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Electric Lamp and Tube Manufacture

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Lamps consist of two basic types: filament (or incandescent) lamps and discharge lamps. The basic components of both lamp types include glass, various metal wire pieces, a fill gas and usually a base. Depending on the lamp manufacturer, these materials are either made in-house or may be obtained from an outside supplier. The typical lamp manufacturer will make its own glass bulbs, but may purchase other parts and glasses from speciality manufacturers or other lamp companies.

Depending on the lamp type, a variety of glasses may be used. Incandescent and fluorescent lamps typically use a soda-lime glass. Higher temperature lamps will use a borosilicate glass, while high-pressure discharge lamps will use either quartz or ceramic for the arc tube and borosilicate glass for the outer envelope. Leaded glass (containing approximately 20 to 30% lead) is typically used for sealing the ends of the lamp bulbs.

The wires used as supports or connectors in lamp construction may be made from a variety of materials including steel, nickel, copper, magnesium and iron, while the filaments are made from tungsten or tungsten-thorium alloy. One critical requirement for the support wire is that it must match the expansion characteristics of the glass where the wire penetrates the glass to conduct the electrical current for the lamp. Frequently, multi-part lead wires are used in this application.

Bases (or caps) are typically made from either brass or aluminium, brass being the preferred material when outdoor use is required.

Filament or Incandescent Lamps

Filament or incandescent lamps are the oldest lamp type still being manufactured. They take their name from the way these lamps produce their light: through the heating of a wire filament to a temperature high enough to cause it to glow. While it is possible to manufacture an incandescent lamp with almost any type of filament (early lamps used carbon), today most such lamps use a filament made of tungsten metal.

Tungsten lamps. The common household version of these lamps consists of a glass bulb enclosing a tungsten wire filament. Electricity is conducted to the filament by wires which support the filament and extend through the glass mount which is sealed to the bulb. The wires are then connected to the metal base, with one wire soldered at the centre eyelet of the base, the other connecting to the threaded shell. The supporting wires are of special composition, so that they have the same expansion characteristics as the glass, preventing leaks when the lamps become hot during use. The glass bulb is typically made from lime glass, while the glass mount is leaded glass. Sulphur dioxide is frequently used in preparing the mount. The sulphur dioxide acts as a lubricant during high-speed lamp assembly. Depending on the design of the lamp, the bulb may enclose a vacuum or may use a fill gas of argon or some other non-reactive gas.

Lamps of this design are sold using clear glass bulbs, frosted bulbs and bulbs coated with a variety of materials. Frosted bulbs and ones coated with a white material (frequently clay or amorphous silica) are used to reduce the glare from the filament found with clear bulbs. The bulbs are also coated with a variety of other decorative coatings, including coloured ceramics and lacquers on the outside of the bulbs and other colours, such as yellow or pink, on the inside of the bulb.

While the typical household shape is the most common, incandescent lamps can be made in many bulb shapes, including tubular, globes and reflector, as well as in many sizes and wattages, from subminiature through to large stage/studio lamps.

Tungsten-halogen lamps. One problem in the design of the standard tungsten filament lamp is that the tungsten evaporates during use and condenses on the cooler glass wall, darkening it and reducing the light transmission. Adding a halogen, such as hydrogen bromide or methyl bromide, to the fill gas eliminates this problem. The halogen reacts with the tungsten, preventing it from condensing on the glass wall. When the lamp cools, the tungsten will re-deposit back on the filament. Since this reaction works best at higher lamp pressures, tungsten-halogen lamps typically contain gas at several atmospheres pressure. Typically the halogen is added as a part of the lamp fill gas, usually at concentrations of 2% or less.

Tungsten-halogen lamps may also use bulbs made from quartz instead of glass. Quartz bulbs can withstand higher pressures than those made from glass. The quartz bulbs present a potential hazard, however, since the quartz is transparent to ultraviolet light. Although the tungsten filament produces relatively little ultraviolet, prolonged exposure at close range can produce reddening of the skin and cause eye irritation. Filtering the light through a cover glass will greatly reduce the amount of ultraviolet, as well as provide protection from the hot quartz in the event the lamp ruptures during use.

