US3353135A - Polytetrafluoroethylene lubricant for carbon composition resistors - Google Patents

Description

Nov. 14, 1957 e. F. CH WICK 3 353,]35v POLYTETRAFLUOROETHY E LUBRICANT FOR I CARBON COMPOSITION ISTORS Filed June 14 65 NVENTo GEORGE F. CHADw BY MW ORNEY United States Patent 3,353,135 POLYTETRAFLUOROETHYLENE LUBRICANT FOR CARBON COMPOSITION RESISTORS George F. Chadwick, North Tonawanda, N.Y., assignor to Air Reduction Company, Incorporated, New York,

N.Y., a corporation of New York Filed June 14, 1965, Ser. No. 463,846 7 Claims. (Cl. 338226) ABSTRACT OF THE DISCLOSURE A carbon composition resistor including by weight 0.2% to 1% polytetrafluoroethylene interdispersed in the core and shell thereof. The additive cited increases heat stability and load life properties of the resistor and provides lubrication during resistor molding.

The present invention relates to electrical resistors and more particularly the invention relates to electrical resistor compositions having high heat stability and other advantageous properties.

Carbon composition resistors are widely used in a great number of industries, and particularly in the radio and television industries. Such resistors must be able to withstand high temperatures for long periods of time without a great variation in resistance values. To this end, efforts are constantly being :made by the resistor manufacturing industry to improve the operating characteristics of such resistors.

Prior art carbon composition resistor conductive cores generally consist of a nonconductive filler material, such as silica, a conductive carbonaceous material, such as carbon black, lampblack, graphite, etc., and an organic binder which may include coal pitch phenolic resins, silicone resin, acetone, etc. The proportions of the ingredients are varied to obtain the desired resistance values and electrical properties. An insulating shell usually en capsulates the conductive core, the shell generally comprising a binder of organic resins such as phenolic or silicone resins, and a filler material such as silica sand.

I have found that by the use of small percentages of polytetrafluoroethylene dispersed in such resistor cores and shells, I achieve a carbon composition resistor having increased heat stability and greatly extended load life.

It is an object of the present invention to provide an improved resistor composition. It is another object of the invention to provide an improved carbon composition resistor.

Another object of the invention is to provide a carbon composition resistor having superior heat stability and longer load life.

Other objects, features, and advantages of this invention will be readily apparent from consideration of the following specification relating to the annexed drawing in which:

FIG. 1 is a longitudinal cross-sectional view of the carbon composition resistor of this invention;

FIG. 2 is a plan view of the shell portion of the resistor of FIG. 1 prior to insertion of the core portion of the resistor into the shell; and

FIG. 3 is a plan view of the core portion of FIG. 1.

FIGURES 1-3 are representative showings of carbon composition resistors which may employ my novel resistor compositions. In the manufacture of a carbon composition resistor it is conventional to preform a carbon composition core portion such as shown at 1 in FIG. 3. The core is provided with terminal sockets as shown at 7. The core is then inserted in a preformed resistor shell such as shown at 2 in FIG. 2. Suitable electrical terminals are inserted into sockets 7 and the assembled shell and terminals are then pressed in a conventional manner,

either with or without the presence of heat to form a resistor as shown at 3 in FIG. 1. The completely formed resistor thus has terminals as shown at 4, a core portion as shown at 5, and a shell portion as shown at 6 in FIG. 2. The core is the conductive portion of the resistor and the shell serves as an insulating and protective nonconductive jacket. In some instances the shell may not be necessary or desired, in which case the resistor consists merely of the core portion and terminals of FIG. 1. A satisfactory method of producing the resistor of FIG. 1 is fully described in United States Patent No. 2,471,592. Other methods of manufacturing a carbon composition may also advantageously employ my novel carbon resistor core and/ or shell compositions.

A serious problem encountered by the use of prior art carbon composition resistors has been the inability of the resistor to maintain its original resistance value under high operating temperatures for long periods of time. I have discovered that the addition of 0.2% to 1% of micronized polytetrafluoroethylene to the resistor core mix and 0.5% to 2% micronized polytetrafluoroethylene added to the resistor shell mix greatly increases the heat stability and load life of carbon composition resistors.

It should be noted that all references to percentages in this application refer to percentages by weight of the total composition of the core or shell.

Amounts of polytetrafluoroethylene in substantial excess of 1% in the resistor core mix are detrimental to the resistor characteristics in that the RTC (resistance/ temperature characteristics) requirements of the resistor are exceeded. At the same time, an additive of greater than 2% polytetrafluoroethylene in the resistor shell mix does not appear to improve the heat stability and load life of the resistor.

It should be noted that although the percentages of polytetrafluoroethylene set forth hereinabove are the percentages added to the resistor core and shell mix respectively, the percentage of polytetrafluoroethylene is substantially the same in the finished resistor core and shell. The resistor loses only approximately 2% by weight in volatiles during the curing and other finishing processes and since the polytetrafluoroethylene is such a small percentage of the overall resistor formulation, the proportion of polytetrafluoroethylene to the total resistor weight does not substantially vary from the time it is added to the core and shell mix until such time as the resistor is finished. Therefore, the percentages set forth in this application are intended to refer to the proportion of polytetrafluoroethylene present in either the finished core or shell.

Table 1 compares the percentage change in the resistance of carbon composition resistors having centage of micronized polytetrafluoroethylene added to the resistor core and resistor shell mixes with the percentage change in the resistance of resistors having the same composition but without polytetrafluoroethylene. In the test, a number of identically manufactured resistors, except for the presence or absence of polytetrafluoroethylene as indicated, were tested for the specified time and temperature and theresistance change shown in the table is the average resistance change for the total number of resistors tested.

