Placing a tightly wound coil of metal foil in a vacuum furnace as it is received from a rolling mill with a film of rolling oil on its surfaces, subjecting it to a partial vacuum so as to increase the rate of evaporation of the rolling oil, heating the coil to an annealing temperature in a protective atmosphere, and subsequently cooling the coil in a protective atmosphere under at least about atmospheric pressure.
Perry Sr., Robert J. (Havertown, PA)
Thome, William L. (Toledo, OH)
1. A method of annealing a tightly wound coil of aluminum foil and scavenging gases and vaporizable liquids therefrom comprising the steps of placing said coil in a vacuum furnace, sealing said furnace, partially purging said furnace and said coil of contaminating gases by evacuating said furnace to about 1.0 to 0.1 p.s.i.a. in backfilling said furnace with a protective atmosphere to about 14.7 p.s.i.a. heating said coil to a temperature about equal to the boiling point temperature of said vaporizable liquids at standard conditions while circulating and replenishing said protective atmosphere, reducing the pressure in the furnace to between about 1.0 and 0.1 PSIA in a regulated manner so as to control the rate of evolution of gases of vaporization from said coil while continuing to supply heat to said coil until the desired annealing temperature of between 500° F. and 750° F. is reached, increasing the pressure within the furnace to at least atmospheric pressure by filling it with a protective atmosphere, cooling said coil by directing a cooled stream of protective atmosphere against and around the foil coil and removing said coil from said furnace.
2. A method according to claim 1 wherein said annealing temperature is between 500° F. and 700° F. but is less than the temperature required for a full anneal of said foil.
BACKGROUND OF THE INVENTION
Metal foil, as it is received from a rolling mill, has a coating of rolling oil deposited on it during the rolling operation. This oil coating or film must be removed totally or partially from the foil, particularly from aluminum foil, to render it suitable for many of its uses, such as, for example, household uses and use as a vapor barrier in packaging materials. Under prior art methods the oil film was removed by heat alone during an annealing process. Generally the coils of foil were placed in an annealing furnace, having a protective atmosphere, and heated to a temperature which was sufficiently high to vaporize the oil film and drive the vapors from the coil. For most rolling oils a temperature above 500° F. was required and usually the coil was brought to a temperature of about 700°-750° F. before the oil was substantially completely driven off. A typical rolling oil has a critical temperature of 550° F. at 14.7 pounds per square inch absolute (PSIA) and a critical temperature of 340° F. at 1 PSIA.
In prior art methods which required coil temperatures of between about 650°-750° F., the control of processing time and temperature became very critical, especially if a specific amount of the rolling oil was to be left on the foil. Other problems were prevalent with the prior art methods, particularly when practiced on a typical coil of aluminum foil which may have a diameter and length up to about 30 inches and 80 inches respectively, and may be made of a tightly wound strip of foil having a thickness of about 0.00025 to 0.0045 of an inch. Fluids entrapped in the wraps of such a coil were extremely difficult to remove. The temperature required to effectively remove the rolling oil from the coil was a temperature which fully annealed the foil. Therefore a cleaned aluminum foil having less than a full anneal could not be produced. At this relatively high temperature the aluminum foil was readily stained or tarnished by components of the oil film and other vapors or moisture within the coil. Also, at this temperature the evolution of the oil vapor from the coil was often rapid and uncontrolled. Sometimes the vapor pressure was sufficiently excessive to cause the foil to become wrinkled or distorted.
The present invention solves the above mentioned by providing a process which promotes the scavenging of gases and vaporizable liquids such as rolling oil from a coil of foil at a temperature which is lower than the normal boiling point temperature of the oil. By subjecting a coil of foil to a partial vacuum during at least a portion of the cleaning and annealing process, a partially annealed foil which is substantially free from oil film may be readily produced. Partially annealed foil is stronger than fully annealed foil and is therefore superior to fully annealed foil for many purposes. There are also advantages in using the process of this invention for the production of fully annealed aluminum foil and for heat processing other metal foils at elevated temperatures. Since potential contaminants such as the rolling oil and minute quantities of moisture and entrapped air are removed from the foil coils at relative low temperatures, the danger of staining or tarnishing the foil is reduced even though the coils are ultimately subjected to elevated temperatures up to their full annealing temperatures. Another advantage of the process is that some of such contaminating liquids or vapors may be removed from the coil prior to filling the furnace with a protective atmosphere and prior to the application of heat to the coil. Thus, the amount of contaminants flowing from the coil into the protective atmosphere, during the subsequent heat processing, is reduced. The time required for purging the furnace of ambient air may be reduced by using a vacuum means to reduce the oxygen content in the furnace to an appropriate level and then backfilling the furnace with a protective atmosphere. Also, by varying the pressure within the furnace between about 14.7 and 0.1 p.s.i.a., the rate of evolution of the vapors from the coil may be readily controlled during the heating process since the rate of evolution is no longer entirely dependent upon the coil temperature.
