US2893900A - Process of condensing polytetrafluoroethylene vapors onto a substrate and sintering the condensate - Google Patents

Description

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C OLD MWF INVENTOR. fuss/v4' 3 MAcm/N 2,893,900 Patented July 7, 1959 PRUCESS F CONDENSEG POLYT i f )ETI-HELENE VAPORS GNT() A AND SiNTERlNG THE CONDENSATE My invention relates generally to polymeric-coated articles and more particularly to improved techniques for producing sintered polytetraiiuoroethylene films and coatings on solid surfaces.

Poly-tetrafluoroethylene is a polymeric thermoplastic material whose exceptional stability with respect to light and thermal radiation, solvents, electrical stresses and chemical attack renders it highly desirable in many important applications. The material which is also known by the trademark Teflon is inert to all corrosive agents including aqua regia and no solvent will dissolve or even swell it. A metallic surface coated with this polymer is protected from corrosion and chemical attack. lts unusual dielectric strength is of great value when polytetraliuoroethylene is coated on wire or other current conveying elements. Polytetrauoroethylene has exceedingly `low adherence properties which is of great advantage as a coating in resisting the sticking of other materials thereto. Moreover, its very low coefficient of friction in the dry state approaches the characteristics of oil, thereby obviating the need for a lubricant when polytetrafluoroethylene is coated on wearing surfaces.

Tetrafluoroethylene is made by converting chloroform to diiiuorochloromethane, two molecules of which are pyrolized to form tetrafluoroethylene. Polymerization is effected by peroxide catalysts, the solid polymer being formed directly from the gaseous monomer.

In existing coating techniques, concentrated aqueous colloidal dispersions of polytetraiiuoroethylene are applied to a substrate by spraying, dipping or brushing methods and sintered to form an adhering film. But in order to obtain eifective adhesion with known techniques, it is essential that the substrate rst be roughened before applying7 the coating.

Thus in the New Products Bulletin No. l, th ed., published by the Fabrics and Finishes Dept., Finishes Div., E. I. du Pont de Nemours & Co., it is stated that nickel plate must iirst be scuffed with fine emery paper before finishing with Teflon and glass must first be sandblasted to afford a surface to which Teflon will adhere. Similarly in the article Fluorocarbon Dispersion Coatings appearing in the November 1955 issue of Materials and Methods, p. 93, it is stated that all surfaces to be coated must be -roughened to provide a good tooth for the coating.

Another and more serious drawback is that the resul-tant iilrn is relatively thick and porous and fails to afford the desired measure of protection. Indeed, in order to obtain protection for the underlying substrate it is recommended in the Du Pont reference cited above that for best corrosion resistance, priming coats should be limited to about 0.3 mil, the use of intermediate coats should be avoided, and 4 to 6 coats of Teonf approximately one mil each, should be applied. This requires a minimum thickness of 4.3 mils of expensive polytetrauoroethylene. Thus it has heretofore not been possible to take full advantage of the protective characteristics of polytetraiiuoroethylene.

Accordingly, it is the principal object of the invention to provide on a solid substrate an impermeable coating of polytetraiiuoroethylene which is relatively thin as compared to coatings Iattainable with methods known heretofore.

More particularly, it is an object of the invention to provide a simpliiied and reliable technique for producing a sintered coating of polytetraiiuoroethylene on a solid surface which entails a more economical source of this substance than the aqueous dispersion presently used.

A fsignicant feature of the invention resides in the fact that the films or sheathings produced in accordance with the invention can be made in thicknesses ranging from less than one-hundredth of the minimum dimension produced by previous techniques to at least the same dimension. More specifically, one is able with the invention to produce polytetralluoroethylene coatings varying from about 0.000005 inch to `about 0.005 inch.

Another object of the invention is to produce a polytetrailuoroethylene iihn or coating which is materially less porous than those produced by spraying, dipping or brushing dispersions of polytetrauoroethylene particles whereby extremely thin impermeable films are developed which afford a high degree of protection at a substantially reduced cost.

