US 2679569 A
Description (OCR text may contain errors)
May 25, 1954 R. D. HALL ELECTRICALLY coNDucTIvE FILM Filed Aug. 25, 1951 BAUJHD. HALL,
Patented May 25, 1954 UNITED STATES PATENT OFFICE ELECTRICALLY CONDUCTIVE FILM Ralph D. Hall, Pacoma, Calif., assignor to Electroflm Corporation, Los Angeles, Calif., a corporation of California Application August 25, 1951, Serial No; 243,733
This invention relates to improved electrical resistorv elements of a type comprising a thin electrically conductive film or coating applied to the surface oi a base or carrier member. Conductive films formed in accordance with the invention are adapted for a Wide variety of heating and'electricalresistance uses, as for instance for heating the surface of an aircraft part to preventits icing, or for radiating heat from a wall to'which the iilm is applied to warm a room.
vThe resistor iilms of the present invention include an electrically insulative binder material, within which is distributed a preferably finely divided conductive material of a nature to give the composite nlm the desired resistance characteristics. In use, electricity is supplied to the film through a `pair of electrodes or terminals electrically connected to the lm at spaced locations.
A particular difficulty encountered in many prior iilms of the above general type has been their tendency to badly overheat or burn due to `changes in their electrical resistances. One irnportant object of the present invention lis to provide'a lm so constituted as to prevent such excessive overheating' or burning. This result may be 'achieved in large part by predetermination of the temperature coefcient of resistance of the l'm, and specifically by so forming the lm that its coeicient has a positive value over the usable temperature range. When the lrn is given such a positive temperature coefcient, its resistance increases with a rise in temperature, so that the lm 'itself acm to limit accumulative current increases, and prevent the cumulative heating which characterizes a'negative coefficient. Best operating results have-been achieved when the positive temperature coefficient is of a value below about -l-.004 per degree C.
For purposes of deniteness, the term temperature coefficient of resistance, as used in this application', is defined as the value of aref in the following formula:
R=Rref l-I-areft) where Rt=resistance at tc Rief=0riginal resistance at a (diferent) reference temperature, and
aref-:TI C. R. at the reference temperature (for instance 0 C.)
To attain a composite nlm having both a predeterminedresistance value vand a predetermined temperature coeiiicient, two or more different conductive materials are distributed through the' binder in proportions to produce the desired characteristics. In accordance with the invention, one of these conductive materials is a metal, normally having a positive coefiicient, while a second of the materials may have a lower or negative coeiiicient partially counteracting the positive coeiiicient of the metal. As the metal, I may typically use elementall silver, nickel, or zinc, or a metallic alloy, such as stainless steel. For best results, the second material should be carbon, preferably in the form of finely divided graphite having a negative coefcient.
In many instances, it has been found that incorporation of only the above materials in a kbinder cannot produce a film of a particular resistance without at the same time giving to the film a negative temperature coefficient. For this reason, I often find it desirable to distribute within the binder an additional resistance material adapted to increase the resistance of the composite lm whileV maintaining its positive coefiicient. This third material may be characterized broadly ashavinga greater resistance than the composite lm;` and usually itself has a negative coeicient, thoughnot of a value to give a negative coefficient to the film. Good results have been achieved by using as the resistance materiall a metallic oxide, such as antimony oxide, titanium dioxide, Zinc oxide, chromic oxide or ierric oxide. Of the various oxides, antimony oxide and titanium dioxide have proven most desirable for' the purpose.
