US2825702A - Heating elements in film form - Google Patents

Description

March 9 1953 H. I. SELVERSHER 2,$25,702

HEATQNG ELEMENTS IN FILM FORM Filed Sept. 3, 1953 HERMAN Z S/L l/EPSHE/Q- n e States Patent HEATING ELEMENTS IN FILM FORM Herman I. Silversher, Los Angeles, Calif., assignor to Electrofilm, Inc., Los Angeles, Calif., a corporation of California Application September 3, 1953, Serial No. 378,334

7 Claims. (Cl. 252-503) This invention has to do with improvements in heating elements or heating media, particularly of the film type comprising finely divided particles together having appropriate electrical conductive and resistive properties, held together in a binder having initially a fluid form permitting dispersion of the contained particles, and finally a solid condition fixing the particles in proper contact and orientation. In certain of its aspects, the present invention is an improvement upon the subject matter of a copending application Serial Number 243,733, filed August 25, 1951, by Ralph D. Hall, on Electrically Conductive Films, now Patent No. 2,679,569, issued May 25, 1954.

The objects and advantages of the invention will be understood to best advantage following a brief consideration of the general characteristics of this type of heating element. The latter comprises as its electrically conductive component finely divided particles of metal predominantly under 40 microns, and preferably under ice tion to non-conductivity a necessary combination of properties that will maintain the physical relation between the particles despite use of the film in wide temperature range atmospheres tending to contract or expand the film, or under conditions such that the film is otherwise subjected to deformation. Heretofore various kinds of binders generally classifiable as resins and elastomers, have been proposed and used in heating or resistance elements with varying degrees of success. Among these binders that harden to essentially non-elastomeric form,

- the property found to be most lacking and yet most eeded, is the capacity of the binder to completely or very closely restore to its initial dimension following deformation of an elongative or contractive nature, and to retain constancy of resistance after repeated wide temperature range cycling.

The present invention has for its primary object to provide in electrically energized heating films of the type described, a resinous binder which more completely approaches the ideal requirements for the binder medium than any materials heretofore used for this purpose. From extensive determinations I have found that the commercially known resins sold by Shell Chemical Corporation under the trade-name Epon resins, have properties that give unique superiority as binder and insulative film components in the present type of element. Primarily they are characterized in addition to their adhesive and resistance qualities, by the capacity given the film to restore dimensionally practically completely following temperature changes ranging e. g. from -70 to +350 F.

These Epon or epoxide resins appear to have the general formula:

10 microns, of which silver, aluminum, chromium, copper and nickel, and mixtures thereof, are typical. Extensive experience has indicated silver to be the preferred metal. In addition to their small size, these metal particles are further characterized by their being preferably in-flake form, the particular advantage of which is that in the heating film they assume contacting overlapping or shingle-like relation which stabilizes their contact and the conductivity of the film as a whole against such efiects as contraction, expansion or other deformation of the fihn that might otherwise tend to produce interruptions in the inter-particle contact. And as indicated, the capacity of the flakes for orientation in such relation is assured by the initial fluidity of the binder.

The electrically resistive component of the film is supplied by using a non-conductive binder and using also, if desired, in uniform mixture with the metal particles, finely divided particles which are non-conductive or less conductive than the metal particles. Such non-conductive or less conductive particles may include any of various metallic oxides, of which antimony oxide is typical, as well as carbon or graphite. it is preferred to use flake graphite which, like the metal particles gives best results in sizes below microns, and preferably with the particles predominantly below 10 microns. Any of various oxides may be used with the graphite. Antimony oxide is preferred because of its property of rendering the film fire resistant because of the flame snuffing tendency of the antimony oxide.

However effective may be the conductive and resistive properties of these particles as such, the efiiciency and sustained electrical and thermal qualities of the film composite depends upon the binder and its having in addi- The reactive epoxy and hydroxyl groups of these resins will esterify with organic acids under the influence of heat. Esters of varying properties are obtainable by reaction of compounds corresponding to the formula with long chain fatty acids, say within the 8 to 18 carbon range, obtained from such materials as castor, linseed, soy bean and coconut oils.