Hazards and Precautions

Overall, the greatest hazards in lamp production, regardless of product type, are due to the hazards of automated equipment and the handling of glass bulbs and lamps and other material. Cuts from the glass and reaching into the operating equipment are the most common causes of accidents; material-handling issues, such as repetitive motion or back injuries, are of particular concern.

Lead solder is frequently used on the lamps. For lamps used in higher temperature applications, solders containing cadmium may be used. In automated lamp assembly operations, exposure to both of these solders is minimal. Where hand soldering is done, as in repair or semi-automated operations, the exposures to lead or cadmium should be monitored.

Potential exposures to hazardous materials during lamp manufacturing have consistently decreased since the middle of the 20th century. In incandescent lamp manufacturing, large numbers of the lamps formerly were etched with hydrofluoric acid or bifluoride salt solutions to produce a frosted lamp. This has largely been replaced by the use of a low-toxicity clay coating. While not completely replaced, the use of hydrofluoric acid has been greatly reduced. This change has reduced the risk of burns to the skin and lung irritation due to the acid. The ceramic coloured coatings used on the outside of some lamp products formerly contained heavy metal pigments such as lead, cadmium, cobalt and others, as well as using a lead silicate glass frit as part of the composition. During recent years, many of the heavy metal pigments have been replaced by less toxic colourants. In cases where the heavy metals are still used, a lower toxicity form may be used (e.g., chromium III instead of chromium VI).

Coiled tungsten filaments continue to be made by wrapping the tungsten around a molybdenum or a steel mandrel wire. Once the coil has been formed and sintered, the mandrels are dissolved using either hydrochloric acid (for the steel) or a mixture of nitric and sulphuric acid for the molybdenum. Due to the potential acid exposures, this work is routinely done in hood systems or, more recently, in totally enclosed dissolvers (especially where the nitric/sulphuric mix is involved).

The fill gasses used in tungsten-halogen lamps are added to the lamps in totally enclosed systems with little loss or exposure. Hydrogen bromide use presents its own problems due to its corrosive nature. LEV must be provided, and corrosion-resistant piping must be used for the gas delivery systems. Thoriated tungsten wire (usually 1 to 2% thorium) is still used in some lamp types. However, there is little risk from the thorium in the wire form.

Sulphur dioxide must be carefully controlled. LEV should be used wherever the material is added to the process. Leak detectors may also be useful in storage areas. Use of smaller 75-kg gas cylinders is preferred over larger 1,000-kg containers due to the potential consequences of a catastrophic release.

Skin irritation can be a potential hazard from either the soldering fluxes or from the resins used in the basing cement. Some basing cement systems use paraformaldehyde instead of natural resins, resulting in potential formaldehyde exposure during curing of the basing cement.

All lamps use a chemical “gettering” system, in which a material is coated on the filament prior to assembly. The purpose of the getter is to react with and scavenge any residual moisture or oxygen in the lamp after the lamp is sealed. Typical getters include phosphorus nitride and mixtures of aluminium and zirconium metal powders. While the phosphorus nitride getter is fairly benign in use, handling aluminium and zirconium metal powders can be a flammability hazard. The getters are applied wet in an organic solvent, but if the material is spilled, the dry metal powders can be ignited by friction. Metal fires must be extinguished with special Class D fire extinguishers and cannot be fought with water, foam or other usual materials. A third type of getter includes use of phosphine or silane. These materials can be included in the gas fill of the lamp at low concentration or can be added at high concentration and “flashed” in the lamp prior to the final gas fill. Both these materials are highly toxic; if used at high concentration, totally enclosed systems with leakage detectors and alarms should be used at the site.

Discharge Lamps and Tubes

Discharge lamps, both low- and high-pressure models, are more efficient on a light per watt basis than incandescent lamps. Fluorescent lamps have been used for many years in commercial buildings and have been finding increased use in the home. Recently, compact versions of the fluorescent lamp have been developed specifically as replacements for the incandescent lamp.

High-pressure discharge lamps have long been used for large area and street lighting. Lower-wattage versions of these products are also being developed.

Fluorescent lamps

Fluorescent lamps are named for the fluorescent powder used to coat the inside of the glass tube. This powder absorbs ultraviolet light produced by the mercury vapour used in the lamp, and converts and re-emits it as visible light.