Still another object of the invention is to provide a coating technique for polytetrailuoroethylene whereby a film may be formed on a solid surface of any shape and of any known material, whether metallic, ceramic or of any other type able to withstand the temperatures involved in the coating technique. The coating is applicable to the internal surfaces of cylinders and the like -as Well as to the exterior areas thereof.

For a better understanding of the invention, reference is made to the following detailed description to be read in conjunction with the accompanying drawing, whose single ligure illustrates the structure for carrying out a process in 4accordance with the invention for coating wire.

Briefly stated, I have discovered that the vapors generated `on heating bulk polytetratluoroethylene may be condensed on suiiiciently cool surfaces to form a white powdery condensate which may be sintered at an appropriate temperature to constitute a transparent or translucent lm on the substrate. I have also discovered that no roughening of the substrate is required to make the sintered lm adhere to the substrate despite the well known anti-sticking characteristic of polytetrauoroethylene. The film so produced was found to stick to smooth surfaces such as glass without any prior -ronghening or Sandblasting of the substrate. As presently understood, it is believed that the adherence to the substrate occurs because the lineness of the polymer molecules in the condensed vapor is sufficient -to enable them to penetrate the minute crevices or fissures in solid surfaces however smooth. Consequently, effective and tenacious bonding of the sintered coating to the underlying substrate is obtained simply by condensing the polymer vapor onto the `cleaned surface and the sintering the film.

Polytetrafluoroethylene is waxy in texture and opaque olf-white in color except that sections thinner than 0.005 inch are transparent. The lms or coatings so formed may be made extremely thin, they are resistant to adhesion and chemical attack, and afford highly effective corrosion protection for the underlying substrate. The iilm so produced has an extremely low dielectric loss factor over a wide range of frequencies and has a low coeicients of friction against metals and other substances. Polytetrauoroethylene is `also notable for its zero water absorption and high impact strength.

Despite their extreme thinness, the polytetraiiuoroethylene lms in accordance with the invention, by rea' son of their lack of porosity, provide for greater insulation against corrosion of the underlying surface than has heretofore been possible. For example, a polytetrauoroethylene finish has been formed by the process disclosed herein on low carbon steel, the filmY having a thickness in the order of about 0.00007 inch. This coating when subjected to test was found to possess a salt spray life equivalent to that provided by a water dispersionA polytetrauoroethylene coating, properly applied, havingk a thickness of 0.0007 inch. (A description of this prior coating technique may be found in the du Pont New Product Technical Bulletin No. l, 5th ed.) Thus the invention accomplishes a degree of protection equivalent to the aforesaid du Pont coating at but onetenth the cost of the material. This saving in polytetrailuoroethylene material is of economic importance, since the cost of the bulk is relatively high.

In carrying out the invention, the surface to be coated is preferably first thoroughly cleaned, as by immersion in acetone or any other suitable solvent or cleaner which does not contain chlorine. No need exists to increase, the surface roughness or porosity, as by sandblasting, vapor honing, or etching, as is required by prior methods.

The polytetrauoroethylene which is to be vaporized may be iny any solid or powder form and scrap material may be used if desired. An example of a suitable powder is the du Pont product Tetlon-No. 1. The polytetrafluoroethylene is heated to lthe point of vaporization. The vaporization is effected preferably in the temperature range of about 450 C. to 600 C. Beyond the upper limit of this range, lthe molecular weight of the vaporized material becomes excessively low. The rising vapors are directed onto the cool surface of the article being finished, this being accomplished, for example, by suspending the article over the vapor source or by blowing the vapors around and into the article, as would be the case in the coating of a cylinder. This process may be carried out in an air chamber under controlled pressure conditions, or in vacuo. Under vacuo, a slightly lower temperature range may be used.

The vapors striking the cool surface to form a white condensate powder thereon which is then heated to a sintering temperature in the order of about 300 C. to 400 C. to establish a uniform and continuous film on the substrate. This sintering may be produced by localized radio-frequency heating or` equivalent means. The thickness of the condensate may be controlled by varying the density of the vapors and the time of condensation. The ultimate thickness of the coating may be controlled by applying successive layers thereof through the process of alternate condensation, sintering, etc.

