US 2788296 A
Description (OCR text may contain errors)
April 9, 1957 s. LOUIS 2,788,296 THOD OF APPLYING AN ELECTRICALLY CONDUCTIVE TRANSPARENT COATING TO A NON-CONDUCTIVE BASE Filed Nov. 15. 1951 HIE INiENTOR. HEN ULIJ S. LULIIS FlEiENT United States Patent METHOD OF APPLYING AN ELECTRICALLY CON- DUCTIVE TRANSPARENT COATING TO A NON- CONDUCTIVE BASE Arnold S. Louis, New York, N. Y., assignor to Myron A. Coler, Scarsdale, N. Y.
Application November 15, 1951, Serial No. 256,515
8 Claims. (Cl. 117---211) 1 This invention relates to electrical insulators having transparent electrically conductive surfaces and method of making same. I
In many applications for electrical insulators such as glass and plastics it is desirable that an electrically conductive surface be provided in order to eliminate electrostatic charges. In electrical measuring apparatus electrostatic charges cause disturbances to sensitive meters. Accordingly, a conductive surface is sought for the meter casing observation window. In another typical case, that of aircraft, the electrostatic charging of the plastic cockpit canopy and insulating plastic surfaces covering radio and radar antennas create electrical interference with radio communication, and radar signals.
There is disclosed herein a simple method of coating a thermoplastic article with a transparent conductive surface involving the application of sufficient heat and pressure to secure a relatively firm bond between the conductive material and the insulating base. The conductive materials distinctly pointed out in the examples which follow include carbon which is well known in the prior art as a useful coating material having great hiding power but not as a transparent material.
The prior art shows various partially successful methods of applying carbon and other conductive films. Thus, surfaces possessing sutlicient inherent adherence such, for instance, as that of a wax object, will readily attach to itself a relatively continuous graphite layer which can be electroplated. Such films are rather delicate and usually completely opaque. Again, it is conventional to apply graphite in dispersion in film-forming adhesives to the surface of insulator objects. It has not been possible to obtain such coatings which are sufficiently conductive and, at the same time, reasonably transparent. Furthermore, there are problems connected with peeling and loose adherence of the coating and with crazing of insulator plastics to which they are applied.
Aconventional device to obtain transparent conductive coatings is the application of organic films containing water-soluble electrolytes. Such films are generally very unstable as to their electrical properties and can be easily washed from the insulator base. This invention,
by contrast, is concerned with obtaining a coating which is sufiiciently thin to be transparent and yet sufficiently adherent to withstand considerable abrasion and washing.
i Accordingly, it is an object of this invention to provide an improved method for applying an electrically conductive transparent coating to a thermoplastic insulator.
It is another object of this invention to provide a method for applying an electrically conductive transparent coating to a transparent thermoplastic material.
A still different object is to provide a glass having an electrically conductive transparent surface.
" A' particular object of this invention is a method of obtaining a uniform conductive coating on a plastic surface.
Still another object of this invention is to provide an article molded of a transparent plastic insulator, said article having a transparent electrically conductive coating adherent to its outside surface.
In carrying out the present invention pressure and heat are utilized to cause the conductive material to adhere to the substrate. There is also disclosed in greater detail hereinafter an improved method of securing such coatings by a transfer or offset process in which the conductive material is applied to a relatively inert surface and is then transferred by the action of heat and pressure to the substrate. The resultant film is characterized by thinness, by relatively high adherence to the substrate and by resistance to abrading action.
For a more complete understanding of our invention and for further objects and advantages thereof, reference should now be made to the accompanying description taken in conjunction with the drawing in which:
Figure 1 diagrammatically illustrates, a. system of carrying out one phase of the invention.
Figure 2 shows in elevation a view of elements of this invention assembled in a molding press.
Figure 3 shows in elevation a continuously operating embodiment of the process of this invention.
