US3418155A

US3418155A - Electron discharge control

Info

Publication number: US3418155A
Application number: US491603A
Authority: US
Inventors: Alex D Colvin; Allen H Turner
Original assignee: Ford Motor Co
Current assignee: Energy Sciences Inc
Priority date: 1965-09-30
Filing date: 1965-09-30
Publication date: 1968-12-24
Anticipated expiration: 1985-12-24
Also published as: BE694780A; GB1109023A

Description

1968 L I A. D. COLVIN ETAL 3,418,155

ELECTRON DISCHARGE CONTROL Filed Sept. 30, 1965 2 Sheets-Sheet 1 ALEX D. COL V/N AL L ENH. TURNER INVENTORS A'ITO EYS Dec. 24, 1968 Filed Sept. 30, 1965 D055 PROF/LE5 2 SheetsSheet :2.

47' I7OKV-$PAClNG 2 INCHES (WORK TO NEAREST WINDOW) v 5 1 r 8 .I Q

Pi, R 3 u l0 o-w/rwur HEL/UM CHAMBER mm HEL IUM amuse/e rs nvcfl DEPT/l nv omscr/on/ OF BEAM) POSITION A L EX 0. COL w/v FIG 2 ALLEN H. TURNER ATTORNEYS United States Patent 3,418,155 ELECTRON DISCHARGE CONTROL Alex D. Colvin, Livonia, and Allen H. Turner, Ann Arbor, Mich, assignors to Ford Motor Company, Dearborn, Micln, a corporation of Delaware Filed Sept. 30, 1965, Ser. No. 491,603 9 Claims. (Cl. 117-9331) ABSTRACT OF THE DISCLOSURE Method for irradiating a substance comprising transmitting an electron beam from a zone of lesser pressure through a first electron window, through an expanding zone to a second electron Window and through the latter to the surface to be irradiated. The zone between the first and second electron windows contains a gaseous medium which is maintained at an average density that is substantially below that of air at 1 atmosphere pressure.

This invention relates to an improved electron-discharge device and to a method for eifectively spreading an electron beam issuing from such device while minimizing loss of average beam energy and current.

The use of ionizing energy in the form of high-energy electrons is finding increased application in a variety of processes including those of radiation chemistry, sterilization, preservation, etc. The development of radiationcurable coating compositions, i.e., paints, varnishes, etc., has made possible important advances in the coating field which aside from qualitative benefits provide the advantages of greatly reduced curing times and substantial reductions in space requirements for curing equipment. The degree to which electron-initiated polymerization replaces conventional baking and other curing methods in industrial coating is, however, dependent upon the availability of electron-emission equipment capable of providing efiicient utilization of the power required to provide the polymerization-effecting electrons and effective distribution of the resultant energy in a manner such as to provide a production rate compatible with the intended operation.

A high-energy electron source may be provided by accelerating electrons to high-energy in an evacuated tube, and permitting the high-energy electrons to issue from the tube through an appropriate electron window onto the product to be irradiated. The high-energy electrons may be caused to issue from the tube in the form of a sheet, and the product may be placed on a conveyor which moves the product through the electron sheet transversely thereto. In one such device, electrons are accelerated as a beam within an evacuated tube, and then a rapid scanning movement is imparted to the electron beam before it passes through the electron window and issues from the tube. In another such device an electron beam is focused into sheet form within the tube by a system of cylindrical electron optics. See Robinson, US. Patents 2,602,751 and 2,680,814. Where precise focusing is not essential, the electron-emitting cathode or cathodes may simply be partially enclosed in a suitable housing which restricts and directs the electron sheet to the electron window.

Coating materials which can be cured by electroninitiated polymerization absorb energy eificiently up to a limit inherent in their composition and a further increase in energy reception per unit time will not permit proportionally increased line speed after such limit is reached. Line speed can be substantially increased without increasing the energy output of the electron-discharge device if the beam can be spread without incurring an excessive loss of average beam energy and current. The

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potentials employed in the polymerization of unsaturated coating materials suitable for use in paints, varnishes, etc., are advantageously in the range of about 150,000 to about 450,000 electron volts.

