US2878353A - Electrical resistors - Google Patents

Description

March 17, 1959 B. E. ELY, JR, EI'AL 2,

ELECTRICALRESISTORS Filed Dec. 16, 1954 I7 ZET 111;: z?

FIG. 3

I} El-T 3 4 3 INVENTORS BERTEN E. EL Y, JR.

MM ATTORNEY I United States Patent i ELECTRICAL RESISTORS Berten E. Ely, Jr., Thornton, Ernest Charles Grabill, Wilmington, and Joseph Ross Perkins, Jr., Newark, Del., assignors to E. I. du Pont de Nemours and Company, Wilmington, Del'., a corporation of Delaware Application December 16, 1954, Serial No. 475,698

1 Claim. (Cl. 20163) This invention relates to improved resistors for electric circuits and more particularly to electric resistors made with tetrafluoroethylene resin and the process whereby such resistors are made.

-Polytetrafluoroethylene was first obtained as a solid granular product by processes as described in U. S. Patents 2,230,654, 2,393,967 and 2,394,243. In more recent developments it was shown that concentrated aqueous dispersions of polytetrafluoroethylene could be obtained by polymerizing tetrafluoroethylene in an aqueous mediu'm'in the presence of certain surface active agents as described in U. S. Patents 2,534,058 and 2,559,752. The outstanding resistance of polytetrafiuoroethylene to chemical attack and high temperatures has led to many mechanical applications of the polymer. Solid fillers such as asbestos, silica and coke flour are often added to increase rigidity and compressive strength and decrease creep of molded articles of polytetrafluoroethylene as described in U. S. Patents 2,592,147 and 2,400,099 among others. The outstanding electrical properties of the polymer have led to many applications of polytetrafluoroethylene as an insulator. The use of certain carbon filled compositions of polytetrafluoroethylene in electrical resistors, however, has not been developed heretofore.

There are numerous applications for electric resistors in electric and electronic circuits in the field of communications and other industrial fields. Resistors are,,in general, resistance wires wrapped on ceramic coils and insulated with phenolic resins, phenolic resins compounded with a conductor, and the like. These resistors however have certain disadvantages. The resins in the resistors often are not useful at high temperatures in that the resin decomposes, when high electric loads giving rise to high temperatures are applied. These resins are also often subject to attack by chemicals and moisture. Re-

sistors-made by prior art processes are often too large I and rigid. They will break under impact and cannot be bent.

It is therefore an object of this invention to prepare novel, high-;use-temperature resistors. A further object is to produce resistors from carbon filled compositions of polytetrafiuoroethylene. Another object of this invention is the preparation of electric resistors by simple and inexpensive methods. Other objects will become apparent hereinafter,

.The objects of this invention are accomplished by admixingfrom 2 to 15%, by weight of the polymer, of colloidal carbon with an aqueous dispersion of polytetrafluoroethylene until a uniform mixture is obtained, co agulating and drying the polymer, extruding the resulting curd-like compositions into the desired shape, cutting the continuous extrudate, inserting formed leads into the resistor and 'sintering at a temperature above 327 C. in amold to fuse the polytetrafiuoroethylene and to imbed the lead wires hermetically in the resistor. In a restricted embodiment, this invention resides-in obtainin'ghhtiiogeiieous mixturesofipolytetrafluoroethylene and H 2,878,353 Patented Mar. 17, 1959 carbon useful in resistor applications and in a method of attaching lead wires to polytetrafluoroethylene resistors to give outstanding electric contacts.

It has now been discovered that certain compositions of, polytetrafluoroethylene and carbon will give outstanding resistors. Such compositions are obtained when the carbon content is within 2 to. 15% of the polytetrafiuoroethylene and when the carbon is evenly dispersed throughout the polymer. Polymers having less than 2% carbon are too high in resistance to be useful and are considered insulators, polymers having more than 15% carbon are too low in resistance and could be classified as conductors. In the range of 2 to 15% of carbon the resistance of carbon polytetrafluoroethylene compositions increases almost exponentially as the carbon content is decreased; thus by varying the carbon composition any desired resistance for a standard shape can be obtained. Polytetrafluoroethylene carbon compositions, however, are useful only if the carbon is homogeneously distributed throughout the polymer. inhomogeneities will result in the formation of hot spots causing degradation and carbonization of the polymer and thus permit the current to burn a path through the resistor, thereby lowering the resistance and making the resistor useless. By dispersing colloidal carbon in aqueous dispersions of polytetrafluoroethylene, a homogeneous mixture is obtained, and resistors made from such compositions do not vary in their resistance when used in one circuit.

