The time required for structuring is found to increase rapidly as the annealing temperature is decreased and for compositions which it is desired to crosslink at temperatures significantly below 2000 C, it may be preferred to anneal at 2000C for a short period to reduce the resistance of the composition, optionally cool to room temperature and subsequently hold the composition at Ta for a sufficient time prior to crosslinking to relax the polymer molecules at Tc. Those skilled in the art will recognize that relaxation times for polymers vary widely with molecular weight and temperature (inter alia) and whilst in some circumstances compositions would relax almost instantaneously at Tc, in other circumstances a period of some minutes may be required to relax the molecules.

Moreover, it is possible by the use of plasticizers or other internal viscosity reducers to greatly reduce the relaxation anneal and in many circumstances to reduce the structuring anneal as well. Thus, the optional use of such plasticizers is contemplated in the instant invention. We prefer to use additives which can be polymerized or crosslinked into the polymer or which by some other means have their viscosityreducing effect neutralized after the TC anneal, since such additives can otherwise have a deleterious effect on the resistance stability of polymeric PTC heaters under service conditions. Plasticising additives are particularly useful when peroxide or any of the other thermally activated crosslinking processes are used, as it is not then possible conveniently to subject the composition to structural annealing.The peroxide or curing agents, when used, are often good viscosity reducers for the polymer, and other plasticizers may also be used to advantage.

The density of crosslinking, if above a certain minimum level, does not have a substantial effect on the Ts of the composition. However, the slope of the resistance temperature slope above Ts is a strong function of the crosslinking density; the higher said density the steeper the slope. Thus, the optimum crosslinking level is that which will achieve the desired resistance-temperature slope without undesirably compromising the physical properties of the product.

As briefly noted above, especially with polymer compositions which do not crosslink readily using ionizing radiation or chemical free radical generating additives such as peroxides, it is advantageous to add crosslinking promotors (or "coagents", to use a term known in the art). Such materials are usually unsaturated monomers.

Suitable unsaturated monomers (or mixtures of monomers) generally contain at least two ethylenic double bonds in each molecule and preferably contain at least three.

They should of course preferably be compatible with the polymer and have low volatility under the process conditions. Examples of such monomers include allyl esters of polycarboxylic acids and other acid moieties such as cyanuric acid, e.g., triallyl cyanurate and isocyanurate, diallyl aconitate, maleate and itaconate, and retraallyl pyromellitate; bis and tris maleimides, e.g., N,N'-ethylene- and N,N'-mphenylene-bis-maleimide; acrylic and methacrylie esters of polyhydric alcohols, e.g., dipentaerythritol hexamethacrylate, ethylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate and penta-erythritol tetramethacrylate; vinyl esters of polybasic acids, e.g. trivinyl cyanurate and citrate; vinyl and ether allyl ethers of polyhydric alcohols, e.g. the tetra-allyl and tetravinyl ethers of pentaerythritol; and bis acrylamides, e.g. N,Nl-methylene- and N,N1-p-phenylene-bis-acrylamide. The amount of monomer required to provide a useful amount of crosslinking (as compared to a composition which is the same except for the presence of the monomer and has been processed in the same way) depends upon the particular monomer and the other ingredients, but can readily be determined by those skilled in the art having regard to the disclosure herein. Amounts within the range of 1 to 10% by weight of the composition, are generally satisfactory.

Other factors.

As indicated above, the compositions of the invention may contain conventional ingredients such as antioxidants, flame retardants, inorganic fillers, thermal stabilisers, and processing aids. We have found that the presence of a non-conductive filler (e.g.

in amount 5 to 35%, preferably 10 to 25%, based on the weight of the composition) is often advantageous. Particularly is this so when the polymer is a polysiloxane and the filler is silica. The presence of the non-conductive filler reduces the overall coefficient of expansion of the composition, and the improvement in PTC behavior is therefore surprising in view of the theory put forward by Kohler (see for example U.S. Patent No. 3,243,753) which attributes the PTC effect to the difference between the thermal expansion ratios of the conductive filler and the composition.

For many uses of PTC compositions, e.g., in self-limiting heaters, it is important that the PTC composition should be chemically stable at the operating temperature, which may be substantially higher than Ts, e.g., in the range Ts plus 20 to Ts plus 60. A number of polymers fail to meet this requirement, including in particular poly (epichlorohydrins) which have been cross-linked at elevated temperature, e.g., above 150 degrees C.

