US3484276A - Apparatus for and method of providing a melted insulating coating on the inner surface of a tubular article - Google Patents

Description

URGG OD QF P 3.4842 76 MELTED INSULATING A TUBULAR ARTICLE 2 Sheets-Sheet 1 Dec. 16, 1969 A A. ETAL APPARATUS FOR AND E ROVIDING A COATING ON THE NN R SURFACE OF Filed July 21, 1966 5 3 INVENTOR.

ANTHONIE .uauassmx ERNEST 0.W.VA DER JACOB w.na TE AGENT A J. BURGGRAAF ET AL 3,484,276 APPARATUS FOR AND METHOD OF PROVIDING A MELTED INSULATING COATING ON THE INNER SURFACE OF A TUBULAR ARTICLE Filed July 21, 1966 r 2 Sheets-Sheet 2 QMKJW' AGENT United States Patent US. Cl. 117-95 Claims ABSTRACT OF THE DISCLOSURE An insulating coating resistant to attack by corrosive vapors is applied to the inner surface of a tubular article by applying a suspension of the coating material to the surface and thereafter generating an inductive plasma which develops the heat required for melting the material.

It is known to apply a layer to the inner surface of metal vapor discharge lamps made of glass, quartz or similar substances for protecting the wall material from attack. Such coating layers mostly consist of substances the melting point of which lies at a higher temperature than that of the material of which the tube wall is made.

The attack of the wall surface is due to the presence of strongly electropositive metal vapors such as sodium vapor, cadmium vapor, magnesium vapor, and the like and to the production of metal ions during the electric discharge. Since the replacement of the conventional inexpensive kinds of glass containing or not containing lead by other insulating materials, for example, borate glass or boron silicate glass which are resistant to chemical and electrochemical action and are less likely to be attacked involves difliculties in processing, coating layers are applied consisting of the said substances. Furthermore, a satisfactory protection is obtained by coating with metal oxides capable of withstanding high temperatures and are not attacked by metal vapor such as alumina, zirconium oxide, calcium oxide, magnesia or beryllium oxide.

The protective action of such a coating is not only based on the kind of material used. It is important that a completely uninterrupted layer should be formed. A density such that the metal ions and the harmful vapors absolutely cannot reach the subjacent wall cannot be attained with certainty if the known coating techniques are used in which the layer is formed by sintering or burning the coating material into the subjacent surface. It goes without saying that the formation of completely uninterrupted layers is promoted by melting the applied substance resistant to attack. The generally requires higher temperatures than the material for the tubular envelope is capable of withstanding so that such a protective layer cannot be obtained by the conventional method of application. It must be prevented that the softening temperature of the glass support should be exceeded during the supply of the required heat.

The invention relates to a method of applying such a coating increasing the resistance to chemical and electrochemical attack to the inner surface of a tubular article of glass, quartz or another insulating material, in which an uninterrupted layer is formed by melting the coating material, while the use of insulating substances of a higher melting point than that of the tube wall does not hamper the manufacture of such a layer. According to the invention, the heat required for melting the coating material on the inner surface of the tubular article is obice tained by the presence of an electric gas discharge in the hollow space of the article, which discharge is maintained by an alternating magnetic field of high frequency produced by an induction coil surrounding the article.

The invention further relates to devices for carrying out the method.

In a gas discharge maintained by high-frequency induction, which is referred to as inductive plasma, very high temperatures can be attained which decrease from the core of the discharge towards the surroundings. By a suitable choice of the strength of the high-frequency induction field and the pressure of the gas required for the discharge, the heat required for attaining the desired temperature can be developed in the proximity of the wall. In the method according to the invention, use is preferably made of a plasma at a lower gas pressure, since, when rarefied, the gas ignites more readily and the plasma is spread on a larger sectional area. Gases having a low ionization potential bring about a minimum of difliculties when the electric discharge is caused to continue so that use of preferably made of argon or neon.

