Photoflash Lamp Coating

Abstract

A photoflash lamp having a protective coating over its glass envelope comprising a vacuum-formed thermoplastic sleeve with longitudinally extending ribs disposed between the plastic sleeve and glass envelope for reducing stress levels in the formed sleeve and for providing thermally insulating air gaps between the sleeve and envelope.

Description

BACKGROUND OF THE INVENTION

This invention relates to photoflash lamps and, more particularly, to an improved protective coating for flashlamps.

A typical photoflash lamp comprises an hermetically sealed glass envelope, a quantity of combustible material located in the envelope, such as shredded zirconium or hafnium foil, and a combustion supporting gas, such as oxygen, at a pressure well above one atmosphere. The lamp also includes an electrically or percussively activated primer for igniting the combustible to flash the lamp. During lamp flashing, the glass envelope is subject to severe thermal shock due to hot globules of metal oxide impinging on the walls of the lamp. As a result, cracks and crazes occur in the glass and, at higher internal pressures, containment becomes impossible. In order to reinforce the glass envelope and improve its containment capability, it has been common practice to apply a protective lacquer coating on the lamp envelope by means of a dip process. To build up the desired coating thickness, the glass envelope is generally dipped a number of times into a lacquer solution containing a solvent and a selected resin, typically cellulose acetate. After each dip, the lamp is dried to evaporate the solvent and leave the desired coating of cellulose acetate, or whatever other plastic resin is employed.

In the continuing effort to improve light output, higher performance flashlamps have been developed which contain higher combustible fill weights per unit of internal envelope volume, along with higher fill gas pressures. In addition, the combustible material may be one of the hotter burning types, such as hafnium. Such lamps, upon flashing, appear to subject the glass envelopes to more intense thermal shock effects, and thus require stronger containment vessels. One approach to this problem has been to employ a hard glass envelope, such as the borosilicate glass envelope described in U.S. Pat. No. 3,506,385, along with a protective dip coating. Although providing some degree of improvement in the containment capability of lamp envelopes, the use of dip coatings and hard glass present significant disadvantages in the areas of manufacturing cost and safety.

To overcome these disadvantages, a more economical and significantly improved containment vessel for flashlamps is described in a copending application Ser. No. 268,576, filed July 3, 1973 and assigned to the assignee of the present application. According to this previously filed application, a thermoplastic coating, such as polycarbonate, is vacuum formed onto the exterior surface of the glass envelope. The method of applying the coating comprises: placing the glass envelope within a preformed sleeve of the thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the glass envelope; and, simultaneously heating the assembly incrementally along its length, whereby the temperature and vacuum cause the thermoplastic to be incrementally formed onto the glass envelope with the interface substantially free of voids, inclusions and the like. This method provides an optically clear protective coating by means of a significantly faster, safer and more economical manufacturing process, which may be easily integrated on automated production machinery. The process permits use of the stronger, more temperature resistant thermoplastics, and the resulting coating maintains the glass substrate under a compressive load, thereby making the glass envelope itself more resistant to thermal shock. As a result, this coating reduces the cost of materials by permitting the use of soft glass to meet high containment requirements.

In general, thermoplastic materials have a coefficient of thermal expansion several times greater than the coefficient of thermal expansion of the glass envelope. Hence, as the thermoplastic coating cools from the softening temperature subsequent to vacuum forming, it will exert a compressive load on the envelope to thereby in effect strengthen the glass. For example, the thermoplastic coating may exert a compressive load of from 1,000 to about 4,000 pounds per square inch on the glass envelope. Although the glass becomes stronger with a higher compressive load, an increase in the compressive loading on the glass results in a corresponding increase in the tensile loading on the coating. Typically, these tension stresses in the coating may be approximately 2,000 to 3,000 pounds per square inch. In itself, this loading appears acceptable if uniform throughout the coating. In actual practice, however, higher localized stresses appear to develop, probably due to irregularities in the glass, friction between the plastic and glass, and irregularities on the inner surface of the plastic.