Great difficulty has been encountered in previously proposed iilms of the present type by reason of their very unpredictable; unstable, and non-uniform characteristics. Diierent films made of exactlythe same constituents in exactly the same proportions, butvaryingin the physical relation of thecomponents, have often had widely differing electrical resistances, and as a result it has generally been considered impossible to accurately predetermine the resistance ofv a nlm. In accordance lwith the present invention, I am able to overcome these disadvantages of prior expedients, and to produce elements whose resista'nce characteristicsmay be very closely predicted, so that" an element may be predesigned for any desired use.' This predeterminability of the film resistance, and an accompanying uniformityv and stability of resistance characteristics, are all achieved`in large part by special formation of the various conductive and resistance particles in the lm. For this` purpose, it has been found'desirable that the particles of either the-metalor carbon, andpreferably both,
be in flake form. As will be appreciated, such ake formation of these materials maximizes the likelihood that each individual particle will form an eifective electrical contact with adjacent particles, to provide a uniform and continuous electrical circuit across the entire extent of the film. In addition to improving the resistance characteristics of the nlm, the use of flaky particles serves to strengthen and integrate the nlm, by reason of the intertting (i. e. overlapping or shingle-like) relationship of adjacent flakes, in a manner preventing cracking of the film and resultant breaking of the electrical circuit upon changes in nlm temperature.
The predeterminablity and uniformity of nlm resistance has been further enhanced by forming the particles of some or all of the binder carried materials of such extremely small sizes as to cause their virtually completely uniform dispersion throughout the entire film. For this purpose, it is desirable that the carbon particles have maximum dimensions not greater than about microns, and preferably between about 1 and 5 microns; and that the metal and metallic oxide particles have maximum dimensions not greater than about 40, and preferably less than about microns.
Many of the insulative binder and some of the other materials used in the present films are of an inflammable nature, and it is consequently desirable to provide some means for preventing their burning. For this purpose, I may incorporate in the film composition a fire retardant material, preferably in finely divided uniformly distributed'condition, of a type not chemically reactive with the other film constituents, and adapted to remain stable under normal operating conditions and temperatures, but to produce a name quenching gas when heated to a temperature above about 600 as by burning of the binder. I may employ for this purpose a chlorinated hydrocarbon plasticizer, such as the chlorinated polyphenyl sold by Monsanto Chemical Co., of St. Louis, Mo., under the trade name Aroclor, adapted to release flame smothering chlorine gas when heated above 600 F. Other satisfactory fire retardants include the chlorinated hydrocarbons sold by Monsanto Plastics Co. of St. Louis, Mo., under the trade names Santisizer 141 and Santisizer 160. when the metallic oxide is employed as the resistance material in the present films, it also serves to a certain extent as a fire retarding element for aiding in preventing burning of the lm.
The nonconductive binder may be of any of i" various known materials adapted to withstand the particular conditions to which the film is to be subjected. Where the film is to be applied to a substantially rigid member, the binder may be a suitable resin, preferably a thermosetting alkyd, phenolic, furfural, vinyl, or silicon resin. If it is desired to apply the conductive nlm to a flexible member, such as a tape, the binder may be a suitable elastomer, such as Buna N, neoprene, natural rubber, or a silicone rubber. Best results have been attained by using between about to 110 parts binder by weight to 100 parts by weight of the materials distributed within the binder. In order to prevent the binder from reacting chemically with the metal particles and oxidizing, it is desirable that the binder have a pH between about 6 and 8, or in other words be approximately neutral.
The film composition may be applied to a surface by spraying, knife spreading, or any other It is noted that l kconvenient method. Ordinarily, the composition is first thinned to a spraying or spreading consistency by the addition of a suitable solvent. For this purpose, I may employ any of the conventional solvents for the particular binder being used, as for instance a hydrocarbon, alcohol, ketone, ester or ether solvent. After application, the composition is heated to a temperature sufficient to drive off the solvent and cure the resin or other binder. The conductive nlm exhibits best operating characteristics when of a thickness between about .001 and .025, and is preferably about .'006. A film less than .001 thick has a less uniform electrical resistance than a film within the dened range, and when the thickness is greater than .025", the likelihood of the lms cracking and thus breaking the electrical circuit increases. However, the unit resistance (ohm/Sq.) is governed by the constituents and not the element thickness.