With reference to Figure .1, plate 2 is provided with a surface having low intrinsic adhesion to carbon, mirrorfinished chrome plate being suitable. Glass is also useful. What is essential'is that plate 2 have an exceedingly smooth, non-adherent surface which will not deform at the transfer temperature. A burner 4 is adapted to burn a mixture of a carbon-containing fuel such as acetylene, ethylene, natural gas or city gas with less than the amount of air or oxygen gas required for complete combustion under conditions whereby the undecomposed fuel is thermally cracked in the flame and carbon is formed. The resulting carbon black is deposited on plate 2. Various methods may be used to obtain a uniform coating including use of a plurality of burners and movement of the plate or burner. The method I prefer is the use of a high voltage generator, 6 which charges plate 2 electrostatically so that the carbon particles are attracted to the more thinly coated portions of the plate thus providing a uniform coating.
The coated plate is then placed in close contact with a sheet of plastic 8 so that the coating it) is interposed between the plate 2 and plastic 8. This assembly is then compressed between heated press platens 12 and 14. The proper pressure and temperature will, of course, be determined by the characteristics of the substrate plastic used. In general, the assembly should be heated to the conventional molding temperature of the plastic, if the plastic is of the thermosetting type. If it is a thermoplastic it should be heated at least to its heat distortion point, preferably to its compression molding temperature. Pressure should be about the same as during usual molding operations, 1000 to 5000 pounds per square inch usually being adequate. in the case of thermoplastics, the articles should be held under high pressure for a comparatively short time, preferably only long enough to bring the surface of the article to the temperature indicated above.
When films are formed in this fashion it has been found that if a uniform deposit of carbon is put on the transfer plate a similarly uniform film Will be transferred to the plastic surface. Surprisingly, a very thin deposit of carbon can be laid down and transferred directly to the plastic such that the film is actually transparent but yet retains sufiicient conductivity to discharge static readily.
For each plastic and each temperature and pressure of transfer there is a characteristic amount of carbon which will be very firmly bound to the plastic. If it is attempted to transfer a larger amount of carbon black, the remainder will be more loosely bound and can, if desired, be removed by rubbing or bufiing. A smaller amount of carbon black than this critical amount can be directly and firmly transferred.
Curved surfaces may be similarly treated with suitably shaped compression dies. Alternatively, sheet may be coated prior to shaping; the coating will remain conductive after a reasonable amount of physical work.
Example 1 A mirror-finished chromium plate on steel was lightly but completely covered with carbon black from a flame formed by burning a mixture of acetylene gas with less oxygen than the amount required for complete combustion.
The coated plate was pressed against a sheet of polymethylmethacrylate under a pressure of 4000 p. s. i. for a period of 15 minutes at a platen temperature of 350 F.
When the plastic was removed it was found that a coating of carbon was formed on the surface. The coating was transparent and electrically conductive. It passed about 65% of incident light, being so transparent that newsprint could be read through it with ease. It has a resistivity of less than one megohm per square. The film was sufficiently adherent so that Scotch tape could be applied to it and ripped off without damaging the film. The coating could also stand considerable washing with soap and water and at least a moderate degree of abrasion.
Example 2 A ferrotype plate was coated with carbon black in essentially the same way as described in Example 1 except that a considerably heavier coating was laid down. This coating was next to the top one of several sheets of phenolic impregnated laminating paper. Another ferrotype plate completed the pile. The whole was pressed at about 2000 pounds per square inch and 325 F. for 40 minutes. The carbon was completely and firmly transferred to the resulting laminate yielding a highly abrasion resistant surface with a resistance of 12,000 ohms per square.
Example 3 The procedure of Example 2 was repeated except that a sheet of plate glass was substituted for the first mentioned ferrotype plate. Results were substantially the same as reported for Example 2.
The technique described above is not limited to carbon formed by the pyrolysis of carbonaceous fuel gases. Any similarly finely divided conductive material formed by the pyrolysis of a gasiform substance is equally applicable, as for instance iron or nickel formed by the decomposition of the corresponding carbonyls or boron formed by the pyrolysis of boron hydrides.
Again, conductive material may be formed by the pyrolysis of liquid or solid materials which have been spread on the transfer surface. Thus carbon may be formed by the carbonization of a film of hydrocarbon oil or of. a thin layer of a plastic such as polyethylene or polydivinylbenzene.
