US3819970A - Low pressure ultraviolet lamp - Google Patents

Abstract

A low pressure ultraviolet lamp with an energy output effective in polymeric resin curing comprises a fused quartz envelope and a fill of mercury, argon and neon.

Description

United States Patent Roche LOW PRESSURE ULTRAVIOLET LAMP William J. Roche, Merrimac, Mass.

GTE Sylvania Incorporated, Danvers, Mass.

Filed: Dec. 27, 1972 App]. No.: 318,827

Inventor:

Assignee:

U.S. Cl. 313/225, 313/185 Int. Cl. H0lj 61/12 Field of Search 313/225, 185

References Cited UNITED STATES PATENTS 12/1939 Alterthum et a1. 313/225 June 25, 1974 12/1939 Alterthum et a1. 313/225 8/1954 Heine et al. 313/225 Primary Examiner.10hn Kominski Assistant Examiner-Darwin R. Hostetter Attorney, Agent, or Firm-James H. Grover ABSTRACT A low pressure ultraviolet lamp with an energy output effective in polymeric resin curing comprises a fused quartz envelope and a fill of mercury, argon and neon.

3 Claims, 1 Drawing Figure LOW PRESSU ULTRAVIOLET LAMP BACKGROUND OF THE INVENTION Low, medium and high pressure ultraviolet (UV) lamps, known to be useful bactericidally, are typically manufactured with an elongate tubular envelope of silica glass or fused quartz with a fill or argon gas and mercury. The gas fill pressure of low pressure lamps is at or below approximately 7 torr, of medium pressure lamps up to about 100 torr, and of high pressure lamps one to several hundred torr. At low pressure the UV output is largely (e.g. 90 percent) at the 2537 A (angstrom) line. Increasing fill pressure shifts the UV output toward longer wavelengths and reduces each line to very low proportion (e. g. about l0 percent) of the UV energy.

In addition to bactericidal uses an important industrial application of ultraviolet lamps is in the photopolymeric curing of solventless inks for high speed printers. Such inks, instead of a relatively slow drying solvent, contain a dry resin which cures and sets, photopolymerically on exposure to light. Practically, UV light is used, infrared light having a heating effect strong enough to damage the printing paper and ink. Further, suitable resins are selectively responsive to particular wavelengths. A lamp providing a suitable UV output must also have a long lived continued output near its initial energy maximum to insure uniform photopolymeric curing. With high speed presses running 500 to 1,000 sheets per minute, the lamp cannot extinguish for more than a few seconds due to momentary power interruption or reduction without a prohibitive loss of uncured sheets. The lamps must restart readily, rapidly and reliably.

The above requirements are not met by existing UV lamps in any of the three pressure ranges.

Prior low pressure lamps with a fill of pure argon gas have too low a UV energy output for curing, and the energy drops off rapidly in the first few hundred operating hours. They also are too slow in restarting to be reliable.

Medium and high pressure UV lamps are much too slow in restarting. In lamps of comparable power their spectral energy is spread over several relatively weak wavelengths some of which are ineffective to cure wavelength-selective resins. Thus although medium and high pressure UV lamps have a relatively higher energy output than low pressure lamp, a large portion of the energy may be useless for curing a particular resin. And the higher energy output also includes more infrared energy in proportion to useful UV, thereby creating a heating problem. Further the high energy output is provided by inordinately higher power input requirements and reactive lamp ballasts of weight and expense out of proportion to that of low pressure lamps.

Accordingly the object of the present invention is to provide a low pressure UV lamp having low power input and ballast requirements, which emits substantially greater UV energy at a useful wavelength remote from daylight and infrared wave-lengths than prior low pressure lamps, which is long lived and fast in restarting, and which is therefore highly practical for use in polymeric curing.

STATEMENT OF INVENTION According to the invention a low pressure ultraviolet lamp achieving the above stated objects comprises a fused quartz envelope, electron emissive electrodes spaced apart within the envelope for supporting an arc discharge therein, and a fill in the envelope of mercury and a mixture of neon gas and between 0.l percent and 10 percent by volume of argon gas at a pressure of ap- O proximately seven torr or less.

DRAWING The single FIGURE is an elevation of a low pressure ultraviolet lamp shown partly in section.