As a first example of the present invention, a very stable and uniform resistance film has been formed from the following constituents, mixed in the designated proportions by weight: 27.5 parts amorphous graphite, maximum particle dimension 10 microns, purchased from Joseph Dixon & Co., Jersey City, N. J., under the manufacturers number 200-18; 27.5 parts antimony oxide; 45 parts ake silver, maximum flake dimension 40 microns, sold by Metals Disintegrating Company, Elizabeth, New Jersey, under the product No. MD585; 8 parts fire retardant chlorinated hydrovarbon plasticizer, sold by Monsanto Chemical Company, St. Louis, Mo., under the trade name Aroclor 1254; and parts alkyd resin. These ingredients were intimately mixed together and thinned to spraying consistency with Xylene solvent, after which they were sprayed onto an insulating base film of the alkyd resin. A conductive film of the above composition .006" thick had a uniform electrical resistance of approximately 3 ohms per square. In this connection, it is noted that where the term ohms per square is used in this application, I refer to the resistance of a square element of any size, the resistance being the same regardless of the dimensions of the square. The individual temperature coefficients of resistance of the silver and graphite particles used in the above example were +006 and .004 per degree C. respectively, and the coefcient of the composite film was -[-.00098 per degree C.
In a second example of the invention, the above constituents were mixed in the following proportions by weight: 30 parts amorphous graphite; 30 parts antimony oxide; 40 parts silver; 8 parts Aroclor 1254; and 80 parts resin.
A third typical formula was of the following composition, proportions again being taken by weight: 11.5 parts nickel flake, maximum flake dimension 40 microns, sold by Metals Dsintegrating Company, Elizabeth, New Jersey, under the product No. MD750; 68.6 parts Merrillite zinc precipitate, maximum particle size 40 microns, sold by Alloys Division Metals Disintegrating Company, Berkeley, California; l0 parts amorphous graphite, maximum particle dimension 10 microns; 80 parts alkyd resin; and 4 parts Aroclor 1254. The electrical resistance of a .006 nlm of this last composition was approximately 12 ohms per square. The temperature coefficients of resistance of the nickel, zinc and graphite were +0003, +005 and .004 per degree C. respectively, and the coefficient of the composite lm Was -l.00013/ C.
ber c'arryinga conductvelm of the present ine vention` is itself of an electrically conductive nature. an insulative'layer may be applied to the member. before application of. the conductive film.. Certain particular features of the inventionhave to do with improved means for assuring an adequate and continuous bond between such an insulative base layer and the resistance film. Specifically, the effectiveness of the bond may be maximized by (l) forming the base layer andconductive film to have approximately equal coefficients of expansion, and (2) applying the lms in a manner such that some of the particles in the conductive film diffuse into the insulative film, to reinforce the inter-layer bond. This last feature of the invention and certain others, will be better understood from the following detailed description of the typical embodiments illustrated in the accompanying drawing, in which:
Fig. l is a fragmentary perspective view of a heating or resistance film embodying the invention and shown applied to a rigid base member;
Fig. 2 is an enlarged fragmentary sectional view taken on line 2 2 of Fig. 1;
Fig. 3 is an enlarged fragmentary sectional View on the scale of Fig. 2, and showing a preferred method for reducing the thickness of the applied film to a desired and uniform dimension; and
Fig. 4 shows a exible nlm applied to a flexible woven fabric.
Referring rst to Figs. 1 and 2, I have typically represented at I a rigid base member to which an insulating film II and the heating or resistance film i2 are applied. The base member may be any member whose surface is to be heated, or across whose surface electricity is to be conducted, as for instance an aircraft part to be maintained against icing, or the wall of a room to be heated.
The electrically insulative base layer or film Il is first applied to the surface of the member I0, and then separately thermally cured. The insulative layer may typically comprise an electrically non-conductive binder material containing high dielectric particles, such as mica or vermiculite, distributed throughout the binder. The binder of film II may be any of the binders listed above as usable in the conductive film l2, and in order to assure a permanent bond between films I i and l2, is preferably so chosen as to give the tivo films approximately the same coefficient of expansion. In many instances, the same b-inder is used for both films. formula for the insulative film includes 60% binder, 20% mica and 20% vermiculite.
