US 3227132 A
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Jan. 4, 1966 P. J. CLOUGH ETAL 3,227,132 APPARATUS FOR DEPOSITING COATINGS OF TIN ON A FLEXIBLE SUBSTRATE Filed Dec. 31, 1962 2 Sheets-Sheet 1 L VAC.
7 PUMP VAC. PUMP Fig. I
INVENTORS PHILIP J. CLOUGH BY ROBERT W. STEEVES Maw ATTORNEY Jan. 4, 1966 P. J. CLOUGH ETAL 3,227,132
I APPARATUS FOR DEPOSITING COATINGS OF TIN ON A FLEXIBLE SUBSTRATE 2 Sheets-Sheet 2 Filed Dec. 31, 1962 VAC. PUMP Fig. 2
INVENTORS PHILIP J. CLOUGH BY ROBERT W. STEEVES ATTORNEY United States Patent 3,227,132 APPARATUS FOR DEPOSETING COATINGS 0F TIN ON A FLEXIBLE SUBSTRATE Philip J. Clough, Reading, and Robert W. Steeves, Nahant, Mass, assignors, by rnesne assignments, to National Research Corporation, a corporation of Massachusetts Filed Dec. 31, 1962, Ser. No. 248,731 3 Claims. (Cl. 11849) This invention relates to coating and more particularly to the coating of various substrate materials with tin by vacuum evaporation and deposition of tin.
In the coating of various substrates with a tin film by vacuum evaporation of the tin, considerable difficulty has been experienced in obtaining uniform tin coatings of high purity and high evaporation rates of the tin, which is necessary in producing bright specular metallic tin coatings for use in products requiring a high quality specular metallic appearance. The principal reason for these difficulties results from the fact that the tin metal exhibits an oxide skin on the surface of the molten metal. This oxide results either from residual oxide in the metal itself or is formed by the tin metal in combination with residual gas molecules of oxygen, water or carbon monoxide remaining in the vacuum chamber. This coating of tin oxide forms a thin skin on the surface of the tin preventing evaporation of the tin metal thereunder. Where increased heat is used in order to obtain an increased evaporation rate there results a spora-tic bursting of portions of the oxide skin due to the increased vapor pressure which builds up underneath the skin. This results in uneven or nonuniform coatings and low evaporation rates at the evaporation temperature. Additionally, the tin coatings produced are milky white, hazy, or brown and non specular in appearance.
Accordingly, an object of the present invention is to provide an improved apparatus for evaporating molten tin in a vacuum for the purpose of providing tin coatings having a high degree of purity and uniformity and a high quality specular metallic appearance.
Another object of the present invention is to provide for high tin evaporation rates by the prevention of the formation of oxide scum on the surface of the molten tin being evaporated.
Another object of the present invention is to provide for a clean tin evaporating surface over long periods of operation.
Other objects of the invention will in part be obvious and will in part appear hereinafter.
The invention accordingly comprises an apparatus possessing the construction, combination of elements and arrangement of parts, which are exemplified in the following detailed disclosure, and the scope of the application of which will be indicated in the claims.
For a fuller understanding of the nature and objects of the invention, reference should be had to the following detailed description taken in connection with the accompanying drawing wherein:
FIG. 1 is a schematic, diagrammatic, sectional view of one preferred embodiment of the invention; and
FIG. 2 is a schematic, diagrammatic sectional view of an alternative embodiment of the invention.
Referring now to the drawing, there is illustrated a diagrammatic, schematic, sectional representation of one preferred embodiment of the invention. As illustrated, the apparatus includes .a first vacuum chamber arranged to be evacuated to .a pressure on the order of 1x 10* torr by a high capacity vacuum pumping system 12. The chamber 10 has a vacuum tight cover 14 which can be lifted up to permit loading and unloading of rolls of the substrate 16 to be coated. The apparatus also includes a vacuum coating chamber 18 arranged to be 3,227,132 Patented Jan. 4, 1966 evacuated by a high capacity pumping system 19 to a pressure on the order of 5X10 torr or less. Positioned within the vacuum coating chamber 18 is a source of tin vapors 2d. The tin source 21 is preferably an elongated container having a length substantially equal to the width of the substrate to be coated. In the preferred embodiment the tin source is constructed as shown in the copending application of Clough et 211., Serial No. 130,647, filed August 10, 1961, assigned to the same assignee as is this application and which is now abandoned.