Departing for the moment from reference to the chemical nature of the resin compositions herein employed, and with reference to the physical form of the films used, the invention while being primarily concerned with the composition of the heating element film, also contemplates the use of a resinous film as an electrical insulative coating to be applied to the surface of the object or material to carry the heating film. By way of example, the latter may be used for the surface heating of various etallic or non-metallic airplane parts, e. g. instruments, purge tanks and the kile, as well as for such other purposes as the making of heat radiating panels and the like. Prior to application of the heating element film, I may coat the carrier surface, as by spray application, with a resinous composition to form what may be designated as a base insulative layer.

Broadly considered, the base layer, when used, may have any of various insulative compositions, as in the class of resins such as the alkyd resins. In some instances it may be found desirable to apply the heating film directly to a resistance or insulation base such for example as sheet asbestos. However, because of its various advantages, an epoxide-containing base film composition is preferred.

In compounding the base layer resin, 1 use an epoxide resin of the type formulated above, and which may be resin;

' pigments such as vermiculite, mica, antimony "oxide, etc;

' Satisfactory results have beenobtained by using as'tb'e epoxid'e component 'of the insulative base film resin com position, shelbChernica'l Corporations Epon ester YC3 or D-4 the former being understood to result from estei'ificationof about 75 Epon 1007 with about 25% coconutffatty acid. Resi'n D41 issimilar, being the product resulting from 'esterificjation of about "60% Epon 1004 a with about 40% 'dehydrate'd castor fatty acids.

I For details concerning Epon 1004, reference may be had to the Greenlee Patent No. 2,615,007 wherein the 7 process of making the ester is described, and the ester is particularly defined as the product resulfin'g from react- 7 ing' a mixture of p,p'dihydroxydiphenyldimethyl methane and epichlorhydrin, the molar ratio 'of .p,pdihydroxy- V diphenyldimethyl methane 'to epichlo'rhydrin in said mixturebeing between 1:12 and 1:15, with aqueous caustic soda sufiicient in amount to combine with the chlorine of the chlorhydrin, at. least part 'of the aqueous caustic soda being present in the initial mixture to convert the 1:,p7'dihydroxydiphenyldimethyl methane at least in part intoits sodium salt, the reaction being continued with heating for a s'uificient period of time to effect substanj tially complete reaction of the p,pdihydroxydiphenyldimethyl methane and epichlorhydrin to produce solid, water-insoluble resinous products having, after washing with water, softening points within the range of from about 84 C. to about 112 C. and with an epoxide equivalent within the range of from about 591 to about 1179.

Epon 1007 is described as to its preparation and composi-.

tion in the Greenlee Patent No. 2,615,008 and is particularly'defined therein as the product consisting essentially of a dihydric phenol free from reactive group other than phenolic hydroxyl groups, and a low melting point epox'ide resin resulting from the reaction with'heatin g of .amixtureof approximately 1 mol of dihydric' phenol free from reactive groups other than phenolic hydroxyl groups With from about 1.2 to about 2 mols of a chlorhydrin selected from the group which consists of epichlorhydrin 50 to 80 weight parts of Epon 1007 and between 20 to 50 parts of a formaldehyde 'resin'of the class previously designated, though preferably Beetle 227-8.

in substantial proportions a fusible urea formaldehyde or melamine modified urea resincontaining active hydrogen, and a complex epoxidelwhich is a polyether derivative 7 of a polyhydric phenol containing epoxide groupsand free 'from functional groups other than epo'xide and hyand glycerol 'dichlorhydrin and sufficient caustic alkali to combine withthe chlorine of the chiorhydrin, said low melting point resins being .polyethers of the dihydric phenols with the terminal groups of the .polyethers in- 'cluding terminal epoxide groups, said polyethers being free from functional groups other thanalcoholichydroxyl and epoxide groups, the proportion of dihydric phenol to epoxide resinbeing less than that corresponding'to the epoxide equivalent of the resin, whereby on heating the composition a higher molecular. weight and a higher meltingpoint epoxide resin is formed.