After the insulative layer II has been applied and cured, the conductive film I2 is sprayed onto, or otherwise applied to, the outer surface of layer II. The binder in the conductive film is then thermally cured, to simultaneously integrate the constituents of the film, and bond it to the base layer. The heat of curing the binder in conductive film I2 softens insulative layer II to an extent such that some of the particles I3 migrate downwardly into an upper portion of layer II at I 4, to positively assure a tight mechanical bond between two films.
For passing electrically through the film, a pair of elongated electrodes I 5 and I t may be fastened to the surface of the conductive film at a pair of spaced locations. These electrodes may be attached to the film in any suitable manner, as by bolts I1 passing upwardly through member A typical f I Ufandthe htwo lms;A The upper ends of bolts 4I'I f' methods sometimes do not produce nlms of sufficientlyaccurate and uniform thickness to assure predetermined'resistance characteristics.` Consequently, after the uncured composition for one of the films has been applied, it is sometimes desirable to smooth the film to a desired thickness in the manner represented in Fig. 3. Specifically, an elongated knife I9 extending across the lm be moved along the upper side of the ilm in predetermined spaced relation to base member Ill, to level oi the entire film to a predetermined desired thickness.
Fig. 4 represents the manner in which a conductive film 20 embodying the invention may be applied to a flexible fabric or tape 2l, instead of the rigid base member of Fig. l. In order to permit flexure of the film with the tape, the film binder may be a suitable elastomer in this instance, such as Buna-N, neoprene, natural rubber, or a silicone rubber. The tape may have an adhesive material 22 at its side opposite the film 2B, to facilitate application of the tape to a carrier surface.
1. A heating element comprising a base material carrying a hardened and dimensionally stabilized thin electrically conductive film having resistive heating properties and comprising an initially iiuid binder cured to permanently solid non-conductive form and containing uniformly distributed solid conductive flaky metal particles having relatively higher temperature coiiicient of resistance admixed uniformly with resistive flaky graphite particles having lower temperature coefficient of resistance, said metal particles being predominantly under 40 microns in size and the graphite particles being predominantly under l0 microns in size, and said film resulting from curing a quantity of the binder with the flaky metal and graphite particles in intertting relation therein sufficient to permanently so maintain the flakes in the binder.
2. A heating element as claimed in claim 1, in which the majority of said metal particles are under l5 microns in size.4
3. A heating element as claimed in claim l, in which said film has a positive temperature coefficient of resistance below about +0.50@
4. A heating element as claimed in claim l, including finely divided metal oxide uniformly distributed within the film and having a lower coefficient of resistance than said metal particles.
5. A heating element as claimed in claim l, including antimony oxide uniformly distributed within the film.
6. A. heating element as claimed in claim l, in which said metal particles are flake silver.
7. A heating element as dened in claim l, in which said binder is a thermosetting resinous plastic material and said metal particles are elemental silver.
8. A heating element as claimed in claim. 1, including a chlorinated hydrocarbon fire retardant uniformly distributed within the film.
9. A heating element as claimed in claim 1, in which said binder is a silicone rubber.
10. A heating element as claimed in claim 1, in which said metal particles are silver particles under 15 microns in size, said binder is a thermosetting resinous material, and said lm contains a metallic oxide uniformly distributed therein.
References Cited in the file of this patent UNITED STATES PATENTS Nurnber Name Date Sklar Mar. 19, 1935 Podolsky Nov. 10, 1936 Ruben June 24, 1941 Kappeler Nov. 10, 1942 Plass et a1 Mar. 16, 1948 Haas June 8, 1948 Sussenbach Nov. 2, 1948 Drugmand Aug. 23, 1949 1 Warrick Jan. 17, 1950 Number Number Name Date Podolsky June 17, 1950 Bain Nov. 14, 1950 FOREIGN PATENTS Country Date Great Britain Feb. 7, 1921 Great Britain Aug. 11, 1938 OTHER REFERENCES Nat. Bureau of Std. Circular 468, Nov. 15, 1947,
Zimmerman & Lavine, Handbook of Material Industrial Research Service,