The PTC compositions of the invention are preferably prepared by melt-shaping, e.g., by molding or extrusion, a suitable cross-linkable composition, followed by crosslinking the shaped product.

The invention is illustrated by the following Examples in which percentages are by weight and temperatures are in "C. The Examples are summarised in Table 3 below.

Except where otherwise stated, in each Example, the indicated polymer and carbon black were blended on a 4 inch (10 cm) two roll mill to give a blend containing the indicated percentage of carbon black, and the blend pressed into a sheet 6 x 6 x 0.03 inch (15 x 15 x 0.075 cm.). Strips 1.5 x 1- x 0.03 inch (3.8 x 2.5 x 0.075 cm.) were cut from the sheet. Electrodes were created on opposite ends and opposite faces of the long dimension of each strip by painting thereon 0.25 inch (0.64 cm.) wide bands of silver paint.The strip was annealed for 10 minutes at 2000 C. It was then placed on a metal plate maintained at the indicated temperature (+10"C) and (when the plate temperature was greater than room temperature) was allowed to equilibrate with the plate, giving the polymer time to relax. The strip was then cross-linked, while maintaining the plate at the indicated temperature, by exposure to the indicated irradiation dose in Megarads from a 6 mA beam of about 0.7MV electrons; the strip was thereby heated to a somewhat higher temperature, usually 10 to 200C higher at the higher dose. After cooling, the strip was heated slowly from room temperature to its peak temperature, measuring its resistance between the electrodes at 1500 intervals.The resistances in the steepest portion of the resistance/ temperature curve were fitted to an expression of the form Ln (RT/Ro) = = (T-To) (where RT was the resistance at Temperature T and Ro was the resistance at the temperature T, at the start of this portion of the curve), and a was calculated. For a curve having an R14 greater than 2.5 and an R30 greater than 6, a is at least about 0.6.

Details of the carbon blacks used in the Examples are given in Table 1 below.

Details of the polymers used in the Examples are given in Table 2 below. Details of the Examples themselves are given in Table 3, which also gives the significant resistance/temperature characteristics of the products.

In Table 3, the polymers and carbon blacks are identified by their numbers as set out in Tables 1 and 2; the trade names of the polymers are also given and for the carbon blacks the particle size, nitrogen absorption and dibutyl phthalate absorption values are given in parentheses after the identification number.

TABLE 1 CARBON BLACKS Type No. Trade Names Industry ASTM Size N2 DBP D1976--67 Class Class Mu MMg cc/100g 1. Vulcan XC-72 XCF N472 30 254 178 2. Sterling SO FEF N550 42 42 120 TABLE 2 POLYMERS Type No. Trade Name Polymer Type Filler Type 1. Nordel 1470 Ethylene/propylene/ None diene terpolymer 2. Epsyn 5508 Ethylene/propylene/ None diene terpolymer with high green strength 3. Nysin 35-8 An acrylonitrile/ None butadiene rubber containing 35% acrylonitrile TABLE 3 Carbon Black Radiation Curing Resistance (Megohms) Peak Example Polymer Type % Dose Temp. C at 250 C at Peak Ts To Temp. α R14 R30 R100 1. 1. Nordel 2. 30 20 25 0.003 > 1000 40 70 -160 0.23 25 -1000 (42/42/120) 2. 100 0.0002 100 105 115 220 0.15 7.7 80 > 1000 3. 2.Epsyn 2. 48 10 100 0.0001 > 10 30-100 160 -200 0.12 5.4 35.2 1000 5508 (42/42/120) 4. 45 10 100 0.0002 50 100 130 250 0.09 3.5 10 -100 see note 5. 45 20 140 0.0002 15 130 160 240 0.11 4.5 15 > 100 6. 3. Nysin 1. 30 20 25 0.01 -10 25-40 70 145 0.09 3.5 15 > 100 35-8 (30/254/178) 7 140 0.002 -0.035 150 175 235 0.06 2.5 6 17 8. 180 0.001 0.007 120 205 250 0.046 1.9 4 10 Notes on Table 3.

Examples 1 and 2.

The Nordel-containing compositions were prepared in a Banbury mixer. Both the Nordel- and Epsyn-containing compositions were annealed at 200 C for 5 minutes after they had been irradiated. The resistance of the second Nordel-containing composition, which had been irradiated at 100 C, decreased from 25 to 100 C.

