US3243753A - Resistance element - Google Patents

Description

March 29, 1966 F. ,KOHLER RESISTANCE ELEMENT Filed Nov. 15, 1962 {50 250 fE'MPEPATt/RE- "F PEf/STAA L'E-OHMJ INVENTOR. 59:0 (0w. 0

ATTORNE Y5 United States Patent 3,243,753 RESISTANCE ELEMENT Fred Kohler, 113 E. 31st St., New York 16, N.Y. Filed Nov. 13, 1962, Ser. No. 236,943 10 Claims. (Cl. 338-31) The present invention relates to an improved elect-rical resistance element which varies in resistance in response to changes in temperature and an improved means for manufacturing same. More specifically, the present invention deals with an improved resistor containing a thermoplastic and finely divided conducting particles, said resistor exhibiting a relatively small increase of resistance, for an increase of temperature below a selected temperature value, and which exhibits an extremely high temperature coefiicient of resistance at temperatures above said selected temperature value, such that the resistance of said resistor increases sharply when the selected value of temperature is exceeded.

Heretofore, it has been suggested that a resistance element could be made from plastics, mixed with conductive materials However, in general, such materials have not been put to wide use. The plastic body often will melt during its use or the plastic will deteriorate at elevated temperatures. Further, they have not shown such particular properties which would make their use advantageous.

In accordance with the present invention, means are taught for forming a resistor element which is characterized by its unusual change of resistance with change in temperature. More particularly, the present resistor is in the form of a plastic e.g., a thermoplastic or suitable thermosetting material, preferably a polyolefin plastic, containing a finely divided conductive powder intimately dispersed through the plastic matrix. The relative quantities, types and arrangement of the finely divided conductive particles and the plastic are such that below a certain selected temperature level, particle to particle contact exists between the finely divided particles of the conductive constituents, thus giving the total composition a relatively low resistance. Above said selected or critical temperature the substantial difference in thermal coefiicient of expansion between said conductive particles and the thermoplastic material results in the breaking down of contact between the conductive particles with a resultant sharply increased resistivity. By way of example, a resistance element of the present invention normally exhibits an increase in resistance of more than 250% over a temperature range of less than 25 F., at about said selected critical temperature value. A 1000% increase in resistance or even higher percentage over a temperature range of less than 25 F. at about said critical temperature value has been obtained in experimental samples.

As noted previously, the non-conductive plastic polymer which serves as thematrix holds said conductive particles together and is preferably a polyolefin, such as polyethylene, polypropylene, polyisobutylene, halo-derivatives of polyethylene, such as polytetrafluoroethylene, trifluoromonochloroethylene, copolymers or admixtures thereof. The polyolefin plastic must have 'a substantially greater coefficient of thermal expansion than the conductive particles. The polyolefin polymers may be produced by any of a wide variety of conventional processes now in industrial use, such as high pressure polyolefin processes, relatively low temperature, low pressure polymerization reactions such as the use of titanium halides in combination with alkyl aluminum compounds, etc. The polymer generally has a thermal coefificient of expansion of about 2 l0 C. to 25 10 C., preferably l2 10 C. to '22 10 C., said temperature coeflicient of expansion being at least 3, preferably 30 times greater than the thermal coefficient of expansion of the conductive materials. An example of this is the combination of carbonp woder with a coefiicient of thermal expansion of 5 .4 10- C. and polyethylene which has a coefl'icient of thermal expansion of about 16 lO C.

The finely divided conducting particles are preferably carbon, such as carbon black, graphite, etc. Alternatively the conducting particles may be finely divided metals, alloys, non-metallic conductors, metal salts, etc. Examples thereof include iron, copper, chromium, titanium, tungsten, platinum metals, boron, silicon, silver, gold and aluminum. The conducting particles are normally of a size range of about 0.003 to 1.0 microns, preferably 0.010 to 0.080 micron. In general, the present compositions will contain, based on thermoplastic and conducting particles, 25% to preferably 35.0 to 60.0% thermoplastic, the remainder being finely divided conductive particles. These percentage figures are based on the volumes respectively of the thermoplastic and of the particles in their finely-divided form, such as carbon black.

