Photoflash lamp and method of coating same

Abstract

In a photoflash lamp having a vacuum-formed thermoplastic coating over its glass envelope for providing reinforcement, a silicone mold release agent is disposed between the plastic coating and glass envelope to relieve high localized stresses and provide a more uniform compressive loading on the glass. In a method for applying the thermoplastic coating, the silicone release agent is sprayed onto the glass envelope of the lamp before it is located in a preformed sleeve of the thermoplastic material, which is subsequently vacuum-formed onto the glass envelope.

Parent Case Text

This is a division, of application Ser. No. 287,724, filed Sept. 11, 1972 now U.S. Pat. No. 3770366.

Description

BACKGROUND OF THE INVENTION

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

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 more volatile types, such as hafnium. Such lamps, upon flashing, appear to subject to 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, 1972 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 thermplastics, and the resulting coating maintains the glass substrate under a compressive load, thereby making the glass envelope itself more resistant to failures. As a result, this coating reduces the cost of materials by permitting the use of soft glass to meet high containment requirements.

The thermoplastic material is selected to 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 1000 to about 4000 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 2000 to 3000 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.

SUMMARY OF THE INVENTION

In view of the foregoing, a principal object of this invention is to provide a photoflash lamp having a vacuumformed thermoplastic coating to reinforce the glass envelope wherein high localized stresses in the coating are relieved and a more uniform compressive loading on the glass is provided.

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

A further object is to provide an improved method for coating the glass envelope of a photoflash lamp with a thermoplastic material.

These and other objects, advantages and features are attained, in accordance with the invention, by disposing a lubricating release agent between the vacuum-formed thermoplastic coating and the glass envelope. The improved method comprises; applying a thin film coating of mold release agent on the exterior surface of the glass envelope; placing the film coated glass envelope within the preformed sleeve of thermoplastic material; drawing a vacuum in the space between the thermoplastic sleeve and the film coated 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 film coated glass envelope.

Alternatively, the mold release agent may be coated on the interior surface of the preformed thermoplastic sleeve prior to assembly with the glass envelope.

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 a percussive-type photoflash lamp having a protective coating in accordance with the invention;

FIG. 2 is an enlarged sectional elevation of a preformed sleeve of thermoplastic adapted for assembly and vacuum forming onto the film coated glass envelope of a percussivetype photoflash lamp;

FIG. 3 illustrates the step of aerosol spraying a thin film coating of mold release agent on the exterior surface of the lamp envelope, in accordance with the invention;

FIG. 4 is an enlarged elevation, partly in section, showing a percussive flashlamp assembled in the thermoplastic sleeve of FIG. 2 prior to vacuum forming;

FIG. 5 is a simplified fragmentary elevation, partly in section, of the vacuum forming apparatus, this view illustrating the simultaneous vacuum drawing and heating steps;

FIG. 6 illustrates the constricting step carried out by the apparatus of FIG. 5; and

FIG. 7 illustrates the tipping off step carried out by the apparatus of FIG. 5.

DESCRIPTION OF PREFERRED EMBODIMENT

The present invention comprises an improvement over the lamp and method described in the aforementioned copending application Ser. No. 268,576, and in like manner, its teachings are applicable to either percussive or electrically ignited photoflash lamps of a wide variety of sizes and shapes. For purposes of example, FIG. 1 illustrates a percussive-type photoflash lamp embodying the principles of the invention.

Referring to FIG. 1, the percussive lamp 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 lubricating release agent 47 disposed between the thermoplastic coating and 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 protuberance, at the lower extremity of the wire anvil. Additional means, such as lobes 42 on wire anvil 32 for example, may also be used in 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.

Typically, the lamp envelope 22 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.

Operation of the percussive-type lamp of FIG. 1 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.

The improved coating method will now be described with reference to FIGS. 2-7. For purposes of example, the method will be described with reference to vacuum forming a thermoplastic coating on the percussive lamp of FIG. 1, although it will be understood that a similar method may be employed with an electrically ignited lamp. Referring first to FIG. 2, the thermoplastic material to be coated on the exterior surface of the lamp envelope is initially provided as a preformed sleeve 48 having the shape of a test tube. To facilitate passage of the coaxially projecting primer tube 30, sleeve 48 is provided with a single coaxially disposed hole 50. Sleeve 48 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 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 accordance with the present invention, before assembling the sleeve on the envelope, a thin film coating of a clear mold release agent 47 is applied on the exterior surface of the glass envelope 22. Preferably, the release agent 47 is applied by means of an aerosol spray 51, as illustrated in FIG. 3, although other methods of application, such as by dipping in a solvent solution or by agitating in a dry release agent powder may be employed. The material 47 may comprise any lubricating release agent which is clear and substantially inert with respect to the glass envelope 22 and the thermoplastic sleeve 48. A silicone mold release agent has been found to be particularly suitable for this application.