Examples 3 and 4.

Example 3 was irradiated within a few minutes of being placed on the hot plate and had a very indefinite Ts in the range from 30100 C. Example 4 was allowed to equilibrate and relax longer before irradiation and had a more clearly marked Ts at 1000 0. Furthermore, the resistance of 4 was relatively constant from 400 to Ts while that of 3 doubled in the 30 range from 25 to 70"C.

WHAT WE CLAIM IS: 1. A composition comprising: (1) a cross-linked elastomeric polymer; and (2) conductive particles enclosed within the cross-linked network of said polymer, the particles having a size of at least 18 millimicrons; the polymer component of which composition has a gel fraction of at least 0.6, and has at least one useful Ts (as hereinbefore defined), wherein the polymer is derived from a gum exhibiting at 250C a tensile stress of at least 5 psi at 20% elongation, and the composition has a Ts not more than 45" below Te (as hereinbefore defined).

2. A composition as claimed in claim 1, wherein the composition has a T5 not more than 350C below Tc.

3. A composition as claimed in claim 1 or claim 2, wherein the conductive particles have a size of less than 50 millimicrons.

4. A composition as claimed in claim 3, wherein the particles have a size of less than 42 millimicrons.

5. A composition as claimed in claim 3 or claim 4, wherein the polymer is a butyl rubber, a halogenated butyl rubber, an ethylene-propylene rubber, or a mixture of any two or more such rubbers, cross-linked by a chemical cross-linking agent.

6. A composition as claimed in any one of claims 1 to 4, which has at least one critical range (as hereinbefore defined) having an R,0 value (as hereinbefore defined) of at least 6.

7. A process for the preparation of a composition as claimed in any one of the preceding claims, which process comprises: (1) dispersing conductive particles having a size of at least 18 millimicrons in a polymer, and annealing the dispersion; (2) cross-linking the dispersion from step (1) to obtain a composition the polymer component of which has a gel fraction of at least 0.6 and which comprises conductive particles enclosed within the cross-linked network of the amorphous polymer; the particles, the polymer, and the cross-linking conditions being such that the crosslinked composition has at least one useful Ts.

8. A process as claim in claim 7, wherein the annealing is carried out at an elevated temperature for at least one minute.

9. A process as claimed in claim 8, wherein annealing is carried out for at least two minutes.

10. A process as claimed in claim 8, wherein annealing is carried out for from 3 to 15 minutes.

11. A process as claimed in claim 8, wherein annealing is carried out for from 3 to 10 minutes.

12. A process as claimed in any one of claims 7 to 11, wherein the dispersion also contains a cross-linking agent and wherein the annealing is carried out at a temperature and for a time such that excessive cross-linking is avoided.

13. A process as claimed in any one of claims 7 to 12 which also comprises melt-shaping the cross-linkable dispersion.

14. An electrical device comprising a pair of electrodes separated by an electrically conductive element comprising a composition as claimed in any one of claims 1 to 6.

15. An electrical device comprising at least one electrode and an element comprising a composition as claimed in any one of claims 1 to 6.

16. An electrical device as claimed in claim 14 or claim 15, which is a heating device.

17. A method of controlling the size of an electrical current flowing through an electrical circuit containing a PTC element by maintaining the temperature of the

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. Examples 3 and 4. Example 3 was irradiated within a few minutes of being placed on the hot plate and had a very indefinite Ts in the range from 30100 C. Example 4 was allowed to equilibrate and relax longer before irradiation and had a more clearly marked Ts at 1000 0. Furthermore, the resistance of 4 was relatively constant from 400 to Ts while that of 3 doubled in the 30 range from 25 to 70"C. WHAT WE CLAIM IS:

1. A composition comprising: (1) a cross-linked elastomeric polymer; and (2) conductive particles enclosed within the cross-linked network of said polymer, the particles having a size of at least 18 millimicrons; the polymer component of which composition has a gel fraction of at least 0.6, and has at least one useful Ts (as hereinbefore defined), wherein the polymer is derived from a gum exhibiting at 250C a tensile stress of at least 5 psi at 20% elongation, and the composition has a Ts not more than 45" below Te (as hereinbefore defined).

PTC element within a critical range thereof or by changing the temperature of the PTC element from a value outside a critical range thereof to a value inside that critical range (or vice versa), wherein the PTC element comprises a composition as claimed in any one of claims 1 to 6.