The present compositions are made in a manner which insures substantial contact among the conductive particles at temperature below the critical temperature of the resistor, with the consequent separation of the conductive particles at the critical temperature due to the substantially greater thermal coefiicient of expansion of the thermoplastic material. The resistor, of course, has a positive resistance-temperature characteristic. As the temperature is raised, the thermoplastic expands more than the conductive particles, thus reducing the compressive force on the particles. As the temperature increases a point is reached where there has been a very substantial decrease in compressive pressure on the conducting particles due to the greater rate of expansion of the thermoplastic. At this point, there is encountered the very sharply increased resistance characteristic of the present resistance element.

The present resistors are preferably produced in a manner characterized by the conductive finely divided particles being under compressive stresses while simultaneously the thermoplastic is formed by in situ polymerization of the appropriate monomer. Thus, for example, the finely divided conductive particles, e.g., carbon black of less than 200 mesh may be mechanically compressed, e.g., pressures of 1 to p.s.i., in a reaction vessel. Conventional catalytic agents for promoting the polymerization of the monomer, e.g., ethylene gas, may be dispersed about said compressed carbon particles. The ethylene gas is then pumped preferably at high pressures of 1000 to 2000 atmospheres and at normal polymerization temperatures into contact with said finely divided conductive particles. At the conventional polymerization temperature range, e.g. 300 to 400 F. the ethylene is polymerized to polyethylene which forms a matrix about and between the carbon particles without substantially disturbing the carbon-carbon particle contact due to the compressive forces exerted by mechanical means on the conducting particles. Very intimate contact between the carbon and polyethylene thus formed is assured and the conducting carbon lattice and plastic matrix will be relatively densely packed together. Under these conditions, the polymerizing monomer will tend to cross-link and intimately line the surface of the carbon particles. This upon cooling will assure particle to particle contact under pressure, provided that enough carbon had been used to form a coherent lattice. The mechanical compression means may take the form of a reaction chamber substantially fully packed with conducting particles, a pistonended reaction zone.

the conducted particles is preferably carried out in a gas phase is in situ polymerization, it may also take the form of a liquid phase reaction zone. As an example of this, the plastic material may be a liquid epoxy resin. As soon as practicable after addition of the catalyst, the liquid mixture is injected under pressure into a reaction vessel tightly packed with carbon particles. The polymerization now takes place around the conductive particles, while particle to particle contact remains relatively undisturbed.

It is also possible to produce the resistance element by solid phase polymerization of a polyolefin of low molecular weight with carbon particles of a very small diameter. For this purpose carbon blacks with a particle size in the order of 3-25 millimicrons are preferred.

To accomplish this, about 60% carbon by weight must be used. This carbon is then intimately mixed with a partially polymerized polyolefin. The mixture is then heated to a temperature where the plastic forms a viscous fluid and then thoroughly blended. This blending is preferentially carried out in a pressure vessel under a pressure of 100 lbs./in. or even higher. Under these conditions, and especially under'the influence of a suitable catalyst such as an organic persulfate, the low molecular weight chain will grow in length along the great amount of surface area, provided by the carbon black. As the percentage of carbon is in the order of 60% by weight of the composition, carbon to carbon contact exits throughout the composition. At a temperature above 350 F. extensive cross-linking of the polymer chains takes place, as well as linkages with the carbon atoms are formed.

The above processes for forming the resistance element of the present invention are distinguished from merely admixing finely divided conductive particles with particles of preformed polymer. Further the concept of mechanically compressing the conductive particles while forming a thermoplastic matrix by in situ polymerization, distinguishes the present process from merely conducting polymerization in a solution containing finely divided particles.