The average energy level of electrons striking the workpiece will be the average energy level they possess upon emerging from the window of the substantially gas-evacuated accelerator minus the energy loss sustained in passing between this window and the workpiece. If the intervening medium is air, the average beam energy falls rapidly, i.e., a loss equivalent to reducing the potential several thousand volts per inch of intervening air, and an increasing proportion of the beam is completely attenuated before reaching the workpiece.

It now has been discovered that in the irradiation of materials having time-limited absorption characteristics, e.g., radiation-curable paints, the line speed of the workpiece can be substantially increased while achieving the desired degree and uniformity of polymerization by interposing a second electron window between the accelerator window and the workpiece and causing the electron beam to pass for a distance sufficient to provide the desired degree of spreading through a gaseous medium the average density of which is maintained substantially below that of air at 1 atmosphere pressure, so as to provide a controlled resistance to uninterrupted flow of electrons that is substantially less than that of 1 atmosphere of air.

In a preferred embodiment, an enclosed zone between the primary and secondary electron windows is charged with a gaseous medium, which consists essentially of a gas that is substantially lighter than air of equal pressure, e.g., hydrogen or helium. At a given pressure, hydrogen provides the least resistance of any gas and hence will permit positioning the gas-evacuated accelerator further from the workpiece at a given potential. Helium, on the other hand, has the advantage of being essentially inert.

In another embodiment, the enclosed zone between the primary and secondary electron Windows is substantially gas-evacuated. Since the vacuum pulled will be at most a partial one, some gas will, of course, be left in the zone. This embodiment provides even less interference to electron flo'w in that it provides an even smaller loss of average energy through collision with gas molecules and the resultant scattering. However, the use of a vacuum chamber introduces the requirement of providing support for the secondary window which in order to be compatible with beam spreading will be greater in area than the window through which electrons emerge from the gas-evacuated emission zone. Reduced pressure operation also necessitates complex pressure controls and gas-tight construction.

In a third embodiment, the gaseous medium consists essentially of a heavier gas, e.g., CO employed at a pressure sufficiently reduced to lower the resistance of such gas to electron flow through the enclosed zone to a level substantially below that of 1 atmosphere of air.

The straight-line distance between the primary and secondary windows is advantageously in excess of about 2 inches and in excess of about 50% of the total distance between the primary window and the workpiece.

To avoid repetitious detail that would be modified only in obvious changes necessitated by a change in the composition or pressure of the gaseous medium in which beam spreading is effected, this invention is hereinafter primarily described with relation to that embodiment in which such medium conesists essentially of helium.

This invention is carried out effectively by providing an enclosed chamber of helium between the metal window to the gas-evacuated, electron-emision chamber and a.

second and larger window positioned nearer the workpiece. The second window is a sheet material offering a resistance to the transmission of high-energy electrons therethrough that is not substantially in excess of that of the first window to the same.

The average energy loss through helium is only about of that in air at equal pressure and the degree of scattering is also reduced. Although gains can still be made at helium pressures approaching but significantly below that pressure level at which passage through the helium reduces the average beam energy to a degree equal to the loss sustained in 1 atmosphere of air for the same distance, it is advantageous to employ a helium pressure of less than about 1.5 atmospheres. While the energy loss is even less in a zone of subatmospheric helium pressure, the distance-pressure combination must be such as to provide the desired degree of beam spreading. A pressure of about 1 atmosphere is preferred since it introduces no additional stress upon the primary and secondary electron windows and eliminates the need for complex pressure-control equipment. These pressures also refer to the total gas pressure within the enclosed chamber.

It is one object of this invention to provide method and means for polymerizing a radiation-curable coating material which permits a significant increase in line speed without significantly increasing the energy transmittal from the irradiation means.

It is another object of this invention to provide method and means for polymerizing coating materials with polymerization-effecting electrons in which the efliciency of energy utilization is maximized.