To confer anti-sticking properties to a metal substrate, one coat of polytetrauoroethylene is all that is necessary, the coat being no more than 0.00001- inch thick. A coating of this thickness, when made in accordance with the invention, has anti-sticking properties tantamount to those obtained with coatingsv of 0.0007 inch thick produced by prior techniques. For. example, the contact angle of water on` a sintered polytetralluoroethylene coating'of 0.00001 inch thick produced by the instant processis approximately 108. This value closely approaches that obtained on far thicker coatings produced by previous techniques. The anti-sticking properties of the coating produced by the invention has been demonstrated by coating an aluminum pan with a sintered polytetrafluoroethylene film of 0.00001 inch thick. A cake baked in a pan so coated was found to separate readily from the surfaces thereof very` much as it would in a greased pan. In contradistinction a similar cake adhered to 4the. surface of a non-greased and non-coated panemployed as a control. Itis to be understood that Coating: Pulling force in kilograms1 A. Uncoated copper wire (control sample) 7.5 to 8.0 B. A soap-coated copper wire (control sample) 6.5 to 7.0

C. A `copper wire coated by instant process with film 0.00001 inch thick 6.5 to 7.0 D. The coated wire of C plus a soap film 5.5 to 6 1 Average of at least four separate samples.

From the above table it is clearly evident that the coefficient of friction of the instant coating is the same as soap under the extreme pressure conditions of wire drawing. Thicker coatings produced in accordance with the invention yield even lower coeicients of friction.

Even more striking results have been obtained in drawing of cold worked stainless steel wires through a steel die for a total deformation of 36.5%. In this case the pressure was so large that soap afforded no lubrication. However, the instant coating reduced the drawing force rom above 20 kilograms down to 12 to 14 kilograms. This result was obtained consistently for 10 specimens.

It may be desirable to produce thin sintered lms of polytetrafluoroethylene which are removable from the substrate surfaces on which condensation takes place. Such films may be produced by rst covering the smooth surface of a metal or glass sheet with a liquid that will not vaporize markedly at the sintering temperature. This liquid serves. to lill up any crevices, pores or fissures in the surface ofthe substrate. Suitable for this purpose is silicone oil or Isome other parting compound such as graphite and the like. The polytetraluoroethylene is then vaporized and condensed on the previously coated glass or metal sheet to form a powdered layer thereon in the manner hereinabove described, and this layer is sintered to form a continuous and uniform lrn of polytetrafluoroethylene which may then be stripped from the surface on which condensation occurred.

As will be evident from the following examples, the invention not only affords economic advantages but makes it possible to coat products which could not be coated by known techniques, thereby extending the limit of utility of existing products.

(l) Continuous production of magnet wire insulation coatings: In this case, the instant process makes possible, by the continuous production of very thin but adequate insulation coatings, the achievement of more turns per volume of winding. By producing the coating in an economic fashion, a magnet wire polytetrauoroethylene coating is obtained which at the same time is less expensive thanprior polytetrauoroethylene coatings, but which also enables the coated magnet wire to be put into wider use. The continuous coating of magnet wire by the instant process is effected by passing the cold wire at a controlled speed continuously through a chamber containing the polytetrauoroethylene vapors of desired density and then through a zone in which sintering is accomplished.V The structure for carrying out this technique is illustrated in the drawing, wherein a wire 10 drawn from a supply reel: 11- is fed continuously through a. condensing chamber 12 which is surrounded by a vaporization chamber 1 3 into which is supplied solid polytetrafluoroethylene through a hopper 14. The material in vaporization chamber 13 is heated by a suitable electrical element 15, the resultant vapors passing through openings into the condensing chamber 12 to condense on the cool wire passing therethrough. The wire with the powdered condensate is then fed continuously through a sintering furnace 16 and the wire having the sintered film thereon is nally wound on a reel 17.

(2) Continuous production of transformer steel sheet insulating coatings: This may be accomplished as described in Example 1. The same advantages apply in this case as well, namely, by enabling a decrease to be obtained in the thickness of the insulation coat, transformer characteristics are significantly improved. Also, by making feasible an economic coating of polytetraiiuoroethylene, the high temperature properties of polytetraffuoroethylene can be put to use to increase the limiting operating temperature of transformers, etc.