' The resistance elements of the present invention find "a wide variety of interesting applications. For example, they may be employed in space heaters or radiant type wall heaters in which it is. desired that the maximum temperature will be self-limiting. Because of their extremely rapid and sharp increase in resistivity at a selected temperature level, the resistors of the present invention may readily be employed in combustible panels or walls where a maximum temperature may not be exceeded even in the event of, a short circuit. These resistance elements used as heaters have the property of drawing more current as the ambient temperature decreases. This is due to the fact that in colder surroundings the maximum temperature, which these resistor elements reach is a few degrees lower than the maximum temperature reached in a warmer ambient. Consequently, as the resistance body is a few degrees cooler, the resistance is considerably lower. This means that in cooler weather more power is automatically drawn from the power lines, without the use of a thermostat. A heating system using the said resistance elements will consequently be largely self-regulating. They may be employed as a thermal-control element which would permit the construction of time delay relays. In one application of the present invention the resistor may be placed in series with a source of electrical power in the load. Under normal conditions, the resistance of the load is such that relatively little current flows through -Ithe resistor and thus the temperature of the resistor remains below the critical value. If, for example, the load is short circuited, the current in the resistor increases and its temperature is rapidly raised to about its critical value. As a result, the resistance of the thermoplasticfine particle resistance element greatly increases. This means that a great deal of power is dissipated in the resistance element. This power dissipation, however, can only take place for a very short time, such as a fraction of a second, because the dissipated power will raise the temperature of the element to a value where the resistance has become so high, that the original current is limited to a negligibly small value; in fact, a value of current just high enough to maintain the element at its new high temperature-high resistance equilibrium point. Accordingly, the resistor may act as a form of fuse which reduces the current flow through the short circuit load to a safe extremely low value when the resistor is heated to the critical temperature range. Normally, when employed in this manner, it is necessary to first interrupt the current in the circuit 50 as to allow the resistor to cool below its critical temperature before reapplying the current. The electrical circuit thus described is quite advantageous in that it eliminates the need for replacement of fuses, costly circuit breakers or other fuses'characterized by moving parts and offers a highly effective means of controlling maximumcurrent to the circuit. Depending on its mass, and the type of heat-sink it is mounted on, various inverse time vs. current characteristics are possible.

When employed in the above uses, one or more conductive materials are fixed, e.g., clamped, screwed, etc., to the plastic element to form the resistor, the plastic being between two conducting surfaces. The select value of current corresponding to the rapid increase of resistance will depend on and can be varied by, choice of cross-sectional area and surface to value ratio of the resistor, the specific resistivity and thermal characteristics of any heat sink to which the plastic resistor is attached, etc.

With reference to the drawings;

FIGURE 1 depicts the greatly enlarged segment of the present resistor under normal temperature conditions.

FIGURE 2 illustrates the structure of the present resistor upon reaching and exceeding the critical temperature range.

FIGURE 3 graphically illustrates the temperature vs. resistance characteristics of an experimental resistor produced as per the teachings of this application,

FIGURE 4 shows a practical design for a recycling fuse,- which will reduce the current in the circuit to a negligibly small value in a short interval after a certain critical value of current in the circuit has been exceeded.

The various aspects and modifications of the present invention will be made more clearly apparent by reference to the following descriptive examples and accompanying drawings.

Example 1 The following examples illustrate a typical procedure for forming a resistance element in accordance with the present invention.

Finely divided carbon black having a size range of about 0.010 to 0.080 micron is placed in a tubular reaction zone having a movable end segment which can act as a piston. The reaction zone is substantially packed with the carbon particles and the position of the piston and the reactor set so as to employ about 10 pounds per square inch of mechanical pressure across the body of carbon particles. Thereafter, ethylene gas is introduced into the reaction zone which is maintained under reaction conditions of temperature and pressure, e.g., 350 F. and 1000 atmospheres. A suitable catalyst such as an organic persulfate may be dispersed about the packed carbon particles or alternatively injected with the ethylene monomer. The ethylene is thus polymerized in situ upon the carbon particles while simultaneously forming a matrix between and among the conducting particles- Because of the mechanical compression during the polymerization reaction and subsequent cooling, the resulting product recovered is characterized by a highly filled polyethylene plastic wherein the conducting particles are in substantial contact with one another. In the present example the resulting product may contain 40% polyethylene, 60% carbon particles and have a resistance at room temperature of about 1 ohm/inch.

With particular reference to FIGURE 1, shown therein isa greatly enlarged and simplified view of the resultant thermoplastic-carbon particle resistance element thus produced. It is noted that carbon particles 10, although surroundedby thermoplastic matrix 11- are in relatively intimate contact. Both the mechanical compression exerted during the polymerization and the contraction of the polyethylene during cooling of the reaction temperature to-room temperature, insure the close contact of the conducting finely divided particles and thus the relatively low resistance of the resistor unit.

