US3621323A

US3621323A - Coated incandescent electric lamp

Info

Publication number: US3621323A
Application number: US860139A
Authority: US
Inventors: Frank W Thomas; William R Tooke Jr
Original assignee: Thomas Manufacturing Co
Current assignee: Thomas Manufacturing Co
Priority date: 1968-07-01
Filing date: 1969-08-14
Publication date: 1971-11-16
Anticipated expiration: 1988-11-16
Also published as: GB1208528A

Abstract

An incandescent electric lamp including an elastomeric coating extending continuously over the glass envelope of the lamp and down over the upper portion of the metallic base by at least 1/8 th of an inch. Such a coating creates an electrically safe, substantially shatterproof pod which is affixed to the metallic base. Upon accidental, inadvertent or intentional shattering of the glass envelope, the coating has sufficient strength and body to retain its shape as an electrically safe pod which is attached to the metallic base. The pod thus contains the glass fragments, prevents exposure of the lamp filaments to human contact and provides a tough, homogenous body which may be rotated to safely remove the broken lamp from a socket.

Description

United States i atomg [72] Inventors Frank W. Thomas Baltimore, Md.; William R. Tooke, Jr., Atlanta, Ga.

[21] Appl. No. 860,139

[22] Filed Aug. 14, 1969 [45] Patented Nov. 16, 1971 [73] Assignee Thomas Manufacturing Company Parkton, Md. Continuation-impart of application Ser. No. 725,568, Apr. 12, 1968. This application Aug. 14, 1969, Ser. No. 860,139

[54] COATED INCANDESCENT ELECTRIC LAMP 3,137,804 6/1964 Rubens 313/312 FOREIGN PATENTS 755,838 8/195'6 Great Britain .1 313/317 Primary Examiner-David Schonberg Assistant Examiner-Paul A. Sacher Attorney-Jones & Thomas ABSTRACT: An incandescent electric lamp including an elastomeric coating extending continuously over the glass envelope of the lamp and down over the upper portion of the metallic base by at least 16th of an inch. Such a coating creates an electrically safe, substantially shatterproof pod which is affixed to the metallic base. Upon accidental, inadvertent or intentional shattering of the glass envelope, the coating has sufficient strength and body to retain its shape as an electrically safe pod which is attached to the metallic base. The pod thus contains the glass fragments, prevents exposure of the lamp filaments to human contact and provides a tough, homogenous body which may be rotated to safely remove the broken lamp from a socket.

IATENTED 16 \971 $621,323

INVENTORS Frank W. Tfwmas w w ATTORNEYS BY William RTouke, Jr.

COATED INCANDESCENT ELECTRIC LAMP CROSS-REFERENCE TO RELATED APPLICATION This application is a continuation-in-part of our copending US. Pat. application for a Method of Coating an Incandescent Electric Lamp", filed Apr. 12, 1968 and having U.S. Ser. No. 725,568.

This invention relates to an electrically safe, substantially shatterproof incandescent electric lamp. More particularly, the present invention concerns an electric lamp having a continuous elastomeric coating over the glass envelope and the upper portion of the metallic base.

Conventional incandescent electric lamps are subject to breakage by either physical impact or thermal shock. Once the glass envelope is destroyed by such breakage, the incandescent lamp filament is exposed. If the filament is not destroyed during breakage of the envelope, it soon overheats and melts without the regulated atmosphere of the envelope. Once the filament is destroyed, it is conventionally assumed that the lamp is burned out and harmless. To the contrary, contact with the exposed filaments will produce an electrical shock sufficient to cause serious damage and possibly death.

Thus, it becomes apparent that a need exists for a shatterproof and electrically safe incandescent electric Ramp. it is an object, therefore, of the present invention to provide an incandescent electric lamp which is both electrically safe and sub stantially shatterproof.

These and other objects, features and advantages of the present invention will become apparent from a review of the following detailed description of one embodiment of the invention and the accompanying drawings wherein like characters of reference refer to the same parts throughout and:

FIG. 1 is a vertical elevational view of the lamp of the present invention;

FIG. 2 is a sectional view taken through a wall of the glass envelope of the lamp; and

FIG. 3 is a vertical elevational view of the lamp in a shattered condition.