It is, therefore, a broad object of this invention to provide an improved process for heat treating coils of metal foil.
It is another object of this invention to provide an improved process for bright annealing coils of metal foil, particularly foil having a highly reflective surface.
It is still another object of this invention to provide an improved process for removing vapors and vaporizable liquids from coils of metal foil.
DETAILED DESCRIPTION OF THE INVENTION
The above-mentioned objects and advantages are attained by placing one or more coils of metal foil, such as aluminum foil, in a vacuum furnace. After closing the furnace door, the furnace is purged of air and filled with a protective atmosphere or other appropriate atmosphere. Preferably this is accomplished by reducing the pressure within the furnace to about 1.0 to 0.1 p.s.i.a. and then backfilling the furnace with a protective atmosphere until the pressure inside the furnace is about 14.7 p.s.i.a. This reduces the O2 content of the furnace atmosphere to as little as about 0.15 percent. When this preferred method of purging is used, some of the fluids inside the foil coil are vaporized during the purging pumpdown and they are withdrawn from the coil along with other entrapped fluids which may be already in the vapor state.
The coil is then heated so that it attains an annealing temperature substantially uniformly throughout the coil. Annealing temperatures vary with the type of metal foil being processed and the amount of annealing desired. The annealing temperature range for aluminum extends between about 500° F. and 750° F. A coil of aluminum foil heated to 750° F. ± 20° F. will be fully annealed. Preferably heat should be applied at a rate which keeps the temperature difference small between the interior portion of the coil and the surface portions of the coil, particularly when the interior portions of the coil reach a temperature at which the vapor pressure of the rolling oil exceeds the pressure of the atmosphere within the furnace. Also, it is preferred to apply the heat so that the difference between the furnace temperature and the coil temperature is reduced as the coil temperature increases. This may be accomplished by gradually reducing the difference or by reducing it in a stepwise fashion. If an exceptionally long coil is being processed, for example a coil longer than 60 inches, the heating cycle may include a soak period during which the coil is subjected to a substantially constant heat environment. It is generally advantageous to provide such a soak period when the coil temperature is approximately at least equal to the boiling point of the rolling oil at the existing furnace pressure. The heat transfer in a coil of foil, particularly in a coil of highly reflective foil, is primarily through the end surfaces of the coil longitudinally towards the center of the coil. As a result, the longitudinal center of the coil is the last portion to attain heat. The oil film on this portion is therefore the last to be vaporized and removed. In being removed, the vapor produced from the oil film must pass the hotter outer ends of the coil. Therefore, if the end portions of the coil are allowed to become excessively hot before the fluids are removed from the coil, the foil in these end portions may react with the exiting fluids and become stained or tarnished by them.
During the heating cycle, the protective atmosphere in the furnace preferably is circulated and continuously replenished or maintained. If the coil is heated principally by radiation, then the entire heating cycle may be performed with a partial vacuum in the furnace. However, if a substantial portion of heat is transferred to the coil by means of convection, the first portion of the heating cycle may be performed at about atmospheric pressure and the last portion of the heating cycle may be performed at a reduced pressure, such as about 1.0 to 0.1 p.s.i.a. The first portion of the heating cycle is the portion wherein the coil temperature is raised from ambient temperature to about the boiling point of the rolling oil contained in the coil. The remaining portion of the heating cycle is the last portion wherein the coil temperature is raised to the desired annealing temperature.
After the desired annealing temperature is reached, the heating means may be turned off and then the coil may be cooled to about 400° F. or less and removed from the furnace. Preferably, the cooling is accomplished in a protective atmosphere at about atmospheric pressure or above. A cooled stream of protective atmosphere may be directed against and around the foil coil and circulated past a cooling means, such as a water-cooled heat exchanger.
Various modifications of the above-described embodiments of the invention will be apparent to those skilled in the art, and it is to be understood that such modifications can be made without departing from the scope of the invention, if they are within the spirit and tenor of the accompanying claims.