When the temperature of the resistance unit 12 is raised to a critical temperature value, there is a substantial and relatively sharp decrease -in the contact'among the conducting particles due to the substantially greater coefficient of thermal expansion of the thermoplastic as compared to finely divided conductors. This is shown in exaggerated form. in FIGURE. 2. As a consequence, upon r'eachirig'a temperature level at which this elfect becomes pronounced the resistance of the resistor very sharply increases in value.

Example 2 The dramatic increase in resistance of the elements of the present invention is graphically shown in FIGURE 3.. FIGURE 3 illustrates the effect of temperature on a three-inch long by-one-inch diameter tube of a polyethylene carbon material similar to that described above.

This tube had a wall thickness of about .150". It was clamped with a metal ring, similar to a hose clamp at each end, so that the distance between the edges of the rings was about three inches.

' The resistance of the resistor was then measured while varying its temperature from about 80 to 395 The testwas performed by placing the resistance element into an oven,"the temperature of which could be adjusted and measured. A thermometer was placed inside the resistor tube, 'while an ohm-meter was connected to leads connecting to the ends of the resistor. 'Results are set forth in Table 1 and graphically illustrated in FIGURE 3. The specific resistance in terms of ohms-inch is obtained by multiplying the resistance in ohms by thecros's sectional area divided by the length of the strip desired, i.e..47 /3 -0.157.

- TABLE 1 Temp., Resistance, Temp., Resistance,

F. ohms F. ohms As shown in FIGURE 3, the resistor of the present invention exhibits a normal small and positive resistance temperature relationship. This is so under normal temperature conditions to be encountered in its use. In the present example, this could be considered to be temperatures of up to about 250. However, as depicted in FIGURE 3, at this relatively critical temperature level a very sharply increased change in resistance with increased temperature occurs, e.g., a 600% increase in resistance from a temperature of about 255 F. to 266 F. At temperatures beyond this point the resistance increases many fol d beyond that at lower temperatures. It thus becomes apparent that the resistance elements used in the test are characterized by a dramatically increased resistance over a relatively narrow temperature range.

Example 3 The following illustrates a typical application of the present resistance element in terms of its use as a circuit fuse.

By way of example, as shown in FIGURE 4, a simple series cincuit containing a load which may, for example, be a IOU-watt lamp and a resistance element of the present invention is connected by wiring 15 to a standout AC. or DC. voltage source of 110-120 volts. The resistance element may be a polypropylene carbon particle corn position 12 of the present invention wedged or threaded between two metal cylinders 13 and 14 which are connected so as to be in series with the remainder of the circuit. The wedge has a resistance of about 0.05 ohm at room temperature and the resistance of the load element is about 85 ohms. 4

During the course of normal use the temperature of the overall circuit may rise to, for example, 200 F. However, the resistance of the polyethylenecarbon par ticle wedge will only increase to about 0.10 ohm. Heat dissipation in the composition will be about .1 Watt. However, when the load element is short circuited, there is initially a very substantial increase in current passing through the circuit and due to the I R volt-age factor the fuse of the present invention is heated to a temperature within the critical range. This causes the resistance of the fuse element to increase to the order of 1000 ohms at 300 or 50,000 ohms or "greater at 350. Thus a new equilibrium point is 'reachedwhere heat dissipation due. to PR, equals thermal losses. Thus, while normal current flow would not cause heating to the temperature above which there is a rapid increase in resistivity, the increase in current due to the short circuiting gives a sudden increase in heat generation and greatly increases the resistivity of the fuse to such ahigh value that the present composition itself will thus become the current limiting device, thereby limiting the final equilibrium current to a final fraction of its initi-al safe value.

Uponremoving the power source, or opening the circuit on the load side of the fuse of the present invention,

i the initial short circuiting. difficulty has been eliminated the cycle will again be rapidly repeated and the current reduced to a safe value.

When using the electric resistance element of the present invention to control current flow, it generally will comprise less than 5% of the total resistance of the circuit at room temperature. When the current increases to heat theelement to the temperature range of markedly increasing resistance with temperature, the element will comprise more than 95% of the total resistance value of the circuit. The current may thus be limited to less than 5% of its normal value.

Various modifications may be made to the present invention. For example, the following are a number of additional applications of the unusual characteristics of l the present material for useful and practical purposes.

(1) A water heater, which upon evaporation of the water will shut itself off automatically by virtue ofthe material essentially becoming a non-conductor.