In compounding the insulative base film resin composition, the epoxide resin may be admixedin the stated 7 to 3 part ratiojwith desirably a urea or melamine formdroxyl -'gr ups, the proportions of complex iepoxideand 7 formaldehyde resin being between 5 to 9 (preferably about 7) parts by Weight of complex epoxide to from 2 to 5 (preferably about 3) parts by weight pf urea formaldehyde. For further details concerning resin com- 7 positions of this 'type, 'referen'ce may be had to Greenlee Patent No. 2,528,360.

Epon 1007 is characterized in Example 1 of -U. S. Patent 2,742,448 as a condensation product of a bisphenol and epichlorhydrin having an epoxy equivalent of 0.05.

Additional details as to Epon 100? are included in the publication by Shell Chemical Corporation entitled Epon Surface Coating Resins copyright 1948; For example, page f said publication mentions that Epon 1007 has a melting point of 125-135 C., a viscosity (Gardner) of X-Z, in solution in butyl jcar'bitol and an equiv alent weight of 188; I ,7 j

1 Beetle 227-8 ,is identified as urea-formaldehyde resin in the publication Annotated Comprehensive List of Trade Names vof synthetics}? Research Association of British Rubber Manufacturers, circular No. 372, page 26 (October, 1949).

As in forming the base or insulative layer, the heating element layer typically is formedand deposited by spray:

ing the resin mixture together with thel'later described solid particles .or pigments which give the desired electrical and heating properties, thinned to ajsprayable consistency using an appropriate solvent such as the solvent mixture used to thin the base layer forming resins. Typically,

the epoxide and formaldehyde resin mixture may be thinned with about equal proportions 'ofdioxane and toluene.

The heating element filrn thickness typically may vary fing' drawing showing in cross-section the film layers applied to an object or surface to be heated.

The body 10 may be a metallic or non-metallic part {such as an instrument housing, panel :or the like, onto which is sprayed or otherwise applied at uniform thickness :1 layer 11 ,previouslyreferred to as the insulation base layer. After application, this film may be heated to about 1'50" F. for around one-half hour to volatilize the solvent-and cause the resins to assume solid and stable form. The heating element layer 12 is then deposited on the base layer by spray applicatio'n'or ether means, and is then heated at around 350 F. for one-half hour so that the resins become completely polymerized coating 13'which preferably consists of Epon YC-3 i.-e.

aldehyde resin, such as American 'Cyanamid Companys .urea resin Beetle 2278, together with a solvent and thin 'nerwhich generally may be selected from individual "or -mi'xed known epoxide resin solvents, suchas methyl iso- "butyl carbinoi, methyl isobutyl ketone, toluene, xylene,

and dioxane. The resin mixture is thinned sufficiently-to permit even filming by spray application to the surface to "be coated. The thickness of the coating may vary 'as desired, typically between .0025 to .02 inch. The resin mixture may range between 90 to '50 Weight parts of the e'poxide resin to from 10 to parts of the formaldehyde for purposes of the binder component of the heating elementfilm, I preferably use amixture of between about around 75% Epon 1007 esterified with about-25% coconut fatty acid, this ester being mixed with 10% to 50% formaldehyde resin typically Beetle 227-8, and the mixture thinned with suitable solvent for spraying...

As willbe understood, electrical current may bep'as sed between conductors 0 applied to locations.

i Referring further ,to the compositionof the heating filrn 12, the latter contains a dispersion of itheffinely divided metal particles, preferably 'flake silver predomianately under 1.0 microns in size, in the range of about 3'to'80 parts of metal particlesto betwee'n'20. to 550 .par-ts (solid 'contentlafte'r evaporation of solvent) ofthcrfisihs.

Ordinarily it is preferred 10 use finely divided. resistance The re-. sulting resin may be defined as a, composition containing particles together with the conductive metal particles, and for this purpose, I prefer to use between about 3 to 65 parts of flake graphite predominately under microns in size.

