US 2791113 A
Description (OCR text may contain errors)
May 7, 1957 R. M. ANDERSON 2,791,113
COATED FLASH LAMP AND MANUFACTURE THEREOF Filed April 7, 1954 w M, 4 I ,1
Robert M. Anderson,
His Attorney COATED FLASH LAIWP AND MANUFACTURE THEREOF Robert M. Anderson, Euclid, Ohio, assignor to General Electric Company, a corporation of New York Application April 7, 1954, Serial No. 421,571
6 Claims. (Cl. 6731) My invention relates to flash lamps in general and to the manufacture thereof, and particularly to flash lamps of the type comprising a bulb or envelope of glass or similar material and containing a charge of readily combustible light-giving material and combustion-supporting means which, on ignition, burns with great rapidity to produce a momentary flash of actinic light, and more par ticularly to light-transmitting protective lacquer coatings for said lamps.
In flash lamps of the above-indicated type, the bulb is customarily provided with an interior, light-transmit ting, protective coating of a suitable material, such as a lacquer for instance, in order to strengthen the bulb and prevent the cracking thereof, upon charge flashing, by hot burning particles of the combustible material. A similar coating is applied to the exterior surface of the bulb to provide additional protection against explosion and flying glassparticles.
It is desirable in ordinary flash lamps, and virtually necessary in miniature flash lamps, particularly those having a butt seal, to eliminate the interior coating. For example, omitting the interior coating in ordinary flash lamps such as that commercially designated as No. 5, and similar lamps, will effect a reduction in manufacturing costs since the expense of spraying the bulb interior and drying the lacquer coating is eliminated. In sub-miniature flash bulbs of very small volume (about 6 cc. for example), such as disclosed and claimed in co-pending U. S. application Serial No. 421,510, Anderson et al., filed of even date herewith and assigned to the assignee of the present invention, the neck opening is usually too small to admit a lacquer spraying nozzle and, although the lacquer may be poured into the bulb, it is impossible to drain a liquid-filled bulb of the size contemplated due to the small opening and the surface tension of the liquid. Even water will not drain from such a miniature lamp unless an opening is made in the bulb other than the neck opening.
When the interiorprotective lacquer coating is omitted for any ofthe above reasons, several problems arise. The outside lacquer remaining may not be sufficiently strong to contain the forces generated when the lamp is flashed, thus increasing the possibility of explosion of the lamp and injury to the user. Applying a thicker outside lacquer film to increase strength does not solve the problem since lacquers of the type customarily used, such as cellulose acetate, exhibit considerable smoking, charring and even burning when a lamp is flashed, thus drastically reducing tensile strength of the film. Charring, burning or any heat discoloration of the outer lacquer film also reduces the efiective light output of the lamp. Increased film thickness is also objectionable from the tive coating ofsuflicient strength to Withstand the forces generated on charge-flashing of the lamp in the absence of any coating on the interior bulb surface.
Another object of my invention is to provide a bulb for a photoflash lamp or similar device provided with an exterior coating capable of absorbing the thermal shock produced on combustion of the charge with little or no smoking, charring or burning of the coating in the ab- .ieliCe of any protective film on the interior surface of the Still another object is to provide an exterior composite protective coating for flash lamps including one component which softens and adheres to the glass at the temperature produced on charge-flashing of the lamp, to contain glass fragments should shattering of the bulb occur, and a second component which retains adequate tensile strength to minimize the possibility of an explo- Briefly stated, the above objects may be accomplished, in the absence of any interior coating, when the exterior glass bulb surface is provided with a double-layer coating consisting of two distinct superimposed lacquer films possessing different properties as hereinafter decribed. A lacquer film, hereinafter called the bufler lacquer or layer, is applied directly to the exterior bulb surface and dried. The buffer layer is of such composition that it will resist thermal shock, absorbing the heat generated on flashing of the lamp, thus insulating the outer lacquer from intense heat, and has a relatively low softening point becoming soft and tacky at the temperatures present so as to adhere to the glass and prevent rapid dispersion of glass particles should an explosion take place. After the buffer layer has dried, a second lacquer layer is applied. This second or outer lacquer is composed of a suitable material such that the dried film possesses a tensile strength of about 8000 to 10,000 p. s. i. and exhibits a tendency to retain some effective strength during the short interval of high temperature (at least 300 C.) encountered on charge-flashing of the lamp, thus provid ing the strength necessary to contain effectively the internal pressures generated. In addition, the outer layer should have some toughness, that is, abrasion and tear resistance, and should not yellow or discolor unduly with age.