(2) A positive temperature-coefiicient thermistor (resistor with high temperature coefiicient of resistivity). (3) A sensing element for a heat or fire detection system.

(4) A protective element for dry-cell batteries, opening the circuit upon any short circuiting, but passing current again upon removal of the short.

(5) An ambient temperature sensitive timing element,

Where the time delay is a function of ambient tempera mm.

(6) A temperature limiting resistance element. This would be useful for applications such as a baseboard heater, baking ovens, electric blankets, electric irons, etc.

(7) A sensing element for an air or water flow meter, in the manner in which the conventional negative thermistor elements are presently used.

(8) In tubular form a heating element for liquids passing through it.

(9) A thermostatic element without moving parts for automatic control systems.

(10) In the form of thin fibres, a memory or storage element for computers (or in any form in which the thermal mass involved is small).

(11) As an element exhibiting negative resistance characteristics over a portion of its operating range.

Having described the present invention, that which is sought to be protected is set forth in the following claims.

I claim:

1. An electrical resistor element comprising a resistor provided with conductive connecting terminals for forming portions of an electrical circuit, said resistor comprising finely divided conductive particles dispersed in a matrix of a relatively non-conductive plastic, said plastic having a coefficient of thermal expansion at least three times greater than that of said conductive particles, said conductive particles being in substantial particle to particle contact, said resistor containing sufiicient conductive particles amounting from about 25% to about 75% of the total volume of the resistor material and dispersed through said plastic matrix so that below a critical temperature level immediate particle to particle contact exists between the conductive particles, whereas above said temperature level and while the resistor remains a solid, said contact between conductive particles is substantially broken due to the differences in thermal coeflicient of expansion between said particles and said non-conductive matrix, said critical temperature being substantially above room temperature and below the temperatures of melting and of substantial deterioration of the plastic, said resistor being characterized by a relatively flat curve of electrical resistance versus temperature below said critical temperature level and by a sharply rising curve beginning at said critical temperature level, said resistor further being characterized by an increase in resistivity of more than 250% over a temperature range of 25 F. at about said critical temperature level.

2. The resistor element of claim 1 wherein said plastic is a polyolefin.

3. The resistor element of claim 1 wherein said finely divided conductive particles comprises carbon.

4. The resistor element of claim 1 wherein said conducting particles range in size from .003 to 1.0 micron.

5. The resistor element of claim 1 wherein said plastic is polyethylene and said conducting particles are carbon particles.

6. An electrical resistor element comprising a resistor provided with conductive connecting terminals for forming portions of an electrical circuit, said resistor comprising a polyolefin polymer containing finely divided carbon particles distributed Within a matrix of said polyolefin polymer, said carbon particles being present in about 25 to 75 volume percent based on total volume of said polyolefin polymer and particles and. being in carbon particle to carbon particle contact at room temperature, said carbon particles having such a substantially lower coefiicient of thermal expansion than said polyolefin and being distributed through said polyolefin matrix, whereby below a critical temperature level immediate carbon particle to carbon particle contact predominates whereas above said temperature level and while the resistor remains a solid, such carbon particle contact is substantially broken so that the resistor exhibits an increase in electrical resistivity of more than 250% over a temperature range of 25 F. at about said temperature level, said resistor being characterized by a relatively fiat curve of electrical resistance versus temperature below said critical temperature level and by a sharply rising curve beginning at said critical temperature level.

7. The resistor element of claim 6 wherein said polyolefin is polyethylene and said carbon particles have a size range of 0.003 to 1.0 micron.

8. An electrical resistor element comprising a resistor provided with conductive connecting terminals for forming portions of an electrical circuit, said resistor comprising finely divided conductive particles having an average particle size of 0.01 to 0.08 micron dispersed in a matrix of a relatively non-conductive plastic, said plastic having a coefficient of thermal expansion at least three times greater than that of said conductive particles, said conductive particles being in substantial particle to particle contact, said resistor containing suflicient conductive particles amounting from about 25% to about of the total volume of the resistor material and dispersed through said plastic matrix so that below a critical temperature level immediate particle to particle contact exists between the conductive particles, whereas above said temperature level and while the resistor remains a solid body, said contact between conductive particles is substantially broken due to the differences in thermal coefficient of expansion between said particles and said non-conductive matrix, said critical temperature being substantially above room temperature and below the temperatures of melting and of substantial deterioration of the plastic, said resistor being further characterized by an increase in resistivity of more than 250% over a temperature range of 25 F. at about said critical temperature level.