The following examples are illustrative of typical heating film compositions embodying the invention and having the indicated resistance characteristics:

As previously indicated, extensive tests have shown that the resinous binders used have the highly important and distinctive advantage in heating film compositions, of restoration to an initial condition, after undergoing expansion or contraction in response to even wide tempera ture ranges. Tested for example as to its restorative ability after being subjected to temperatures between 70 F. to +350 F., the film was found to undergo well under 1% contractive or expansible departure from its dimension at the selected initial or reference temperature.

I claim:

1. A heating element composition comprising a mixture of finely divided electrically conductive metal particles and also graphite particles and a heat setting resinous binder consisting essentially of a formaldehyde resin of the class consisting of urea formaldehyde and melamine formaldehyde resins together with a complex epoxide which is a polyether derivative of polyhydric phenol containing epoxide groups and free from functional groups other than epoxide and hydroxyl groups, said composition containing said metal, total resins and graphite in the Weight proportion of about 3 to 80 parts metal particles, to 50 parts (solids basis) of resin 6 (the resin itself being composed of from about 5 to 9 parts of said epoxide to from about 2 to 5 parts of said formaldehyde resin), and 3 to parts of graphite.

2. A composition as defined by claim 1, in which said metal particles include finely divided silver.

3. A composition as defined by claim 1, deposited in thin hardened film form having a thickness between about 0.0025 and 0.02 inch.

4. A composition as defined by claim 1, deposited in thin hardened film form having a thickness between about 0.0025 and 0.02 inch, and in which the metal and graphite particles are predominately under 10 microns in size.

5. A composition as defined by claim 1, deposited in thin hardened film form on an electrically insulative resinous base layer.

6. A composition as defined by claim 1, deposited in thin hardened film form on an electrically insulative base layer composed of a mixture of said epoxide and formaldehyde resin.

7. A composition as defined by claim 1, deposited as a thin hardened conductive layer on an electrically insulative resinous base layer, and a thin resinous coating applied to the surface of said conductive layer.

References Cited in the file of this patent UNITED STATES PATENTS 1,872,581 Haroldson Aug. 16, 1932 2,177,484 Fruth Oct. 24, 1939 2,386,095 Edgar et al. Oct. 2, 1945 2,472,801 Barfield June 14, 1949 2,528,360 Greenlee Oct. 31, 1950 2,559,077 Johnson et al. July 3, 1951 2,679,569 Hall May 25, 1954 2,692,321 licks Oct. 19, 1954 FOREIGN PATENTS 490,232 Great Britain Aug. 11, 1938 OTHER REFERENCES Printed Circuits, National Bureau of Standard Circular 468, pages 6, 7, 8, l6 and 18.

Electrical Mfg. July 1949, pages 78-81, Paint, Oil and Chemical Review, November 20, 1952, pages 24-26.

Claims (1)

1. A HEATING ELEMENT COMPOSITION COMPRISING A MIXTURE OF FINELY DIVIDED ELECTRICALLY CONDUCTIVE METAL PARTICLES AND ALSO GRAPHITE PARTICLES AND A HEAT SETTING RESINOUS BINDER CONSISTING ESSENTIALLY OF A FORMALDEHYDE RESIN OF THE CLASS CONSISTING OF UREA FORMALDEHYDE AND MELAMINE FORMALDEHYDE RESINS TOGETHER WITH A COMPLEX EPOXIDE WHICH IS A POLYETHER DERIVATIVE OF POLYHYDRIC PHENOL CONTAINING EPOXIDE GROUPS AND FREE FROM FUNCTIONAL GROUPS OTHER THAN EPOXIDE AND HYDROXYL GROUPS, SAID COMPOSITION CONTAINING SAID METAL, TOTAL RESINS AND GRAPHITE IN THE WEIGHT PROPORTION OF ABOUT 3 TO 80 PARTS METAL PARTICLES, 20 TO 50 PARTS (SOLIDS BASIS) OF RESIN (THE RESIN ITSELF BEING COMPOSED OF FROM ABOUT 5 TO 9 PARTS OF SAID EPOXIDE TO FROM ABOUT 2 TO 5 PARTS OF SAID FORMALDEHYDE RESIN), AND 3 TO 65 PARTS OF GRAPHITE.

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