Thus it can be seen that a bulb exteriorly coated with the double-lacquer films possessing the above-mentioned properties adequately surmounts the problems arising upon omission of an interior lacquer film.
Further objects and advantages of my invention will appear from the following description of species thereof and from the accompanying drawing in Which:
Fig. 1 is an elevation of a sub-midget flash lamp of buttseal type which may be coated in accordance with my invention;
Fig. 2 is an elevation of a flash lamp of conventional stem seal type which may also be coated in accordance with the invention; and
Fig. 3 is a fragmentary sectional view of a flash lamp bulb illustrating the application thereto of a coating in accordance with my invention.
Referring to the drawing, Fig. 1 illustrates a submidget flash lamp of the type generally known as a butt-seal lamp comprising a bulb or envelope 1 of glass or other suitable material having positioned therein a mount comprising a pair of lead-in wires 2, 2 tied to gether by a glass head 4, said lead-in wires connected at their inner ends to the opposite ends of a filament 3 of tungsten wire, the outer ends of said leads 2, 2 being connected respectively to the shell contact 7 and center contact 6 of the base 5. The inner ends or tips of the lead-in wires 2, 2 are provided with small beads or coatings 9, of a suitable primer or fulminating material such as, for example, a mixture of zirconium, magnesium and potassium perchlorate powders bonded together by a suitable binder such as nitrocellulose. The primer beads 9, 9 are located on the lead-in wires 2, 2 at the region of the connection of the latter to the filament 3 so as to embed at least the end portions of said filament and thus become ignited thereby upon passage of an electric current through the filament.
Disposed within the bulb 1 and around the filament 3 and primer beads 9, 9 so as to be Within effective lgllltion range thereof, is a charge of readily combustible light-giving material 10, such as leaf foil, wire, shredded foil, or ribbon which, on ignition by the igniter means 3, 9, 9, burns with great rapidity to produce a momentary flash of actinic light. In the miniature lamp illustrated, the light-giving charge 10 comprises a loose filling of filamentary material, such as shredded foil, of a readily combustible composition, such as aluminum, magnesium or alloys thereof. The bulb 1 further contains a gas filling, such as oxygen for instance, at a suitable pressure for supporting combustion of the charge 10 of lightgiving material.
Illustrated in Fig. 2 is a flash lamp comprising a glass bulb 11 which may have a volume of approximately 25 cc. and is provided at its neck end 12 with a re-entrant stem 13 which extends into the bulb and terminates at its inner end in a press or seal portion 14 through which is sealed a pair of lead-in wires 15, 16 connected at their outer ends to the center contact 17 and shell 18, respectively, of a base 19 cemented or otherwise secured to the neck end of the bulb. The said lead-in wires 15, 16 extend into the bulb in more or less side-by-side relation and are provided at their inner ends with an ignition means 20 substantially the same as that shown in Fig. l. The bulb 11 also contains a charge of readily combustible material 21 and combustion-supporting gas as described above which, upon charge-flashing of the lamp, provides a momentary flash of actinic light.
Fig. 3 is a fragmentary sectional view of a flash lamp bulb showing the application of the protective coating of my invention thereto. As illustrated in Fig. 3, the exterior bulb surface 23 is first coated with a layer of buffer lacquer 22. After the buffer lacquer has dried, the outer lacquer layer 24 is applied directly over the buffer layer.
According to the invention, the buffer layer comprises a thin film of a suitable material having efficient heat in sulating properties and being tenacious with regard to acetone alcohol.