(3) Continuous production of metal-working lubrication coatings. Coatings on wires prior to the drawing stages can be achieved as in Example l. Coatings on parts to be cold extruded, cold rolled, cold roll formed, cold deep drawn, etc., may be obtained by the instant process. These coatings are especially useful where the pressures on the lubricant are relatively high (100,000 p.s.i.). To permit commercial use, however, these coatings must also be inexpensive and the instant process makes possible Such inexpensive coatings.

(4) Economic production of antisticking films of polytetraffuoroethylene: For example, as coatings for baking pans, pots, etc., for consumer use. In these cases, the coatings must be inexpensive if they are to be widely used. Coatings by the instant process are thus inexpensive. The material cost alone for previous coatings has been too high to make use practical. Another example, is the use as a coating for the spouts of bottles, containers and the like, to provide free pouring qualities (pouring without a residue of liquid on the outside of the bottle). The latter application is also limited by economic considerations for the previous processes but is not so limited for the instant process.

(5) The coating of internal surfaces: For example, corrosion protection for assembled automobile mufflers and tail pipes can be achieved by the instant process.

A further development of this invention is the production of a coating that comprises the active products of thermal decomposition of high molecular weight polymers of tetrafluoroethylene. One way of achieving this coating involves the steps of first Vaporizing the bulk material and permitting it to condense on a solid surface, such as glass. Thereafter, the condensate formed on the glass surface is scraped off and placed in a suitable container. Acetone is added to the condensate to form a saturated solution thereof in a mixture with the remaining condensate. The decanted portion of the acetone-condensate combination is then applied to the solid surafce to be coated and finally is sintered to form the film.

The film so formed differs in properties from the high molecular Weight polytetrauoroethylene coatings in that Water wets said film but does not Wet the latter. One characteristic property of this film is that it maintains the substrate substantially free of oxidation up to temperatures at least as high as 400 C. for at least ten hours. This characteristic is of value in the process of heat treatment where said coating substantially prevents decarburization of steel and/or maintains dimensional stability of finished substrates. Use of the instant film obviates the need for the use of expensive atmosphere or salt bath furnaces in the heat treatment or annealing of metals and alloys up to at least 400 C. Another characteristic of the decanted liquid portion of the saturated acetone and the polytetrauoroethylene condensate is that when said liquid is applied to a substrate which has an oxidized surface, the oxidation thereon will be removed. Alternatively, to produce the combination of a saturated acetone solution and condensate, the polytetrafiuoroethylene vapors may be washed through the acetone.

From the foregoing it is evident that applicauts invention enables the economic production of extremely thin polytetrafluoroethylene films having superior characteristics to thicker films of the same material produced by prior art techniques. While there has been shown what at present is considered to be a preferred technique for making such lms, it is evident that many changes and modifications maybe made therein without departing from the essential spirit of the invention. It is intended, therefore, in the annexed claims to cover all such changes and modifications as fall within the true spirit of the invention.

What is claimed is:

l. A process for forming a film of polytetrafiuoroethylene comprising the steps of condensing polytetrafluoroethylene vapors onto a relatively cool surface to form a condensate including a layer of powdered polytetrafiuoroethylene thereon, and subsequently sintering said condensate in situ to form a continuous substantially homogeneous film on said surface.

2. A process for forming a film of polytetrauoroethylene comprising the steps of vaporizing polytetrauoroethylene, condensing said vapors on a relatively cool surface to form a condensate including a layer containing powdered polytetrafiuoroethylene thereon, and subsequently sintering said condensate in situ to form a continuous film.

3. The process for producing a sintered film of polytetrafluoroethylene bonded to a solid substrate comprising the steps of heating bulk polytetrafluoroethylene to vaporize same, directing said vapors onto the relatively cool surface of the substrate to be coated to condense said vapors thereon and thereby form a condensate including a layer containing polytetrafiuoroethylene condensate in situ, and then sintering said powder t-o form a continuous and uniform film.