/ The invention is more particularly intended for applying a thin protective layer. With a view to the thickness of the layer, the difference in expansion between the material of the wall of the article and the material of the coating must frequently be taken into account. Therefore, it is sometimes preferred to apply to a first coating a second layer, the intermediate layer consisting of another material which has a coefficient of expansion lying in the transitional area between the expansion coeflicients of the material of the wall and that of the protective coating.

Such an intermediate layer may be sintered, for exf ample, and may form an opaque layer if a diffuse transparency is required.

The conventional kinds of glass used for the manufacture of discharge tubes have an expansion coefiicient of from 50-75 10" cm. cm. degree C7 and are not particularly resistant to the unequal expansion under the influence of the heating, if produced locally, for example, with the use of the inductive plasma. The risk of mechanical stresses being produced which exceed the strength of the material can be avoided by the use of a heating furnace by which heat is supplied from the out side to the wall until a temperature is reached at which the material has a viscosity suitable for the neutralization of the mechanical stresses. An efiicient combination therefore consists in the use of an elecrtic resistance furnace and a high-frequency coil located immediately adjacent one another, while the tube is slipped into the furnace and is moved uniformly in the direction of the coil.

For the treatment of tubular articles of glass having an expansion coefiicient of less than 6X10 cm. cm.- degree C.1 a separate preheating furnace is superfluous.

The coating material may be sucked up into the tube in the form of a suspension and the excess material can be removed after a quantity of this coating material has been deposited on the tube wall. The thickness of the layer after the suspension has been conducted away depends upon the viscosity which can be chosen in accordance with the desired thickness of the layer. The suspension is made of powder intended for the coating having a given grain size which is mixed with alcohol and a solution of nitrocellulose in suitably chosen quantities, whereupon the mixture is ground in a ball-mill for a comparatively long time, for example, for 24 hours.

The suspension layer can be processed to a melted layer adhering to the wall in different ways. For tubes of not too small lengths, the method in acocrdance with the invention can be carried out in that the gas, for example, argon is supplied at one end of the tubular article,

while the gas is conducted away at the other end by means of an exhaust device, the pressure in the tube being adjusted to approximately l to torr. (1 torr is a pressure of 1 mm. of Hg.) This arrangement has the advantage that the outer wall can readily be slightly cooled in order to prevent the admissible temperature from being exceeded. The strength of the tube wall must remain sufficient to avoid the compression of the wall due to the decreased internal pressure. Therefore, the admissible temperature is lower than if a Separate jacket of quartz glass or another insulating material capable of withstanding a sufliciently high temperature is used and the article to be treated is placed in the space surrounded by this jacket. Such an arrangement is more particularly intended for the treatment of tubes of small lengths. In this case, the gas is supplied and conducted away at the ends of the jacket of quartz glass. Contamination of the argon or neon gas required for the discharge must be avoided as far as possible so that gases must be prevented from being released from the suspension on the wall of the tube. The heating of this suspension until the material melts is therefore preceded by a drying period followed by a heat treatment such that the gasforming constitutents are removed therefrom.

The meltable layer may be applied in a manner differ ent from that in which a suspension is used. For this purpose, the tube is rapidly rotated about its longitudinal axis during the activity of the inductive plasma while simmultaneously with the gas current finely divided coating material is allowed to enter the tube. The centrifugal forces produced by the rotary movement are imparted to the powder which is heated in the gas discharge and is pushed by these forces to the outside and reaches the wall in the melted state. The formation of a layer completely coating the wall requires a relative displacement of the tube with respect to the plasma and the inlet aperture of the pulverulent material.

Further details of the invention will now be described with reference to the drawing, in which:

FIG. 1 shows an arrangement for applying a coating to the walls of tubes of considerable lengths,

FIG. 2 shows such an arrangement for short tubes,

FIG. 3 shows diagrammatically a suitable device for carrving out the invention, and

FIG. 4 shows the device for rotating the tube.