37 /z% by weight resin in about 62 /2% by weight acetone. Most of the acetone evaporates on drying of the lacquer leaving a thin, uniform, closely adherent film of isobutyl methacrylate which provides the desired properties. The lamp bearing the buffer film is again immersed, as above, in a solution of outer lacquer. This lacquer is preferably cellulose acetate dissolved in acetone, alcohol and/or other suitable solvents. For example, the preferred outer lacquer comprises about 15% by weight cellulose acetate dissolved in a mixture of solvents containing about 80% by weight acetone, 10% by weight denatured alcohol, and 10% by weight di- Drying this outer lacquer provides a film of cellulose acetate possessing the desired tensile strength.
In this manner the outer bulb surface of the lamp is provided with the double-layer protective coating of my invention comprising a buffer layer adjacent the glass to absorb heat and the outer layer to provide additional structural reinforcement.
. It will be noted that many materials exhibit the properties desired in a suitable buffer layer. polyvinyl resins, such as polyvinyl acetate, polyacrylates, and polymethacrylates, provide suitable heat insulating films. Of the polymethacrylates I have found that ethyl, methyl, butyl and isobutyl methacrylates and copolymcrs or mixtures thereof provide excellent buffer films.
The most widely used outer lacquer in the photoflash art is cellulose acetate due to the fact that the material is so well adapted for this particular use and relatively inexpensive. Acetone is the principal solvent for cellulose acetate, being very active and possessing a high evaporation rate. Suitable solvents possessing similar characteristics, such as ethylene dichloride or tricresyl phosphate for example, may be substituted. However, any suitable material may be applied as an outer layer which exhibits sufiicient tensile strength to structurally reinforce the bulb during charge-flashing of the lamp. For exthe bulb surface. It preferably should be of a material which softens on charge-flashing of the lamp so that the lacquer becomes tacky preventing dispersion of glass particles should the bulb shatter. The material is dissolved in a suitable solvent having a fairly high evaporation rate providing a solution which in its initial form issmooth flowing and adapted for application by dipping or spraying against the exterior of the bulb shell forming thereon a smooth rapid-drying coating of uniform thickness.
After the buffer coating is dry, the bulb is sprayed or dipped in a solution of asuitable material in a suitable solvent to provide the outer structurally reinforcing layer. A suitable outer layer material is a tearand abrasion-resistant plastic which in the dry solid state exhibits a tensile strength of about 8000 to 10,000 p. s. i. and which retains suficient strength during the short interval of elevated temperature, occasioned by flashing of the lamp, to effectively reinforce the glass bulb.
By way of specific example of an embodiment of my invention, a completely assembled flash lamp is dipped in the buffer lacquer so that the bulb is completely immersed as well as a small portion of the base in the area of the base to bulb juncture. The lamp is then with drawn from the lacquer and dried. As a buffer lacquer I prefer to use a solution of isobutyl methacrylate in ace tone. A suitable composition for this lacquer is about ample, cellulose acetate-butyrate is a material suitable foruse as an outer coating possessing, as a dry film, the requisite strength and toughness. Combinations of cellulose acetate and cellulose acetate butyrate are also suitable for the desiredpurpose.
- The materials suitable for the buffer layer, as well as those suitable for the outer lacquer film, may be dissolved in suitable solvents in any proportion which will provide a solution of satisfactory viscosity so that, in the method chosen to apply the coatings, smooth uniform films of the desired thickness will be obtained. The lacquer films should be relatively thin. For example, a suitable butfer film would be at least about 0.002 to 0.003 inchthick and the outer films at least about 0.003 to 0.004 inch thick.
The two films may be applied by any suitable means such as dipping or spraying. Although a common solent may be used in both lacquers, there is no bonding between the two films. I have found that the buffer and outer layers are completely separate and distinct. This feature may, partially at least, account for the particular efficiency with which the double lacquer coating performs the desired functions.
Since the lacquer films draw down tightly over the bulb on drying, and the coefficients of thermal expansion of the lacquer materials and the glass ofthe bulb are fairly close, there is little, if any, difliculty of separation of the lacquers from the bulb surface or from each other. Further, plastic films are generally somewhat elastic and intense heat is present for an extremely short period of time so there is little danger of rupture of the filmsor their separation from the bulb, surface due to thermal expansion of either the glass orthe films.