In order to relieve these local points of high stress and provide a more uniform compressive loading on the glass envelope, a copending application Ser. No. 287,724, filed Sept. 11, 1972 now U.S. Pat. No. 3,770,366 and assigned to the assignee of the present application, describes the use of a thin layer of silicone mold release between the glass and thermoplastic material. The mold release agent appears to lubricate the glass-plastic interface and permit the adjacent surfaces to slide over one another, thereby tending to equalize the stresses.

It is also desirable, however, to predictably control the overall compressive loading on the glass and the corresponding tensile loading in the coating, i.e., the average level of the stresses remaining after the thermoplastic has been vacuum-formed onto the glass envelope of the lamp and cooled. A plasticizer may be added to the composition of the thermoplastic to accomplish this purpose; however, in certain of the thermoplastics particularly suitable for this application, it has been found difficult to control the behavior of the plasticizer so as to avoid producing adverse effects. An improved method for controlling the average stress level is described in a copending application Ser. No. 289,446, filed Sept. 15, 1972 and assigned to the present assignee, wherein a secondary stress relief operation is added to the coating process. After the thermoplastic coating has cooled and contracted from the vacuum-forming process, a narrow band of the coating is heated longitudinally and/or circumferentially. This heated band of coating yields to relieve the stresses generated by contraction of the remainder of the coating which remains cool. Upon subsequent cooling of the heated band, it contracts and again generates tensile stresses in the coating. In this instance, however, the developed stresses are lower than those produced from cooling of the entire coating. As a result, the average tensile stresses in the coating are relieved to some degree. In many applications, however, additional stress relief is required to assure that the structural integrity of the coating is maintained over long term aging.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a photoflash lamp having a stronger reinforcing coating with enhanced aging characteristics.

It is a particular object of the invention to provide a vacuum formed thermoplastic coating for a flashlamp which includes a means for significantly self-relieving the stresses in the coating resulting from the vacuum forming process.

Another object of the invention is to provide an improved containment vessel for a flashlamp.

These and other objects and features are attained, in accordance with the invention, by disposing one or more longitudinally extending ribs between the vacuum-formed thermoplastic coating and the glass envelope. For example, the ribs may be molded on the inner surface of the plastic sleeve which is formed onto the envelope. Preferably, only the ribs contact the glass, and upon cooling, contraction of the plastic sleeve is accommodated by flexure of the sleeve wall between adjacent ribs, thereby self-relieving stresses in the coating. In addition, the ribs cause small air gaps to be formed between portions of the coating and the glass surface of the flashlamp envelope which tend to insulate the plastic coating and keep it somewhat cooler, and thus stronger, during flashing.

BRIEF DESCRIPTION OF THE DRAWINGS

This invention will be more fully described hereinafter in conjunction with the accompanying drawings, in which:

FIG. 1 is an enlarged sectional elevation of an electrically ignitable photoflash lamp having a protective coating in accordance with the invention;

FIG. 2 is an enlarged sectional elevation of a percussive-type photoflash lamp having a protective coating in accordance with the invention;

FIG. 3 is a cross-section of the coated envelope wall of the lamp of FIG. 1 or FIG. 2 showing the ribs disposed between the coating and envelope in accordance with the invention;

FIG. 4 is an enlarged sectional elevation of a preformed sleeve of thermoplastic having internal ribs according to the invention and adapted for assembly and vacuum forming onto the glass envelope of a percussive-type photoflash lamp, the section of FIG. 4 being taken on line 4--4 of FIG. 5;

FIG. 5 is an end view of the preformed sleeve of FIG. 4; and

FIG. 6 is an enlarged elevation, partly in section, showing a percussive flashlamp assembled in the thermoplastic sleeve of FIGS. 4 and 5 prior to vacuum forming.