In the next step, shown in FIG. 4, the film coated glass envelope 22 of the percussive lamp is placed within the preformed thermoplastic sleeve 48, with the primer tube 30 projecting through hole 50. It will be noted that both the sleeve 48 and the lamp envelope 22 have generally tubular sidewalls. To facilitate the vacuum forming process, the fit should be as close as possible. Accordingly, the outside diameter of the tubular envelope 22 and the inside diameter of the tubular sleeve 48 are dimensioned so that, when the envelope is placed within the sleeve, there exists a clearance x of from 0.001 to 0.015 inch between the tubular sidewalls thereof prior to heating and vacuum forming.

The next step, heating and vacuum forming, is illustrated in FIG. 5. The envelope and sleeve assembly 22, 48 is held during the evacuating and heating processes by means of a chuck 50 gripping the primer tube 30. Another chuck 52, having an evacuating tube 54, grips the open end of the thermoplastic sleeve 48. One or more localized sources of heat, represented by heaters 56, 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 48 and envelope 22, while simultaneously heating the envelope and sleeve assembly incrementally along its length. More specifically, the vacuum is drawn through the tube 54, in the direction of the arrow, at the open end of sleeve 48. At the same time, the heaters 56 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 48 through which the primer tube 30 projects, and then proceeds toward the open end of the sleeve from which the vacuum is being drawn. In this manner, the temperature and vacuum cause the thermoplastic sleeve 48 to be formed onto the film coated glass envelope 22 with the interface therebetween substantially free of air voids, inclusions and the like.

Referring to FIG. 5, this incremental heating process may be accomplished at one station by either moving chucks 50 and 52 downward with respect to a set of stationary heaters 56, or by moving heaters 56 upward with respect to a set of stationary chucks 50 and 52. A preferred method of effecting the incremental heating, however, is to index the envelope-sleeve assembly through a plurality of heating stations, with the heaters at each station positioned at successively higher elevations.

At the conclusion of the incremental heating process, the sleeve 48 is constricted at portion 58, as shown in FIG. 6, by slowly pulling chucks 50 and 52 away from each other, while continuing to apply heat and draw a vacuum. Finally, as shown in FIG. 7, the vacuum formed sleeve 48 on the lamp is separated from the portion 60 of the sleeve held in chuck 52 and tipped off at point 62, thereby completing the encapsulation of glass envelope 22 in the thermoplastic coating 46.

The composition of sleeve 48, 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, acylonitrile-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 (10-20%) of compatible plasticizer to the composition. Further, commercial blue dyes can be used in the sleeve for color corrections desirable with various photographic color film.

As described in the aforementioned copending application, the thermoplastic material preferably is selected to have a coefficient of thermal expansion several times greater than the coefficient of thermal expansion of the glass envelope. In this manner, the coating 46, provided by the above described vacuum forming process, will exert a compressive load on the glass envelope 22 to thereby in effect strengthen the glass and make it more resistant to the effects of thermal shock and impact. For example, with a coefficient of thermal expansion at least six times greater than that for the glass, the thermoplastic coating may exert a compressive load of from about 1000 to about 4000 pounds per square inch on the glass envelope.

The added containment strength provided by this compressive loading may be better understood by briefly considering the effects of the combustion process. Upon flashing the lamp and igniting the shreds of combustible, the inner surface of the glass envelope is subjected to severe thermal shock in the form of impact from hot globules of metal oxide; for example, zirconium oxide has a melting point of 2715.degree.C. Each thermal impact against the internal glass surface produces a thermal stress gradient through the wall of the glass envelope, which serves as an insulator to the conducted heat, and causes expansion of the glass. Any thermoplastic coating on the glass will be under tension (T.sub.D) and there will be a localized tensile stress (T.sub.x) at the interface of the coating and glass, opposite the point of globule impact. The build up of the localized tensile stress T.sub.x by the thermal stress gradient is what can eventually cause a crack through the glass wall. On the other hand, the compression loading (C) which is exerted on the glass envelope by the coating functions to counteract the tensile loading of T.sub.D and T.sub.x by delaying the thermal stress gradient through the glass wall; this may be illustrated as T.sub.D + T.sub.x - C. Accordingly, the higher the compressive loading, the stronger the glass. Also, however, an increase in the compressive loading on the glass results in a corresponding increase in the tensile loading on the coating. Hence, a compressive load that is too high can be detrimental to the thermoplastic. Where necessary, the compressive loading can be relieved by the inclusion of a small amount of plasticizer in the coating composition and/or filters that alter the thermal expansion coefficient.