In case of not too small a length. the tube 1 may be provided at both ends with plugs 2 and 3 which both have an aperture 4 and 5, respectively. The aperture 4 in the plug 2 communicates with a contaniner containing a quantity of required gas, for example argon or neon. The aperture 5 in the plug 3 is connected with an exhaust device. The inner surface of the tube 1 is coated with a thin layer 6 of the suspension of the material intended for the coating. Around the wall of the tube, provision is made of a heating furnace 7 and the induction coil 8 of a high-frequency generator.

In case of a tube of shorter length, a separate cylindrical jacket 9 (FIG. 2) may be provided consis ing of quartz glass or of another material melting with diflicultv and having insulating properties. The plugs 2 and 3 and the apertures 4 and 5 provided therein are disposed at the ends of the jacket 9, while the heating furnace 7 and the induction coil 8 surround the jacket 9. The len h of tubing 10 to be treated is placed inside the jacket 9, the inner surface of which length of tubing is coated with the suspension layer 6. The length of tubing 10 is supported by a tubular support 11 of material melting only with difficulty, for example, of quartz glass.

When carrying out the method in accordance with the invention, the tube 1 or the jacket 9 is placed on a platform 12 (FIG. 3). Through a flexible hose 13 the aperture 4 in the plug 2 communicates with a needle valve 14 by means of which the supply of gas from a vessel 15 is controlled. The aperture 5 in the plug 3 serving for conducting away the gas communicates through a flexible hose 16 with the exhaust pump 17. With the needle valve 14 a gas pressure between 10- and 10" torr is maintained.

The heating furnace 7 can be used for drying the suspension and serves at the same time for heating the tube glass in order to avoid impermissibly high mechanical stresses. The furnace 7 and the jacket 9 can be displaced along each other in the longitudinal direction of the tube. In order to carry out this displacement, the platform 12 is provided with a helical rod 18 and a device cooperating therewith for converting a rotary movement into a rectilinear movement, for example, constituted by a transmission 19 with the aid of a worm and a wormwheel, the drive being effected by the motor 20.

The voltage control member 21 of the autotransformer 22 connected to the supply mains serves for adjusting the temperature of the heating furnace 7.

Prior to the ignition of the inductive plasma, the heating furnace 7 is brought to the correct temperature. At a low gas pressure in the tube, the movability of the gas molecules resulting from this heating is suflicient to give rise to ionization of the gas under the influence of a high-frequency induction field. This field is obtained by the energization of the induction coil 8 which for this purpose is connected through a coupling transformer 23 to the high-frequency generator 24.

Initially the tube 1 is lowered as far as possible so that the upper end lies in the heating furnace 7. The platform 12 is set into rotation by putting the motor 20 in operation and as soon as the zone heated by the furnace 7 has got within the reach of the induction coil 8, the coil is energized and the gas discharge is effected. Owing to the resultant developed heat, the suspension layer melts and the temperature required for melting can be adjusted by control of the high-frequency energy supplied to the induction coil 8. The heat of the tube wall can be satisfactorily conducted away to the open air if a rapid cooling is desirable for preventing softening of the glass or crystallization.

When the platform is gradually raised, the tube wall is displaced through the high-temperature range and provided with a coating throughout its length. The shorter lengths of tube of FIG. 2 are treated in a corresponding manner and the displacement by means of the platform is adapted to the length of the tubular glass. The tube to be treated is not subjected to a difference in pressure and the maximum permissible viscosity can be lower without leading to deformation so that the admissible heating temperature is higher than in the case of FIG. 1. In order to avoid excessive heating of the tubular glass, the heat can be conducted away more satisfactorily when a cooling gas is used which is passed through the intermediate space between the length of tubing 10 and the jacket 9.

In another method of applying a melted layer to a tube wall, a jacket 9 of high melting-point insulating material is rotatably arranged between two caps 25 and 26 which are rigidly connected to each other by means of rods 27 and 28 (FIG. 4). A pulley 29 serves to set the jacket 9 into rotation. The rotatable object is supported by ball bearings 30 and 31. Gaskets 32 and 33 serve to prevent the open air from penetrating into the gas-filled space.