These and other objects and advantages of this invention will be more easily understood by reading the following detailed description in connection with the accompanying drawings, wherein:

FIGURE 1 is a partial, schematic view of an electrondischarge device comprising an electron accelerator and a gas-containing electron distributor, portions of said device being shown in section, in the process of transmitting a beam of high-energy electrons to the surface of a sheet material passing transversely thereto upon a moving conveyor; and

FIGURE 2 is a graphic representation of radiation distribution with and without the use of the electron distributor of this invention.

'Referring now to the drawing, there is shown the lower end of an electron-accelerator tube 11 comprising a main housing 13 containing a cathode assembly 15. Cathode asembly 15 comprises a cathode housing 17 having an elongated aperture 19 extending along a major portion of its lower side. Positioned within housing 17 is a pair of spaced-apart bus bars 21 and 23 which hold between them in electrical connection therewith a plurality of tungsten-wire filaments 25 which serve as cathodes. Aperture 19 is of a size and configuration such as to direct a sheet of electrons emitted by filaments 25 to the first window area. In embodiments employing a scanned beam, a changing magnetic field is employed to direct the electron beam so as to achieve the desired distribution of electrons at the window surface. In electrical connection with bus bars 21 and 23, respectively, are conductors 27 and 29 each of which in operation are in electrical connection with the negative terminal of a direct current electrical power source, not shown, and insulated from housing 13 and housing 17. The energy delivered to the negative leads 27 and 29 is controlled by conventional electrical means so that a-slight difference of electrical potential, e.g. 5 volts, is maintained between negative leads 27 and 29 to establish a current through the filaments 25.

A conductor 31 provides the positive lead and is in electrical connection with housing 13 and with ground.

Afiixed to the bottom end of housing 13 by suitable fastener means, e.g. belts, clamps, screws, etc., is window retainer 35 which supports a window-forming sheet 33, a thin metal sheet which may be of aluminum, lithium, titanium, beryllium, an alloy, e.g. aluminum and copper, aluminum and beryllium, magnesium and thorium, stainless steel, etc. Window retainer 35 is provided with a centrally-positioned aperture 37 which frames and defines the accelerator window. Window-forming sheet 33 is in electrical connection with housing 13. Window retainer 35, window-forming sheet 33 and housing 13 are fastened together as hereinbefore described using, Where necessary, suitable sealing means, e.=g. gaskets, sealing rings, etc., so as to form a vacuum-tight seal of the lower end of housing 13. Housing 13, window-forming sheet 33 and window retainer 35 form an emission chamber 39. Emission chamber 39 is essentially gas-tight and for electron transmission is substantially gas-evacuated by conventional conduit and pumping means, not shown, e.g. to an air pressure as low as about 10 Hg. The metal electron window through which the high-energy electrons issue from the acceleration tube is as thin as feasible in order that the electrons may pass therethrough with a minimum loss of energy. On the other hand, the window must have a sufiicient mechanical strength to withstand a pressure differential of about 1 atmosphere since the interior will be exposed to the evacuated emission chamber 39 and the exterior may be exposed to a pressure of about 1 atmosphere.