According to the present invention, there is provided an electrically safe and substantially shatterproof incandescent electric lamp. The glass envelope and the upper portion of the metallic base of the present lamp are continuously coated with the elastomeric substance to produce a tough, connective insulating pod around the envelope and metallic base which eliminates the hazard of thermal shock to the glass envelope and will not allow glass splinters to be spread by the implosion or explosion of the bulb whether caused by thermal shock or by physical impact. A lamp of this type is useful in any location where rain, snow, water spray, or water condensation might come in direct contact with the lighted lamp. Additionally, a shatterproof lamp is useful in any location or activity where breakage is caused by welding splatter, accident, use of small tools, vandalism, gunshot, high winds, or by unusual vibration, where such breakage creates a health or fire hazard to persons in the immediate vicinity. Such hazards may cause fatal electrical shock, eye injuries, cuts, presence of broken glass in foods, explosions and/or fires caused by ignition of volatile gases and dusts, burns and other injury to life, limb and property. Additionally, a shatterproof lamp is useful in assuring maximum life of the lamp by rendering the lamp more resistant to impact or thermal shock, thus eliminating the need for replacement due to causes not related to filament life. This is considered particularly important in hard to-get-to installations where the labor required to replace a burneri-out lamp is costly and/or dangerous.

By coating the glass envelope and the upper portion of the metallic base with a continuous coating of elastomeric material, a tough, insulating pod which is joined to the metallic base is created. The pod serves to prevent contact with lamp filaments when broken, will contain glass splinters upon shattering of the envelope, and will provide an intact, homogenous body which can be rotated to safely remove the lamp from its base socket.

Referring now in more detail to the drawings, FIG. i shows an incandescent electric lamp 10 having a coated glass envelope 12 comprising a spheroidal glass head portion 14 and a hollow glass neck portion 16 integrallly attached to said spheroidal portion. The hollow neck 16 is closed by a metal base 18 at its proximal end and conventional electrical incandescent elements 19 extend into the glass envelope from the metal base in a conventional manner.

The glass envelope 12 is coated with an elastomeric material that retards and prevents shattering of the glass envelope of the bulb by implosion or explosion due to thermal shock or physical impact. As is shown in more detail in FIG. 2, a uniform coating 20 is applied over the surface of the glass envelope and the coating adheres to the envelope. Coating 20 extends continuously over the upper portion of the metallic base (shown in dotted lines in FIG. I) to create a continuous pod which surrounds the glass envelope and is affixed to the metallic base. It is preferred that coating 20 extend approximately one-eighth of an inch over the upper portion of the base so as to attach the coating 20 firmly to the base 18 while at the same time not interfering with the insertion of the base 18 into an electrical outlet.

Since incandescent lamps when lighted become hot, with surface temperatures of the envelope often exceeding 500' F. and because of other required properties of high elasticity, toughness, minimum light transmission impairment, and good bonding, it is found that an organopolysiloxane in a hydrocarbon solvent, such as toluene, provides excellent results as a coating when applied to the outer surface of the lamp. The particular organopolysiloxane chosen is a methylpolysiloxane containing terminal silicon-bonded hydroxy groups which is cured with a metallic salt selected from the class consisting of dibutyl tin dilaurate and dibutyl tin diacetate. Lamps coated with this methylopolysiloxane withstand temperatures approaching 600 F. for over 1,000 hours, which is an acceptable period of time for a lamp to operate. The coating withstands this temperature without appreciable loss of strength or elasticity, and also maintains a uniform appearance. Coating 20 also adheres to the glass envelope to such a degree that it does not peel from the envelope and is quite difficult to remove. In addition, the envelope and its coating are substantially translucent throughout their usable life, and the coating remains tough and tear-resistant after long use. With these qualities, the coating adheres to the glass in such a manner that if the glass ever cracks or separates, as is shown in FIG. 3, the protective coating will hold the glass particles in their original unbroken positions and any chips or fragments not held in place by the coating fall inside the bulb. The toughness of the coating enables it to maintain a temporary airtight envelope although the glass underneath is broken. The elasticity of the coating permits implosive or explosive flexing of the coating without fracture thereof so that the coat remains intact after physical damage to the envelope.

Preparation of the lamp and application of the coating are basically the same for all sizes of lamps. Before applying the coating, the lamps must be cleaned so that no foreign matter adheres to their surface. One method of cleaning the lamps is wiping the bulb with acetone; however, any method of cleaning is acceptable as long as no residue or film is left on the surface of the lamp.