As disclosed therein the tin vapor source comprises a refractory base of carbon or graphite having the inner surface which is to be exposed to tin vapors coated with molybdenum metal. The outer surface of the refractory base is provided with a plurality of layers of refractory materials which are effective to prevent heat loss from the outer surface of the crucible source. The tin source is preferably heated by induction heating coils 22. Between the first vacuum chamber 10 and the vacuum coating chamber 18 there is provided in a preferred embodiment 2. large drum 24 which almost completely fills an opening 26 between these two chambers. This opening is defined in part by curved surfaces 28 which cooperate with the drum surface to form high impedance paths substantially preventing large quantities of gas such as air, oxygen, water and carbon monoxide from passing from the first chamber Ill to the high vacuum coating chamber 18. Preferably the drum Z4 is so positioned, with respect to a pair of guide rolls 39, that the substrate 16 is pressed into contact with the drum 24 and held in contact with this drum while it is carried into the coating chamber 18, through the tin vapors and back out of the coating chambet. The impedance paths created by the curved surfaces or shrouds and the drum thus constitute apertures through which the substrate to be coated must pass. For highest impedance to gases the aperture must be minimized; however, there must be sufficient clearance for the paper to pass While in contact with the drum, without touching the shrouds. Moreover, when it is necessary to join one length of substrate with another by means of a lap joint, the aperture must be large enough to allow two thicknesses of the substrate to pass without interference. Accordingly, it is preferred that the spacing between the shrouds and the drum be made adjustable to permit optimum spacing for different thicknesses of the substrate.
In the preferred embodiment of the present invention, the vacuum coating chamber is provided with a source of inert gas 32 for filling the vacuum coating chamber with an atmosphere of an inert gas such as nitrogen, argon, helium and the like. When the substrate is to be loaded or unloaded vacuum chamber In is vented to the atmosphere and the cover 14 removed to permit access to chamber 10. In order to prevent contamination of the molten tin and formation of an oxide scum on the surface ofthe molten tin by leakage of gases from chamber 10 into the vacuum coating chamber 18, the vacuum chamber 18 is filled or flooded with an inert gas. Preferably an inert gas pressure in excess of atmospheric is provided in the vacuum coating chamber so that any leakage through the opening 26 will be in a direction into chamber 10. After the unloading and loading of the substrate is completed, chamber 10 is evacuated to operating pressure and then the coating chamber 18 is evacuated. The tin source is then brought up to operating temperature and the coating resumed. In this manner a clean tin evaporating surface is maintained whereby vola-t-ilized t-in is rapidly deposited onto the substrate to provide tin coatings having a high degree of uniformity and purity and a high quality specular metallic appearance.
In the preferred embodiment of the invention shown in FIG. 1 the vacuum coating chamber 18 is provided with a removable bottom 38 which provides access to the coating zone or coating .chamber 18. A shield 34 is also preferably provided to permit the tin source to be brought up to operating temperature without exposing the substrate to radiation from the crucible until the tin is at the desired high evaporating temperature on the order of at least 1400 C. to 1600 C.
In the preferred embodiment of the present invention, the molten tin is preferably fed to the source 20 from a supply 41 positioned outside of the vacuum coating chamber 18. In this manner the tin metal can be heated by suitable means to a molten state and pure tin metal 42, free from oxide impurities, introduced into the source 20 by drawing the molten tin metal from below the surface on which the oxide scum 44 forms.
Referring now to FIG. 2 of the drawing, wherein like numbers refer to like elements, there is shown an alternative embodiment of the present invention. In this embodiment of the invention the vacuum coating chamber comprises a single vacuum coating chamber 50. In ac- 'co'rdance with this embodiment of the invention there is provided a movable cover or lid member 48 positioned adjacent the surface of the molten tin and defining a space 52 therebetween. Preferably this cover has an area greater than the area of the upper end of the tin source 20. Preferably the edge of the cover 48 is adapted to extend downwardly beyond the upper edge of the tin source so that when the cover is positioned over the source a gas outlet or passage 54 is provided between the cover and the source. Associated with the cover 48 is conduit means 36 which provides connection to the source of inert gas 32. In operation of this embodiment of the invention, when the chamber 50 is to be opened to load and unload the substrate, for example, the heating of the source 20 is discontinued. The cover 48 is then moved by suitable means not shown from a position shown by the dotted lines at b to the superimposed position over the source 20. The chamber 50 is then vented to the atmosphere and preferably simultaneously a stream of inert gas such as nitrogen is passed into the area defined by the cover and the tin source. The inert gas fil'ls the area defined by the cover and the source, and flows over the surface of the molten tin and out of the passage or outlet 54 formed between the cover and the source in a direction shown by the arrows. A suiiicient flow of nitrogen is provided to maintain a sufiiciently great concentration of nitrogen over the surface of the molten tin to prevent atmospheric gases from contacting the surface of the tin and forming oxide impurities. The cover 14 is then removed and the coated substrate unloaded and a new substrate loaded. The chamber cover 14 is then replaced and evacuation of the chamber to operating pressure begun. When the contaminating atmospheric gases have been removed, the flow of inert gas is stopped and heating of the tin to vaporization temperatures is begun. At this point the cover 48 is preferably moved to a position shown by the dotted lines at a to prevent any coupling to the cover during induction heating. In this position the cover serves as a shield to protect the substrate from radiation from the tin source while the tin source is being brought up to operating temperature. When the operating temperature is reached cover 48 is then moved to a position shown by the dotted lines [2 and coating of the substrate is commenced.