DESCRIPTION OF PREFERRED EMBODIMENT

The teachings of the present invention are applicable to either percussive or electrically ignited photoflash lamps of a wide variety of sizes and shapes. Accordingly, FIGS. 1 and 2 respectively illustrate electrically ignited and percussive-type photoflash lamps embodying the principles of the invention.

Referring to FIG. 1, the electrically ignitable lamp comprises an hermetically sealed lamp envelope 2 of glass tubing having a press 4 defining one end thereof and an exhaust tip 6 defining the other end thereof. Supported by the press 4 is an ignition means comprising a pair of lead-in wires 8 and 10 extending through and sealed into the press. A filament 12 spans the inner ends of the lead-in wires, and beads of primer 14 and 16 are located on the inner ends of the lead-in wires 8 and 10 respectively at their junction with the filament. Typically, the lamp envelope 2 has an internal diameter of less than 1/2 inch, and an internal volume of less than 1 cc., although the present invention is equally suitable for application to larger lamp sizes. A combustion-supporting gas, such as oxygen, and a filamentary combustible material 18, such as shredded zirconium or hafnium foil, are disposed within the lamp envelope. Typically, the combustion-supporting gas fill is at a pressure exceeding one atmosphere, with the more recent subminiature lamp types having oxygen fill pressures of up to several atmospheres. As will be detailed hereinafter, the exterior surface of glass envelope 2 is covered by a vacuum-formed thermoplastic coating 20, with a plurality of longitudinally extending ribs 21 disposed between the thermoplastic coating and the glass envelope in accordance with the invention.

The percussive-photoflash lamp illustrated in FIG. 2 comprises a length of glass tubing defining an hermetically sealed lamp envelope 22 constricted at one end to define an exhaust tip 24 and shaped to define a seal 26 about a primer 28 at the other end thereof. The primer 28 comprises a metal tube 30, a wire anvil 32, and a charge of fulminating material 34. A combustible 36, such as filamentary zirconium or hafnium, and a combustion supporting gas, such as oxygen, are disposed within the lamp envelope, with the fill gas being at a pressure of greater than one atmosphere. As will be detailed hereinafter, the exterior surface of glass envelope 22 is covered by a vacuum-formed thermoplastic coating 46, with a plurality of longitudinally extending ribs 48 disposed between the thermoplastic coating and the glass envelope in accordance with the invention.

The wire anvil 32 is centered within the tube 30 and is held in place by a circumferential indenture 38 of the tube 30 which loops over the head 40, or other suitable protuberances, at the lower extremity of the wire anvil. Additional means, such as lobes 42 on wire anvil 32 for example, may also be used stabilizing the wire anvil, supporting it substantially coaxial within the primer tube 30 and insuring clearance between the fulminating material 34 and the inside wall of tube 30. A refractory bead 44 is fused to the wire anvil 32 just above the inner mouth of the primer tube 30 to eliminate burn through and function as a deflector to deflect and control the ejection of hot particles of fulminating material from the primer. The lamp of FIG. 2 is also typically a subminature type having envelope dimensions similar to those described with respect to FIG. 1.

Although the lamp of FIG. 1 is electrically ignited, usually from a battery source, and the lamp of FIG. 2 is percussion-ignitable, the lamps are similar in that in each the ignition means is attached to one end of the lamp envelope and disposed in operative relationship with respect to the filamentary combustible material. More specifically the igniter filament 12 of the flash lamp in FIG. 1 is incandesced electrically by current passing through the metal filament support leads 8 and 10, whereupon the incandesced filament 12 ignites the beads of primer 14 and 16 which in turn ignite the combustible 18 disposed within the lamp envelope. Operation of the percussive-type lamp of FIG. 2 is initiated by an impact onto tube 30 to cause deflagration of the fulminating material 34 up through the tube 30 to ignite the combustible 36 disposed within the lamp envelope.