Positioned below window retainer 35 is a gas-containing electron distributor 41 comprising a flared or frustrum-like housing 43, a window-forming sheet 45 and a window retainer 47. Window retainer 47 is provided with a centrally-positioned aperture 49 which frames and defines the secondary or electron-distributor window. Housing 43 is aflixed to housing 13 and window retainer 35 by suitable fastener and/or sealing means, so as to form a vacuum-tight seal with window retainer 35 and/ or housing 13. Window retainer 43 has an outwardly-extending flange 51. Window-forming sheet 45 is preferably a thin metal sheet the composition of which may be the same or different from that of window-forming sheet 33, or, it may be formed from a thin sheet of organic polymeric material. The metal windows are preferred, primarily because of their greater operational life span. When a beam of electrons passes through a solid sheet material, a redistribution of the electron pattern inevitably results. This redistribution effect differs with both the thickness and composition of the window-forming sheet material. The window of an electron-distribution chamber and its position in relation to the substance to be irradiated may be tailored to maximize the efliciency of a given irradiation process. Window-forming sheet 45 and window retainer 47 are affixed to flange 51 of housing 43 by suitable fastener and/or sealing means so as to close the lower and larger opening into housing 43 and, preferably, to form therewith a vacuum-tight seal. Housing 43, window-forming sheet 33, window retainer 35, window-forming sheet 45 and window retainer 47 form electron-distribution chamber 53. Positioned within the sides of housing 43 is an inlet conduit and an outlet conduit 57. Chamber 53 may be evacuated by pumping means, the desired amount of gas admitted and conduits 55 and 5'1" sealed, or a continuous flow of gas through inlet conduit 55, electron-distribution chamber 53 and outlet conduit 57 may be regulated to maintain the desired gas pressure in chamber 53.

Also shown, in FIGURE 1, is a conveyor belt 61 and a sheet of plywood 63 passing through an indicated electron beam.

The advantages of the hereinbefore and hereinafter described invention will be more fully understood from the following examples:

Example 1 A silicone-modified polyester, paint-binder resin is prepared in the following manner.

To a reaction vessel are charged the following materials:

Neopentyl glycol grams 1250 Polysiloxane do 625 Xylene ml 200 The polysiloxane employed was a commercially available (Dow Corning Z-60l8) hydroxy-functional, cyclic, polysiloxane having the following properties:

Hydroxy content, Dean-Stark:

Percent condensible 5.5 Percent free 0.5 Average molecular weight 1600 Combining weight 400 Refractive index 1.531 to 1.539 Softening point, Durrans Mercury Method, degrees At 60% solids in xylene Specific gravity at 77 F. 1.075 Viscosity at 77 F., centipoises 33 Gardner-Holdt A-l The charge is heated to 325 to 350 F. and maintained in such temperature range for about 2 hours.

Then, at about 325 F., 245 grams of maleic anhydride, 967 grams of tetrahydrophthalic anhydride and about 3.05 grams of dibutyl tin oxide are added to the charge and the temperature is maintained at about 325 F. for 1 hour.

The temperature is t en increased to about 425 F. and the xylene is stripped off with CO This temperature is maintained until the acid number of the resulting resin is about 25. A vacuum is pulled to about 5-10 mm. Hg. When the acid number is less than about 10, there is added about 0.58 gram hydroquinone.

The charge is cooled to about 180 to 190 F. and 725 grams of styrene are added.

This binder is sprayed upon metal panels to an average depth of about 0.75 to 1.0 mil and irradiated under the conditions hereinafter set forth both with and without employment of a helium-charged (1 atmosphere) electron-distribution chamber.

The tests are carried out with the electron accelerator operating at 170 kv. The interior pressure of the gasevacuated emission chamber is in the range of about 2.5 to about 5 X10 mm. Hg. The accelerator window is an aluminum-copper alloy of 0.001 inch thickness containing 4.5% copper, 1.5% magnesium, 0.6% manganese, reminder essentially all aluminum. This window is essentially rectangular in shape and measures about 1 x 12 inches. A helium-charged, electron-distribution chamber extends 6 inches from the accelerator window. The distributor window is a sheet of aluminum about 0.001 inch in thickness. This window measures about 11 x 20.5 inches. The exterior window of the electron-discharge device is in each test positioned about 2 inches from the workpiece. The workpiece is in air. The other conditions employed and the curing results obtained are set forth in the following table.

Dose profile studies are made with and without the helium-charged chamber under the same conditions as employed in Example 1 except that the workpiece is stationary. The data points are determined by exposing to the beam a radiation-sensitive sheet material (Cellophane300 MSC BlueDupont) the light transmission' characteristics of which undergo change relatable to radiation dosage. This material is subjected to radiation until measurable change is provided in regions of high and low energy absorption. These changes are determined by spectrophotometer and the data obtained is normalized to provide coincidence of peak intensities. The results of these studies are graphically illustrated in FIGURE 2 of the drawings.