After cleaning, each lamp 10 is positioned in a specially designed tray containing from I l to 60 lamps depending upon lamp size and is dipped into an elastomeric mixture with the metallic base of the lamp uppermost so that the glass envelope 12 and approximately one-eighth of an inch of the base are im mersed in the coating mixture. The lamp tray is then withdrawn from the mixture at a controlled rate of approximately 6 inches per minute. It has been found that this rate results in a preferred uniform distribution of elastomer without undesirable excess on the glass envelope. The rate at which the lamp 10 is withdrawn from the resin mixture may be adjusted for each particular lamp size to control the size of the tip of resin which extends downwardly from the spheroidal portion of the lamp immediately after the lamp is withdrawn from the mixture. The rate should be such that the lamp is adequately coated and the tip of the material is sufiiciently small to disappear during rotating of the lamp as described below. Using an elastomeric mixture blended as described below, the tip of elastomer should be about the size of a 1/4- inch diameter sphere.

After the lamp tray is withdrawn from the elastomeric mixture, the tray containing coated lamps is rotated end-over-end by insertion into a rotating tumbler which is designed to carry the tray in rotation. The rotation of the tumbler around an axis parallel to the earth's surface at this point creates a constantly changing gravitational pull upon the elastomeric coating of the lamps. The changing gravitational pull serves to level the coating over the glass envelope and produce a substantially uniform coat on the lamp. It is preferred that the lamps be rotated for approximately 10 minutes at a rate of one revolution per minute in air maintained at approximately 70 F. in addition to leveling the coating, the rotation of the lamp in an atmosphere maintained at 70 F. serves also to uniformly dry the coat sufficiently to fix the coating so that the lamp may be further processed without the coat creeping on the lamp surface.

Upon completion of the rotation step, the lamp is placed in a continuous infrared oven through which air is passed and in which the temperature is maintained at 190 F. The lamp remains in the oven for approximately 15 minutes to dry the resin by driving ofi all of the solvent, such as toluene, in the elastomeric coating 20. It has been found that in the absence of this drying step, blisters and other defects occur in the coatmg.

Following this drying step, the elastomer is cured by placing the lamp in an oven at 320 F. for approximately l minutes.

At this point, the coating is not sufficiently thick to insure a shatterproof incandescent electric lamp and, thus, after being cooled to room temperature, the lamp is once again dipped into the mixture as described above. Following this second clipping of the lamp into the mixture, the lamp is again subjected to the rotating, drying and curing steps described above, but with the duration of the curing step being extended from approximately to approximately minutes.

Lastly, it is necessary to drive off a residual source of mildly objectionable odor which is accomplished by a final heat exposure for minutes at 400 F.

It is found that a coating having a thickness of approximately 0.005 to 0.010 inches with an average thickness of 0.0075 inches is necessary to provide shatterproof characteristics and can be obtained after two dips of the lamp into the resin mixture with the first dip contributing approximately one-third and the second dip contributing approximately two-thirds of the thickness of the coating. It is also found that the foregoing steps are indispensably necessary to provide a coating which is substantially free of wrinkles, attractive in appearance and siiatterproof.

In preparing the coating mixture of the present invention, an elastomer such as the hydroxy chain stopped polydimethylsiloxane described in U.S. Pat. No. 2,985,545, issued to Herbert J. Leavitt and entitled Method of Rendering Cellulosic Material Non-Adherent and Article Produced Thereby, is blended with pyrogenic amorphous silica as a strengthener and toluene as a solvent. in the mixture, the elastomer is approximately 20 percent by weight, the amorphous silica is approximately 2 percent by weight, and the solvent is approximately 78 percent by weight.

It is to be noted that the amorphous silica contributes to the strength and the feel of the coating and the addition of that material to the formulation is critical. Overaddition will affect viscoscity and thixotrophy of the coating mixture as to severely impair flow of the material during dipping and tumbling stages. This flow property is a critical factor in obtaining desired film thickness, surface distribution, and film strength. Explanation of the phenomenon of strength improvement is believed to be microscopically comparable to addition of aggregate to cement. The feel resulting from amorphous silica addition is to change an otherwise tacky feel to a waxy feel and is readily apparent if films with and without silica addition are compared.

This mixture is blended for approximately 10 minutes by placing the elastomer and amorphous silica in a high-speed blender, or the like, operating at approximately 1,000 revolutions per minute and then adding the toluene and operating the blender at approximately 4,000 revolutions per minute until the temperature of the mixture is approximately F. Following blending of the mixture, a vacuum of approximately 20 inches of mercury is applied to the mixture for approximately 20 minutes.

Next, the mixture is filtered into a container. The step of applying a vacuum to the resin mixture deaerates and draws the vapor phase of the toluene from the mixture and the step of filtering removes from the mixture any gas bubbles which form as a result of the vacuum or other steps in the blending of the mixture. It has been found that in the absence of the vacuum and filtering steps, blisters tend to develop in the coating when the lamp is subsequently used.