As shown in FIG. 3, the plurality of ribs 48 or 21 are equally spaced about the glass envelope 22 or 2, and adjacent ribs support an extent 50 of coating wall therebetween which provides flexure for relieving stresses in the coating. This configuration also has the advantageous byproduct of providing small air gaps 52 between the coating and glass surface which tend to thermally insulate the coating and keep it somewhat cooler (and thereby stronger) during flashing than is the case when the coating is in full thermal contact with the glass surface.

One method of providing such a rib-supported lamp coating is illustrated by FIGS. 4-6. Referring first to FIGS. 4 and 5, the thermoplastic material to be coated on the exterior surface of the lamp envelope is initially provided as a preformed sleeve 54 having the shape of a test tube. To facilitate the one or more metallic members depending from the lamp envelope (i.e. leads 8 and 10, or primer tube 30) one or more holes are provided at the bottom of the test tube shaped sleeve. For purposes of example, the method of FIGS. 4-6 will be described with reference to vacuum forming the thermoplastic coating 46 on the percussive lamp of FIG. 2, although it will be understood that a similar method may be employed with the electrically ignited lamp of FIG. 1. Accordingly, sleeve 54 is provided with a single coaxially disposed hole 56 to facilitate passage of the coaxially projecting primer tube 30. The several longitudinally extending ribs 48 are molded or otherwise formed on the inner surface of sleeve 54 and equally spaced thereabout as shown. Sleeve 54 may be formed by a molding process, and to minimize possible checks and crazes in the plastic upon being vacuum formed to the glass envelope, the preformed sleeve 48 generally should be prebaked at about 125.degree.C for at least 15 minutes to drive away residual moisture prior to assembly with the glass envelope. In most cases, however, such prebaking is not required for thermoplastic sleeves containing a plasticizer.

In the next step, shown in FIG. 6, the glass envelope 22 of the percussive lamp is placed within the preformed thermoplastic sleeve 54, with the primer tube 30 projecting through hole 56. It will be noted that both the sleeve 54 and the lamp envelope 22 have generally tubular sidewalls. To facilitate the vacuum forming process, the fit should be as close as possible.

The next step is heating and vacuum forming. The envelope and sleeve assembly 22, 54 is held during the evacuating and heating processes by means of a chuck gripping the primer tube 30. Another chuck, having an evacuating tube, grips the open end of the thermoplastic sleeve 54. One or more localized sources of heat encircle the envelope and sleeve assembly for uniformly applying heat about the tubular sleeve in a substantially localized elevational plane. In operation, the process comprises drawing a vacuum in the space between the sleeve 54, and envelope 22, while simultaneously heating the envelope and sleeve assembly incrementally along its length. More specifically, the vacuum is drawn at the open end of sleeve 54, while at the same time, the heaters are controlled to heat the sleeve to approximately the softening temperature of the thermoplastic material. A relative incremental axial movement is effected between the envelope-sleeve assembly and the heaters, so that incremental heating in a localized elevational plane starts at the end of the sleeve 54 through which the primer tube 30 projects, and then proceeds toward the open end of the sleeve from which the vacuum is being drawn. By proper adjustment of vacuum and forming temperature, the sleeve conformity to the glass is controlled so that only the sleeve ribs 48 contact the glass. At the conclusion of the incremental heating process, the sleeve 54 is constricted and tipped off at 58 (FIG. 2), thereby completing the encapsulation of glass envelope 22 in the thermoplastic coating 46.

Upon cooling, the contraction of the coating 46 is accommodated by flexure of the coating wall 50 between adjacent ribs 48. FIG. 3 illustrates the final, non-cylindrical form of the coating 46. This accommodation of shrinkage may be viewed in terms of the length difference between a segment of the circumference of a circle and its corresponding chord. Accordingly, the ribbed coating is self-relieving to provide the desired low stress levels without the secondary operations of applying a lubricant to the glass surface or additional stripe heating and cooling steps. In addition, as previously discussed, the physical properties of the thermoformed coating are further enhanced by the small thermally insulating air gaps 52 provided by the rib structure.