The glass used in this lamp is similar to that used in incandescent lamps, using lime glass for the tube and leaded glass for the mounts on each end. Two different families of phosphors are in use currently. Halophosphates, based on either calcium or strontium chloro-fluoro-phosphate, are the older phosphors, coming into wide use in the early 1950s when they replaced phosphors based on beryllium silicate. The second phosphor family includes phosphors made from rare earths, typically including yttrium, lanthanum and others. These rare-earth phosphors typically have a narrow emission spectrum, and a mixture of these are used—generally a red, a blue and a green phosphor.

The phosphors are mixed with a binder system, suspended in either an organic mix or a water/ammonia mixture and coated on the inside of the glass tube. The organic suspension uses butyl acetate, butyl acetate/naphtha or xylene. Due to environmental regulations, water-based suspensions are replacing those that are organic based. Once the coating is applied, it is dried onto the tube, and the tube is heated to a high temperature to remove the binder.

One mount is attached to each end of the lamp. Mercury is now introduced into the lamp. This can be done in a variety of ways. Although in some areas the mercury is added manually, the predominant way is automatically, with the lamp mounted either vertically or horizontally. On vertical machines, the mount stem on one end of the lamp is closed. Then mercury is dropped into the lamp from above, the lamp is filled with argon at low pressure, and the top mount stem is sealed, completely sealing the lamp. On horizontal machines, the mercury is introduced from one side, while the lamp is exhausted from the other side. Argon is again added to the proper pressure, and both ends of the lamp are sealed. Once sealed, the caps or bases are added to the ends, and the wire leads are then either soldered or welded to the electrical contacts.

Two other possible ways of introducing mercury vapour can be used. In one system, the mercury is contained on a mercury-impregnated strip, which releases the mercury when the lamp is first started. In the other system, liquid mercury is used, but it is contained within a glass capsule which is attached to the mount. The capsule is ruptured after the lamp has been sealed and exhausted, thereby releasing the mercury.

Compact fluorescent lamps are smaller versions of the standard fluorescent lamp, sometimes including the ballast electronics as an integral component of the lamp. Compact fluorescents generally will use a mixture of rare-earth phosphors. Some compact lamps will incorporate a glow starter containing small amounts of radioactive materials to aid in starting the lamp. These glow starters typically use krypton-85, hydrogen-3, promethium-147 or natural thorium to provide what is called a dark current, which helps the lamp start quicker. This is desirable from a consumer standpoint, where the customer wants the lamp to start immediately, without flickering.

Hazards and precautions

Fluorescent lamp manufacturing has seen a considerable number of changes. Early use of a beryllium-containing phosphor was discontinued in 1949, eliminating a significant respiratory hazard during phosphor production and use. In many operations, water-based phosphor suspensions have replaced organic suspensions in the coating of the fluorescent lamps, reducing exposure to the workers as well as reducing the emission of VOCs to the environment. Water-based suspensions do involve some minimal exposure to ammonia, particularly during mixing of the suspensions.

Mercury remains the material of greatest concern during fluorescent lamp making. While the exposures are relatively low except around the exhaust machines, there is potential for significant exposure to workers stationed around the exhaust machine, to mechanics working on these machines and during clean-up operations. Personal protective equipment, such as coveralls and gloves to avoid or limit exposure and, where needed, respiratory protection, should be used, especially during maintenance activities and clean-up. A biological monitoring programme, including mercury urinalysis, should be established for fluorescent lamp manufacturing sites.

The two phosphor systems currently in production utilize materials considered to have relatively low toxicity. While some of the additives to the parent phosphors (such as barium, lead and manganese) have exposure limits established by various governmental agencies, these components are usually present in relatively low percentages in the compositions.

Phenol-formaldehyde resins are used as electrical insulators in the end caps of the lamps. The cement typically includes natural and synthetic resins, which may include skin irritants such as hexamethylene-tetramine. Automated mixing and handling equipment limits the potential for skin contact to these materials, thereby limiting the potential for skin irritation.

High-pressure mercury lamps

High-pressure mercury lamps include two similar types: those using just mercury and those using a mixture of mercury and a variety of metal halides. The basic design of the lamps is similar. Both types use a quartz arc tube which will contain the mercury or mercury/halide mixture. This arc tube is then enclosed in a hard, borosilicate glass outer jacket, and a metal base is added to provide for electrical contacts. The outer jacket can be clear or coated with either a diffusing material or a phosphor to modify the colour of the light.