A factor of great importance in the manufacture of electric resistors from compounded materials is the contactbetween the resistor and the circuit, for which lead wires are generally used. If the contact is not steady but changes, undesirable electric disturbances, often referred to as noises, occur in the electric circuit. It has been found that good contacts between the polytetrafluoroethylene resistor and the terminal wires can be established if the resistors are prepared by a technique which will give high shrinkage on sintering the polymer so that the electric resistorcan be shrunk on to the terminal wires and thus give firmly bonded, low noise electric contacts between the polymer and the metal lead wire. This necessary shrinkage occurs when polytetrafiuoroethylene admixed with a lubricant is extruded below the sintering temperature of the polymer. The .extrusion of the polymer will cause an elongation as well as an interlacing of the polymer. In the subsequent sintering operation the polymer will expand slightly on heating up and then shrink up to 20% when cooled down. By

using a mold and restricting the shrinkage, lead wires canthus be firmly imbedded in the resistor. The polymer -is first expandedaround the lead wire and then shrunk persingiagent and then admixing i 'PW 31 9519 593 is causing a hermetic seal.

The electrical resistors of this invention are prepared from aqueous dispersions of polytetrafiuoroethylene con-I tainin'gfromlO to 60% of solid polytetrafiuoroethylene having an average particle size of 0.2 micron. The 'aque ous dispersions are obtained by processes such as. de-- scribed in U. 8. Patents 2,559,752 and 2,534,058. These aqueous dispersions are then stabilized against mechanical .agitation by the addition of some Triton X.100,

finely divided carbon, preferably calcined gas black, isv added and the mixture stirred until a homogeneous distribution has been achieved. An alternate method for.

obtaining homogeneous distributions of the polymer and the carbon black consists of dispersing the carbon .black separately in an aqueous medium with the aid' of 3-,,dlS-

carbon black and polymer. The mixture is coagulated and the wet, curd-like composition is dried and lubricated as described in U. S. Patent 2,586,357. The resulting paste is extruded at room temperature into a beading or a ribbon or any other desirable shape. The continuous extrudate is cut into the desired length and holes are made into each end. Into the holes, formed or straight lead wires are forced. Formed lead wires used in this invention are Wires having some shape at the end, such as a mushroom or a knot or loop, that will anchor the lead wire in the polymer. The lubricant is then removed by evaporation and the resistor is placed in a mold which will permit shrinkage but resist transverse expansion. The mold and the resistor are then heated at a temperature above 327 C. to cause sintering of the polymer and the hermetic sealing of the lead wires. A void-free resistor of constant resistance with good contacts to the lead wire is thus obtained. Under certain conditions, particularly where the resistor is of a massive, self-supporting structure, it is not necessary to use a mold for sintering the resistor and hermetically imbedding the lead wires, as little or no deformation with the exception of shrinkage is observed.

The attached drawings show two resistors made by the process of this invention. The first resistor illustrated was made from extruded beading of the resistor composition Figure 1, the second from extruded tape Figure 2. The terminals (1; 3) are imbedded and sealed into the body of the resistor (2; 4) by the process described hereinabove. Two types of formed lead wires are illustrated in the drawings.

The homogeneity necessary for compositions of polytetrafiuoroethylene and carbon used in the process of this invention is obtained by using aqueous dispersions of polytetrafluoroethylene admixing them with colloidal carbon such as calcined gas black in the presence of a surface active agent and then coagulating the mixture. Other conductors such as metals and other forms of polytetrafluoroethylene such as granular polymer, although useful in resistors do not give the same degree of homogeneity required in electric resistors in which the resistance are to be constant. It is believed that this is due to the larger particle size of the metals and the granular polymer even when finely divided.