Polycarbonate resin has been found to be particularly suitable for the ribbed coating, however, the composition of sleeve 54, and thus coating 46, may be of any vacuum formable light-transmitting thermoplastic material having a reasonably high impact strength and softening temperature. Suitable materials include acrylic, acrylonitrile-butadiene-styrene, cellulose acetate, ionomers, methylpentene polymer, nylon, polycarbonate, polystyrene, polysulfone, or alloys thereof. In the case of some of the harder materials, it may also be desirable to add a small amount (1-20%) of a compatible plasticizer to the composition. Further, commercial blue dyes can be used in the sleeve, or coating, for color corrections desirable with various photographic color film.

The principles of this invention could be used in combination with interfacial lubrication or localized heating to provide stress relief if so desired. The number of ribs is optimally from six to 12, however, even a single rib would provide some flexural relief or stress. Above 30 ribs, the difference between the sum of chords and the circumference of the circumscribed circle is insufficient to accommodate the shrinkage of most thermoplastics over a glass object. Resultant stress level may thereby be controlled by selection of the number of internal ribs in the sleeve. Rib height, or radially inward extension from the sleeve inner diameter, is chosen such that formation of a straight chord would result in tangency or near tangency with the outer wall of the glass envelope midway between the ribs.

An alternative to the use of internal ribs in a cylindrical sleeve would be external ribs on the glass envelope. A second alternative would be insertion of a grid of wire or transparent material between the glass envelope and plastic sleeve to function in like manner to the ribs. A third alternative would be to mold the sleeve in the form of an undulating or corrugated wall rather than as an essentially cylindrical wall with thickened sections or ribs as described herein.

Although the invention has been described with respect to specific embodiments, it will be appreciated that modifications and changes may be made by those skilled in the art without departing from the true spirit and scope of the invention.

Claims

What we claim is:

1. A photoflash lamp comprising an hermetically sealed glass envelope, a combustion-supporting gas in said envelope, a quantity of combustible material located in said envelope, ignition means attached to said envelope and disposed in operative relationship to said combustible material, a vacuum-formed thermoplastic coating on the exterior surface of said glass envelope, and rib means disposed between said thermoplastic coating and said glass envelope.

2. A lamp according to claim 1 wherein said rib means comprises a plurality of longitudinally extending ribs.

3. A lamp according to claim 2 wherein said ribs are equally spaced about said envelope whereby the extent of coating wall supported between adjacent ribs provides flexure for relieving stresses in said thermoplastic coating.

4. A lamp according to claim 2 wherein said ribs provide thermally insulating air gaps between said envelope and said coating.

5. A lamp according to claim 1 wherein the composition of said coating comprises a light-transmitting thermoplastic selected from the group consisting of acrylic, acrylonitrile-butadiene-styrene, cellulose acetate, ionomers, methylpentene polymer, nylon, polycarbonate, polystyrene, polysulfone, and alloys thereof.

6. A lamp according to claim 1 wherein said coating comprises a preformed sleeve of thermoplastic material having a plurality of longitudinally extending ribs molded on its inner surface to provide said rib means, said interiorly ribbed sleeve having been vacuum-formed onto said glass envelope.

7. A lamp according to claim 6 wherein said envelope and said sleeve are substantially tubular in shape, said molded ribs are equally spaced about the inner surface of said tubular sleeve, the radially inward extension of each of said ribs from said sleeve is selected such that if a straight chord were drawn between adjacent ribs on the inside diameter of said tubular sleeve it would lie substantially tangent to the outer wall of said glass envelope midway between the adjacent ribs, and the number of said ribs is selected to provide a desired amount of stress relief in said vacuum-formed thermoplastic coating.

8. A lamp according to claim 7 wherein the thermoplastic material of said coating comprises a polycarbonate resin.

9. A lamp according to claim 1 wherein said combustion-supporting gas in said envelope is at a pressure exceeding one atmosphere, and said combustible material in said envelope is filamentary.