In accordance with this embodiment of the invention the molten tin metal is protected from contaminating gases during opening of the chamber by the concentrated flow of inert gas over the surface of the tin metal. While the cover 48 is shown as being loosely fitted, it naturally is made as tight as is consistent with the large temperature gradients encountered. "In any case the pressure in the space 52 is maintained greater than atmosphere when chamber 50 is open.
While the invention has been described in connection with preferred embodiments, it is apparent that numerous changes can be made without departing from the spirit of the invention. For example, the tin source may be wholly formed of molybdenum metal. However, such crucibles are relatively expensive and are not easily formed or machined. Equally the geometric shape of the source and/ or cover can be varied and other heating systems such as resistance heating may be employed. Additionally the substrate may be passed from the atmosphere into the coating chamber through a series of locks of the type shown in US. Patent No. 2,927,330 to Bugbee.
Additionally, the means 41 for feeding molten tin to the tin source can be enclosed in a suitable outgassing chamber.
Since certain changes may be made in the above process and apparatus without departing from the scope of the invention herein involved, it is intended that all matter contained in the above description, or shown in the accompanying drawing, shall be interpreted as illustrative and not in a limiting sense.
What is claimed is:
1. An apparatus for coating a roll of a substrate with tin metal by vacuum evaporating tin and condensing tin on the substrate, said apparatus comprising means defin ing a vacuum chamber, means for evacuating said chamber, means for unrolling said substrate, moving said substrate past a source of tin metal vapors positioned in said chamber and rerolling said substrate in the vacuum chamber, door means for loading an uncoate-d roll of substrate into, and removing a coated roll of substrate from said vacuum chamber, means for melting the tin in the source and heating the surface of the molten tin to vaporization temperature, means for isolating the surface of the molten tin from the door means, said isolation means comprising an elongated narrow opening, and means for providing a superatmospheric pressure of an inert gas over the molten tin with a flow of inert gas through said opening at a sufiicient rate to prevent ingress of oxygen to the surface of the molten tin while said door means is open.
2. The apparatus of claim 1 wherein the tin vapor source and the substrate roll are positioned in different portions of the vacuum chamber and the narrow opening constitutes a portion of the path of travel of the substrate as it is unrolled, coated and rerolled.
3. An apparatus for coating a roll of a substrate with tin meta-l by vacuum evaporating tin and condensing tin on the substrate, said apparatus comprising means defining a vacuum chamber, means for evacuating said chamber, means for unrolling said substrate, moving said substrate past a source of tin metal vapors positioned in said chamber, and rerolling said substrate in the vacuum chamber, door means for loading an uncoated roll of substrate into, and removing a coated roll of substrate from, said vacuum chamber, means for melting the tin in the source and heating the surface of the molten tin to vaporization temperature, means for isolating the surface of the molten tin from the door means, said isolation means comprising a closure for separating the portion of said vacuum chamber adjacent the molten tin from the remainder of the vacuum chamber, and means providing a superatmospheric pressure of an inert gas over the surface of the molten tin, the closure and the superatrnospheric pressure cooperating to prevent backleakage of oxygen to the surface of the molten tin while said door means is open.
References Cited by the Examiner UNITED STATES PATENTS 2,382,432 8/1945 McManus et a1. 117107 2,639,490 5/1953 Brennan 117-107 2,853,970 9/1958 Novak 118-49 2,971,862 2/1961 Baer et al 1l849 X 2,996,037 8/1961 Eng 117106 X 2,996,418 8/1961 Bleil 117106 X 3,087,838 4/1963 Lubin 204-192 X RICHARD D. NEVIUS, Primary Examiner.