Mercury lamps contain only mercury and argon in the quartz arc tube of the lamp. The mercury, under high pressure, generates light with a high blue and ultraviolet content. The quartz arc tube is completely transparent to UV light, and in the event that the outer jacket is broken or removed, is a powerful UV light source that can produce skin and eye burns in those exposed. Though the typical mercury lamp design will continue to operate if the outer jacket is removed, manufacturers also offer some models in a fused design which will stop operating if the jacket is broken. During normal use, the borosilicate glass of the outer jacket absorbs a high percentage of the UV light, so that the intact lamp does not pose a hazard.

Because of the high blue content of the mercury lamp spectrum, the inside of the outer jacket is frequently coated with a phosphor such as yttrium vanadate phosphate or similar red-enhancing phosphor.

Metal halide lamps also contain mercury and argon in the arc tube, but add metal halides (typically a mixture of sodium and scandium, possibly with others). The addition of the metal halides enhances the red light output of the lamp, producing a lamp which has a more balanced light spectrum.

Hazards and precautions

Other than mercury, potentially hazardous materials used in high-pressure mercury lamp production include the coating materials used on the outer envelopes and the halide additives used in the metal halide lamps. One coating material is a simple diffuser, the same as that used in incandescent lamps. Another is a colour-correcting phosphor, yttrium vanadate or yttrium vanadate phosphate. While similar to vanadium pentoxide, the vanadate is considered to be less toxic. Exposure to the halide materials is normally not significant, since the halides react in moist air and must be kept dry and under an inert atmosphere during handling and use. Similarly, although the sodium is a highly reactive metal, it too needs to be handled under an inert atmosphere to avoid oxidizing the metal.

Sodium Lamps

Two types of sodium lamps are currently produced. Low-pressure lamps contain only metallic sodium as the light emitting source and produce a highly yellow light. High-pressure sodium lamps use mercury and sodium to generate a whiter light.

Low-pressure sodium lamps have one glass tube, which contains the metallic sodium, enclosed within a second glass tube.

High-pressure sodium lamps contain a mixture of mercury and sodium within a high-purity ceramic alumina arc tube. Other than the composition of the arc tube, the construction of the high-pressure sodium lamp is essentially the same as the mercury and metal halide lamps.

Hazards and precautions

There are few unique hazards during manufacturing of high- or low-pressure sodium lamps. In both lamp types, the sodium must be kept dry. Pure metallic sodium will violently react with water, producing hydrogen gas and enough heat to cause ignition. Metallic sodium left out in air will react with the moisture in the air, producing an oxide coating on the metal. To avoid this, the sodium is usually handled in a glove box, under a dry nitrogen or argon atmosphere. For sites manufacturing high-pressure sodium lamps, additional precautions are needed to handle the mercury, similar to those sites manufacturing high-pressure mercury lamps.

Environmental and Public Health Issues

Waste disposal and/or recycling of mercury-containing lamps is an issue that has received a high degree of attention in many areas of the world over the last several years. While at best a “break even” operation from a cost viewpoint, technology currently exists to reclaim the mercury from fluorescent and high-pressure discharge lamps. Recycling of lamp materials at the present time is more accurately described as reclamation, since the lamp materials are rarely reprocessed and used in making new lamps. Typically, the metal parts are sent to scrap metal dealers. The recovered glass may be used to make fibreglass or glass blocks or used as aggregate in cement or asphalt paving. Recycling may be the lower-cost alternative, depending on location and availability of recycling and hazardous or special waste disposal options.

The ballasts used in fluorescent lamp installations previously contained capacitors which used PCBs as the dielectric. While manufacture of PCB-containing ballasts has been discontinued, many of the older ballasts may still be in use due to their long life expectancy. Disposal of the PCB-containing ballasts may be regulated and may require disposal as a special or hazardous waste.

Glass manufacturing, particularly borosilicate glasses, can be a significant source of NOx emission to the atmosphere. Recently, pure oxygen instead of air has been used with gas burners as a means of reducing the NOx emissions.



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