Provided the insulator surface to be coated is sufiiciently resistant to heat, carbon may be deposited directly from a sooty flame of the type described above. Polymonochlorotrifluoroethylcne and glass are suitable for such treatment. The coated object may then be subjected to heat and pressure sufiicient to secure adherence of the applied coating. Thus, a sheet of polymonochlorotrifiuoroethylene may be coated evenly with carbon black and heated in a press between mirror-finished chrome sheets to a temperature above its softening point. A glass object after coating may be heated under pressure of polished metal sheets in a furnace under neutral or mildly oxidizing conditions to a temperature just above its softening point.
Example 4 A sheet of plate glass, 6 inches long and 8 inches wide, was coated with a thin layer of carbon black by the procedure described in Example 1. The sheet of glass was placed between two /2 thick slabs of polished stainless steel and the whole placed on the floor of an electric furnace. A mass of steel weighing 250 pounds was placed on top of the assembly. The furnace was heated to 1100 F. and held for /2 hour. After cooling, a firmly adherent transparent coating was found on the surface of the glass. The resistance of the surface was less than one megohm per square.
A film of the type described herein can be continuously applied to a long sheet or strip of plastic by use of the device shown in Figure 3. Roll 34 is of polished chrome plated steel. A sooty flame from burner 36 deposits carbon continuously on said roll 34. Plastic sheet 30 passes continuously between roll 34 and opposing roll 38 being subjected to pressure during passage therebetween. An electrical heater or equivalent (not shown) heats the area of roll 34 which is in contact with sheet 30 to a temperature somewhat higher than the usual molding temperature of the plastic in question. Cooperation of the parts as shown results in the complete and firm transfer of a carbon film to the surface of the plastic sheet. Continuous sheets of glass may be processed in this man ner by preheating the glass to the softening point. Commercially available apparatus for preheating the glass may be used.
By finely divided material is meant material having a particle size of microns or less and preferably no greater than 20 microns.
While preferred embodiments of the invention have been described, it will be understood that further modifications may be made without departing from the spirit and scope of the invention.
What is claimed is:
1. The method of applying an electrically conductive transparent coating of finely divided solid, non-fusible conductive material having a particle size less than 20 microns to an electrically non-conductive thermoplastic substrate comprising the steps of applying a thin transparent uniform layer of said finely divided material to said substrate followed by the step of applying sufiicient heat and pressure to said finely divided material and said substrate to cause the surface of said substrate to soften and said finely divided material to adhere to said softened surface of said substrate.
2. The method of rendering a thermoplastic article electrically conductive comprising the steps of uniformly coating with carbon having a particle size less than 20 microns of a surface having low adherence to carbon terminating the coating step prior to the point at which the coating becomes opaque, placing said carbon coating into intimate contact with said plastic article and applying sufiicient pressure and heat to cause said carbon to adhere to said plastic sheet.
3. The method of claim 2 wherein said low adherence surface is electrostatically charged.
4. The method of claim 2 wherein said plastic article is polymethylmethacrylate.
5. A process for rendering a thermoplastic article electrically conductive comprising coating a surface having low adherence for carbon by impinging on said surface the products of combustion from a sooty flame until a continuous but transparent layer of carbon having a particle size of less than 20 microns is formed, placing said carbon coating into intimate contact with said plastic article and applying sufiicient heat and pressure to cause said carbon to adhere to said plastic article;
6. The process of claim 5 wherein said sooty flame is produced by the burning of acetylene gas.
7. The method of claim 1 wherein said substrate is 2,221,77 glass. 2,328,422 8. The method of claim I wherein said material is 2,357,809 applied to said substrate by first applying said material ,4 3, 13 to a surface having low adhesion for said material, and 5 0 placing said coated surface into intimate contact with said substrate.
References Cited in the file of this patent UNITED STATES PATENTS 10 1,987,969 Parkin Ian. 15, 1935 6 Carlson Nov. 19, 1940 Christensen Aug. 31, 1943 Carlson Sept. 12, 1944 Von Hippel a. D60. 24, 1946 Simpson May 30. 1950 OTHER REFERENCES Printed Circuit Techniques, Nat. Bur. Stand. Circ. 468, November 15, 1947.