I believe that the desired results are achieved by the buffer layer absorbing most of the heat generated on flashing of the. lamp and also distributing the heat generated evenly over the entire bulb surface, thus prevent- For example,
ing the formation of hot spots which are concentrations of intense heat in a small area causing charring and often burning of the outer layer. The preferred methacrylate buffer materials, such as ethyl, methyl, butyl and isobutyl methacrylate, have a common property which partially accounts, at least in theory, for their particular suitability as buifer films. These materials decompose with heat to monomers which completely volatilize at elevated temperatures rather than burn or char. In contrast, most outer lacquer materials possessing the requisite strength and other properties have been found to be those whose decomposition products are carbon or carbon compounds. Such products are combustible and, at or above the decomposition temperature, will char or actually burn.
Other polyvinyl resins suitable for use in bufier layers may decompose to monomers, although this has not definitely been established. In any event, these resins all absorb the heat generated on flashing of the lamp sufficiently to prevent excessive weakening or other damage to the outer layer. In addition, many of these bufier materials, although they may not decompose to monomers, are heat-decomposable to products which volatilize almost completely at elevated temperatures and leave little if any residue such as carbon or other residual material which might discolor or char at the temperatures encountered. Any one or combination of these properties enables the polyvinyl resins to effectively perform the hereinbefore described functions of a buffer layer.
The drying of the lacquer films on the bulb may be accelerated by passing the bulbs through heated ovens or the films may be allowed to dry naturally by evaporation of the volatile solvents. In addition, suitable well-known plasticizers may be added to the lacquers to increase elasticity and plasticity of the films. However, in the specific examples of lacquer compositions given, no plasticizer was found necessary.
Although a preferred embodiment of my invention has been disclosed, it will be recognized that variations and changes may be made therein within the spirit and scope of the invention as defined by the appended claims. It will be further understood that'the ingredients and their proportions as given above can be varied, independently and in relation to each other, within fairly wide limits to obtain the desired results. Further, it will be particularly recognized that my invention is not limited with respect to principal lacquer ingredients, good results being obtained with any lacquer or similar material possessing the properties stated hereinbefore to be desirable in buffer and outer coatings.
What I claim as new and desire to secure by Letters Patent of the United States is:
l. A flash lamp comprising a sealed glass envelope, a quantity of readily combustible material and combustionsupporting means therein adapted to generate on combustion a flash of actinic light, a double-layer protective coating on the exterior surface of said envelope said coating comprising a butfer layer of light-transmitting heat-absorbing lacquer adjacent the exterior glass surface, and superimposed on said buffer layer an outer layer of structural reinforcing light-transmitting lacquer possessing a dry tensile strength of at least about 8000 p. s. i.
2. A flash lamp of the character set forth in claim 1 wherein the buffer layer comprises essentially a polyvinyl resin.
3. A flash lamp of the character set forth in claim 1 wherein the buffer layer comprises essentially a lacquer selected from the group consisting of polyvinyl acetate, ethyl methacrylate, methyl methacrylate, n-butyl methacrylate, isobutyl methacrylate and copolymers and mixtures thereof.
4. A flash lamp of the character set forth in claim 1 wherein the buffer layer comprises essentially a polyvinyl resin and the outer layer comprises essentially at least one material of the group consisting of cellulose acetate and cellulose acetate butyrate.
5. A flash lamp of the character set forth in claim 1 wherein the buffer layer comprises essentially a lacquer selected from the group consisting of polyvinyl acetate, ethyl methacrylate, methyl methacrylate, butyl methacrylate, isobutyl methacrylate and copolymers and mixtures thereof, and the outer layer comprises essentially at least one material of the group consisting of cellulose acetate, cellulose acetate butyrate and mixtures thereof.
6. A flash lamp of the character set forth in claim 1 wherein the buffer layer comprises essentially isobutyl methacrylate and the outer layer comprises essentially cellulose acetate.
References Cited in the file of this patent UNITED STATES PATENTS 2,046,388 Kurlander July 7, 1936 2,277,776 Patton Mar. 31, 1942 2,324,959 Stevens July 20, 1943