DESCRIPTION The low pressure, high intensity ultraviolet lamp shown in the FIGURE comprises an envelope 1 of fused quartz tubing with, for example, standard T8 dimensions of 1.5 mm. wall thickness, 1 inch outside diameter, and length from a few inches to several feet. Unlike previous low pressure UV lamps the envelope has a press 2 at each end and a side exhaust part 3 which reduce the cost of assembly. Molybdenum leadin wires 4 are welded to molybdenum ribbons 6 within the press 2. Molybdenum wires 7 welded to the ribbons 6 and spaced by a glass insulator 8 extend inwardly to nickel wires 9 forming part of the support structure for one of two electron emissively coated, coiled electrodes 11. The support wires 9 are spaced by a ceramic insulator 12 and hold a disk-shaped heat shield 13 with a feed through insulator 14 and also a rectangular anode 16.

After exhausting and prior to scaling the envelope 1 was filled with 40 milligrams of mercury in a 30 inch lamp and a gas fill of neon and argon at a pressure of between 0.1 and 7 torr.

I have discovered that with a fill of neon gas and the balance of between 0.1 and 10 percent by volume of argon gas a low pressure lamp will operate with a higher input wattage and a markedly higher UV energy output than conventional low pressure UV lamps. A fill of less than 0.1 percent argon does not strike an arc reliably. Above 10 percent the lamp tends to saturate in operation at about 1 to 2 watts per inch power density in a T8 envelope and is limited in energy output to about 65 milliwatts of UV energy per square inch at 1 meter, a level quite impractical for photopolymeric curing. In contrast a fill of 0.1 to 10 percent argon, preferably 0.5 percent, and at 2.5 to 3 torr will not saturate below 9 watts per inch and will emit several times the UV energy, e.g., 416 milliwatts per inch at 1 meter. At or somewhat below 7 torr, emission at 2537A decreases without a compensating improvement in lamp maintenance. This marked increase in output in a low pressure lamp is not predictable from performance of medium and high pressure lamps because of the effect of pressure on spectral distribution, and is wholly effective at one meter or less for photopolymeric curing.

In cooperation with the higher wattage input capacity of the present lamp its quartz envelope affords continued efficient radiation of the emitted UV in a way unrelated to the small difference of UV transmissivity of quartz as compared with silica glass. Quartz can be expected to transmit roughly 10 percent more UV than silica glass at previous wattage input densities. But at the several-fold higher wattage input of the present lamp silica glass discolors at such a rate that external energy output drops off to 65 percent within 500 hours. The useful UV output from the present lamp quartz envelope declines only about 5 percent in 500 hours. This excellent maintenance factor cannot be predicted from either low or higher pressure UV lamp experience of the past.

Of great importance in processes, such as printing, which do not allow lengthy down time is the rapid restarting characteristic of the present UV lamp. In the event of a momentary loss or reduction of electrical power the present lamp will restart in approximately one second on the average. The significantly higher output and faster restarting of the present lamp cannot be explained from experience with medium or high pressure UV lamps which typically require three to five minutes for restarting, a down time which cannot be tolerated in photopolymeric ink curing.

The present lamp compares most favorably with higher pressure lamps in economy of power and ballast. The present lamp is about one-third as intense as a medium pressure lamp up to 3,600 A, but requires about one-fourth the input power, and thus has a better input to output power factor. A standard ballast for a medium pressure UV lamp, however, costs approximately ten times as much and weighs twenty times as much as a ballast for operating the present lamp. Moreover the present lamp, having a low infrared emission, may be placed closer to print being cured without thermal degradation of print or paper, and thus compensate for its lower UV intensity while maintaining the substantial economy in power and ballast cost and bulk.

The foregoing description is illustrative of the several unexpected advantages of the low pressure UV lamp of the present invention falling within the scope of the ap pended claims.

I claim:

1. A low pressure ultraviolet lamp comprising:

a quartz envelope,

electron emissive electrodes spaced apart in the envelope, and a fill of mercury and a mixture of neon and between 0.1 percent and 10 percent argon gas by volume at a pressure of approximately 7 torr or less,

the lamp having an input power density characteristic of approximately nine watts per inch and maintaining an effective external ultraviolet energy output of over milliwatts per square inch at one meter.

has an exhaust spaced from the pressed portion.

Claims (2)

1. 2. A lamp according to claim 1 wherein a conductor through the envelope to the electrode is embedded in a pressed portion of the envelope.
2. 3. A lamp according to claim 2 wherein the envelope has an exhaust spaced from the pressed portion.

Images