4. The process for producing a sintered film of polytetrafiuoroethylene bonded to a solid substrate comprising the steps of heating bulk polytetrafluoroethylene to vaporize same, directing said vapors onto the relatively cool surface of the substrate to be coated to condense said vapors thereon and thereby form a condensate including a layer containing polytetrafiuoroethylene condensate in situ, and then sintering said powder to form a continuous substantially homogeneous adherent film on said surface having a thickness in the order of 0.000005 inch.

5. The process for producing a sintered film of polytetrafluorothylene bonded to a solid substrate having a smooth surface comprising the steps of cleaning said smooth surface, heating bulk polytetrauoroethylene to vaporize same, directing said vapors onto the relatively cool surface of the substrate to be coated to condense said vapors thereon and thereby form a condensate including a layer containing polytetrafiuoroethylene powder, and then sintering the resultant condensate in situ to form a continuous and uniform film.

6. The process as set forth in claim 5 wherein said surface is constituted by glass.

7. The process as set forth in claim 5 wherein said surface is constituted by steel.

8. The process as set forth in claim 5 wherein said surface is constituted by copper wire.

9. The process as set forth in claim 5 wherein said surface is constituted by aluminum.

l0. The process as set forth in claim 5 wherein said surface is constituted by a copper-base alloy.

11. The process for producing a sintered film of polytetrauoroethylene bonded to a solid substrate comprising the steps of heating bulk polytetrauoroethylene within a tempearture range of about 450 C. to 600 C. to vaporize same, directing said vapors onto the rela- 7 tively cool surface ofthe substrate to be coated to condense said vapors thereon and' therebyform a condensate including a layer containing polytetrafluoroethylene powder, and then sintering the condensate in situ within a temperature range of about 300 C. to 400 C. to form a continuous and uniform ilm on said substrate.

12. The process as set forth in claim 11 wherein said substrate is constituted by magnet wire. Y

13. The process as set forth in claim 11 wherein said substrate is constituted by the internal surface of a cylinder. v

14. The continuous process of producing an insulating coating on magnet wire comprising the continuous condensation of polytetrauoroethylene vapors onto the magnet Wire passing through a chamber containing said vapors to form a condensate including a layer of powdered polytetrafluoroethylene thereon, and subsequent continuous sintering of said condensate in situ.

15. The continuous process. of producing an insulating coating on a transformer sheet comprising the continuous condensation of polytetrauoroethylene vapors onto the sheet passing through a chamber containing said vapors to form a condensate including a layer of powdered polytetrauoroethylene thereon, and subsequent continuous sintering of said condensate in situ.

References Cited in the le of this patent UNITED STATES PATENTS 2,028,776 Hibbert Jan. 28, 1936 2,420,222 Benning May 6, 1947 2,617,269 Smith-Johannsen Nov. 11, 1952 2,662,034- Mason Dec, 8, 1953 2,686,738 Teeters Aug. 17, 1954 2,719,093 Voris Sept. 27, 1955 Y2,724,672 Rubin Nov. 22, 1955 Arnett Dec. 27, 1955 OTHER REFERENCES Chemical Age, July 13, 1946, page 44.

ENTTED STATES PATENT OEETCE CERTTUFTQATE @F CRECHGN Patent Noa 2,893,900 July '7, 1959 Eugene S. Machlin It is hereby certified that error appears in the-printed specification of the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Column 2, line 59, for "the sintering" read then sintering column 6, lines 38 and 39, for "condensate in situ, and then sintering said powder" read powder and then sinterinfgr said condensate in situ lines t7 and 48, for "condensate in situ, and then sintering said powder" read powder and then sintering said condensate in situ --B Signed and sealed this 5th day oi January 1.960o

(SEAL) Attest:

KARL Hs, AXLINE ROBERT C. WATSON Attesting Officer Commissioner of Patents

Claims (1)

14. THE CONTINUOUS PROCESS OF PRODUCING AN INSULATING COATING ON MAGNET WIRE COMPRISING THE CONTINUOUS CONDENSATION OF POLYTETRAFLUOROETHYLENE VAPORS ONTO THE MAGNET WIRE PASSING THROUGH A CHAMBER CONTAINING SAID VAPORS TO FORM A CONDENSATE INCLUDING A LAYER OF POWDERED POLY-

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