TABLE 1 Average resist. Average resist. Operating Operating change with change without Temp. C.) Time (i112) PIFE P'IFE (percent) (percent) 150 0. 61 +0.02 150 150 0. S7 +0.16 150 500 0. -4. 73 750 1. 33 I4. 02 I 150 1,000 l. 57 20. 94

*PTFEzpolytetrafinoroethylene.

a small per-.

2' 6 The composition of the core mix for the resistor with polytetrafiuoroethylene tested in Table 1 consists of the following formulation:

The core mix of the resistors without polytetrafluoroethylene was manufactured from the same composition as set forth above except that the polytetrafluoroethylene wasomitted.

The, shell mix'formulation for the resistors with polytetrafluoroethylene tested in Table 1 is as follows:

Base shell mix: Grams 330 lb. silica sand, 120 lb. phenolic resin, .5 lb.

silicone resins 1000 Polytetrafluoroethylene 10 Total 1010 The shell mix' used for the resistor without polytetrae fluoroethylene was simply the base shell mix:set'forth above.

It is apparent from Table 1 that resistor compositions not containing polytetrafluoroethylene dispersed therein are subjected to vastly greater resistance changes when tested at high; temperatures over a long period of time. It .is believed, but not certain, .that.the reason for the. improved heat stability of carbon composition resistors containing polytetrafluoroethylene lies in the slower'oxygen penetration into the. resistor core.

It should be noted that. the. particular formulations recited are conventional except for the polytetrafluoroethylene additive and are intended only to be exemplary ofcompositionsfor carbon composition resistors which may utilize polytetrafluoroethylene as an additive to increase load lifeand heat stability. Polytetrafluoroethylene may obviously be used to improve the operating properties of other carbon composition resistors when added to the resistor. formulations in the quantities. specified in this application.

In addition to the advantages given above, it is found that the addition of micronized polytetrafluoroethylene to the core or shell mix substantially lowers the ejection force required to remove the shell or core from its particular die or mold cavity. For example, Table 2 lists sixcarbon composition resistors'having different known internal lubricants, which lubricants are identified'in the table, and a seventh resistor having polytetrafluoroethylene in the mix. It can be seen from Table 2' that the ejection force requiredto eject the composition containing thepolytetrafluoroethylene is markedly reduced-when compared. to the ejection'pressure necessary to remove the resistor compositions containing other lubricants from the mold.

Table'2 Lubricant: Ejection force Carbowax 1100 Polyethylene 350 Calcium stearate 900' Lithium stearate 750 Aluminum stearate 1100 Zinc stearate 1200 Polytetrafiuoroethylene 70 Obviously many modifications and variations of the is therefore to be understood that within the scope of the appended claims, the invention may be practiced otherwise than as specifically disclosed.

I claim:

1. .A carbon composition resistor comprising a conductive core member having 0.2% to 1% by weight polytetrafiuoroethylene dispersed therein, electrical terminal means attached to said core member, and an insulating jacket surrounding said core member.

2. A carbon composition resistor comprising a conductive member having 0.2% to 1% by weight polytetrafluoroethylene dispersed therein, and electrical terminal means connected to said member.

3. A carbon composition resistor comprising aconductive core member having 0.2% to 1% by weight polytetrafluoroethylene dispersed therein, electrical terminal means connected to said core member, and an insulating jacket having 0.5 to 2%-by weight 'polytetrafluoroethylene dispersed therein .surroundingsaid core member.

4. A carbon composition resistor comprising a conductive core member, electrical terminal means connected to said core member, and an insulating,jacketsurrounding said core member, the composition of said conductive core member comprising a nonconductive filler material, a carbonaceous conducting material, an organic binder, and interdispersed quantities of polytetrafiuor-oethylene, the

total amount of polytetrafiuoroethylene present in said composition being between 0.2%. and 1%" by weight.

5. A'carbon composition'resistor comprising a conductive member and electrical terminal means connected to interdispersed quantities of polytetrafiuoroethylene, the. total amountof polytetrafiuoroethylene present insaid' composition being .between'0.2% and 1% by'weight, and said insulating'jacket having 0.5% to 2% by weight polytetrafluoroethylene dispersed therein.

7. .A carbon composition resistor comprising a conduc-q tive core member, electrical terminal meansconnectedto said core member, and an insulating jacket's'urounding said core member, the-composition of said conductive core member comprising a nonconductive filler materiaLacarbonaceous conducting material, an organic binder,and interdispersed quantities .of, polytetraflu'oroethyle'ne, the total amount of polytetrafluoroethylene presentinsaid composition being between 0.2% and.l% byweight, and

thecomposition of said. insulating jacket comprisingsilica:

sand, an organic binder, and interdispersed:quantities of polytetrafluoroethylene, the total amount of polytetrae fluoroethy'lene present in the composition of saidv insulating jacket beingbetween 0.2% and 1% by weight.

References Cited UNITED STATES PATENTS 2,744,988v 5/1956 Tierman 338-322 X 2,883,502 4/1959 Rudner 338330 3,037,266 6/1962 Pfister 338.-226 X.

RICHARD M. WOOD, Primary Examiner.

I. G. SMITH, Assistant. Examiner.

Claims (1)

1. A CARBON COMPOSITION RESISTOR COMPRISING A CONDUCTIVE CORE MEMBER HAVING 0.2% TO 1% BY WEIGHT POLYTETRAFLUOROETHYLENE DISPERSED THEREIN, ELECTRICAL TERMINAL MEANS ATTACHED TO SAID CORE MEMBER, AND AN INSULATING JACKET SURROUNDING SAID CORE MEMBER.

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