9. An electrical resistor element in accordance with the foregoing claim 1 and in which said matrix comprises a polyolefin polymer which is polymerized with the conductive particles in situ substantially in particle-to-particle contact and under pressure.

10. An electrical resistor element in accordance with the foregoing claim 1, and in which said plastic matrix has been polymerized, and its molecules are cross-linked with said particles in situ.

References Cited by the Examiner UNITED STATES PATENTS 2,273,704 2/ 1942 Grisdale 252-504 2,443,073 6/1948 Knudsen 324- 2,588,564 3/1952 Pawlicki 324-105 2,744,981 5/1956 Spears 200-113 2,774,108 12/1956 Wyllie 264-128 X 2,796,505 6/1957 Bocciarelli 338-20 2,799,051 7/1957 Coler et a1 18-475 2,847,391 8/1958 Wheeler 260-41 2,978,665 4/1961 Vernet et al 338-223 3,008,949 11/1961 Langer et al. 260-949 3,055,843 9/1962 Muller et al. 252-511 3,056,750 10/1962 Pass 252-511 FOREIGN PATENTS 676,090 8/1957 Canada.

RICHARD M. WOOD, Primary Examiner. ANTHONY BARTIS, Examiner.

w. D. BROOKS, H. T. POWELL, G. GREENWALD,

Assistant Examiners.

Claims (1)

1. AN ELECTRICAL RESISTOR ELEMENT COMPRISING A RESISTOR PROVIDED WITH CONDUCTIVE CONNECTING TERMINALS FOR FORMING PORTIONS OF AN ELECTRICAL CIRCUIT, SAID RESISTOR COMPRISING FINELY DIVIDED CONDUCTIVE PARTICLES DISPERSED IN A MATRIX OF A RELATIVELY NON-CONDUCTIVE PLASTIC, SAID PLASTIC HAVING A COEFFICIENT OF THERMAL EXPANSION AT LEAST THREE TIMES GREATER THAN THAT OF SAID CONDUCTIVE PARTICLES, SAID CONDUCTIVE PARTICLES BEING IN SUBSTANTIAL PARTICLE TO PARTICLE CONTACT, SAID RESISTOR CONTAINING SUFFICIENT CONDUCTIVE PARTICLES AMOUNTING FROM ABOUT 25% TO ABOUT 75% OF THE TOTAL VOLUME OF THE RESISTOR MATERIAL AND DISPERSED THROUGH SAID PLASTIC MATRIX SO THAT BELOW A CRITICAL TEMPERATURE LEVEL IMMEDIATE PARTICLE TO PARTICLE CONTACT EXISTS BETWEEN THE CONDUCTIVE PARTICLES, WHEREAS ABOVE SAID TEMPERATURE LEVEL AND WHILE THE RESISTOR REMAINS A SOLID, SAID CONTACT BETWEEN CONDUCTIVE PARTICLES IS SUBSTANTIALLY BROKEN DUE TO THE DIFFERENCES IN THERMAL COEFFICIENT OF EXPANSION BETWEEN SAID PARTICLES AND SAID NON-CONDUCTIVE MATRIX, SAID CRITICAL TEMPERATURE BEING SUBSTANTIALLY ABOVE ROOM TEMPERATURE AND BELOW THE TEMPERATURES OF MELTING AND OF SUBSTANTIAL DETERIORATION OF THE PLASTIC, SAID RESISTOR BEING CHARACTERIZED BY A RELATIVELY FLAT CURVE OF ELECTRICAL RESISTANCE VERSUS TEMPERATURE BELOW SAID CRITICAL TEMPERATURE LEVEL AND BY A SHARPLY RISING CURVE BEGINNING AT SAID CRITICAL TEMPERATURE LEVEL, SAID RESISTOR FURTHER BEING CHARACTERIZED BY AN INCREASE IN RESISTIVITY OF MORE THAN 250% OVER A TEMPERATURE RANGE OF 25*F. AT ABOUT SAID CRITICAL TEMPERATURE LEVEL.

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