The caps 25 and 26 and the interposed jacket 9 may be moved upwards and downwards by means of, for example, a hydraulic drawing device 34 the drawing rod 35 of which is connected through flexible cables 36 and 37 to the cap 25. The induction coil 8 and the heating furnace 7 do not change their places.

The gas supply is located in the upper cap 25, while the lower cap 26 is provided with a connection for the outlet.

The supply duct 38 includes a thin pipe 39 which extends to the outside and is movably arranged in a stuffing ring 40 in a closure member 41 closing the supply duct 38. The pipe 39 should not change its place during the upward and downward movement of the jacket 9 and it should merge in the proximity of the inductive plasma 42 obtained by means of the induction coil 8. The pulverulent material supplied through the pipe 39 which serves to manufacture the coating is pushed towards the wall upon rotation of the jacket 9 and is deposited on the wall in the melted state owing to the heat of the inductive plasma 42. The coating may be providcd throughout the surface of the wall in that the construction is uniformly drawn upwards during rotation of the jacket 9 and the activation of the plasma.

Instead of the Wall of the jacket 9, a length of tubing 43 arranged inside this jacket and supported by a support 44 of high melting-point material may be provided on its inner surface with a coating.

What is claimed is:

1. A method of coating the inner surface of a tubular article of insulating material with a protective coating of a melted second insulating material comprising the steps of applying a suspension of the second insulating material to the inner surface of the article, introducing an ionizable gas in the hollow space of said tubular article, and subjecting the suspension to a plasma formed by a high-frequency induction field which develops the heat required for melting the coating material.

2. A method as claimed in claim 1, in which the discharge is effected at a gas pressure of from 10- to 10- torr.

3. A method as claimed in claim 1 in which the tubular article is moved axially through the high-frequency in duction field to produce the plasma.

4. A method as claimed in claim 3, in which the wall of the article is heated separately.

5. A device for coating the inner surface of a tubular article of insulating material with a protective layer of another insulating material comprising means to connect one end of the tubular article the inner surface of which is covered with a suspension of the other insulating material to a gas supply conductor and the other end to an exhaust device, and an induction coil surrounding the article and being relatively displaceable with respect thereto in axial direction for generating an inductive plasma in the gas supplied to said tubular member for melting the suspension of insulating material on the inner surface thereof.

6. A device as claimed in claim 5, in. which the tubular article is disposed in a space within a. tubular jacket of a high melting-point insulating material one end of which is connected to the supply conductor and the other end to an exhaust device, the jacket being surrounded by an induction coil moveable in the axial direction.

7. A device as claimed in claim 5, in which the wall of the length of tubing to be coated is arranged so as to be separately rotatable about the longitudinal axis of the tube.

8. A device as claimed in claim 6, in which the wall to be coated is arranged so as to be rotatable about the longitudinal axis of the jacket together with the surrounding tubular jacket. 9. A device as claimed in claim 7 in which in the axis of rotation a tube of smaller diameter is provided for supplying pulverulent coating material. from the outside to an area lying in the proximity of the plasma zone.

20. A device as claimed in claim 8 in which in the axis of rotation a tube is provided for supplying pulverulent coating material from the outside to an area lying in the proximity of the plasma zone.

References Cited UNITED STATES PATENTS 2,040,767 5/1936 Dudley 117-93.2 2,643,956 6/1953 Kuebler et al. 117-119.8 X 2,676,894 4/1954 Anderson et al. l17-1 19.8 X 2,940,011 6/1960 Kolb 315-111 3,179,784 4/1965 Johnson.

3,383,163 5/1968 Menashi 315-111 X RALPH S. KENDALL, Primary Examiner CHARLES R. WILSON, Assistant Examiner U.S. Cl. X.R.

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