It has been found that best results are obtained when the container into which the resin mixture is filtered is closed and the resin mixture is aged by storing at a temperature of 70 F. for a period of from 24 to to 48 hours. in addition, it has been found that the coating is improved as to uniformity when a newly aged mixture is mixed with a previously used mixture in the ratio of approximately two parts of newly aged mixture to one part of previously used mixture. The mixing of newly aged mixture with previously used mixture is most conveniently accomplished in a blender.

It will be understood that the previously used mixture is a mixture which has been catalyzed and used for applying a coating to lamps. The pot life of a catalyzed mixture blended as described above is approximately 4 days and the previously used mixture is obtained by draining a tank used for the mixture during the clipping of lamps at the end of each working day. When the resulting blended mixture is catalyzed with 25 grams per gallon resin mixture of a catalyst such as the tin salt catalyst described in the above cited patent, the resulting mixture and the dipping, rotating, drying and curing steps described above provide a coating which is uniform and free of wrinkles.

With such a coating, the lamp has a uniform appearance over its entire glass surface and the envelope is translucent. The color of the lamp envelope is substantially white or graywhite and when illuminated is comparable to the appearance of the so called soft-light bulbs in that the light rays are dispersed so that a diffused lighting effect is maintained. Also, the coated lamp emits more than 97 percent of the amount of light (lumens) per Wattage rating as the uncoated bulb.

Lamps coated according to the present invention are tested in various manners in order to ascertain the effectiveness of the coating. The lamps are illuminated at their maximum rated voltage and left on continuously until the coating fails, or the filament fails, or until the lamps are left on for 1,000 hours without failure. Coating failure is defined as the time at which any visible change is evident at the hot spot" portion of the lamp (the uppermost portion of the lamp that is heated to approximately 500 F in a l00-watt lamp). The great majority of the coated lamps so tested last for over 1,000 hours without failure, and those lamps that do not pass the l,000-hour test fail because of filament failure as opposed to coating failure.

A water quench" test is carried out by illuminating a coated lamp to equilibrium temperature and plunging it under a stream of cold water. All those lamps coated according to the present invention pass this test in that the coating does not split, blister, break or change in any manner, and in no case does the glass envelope break or illumination fail.

An impact test is carried out by illuminating a coated lamp until it reaches its equilibrium temperature, and then dropping the lamp a distance of i0 feet to a bare concrete floor, with care being taken to cause the lamp to strike its head and neck as opposed to its base. Every lamp coated according to the present invention passed the impact test in that while the glass envelope of the bulbs usually cracked, the coating did not split or fracture to expose the lamp elements and the fragmented glass is contained within the elastomeric pod.

The coating of the lamp adheres to the glass surface in such a manner that when the coating is scratched with a relatively sharp object, such as a knife, it is not removed easily; and when it is removed it will not flake or peel from the lamp, but only the portion actually scraped by the sharp object is separated from the lamp.

The fee] or hand" of the coated lamp is of a waxy but not sticky, slightly coarse nature. This feel or hand is such that the lamp is more easily handled by a person engaged in packaging the lamp or inserting it into a light fixture when compared to a conventional noncoated lamp.

We claim:

1. In an incandescent electric lamp, a glass envelope defining an opening at one end, a metallic base closing said opening and constructed and arranged to electrically connect illuminating elements within said envelope to an electrical outlet by insertion of said base into said outlet, and an elastomeric coating comprising amorphous silica and a polydimethylsiloxane continuously covering said envelope and a continuous generally annular portion of said base adjacent said envelope, said coating being constructed and arranged to provide a pod which is rupture resistant, which completely encloses and adheres to said envelope and said portion of said base, which retains pieces of said envelope in their original positions relative to each other after said envelope is broken, and which provides a substantially airtight enclosure for said illuminating elements immediately after said envelope is broken, and said portion of said base being sufficiently large for said pod to remain attached to said base and for said base to be removable from said outlet using said pod when said envelope is broken but not being sufficiently large to cover the threads and prevent said insertion of said base into said outlet.

i *l '3 ll t Disclaimer and Dedication 3,621,323.Frank W. Thomas, Baltimore, Md.; and William R. Tooke, Jr., Atlanta, Ga. COATED INCANDESCENT ELECTRIC LAMP. Patent dated Nov. 16, 1971. Disclaimer and Dedication filed June 8, 1984, by the assignee, Thomas Manufacturing Corp.

Hereby disclaims and dedicates to the Public the entire remaining term of said patent.

[Oflicial Gazette August 27, 1985.]