Polytetrafluoroethylene resistors are, however, not limited to the method of obtaining sound terminals described hereinabove. Other methods such .as a graded conductor-metal terminal may be used, terminals may be clamped on to the polymer, or may be cemented to the polymer. These methods of attaching terminals to the body of the resistor however are believed to be not as convenient and efiicient as the method described herein. If, however, such methods are used it is quite possible to obtain the resistors by compression molding techniques known to those skilled in the art, instead of the extrusion used in the process of this invention.

The process of this invention is further illustrated by the following example in which all parts are by weight.

Example.-To an aqueous dispersion containing 60% by Weight of colloidal polytetrafiuoroethylene and 6% by weight of Triton X-100 was added under agitation, 3%, by weight of the polytetrafluoroethylene, of calcined gas black. The resulting mixture was agitated by a paddle stirrer at 150 R. P. M. until all the carbon had been dispersed. To this mixture was added under agitation sufiicient acetone to cause coagulation of the dispersion. The resulting curd-like coagulated composition was dried at 150 C. to remove the water remaining in the coagulated material. To this dried mixture Was added 20% by weight of polymer of n-decane as a lubricant and the resulting mixture was rolled in a container for a period of 20 minutes, which caused the lubricant to be uniformly distributed. This lubricated paste was then extruded through a tapered die having a /s" diameter orifice at room temperature using a rampressure of '2000 I ever, is not limited to this size or shape.

. 4 p. s. i. The extruded beading was cut into inch lengths. The thus formed unsintered resistor was inserted into a 1 inch long glass tube. Small holes were drilled into the ends of the resistor and upturned lead wires were forced into the holes. The assembly was then placed into an oven at a temperature of C. to remove the lubricant and then in an oven at 360 C. for a period of 30 minutes, to fuse the resistor. Upon removing the sintered polymer, the lead wires were firmly imbedded in the resistor. This operation was repeated for a series of compositions varying only in carbon content. The following results were obtained:

The invention has been described in the example as applied to 4; diameter beading. The invention, how- Depending on the nature of the electric current and the nature of the circuit the size and shape of the resistor may be varied. Thus thicker beading may be extruded. The cross-section of the resistor may assume that of a circle, a square, an oval, a fiat ribbon or any desired shape that is extrudable. The length of the resistor may also be varied for specific purposes. The resistances obtained from such variations are of course subject to the laws of electricity. The resistors may further be enclosed in insulators such as unfilled polytetrafiuoroethylene tubing or film.

From the experimental data presented hereinabove a rapid change of resistance with increase in carbon content is observed in the resistor compositions of this invention. This steep change of resistance may be levelled out by the addition of a semiconductor to the filled composition in the same manner that the carbon was added. Semiconductors useful for such purposes are ferrites, barium titanates, and the like. Large quantities of such semiconductors should be avoided as they may affect the fabrication of the filled composition, give rise to Weak and brittle compositions and may lead to gross inhomogeneities in the composition.

The advantages gained by using polytetrafluoroethylene resistors are due to the nature of the polymer. Polytetrafiuoroethylene resistors are insensitive to all common chemicals. The resistors are flameproof and can be used at temperatures up to 250 C. for continuous operation and higher for short periods. Since the nature of resistors is such that electrical energy is dissipated as heat energy, polytetrafiuoroethylene is well suited for resistors as the polymer is not changed by prolonged heating. Furthermore the resistors of this invention are tough and resilient, they can therefore be flexed, bent, and can sustain high impacts Without breaking. In addition they can be made in very small sizes for all ranges of resistance and are therefore extremely useful in miniature electric and electronic circuits. methods can be used for the fabrication of polytetrafluoroethylene resistors. The resistors of this invention are especially useful in electric and electronic circuits.

We claim:

Simple and economic 6 MerencesCiMinthofileofthispatent UNITED STATES PATENTS Haberberger May 12, 1942 Joyce Jan. 8, 1946 Barfield et a1 June 14, 1949 Llewellyn Dec. 11, 1951 Howatt July 26, 1955 Walker July 3, 1956

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