Example 3 An electron-disdharge device is operated as in Example 1 except that helium-containing electron-distribution chambers having depths of 4, 8, l0 and 12 inches are employed.

Example 4 An electrondischarge device is operated as in Example 1 except that the electron-distribution chamber is essentially evacuated and then charged with helium to a pressure of about 0.5 atmosphere to reduce the stress upon the accelerator window resulting from the pressure differential created by the evacuated state of the emission chamber, the electron window through which the beam exits from the electron-distribution chamber being internally supported at a plurality of sites intermediate its periphery.

Example 5 An electron-discharge device is operated as in Example 1 except that the electron-distribution chamber is essentially evacuated and then charged with helium to a pressure of about 1.5 atmospheres, the electron window through which the beam exits from the electron-distribution chamber being externally supported at a plurality of sites intermediate its periphery.

Example 6 An electron-discharge device is operated as in Example 1 except that hydrogen is employed in lieu of helium in the electron-distribution chamber.

Example 7 An electron-discharge device is operated as in Example 1 except that the electron-distribution chamber is substantially gas-evacuated and sealed, the electron window through which the beam exits from the electron-distribution chamber being supported within said chamber at a plurality of sites intermediate the periphery thereof with a metal grid.

Example 8 An electron-discharge device is employed as in Example 1 except that the window of the electron-distribution chamber is a sheet of organic polymer, a copolymer of ethylene glycol and terephthalic acid, of about 0.0007 inch thickness.

Example 9 An electron-discharge device is operated as in Example 1 except that the electron-distribution chamber is substantially gas-evacuated and then charged with carbon dioxide to a pressure of about 0.1 atmosphere, the electron window through which the beam exits from the electrondistribution chamber being supported from within said chamber at a plurality of sites intermediate the periphery thereof.

The term electron window as employed herein refers to a sheet material through which high-energy electrons may be transmitted without excessive loss of energy. In no instance herein does such term refer to an open aperture. High-energy electrons emerging from a metal window into air promotes the formation of ozone and oxides of nitrogen which may chemically attack the window material and shorten window lift. The enclosed chamber for electron spreading will serve the further function of protecting the metal window to the gas-evacuated electronemission zone from chemical attack, especially where the chamber is charged with an inert gas such as helium. This window is more susceptible to failure through corrosion weakening than is the window tlnough which the beam leaves the electron-distribution chamber especially in those embodiments where the former is subjected to a greater pressure diiferential. In all embodiments, the electron beam is concentrated in a smaller area when passing through the first window. If the electron'distribution chamber is operated at atmospheric pressure, the window through which electrons exit this chamber can be replaced much easier than changing the window to the gasevacuated electron-emission zone.

It will be understood that the invention is not limited to the embodiments illustrated in the foregoing examples and that changes and modifications can be made in the construction and operation of the devices hereinbefore described without departing from the spirit and scope of the invention as defined in the appended claims.

We claim:

1. In a method for irradiating a substance which comprises transmitting a beam of high-energy electrons from a zone of lesser pressure through a first electron window to said substance in a zone of greater pressure, the improvement which comprises interposing between said substance and said first electron window, enclosure means and a second electron window transversely disposed in relation to the longitudinal axis of said beam and presenting to said beam a greater surface area than said first electron window, said second electron window and said enclosure means describing an enclosed zone extending from said first window to said second window that gradually increases in cross sectional area in approaching said second window, and in said enclosed zone maintaining a gaseous medium at a pressure such that its average density is substantially below that of air at 1 atmosphere pressure.