Further, in accordance with the present invention, we have found that by using a thin film layer 47 of silicone mold release between the glass and thermoplastic, as described above, the problem of undesirably high localized stress points in the coating 46 can be alleviated or substantially minimized. These high localilzed stresses are believed to be due to interface irregularities and friction; hence, we feel that the thin film of silicone mold release acts as a lubricant in the glass-plastic interface, thereby allowing for slippage between the materials and having the net effect of reducing high stress points.

Verification of this improvement has been seen by placing the finished lamp in a 17% solution of ethyl acetate and methanol for 3 minutes. This solvent-stress test simulates the effects of accelerated aging. The parts without silicone mold release show multiple checks and crazes, while the parts with mold release are nearly free of cracks and crazes. A long term test of 100 hours at 150.degree.F in a dry oven shows similar results.

In one typical embodiment of the invention, a percussive flashlamp of the type shown in FIG. 1 was provided with a clear vacuum-formed coating 46 of polycarbonate resin having a wall thickness of about 0.020 inch. The lamp contained a combustible fill 36 comprising 19.5 mgs. of shredded zirconium foil and oxygen at a fill pressure of 8 atmospheres. The tubular envelope 22 was formed of G-1 type soft glass and had a nominal outside diameter of 0.325 inch. In the process of coating the lamp, an injection molded sleeve 48 of clear polycarbonate resin having a nominal inside diameter of 0.340 inch and a wall thickness of 0.020 was employed. Before placing the lamp in the sleeve, the glass envelope 22 was sprayed with a silicone mold release agent. During vacuum forming, the molded sleeve was incrementally heated to a temperature of about 400.degree.F by a nitrogen flow serpentine heater. The coefficient of thermal expansion of soft glass of this type ranges from 85 to 95 .times. 10.sup.-.sup.7 in./in./.degree.C between 20.degree. and 300.degree. C, whereas the coefficient of thermal expansion of unfilled polycarbonate between 25.degree. and 140.degree.C is about 660 .times. 10.sup.-.sup.7 in./in./.degree.C. Upon measuring several sections of lamps made as described above, the average compressive stress exerted by the coating 46 upon the glass envelope 22 was found to be about 1300 pounds per square inch. Flashing of a number of these lamps in both the vertical and horizontal position exhibited no containment failures.

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. For example, instead of applying the silicone mold release agent 47 to the exterior surface of the lamp envelope 22, the same results can be achieved by spraying or otherwise applying the mold release agent on the interior surface of the preformed thermoplastic sleeve 48. This step would occur after the step of prebaking the sleeve, but before assembling the glass envelope within the sleeve.

Claims

What we claim is:

1. A method of coating the glass envelope of a photoflash lamp with a light-transmitting thermoplastic material, said method comprising:

applying a thin film coating of a lubricating release agent on the exterior surface of said glass envelope, said release agent being clear and substantially inert with respect to said glass and thermoplastic,

placing said film coated glass envelope within a preformed sleeve of said thermoplastic material, and

drawing a vacuum in the space between said thermoplastic sleeve and said film coated glass envelope, while simultaneously

heating said sleeve and envelope assembly incrementally along the length thereof, whereby the temperature and vacuum cause said thermoplastic sleeve to be incrementally formed onto said film coated glass envelope.

2. The method of claim 1 wherein heat is applied uniformly about said sleeve and envelope assembly by one or more localized sources of heat encircling said assembly, and said assembly is indexed through a plurality of heating stations, each of which has said one or more localized sources of heat positioned at successively higher elevations.

3. The method of claim 1 including the further step of prebaking said thermoplastic sleeve to remove the mositure therefrom prior to assembling said sleeve with said film coated glass envelope.

4. The method of claim 1 including the further steps of constricting and tipping off said vacuum formed sleeve at the conclusion of said incremental heating process.

5. The method of claim 1 wherein said release agent is a silicone mold release agent.

6. The method of claim 1 wherein said release agent is applied by spraying it onto the exterior surface of said glass envelope.

7. A method of coating the glass envelope of a photoflash lamp with a light transmitting thermoplastic material, said method comprising:

applying a thin film coating of a lubricating release agent on the interior surface of a preformed sleeve of said thermoplastic material, said release agent being clear and substantially inert with respect to said glass and thermoplastic,

placing said glass envelope within said interiorly coated sleeve of thermoplastic material,

and drawing a vacuum in the space between said interiorly coated thermoplastic sleeve and said glass envelope, while simultaneously

heating said sleeve and envelope assembly incrementally along the length thereof, whereby the temperature and vacuum cause said interiorly coated thermoplastic sleeve to be incrementally formed onto said glass envelope.

8. The method of claim 7 including the further step of prebaking said thermoplastic sleeve to remove the moisture thereof prior to applying said coating of mold release agent thereon.

9. The method of claim 7 wherein said release agent is a silicone mold release agent.

10. The method of claim 7 wherein said release agent is applied by spraying it onto the interior of said preformed sleeve of thermoplastic material.