2. The method of claim 1 wherein said gaseous medium is helium.

3. The method of claim 1 wherein said gaseous medium is hydrogen.

4. The method of claim 1 wherein said gaseous medium is carbon dioxide.

5. In a method for irradiating a substance which comprises transmitting a beam of high-energy electrons from a zone of lesser pressure through a first electron window to said substance in a zone of greater pressure, the improvement which comprises interposing between said substance and said first electron window enclosure means and a second electron window transversely disposed in relation to the longitudinal axis of said beam and presenting to said beam a greater surface area than said first electron window, said second electron window and said enclosure means describing an enclosed zone extending from said first window to said second window that gradually increases in cross sectional area in approaching said second window, and in said enclosed zone maintaining a gaseous medium at a pressure such that its average density is substantially below that of air at 1 atmosphere pressure, the distance between said first electron window and said second electron window being in excess of about 2 inches.

6. In a method for irradiating a substance which comprises transmitting a beam of electrons having an average potential in the range of about 150,000 to about 450,000 volts from a zone of lesser pressure through a first electron window to said substance in a zone of greater pressure, the improvement which comprises interposing between said substance and said first electron window enclosure means and a second electron window transversely disposed in relation to the longitudinal axis of said beam and presenting to said beam a greater surface area than said first electron window, said second electron window and said enclosure means describing an enclosed zone extending from said first window to said second window that gradually increases in cross sectional area in approachingsaid second window, and in said enclosed zone maintaining a gaseous medium at a pressure such that its average density is substantially below that of air at 1 atmosphere pressure, the distance between said first electron window and said second electron window being in the range of about 4 to about 12 inches.

7. In a method for irradiating a coating upon a substrate which comprises transmitting a beam of high-energy electrons from a zone of lesser pressure through a first electron window to said coating in a zone of greater pressure, the improvement which comprises interposing between said coating and said first electron window enclosure means and a second electron window transversely disposed in relation to the longitudinal axis of said beam and presenting to said beam a greater surface area than said first electron window, said second electron window and said enclosure means describing an enclosed zone extending from said first window to said second window that gradually increases in cross sectional area in approaching said second window, and in said enclosed zone maintaining a gaseous medium at a pressure such that its average density is substantially below that of air at 1 atmosphere pressure, the distance between said first electron window and said second electron window being above about 50% of the distance between said first electron window and said coating.

8. An electron-discharge device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons pass when said emission chamber is substantially gas-evacuated and a difierence of electrical potential is provided between said emission means and said metal window sufficient to initiate said stream, a second electron window presenting a greater surface area to an electron beam passed through said first electron window than that presented 'by said first electron window to said beam and spaced apart from said first electron window opposite said emission means, enclosure means extending from said first electron window to said second electron window and with said first electron window and said second electron window defining an electron-distribution chamber that gradually in creases in cross sectional area in approaching said second electron window, and conduit means communicating with said chamber through which gas may be introduced into and withdrawn from said chamber.

9. An electron-discharge device comprising a housing having an aperture, an electron-emission means within said housing and spaced apart from said aperture, and a metal electron window closing said aperture, essentially defining with said housing an essentially gas-tight emission chamber containing said emission means, and providing exit means from said housing through which a stream of high-energy electrons pass when said emis sion chamber is substantially gas-evacuated and a difference of electrical potential is provided between said emission means and said metal window sufficient to initiate said stream, presenting a greater surface area to an electron beam passing through said first electron window than that presented by said first electron window and a second electron window spaced apart from said first electron window opposite said emission means, enclosure means extending from said first electron window to said second electron window and with said first electron window and said second electron window defining a ubstantially gas-tight electron-distribution chamber, and conduit means communicating with said chamber through which gas may be introduced into and withdrawn from said chamber, said second electron window presenting a significantly greater surface area to said beam than 10 said first electron window and said chamber significantly 2,820,165 1/1958 Robinson 313-74 X increasing in cross sectional area as it approaches said 3,188,229 6/1965 Graham 117-93.31 X second electron window.

' ALFRED L. LEAVITT, Primary Examiner. References Cited 5 J. H. NEWSOME, Assistant Examiner. UNITED STATES PATENTS 2,617,953 11/1952 Brasch 313-44 x 2,724,059 11/1955 Gale 3'1374 X 204158,159.11;313--74