The present invention is directed to a process for manufacturing shields of different size for fluorescent lamps from metallic strip, as well as to the shields produced thereby.

Fluorescent lamps are used in indoor lighting and their spread is increasing also owing to the fluorescent lamps with a low power consumption which have been put on the market.

As is known, the fluorescent lamps are formed of glass tubes innerly lined with powders of fluorescent materials (called phosphors); these lamps also comprise two electrodes, generally defined as cathodes and are filled with a gaseous mixture consisting of a rare gas and mercury vapours. When the lamp is turned on, an electric discharge is generated between cathodes with consequent excitation of the filling gas thus forming a cold plasma; the excited atoms of mercury being present in the plasma emit a UV radiation which is converted by phosphors in the visible light of the lamp.

It is known that in the lamps of this type each cathode has a shield which is generally made of a metallic material strip, bent and welded at the ends to form a cylindrical surface. These shields are placed about the cathodes, axially to the lamp and have the purpose of preventing from degradation the phosphors being nearer to the cathodes, due to a direct electronic radiation from the cathodes themselves.

Within a fluorescent lamp there is generally present a getter material, i.e. a metal or a metallic alloy capable of irreversibly fixing the reactive gases within lamp by means of a chemical reaction. Actually, various gases have a harmful influence on the lamp functioning through different mechanisms: hydrogen requires an increase of the potential difference which is necessary between the two cathodes for maintaining the discharge; oxygen as such or contained in the water takes away mercury from its function to form mercury oxide; carbon monoxide and bi-oxide are decomposed on the cathode thus forming oxygen which has the harmful effect already mentioned, and carbon which

deposits onto the phosphors in the form of a black layer thus reducing the lamp brightness. The most commonly used getter material in the lamps is an alloy of percent composition in weight Zr 84%-AI 16%, manufactured and sold by the applicant with the tradename St 101. Other getter materials which are useful to the purpose are e.g. the alloy having the composition Zr 70%-V 24,6%-Fe 5,4% and the alloy of composition Zr 77,6% Fe 23,4% (percentages by weight) manufactured and sold by the applicant under the tradenames St 707 and St 198, respectively.

It is widespread among the manufacturers of lamps the use of shields having also the function of support for the getter material. In these shields getter material is present as a powder being cold rolled on a surface of the strip forming the shield.

Furthermore the fluorescent lamps require mercury, for their operation, in quantities varying between 3 and 10 mg according to the type of lamp.

A precise and reproducible dosing of small quantities of mercury in the lamps is difficult in practice: using liquid batches of mercury is complicate in consequence of the extremely small volumes required, which are less than one μl, the harmless of the mercury vapours that may develop in the environment and of the fact that the metallic mercury, when entering into contact with air at the outlet of the dispenser, becomes oxidized, thus clogging the system. US patent 3.657.589 discloses, to introduce mercury into the lamps, the use of intermetallic compounds of general formula Zr _x Ti _y Hg _z , among which the compound Ti ₃ Hg is preferred, being manufactured and sold by the applicant with the tradename St 505. Mercury is released from these materials, upon closing the lamp, by heating from the outside at temperatures of about 850-900°C. Copper-tin alloys or copper-silicon alloys, as descπbed in EP-A-0669639 and EP-A- 0691670 in the applicant's name, can be added to these intermetallic compounds with the function of promoting the mercury releasing. Also these materials can be rolled onto the strip by which the shield is produced, in order to obtain a device combining the functions of shield, getter and mercury dispenser, which also provides a very helpful solution for the manufacturing of lamps. As already outlined in the introductory portion of the description, a great number of fluorescent lamps with a reduced power consumption has

been put on the market in the last years, these lamps being different from one another under both the aspects of geometry and the component materials, Such a variety of lamps brings about the requirement of a broad variation of the shield diameters and of the quantities of mercury releasing materials and getter material (in the following all defined with the general term of active materials). However the production of a different strip for every application is burdensome, under an industrial point of view, as it requires a new configuration of manufacturing machines for each strip required. A possible solution in this direction is proposed in JP-A-6-96728 in the name of the company Toshiba, which suggests to provide a complete shield by joining a piece of strip on which the active materials are rolled and a piece of metallic strip without any material. By controlling the lengths of the two different strip pieces it is possible to vary the shield diameter and the total quantity of active materials being introduced. Although this solution allows to overcome the problem deriving from the variety of shield size, it is scarcely functional under a practical point of view.

Object of the present invention is that of providing a solution to the requirement of producing in a simple way strips useful to form shields for fluorescent lamps having whichever size range.

This object is obtained according to the present invention through a process for manufacturing shields for fluorescent lamps, comprising the steps of:

- providing a support for powders in the form of a metallic strip, having a length equivalent to about the circumference of the smallest shield to be produced;

- depositing on said support strip at least one track of getter material and at least another of mercury releasing materials in powdered form;

- producing pieces of the support strip, by cutting the same with parallel cuts, at a pitch and at an angle with respect to the central axis of the strip which are varying according to the desired size of the shield.

The invention will be described in the following with reference to the drawings in which:

Figure 1 shows a possible support strip from which lengths or pieces for producing shields can be obtained;

Figure 2a shows the strip of figure 1 with marked the cuts to be made

(lines l-l, I'-l', l"-l", ...) to obtain pieces like that of figure 2b (it will be noted that the piece of figure 2b and strip of figure 2a are not in the same scale); Figure 2c shows a shield for fluorescent lamps as obtained from the piece of figure 2b; Figure 3a shows the strip of figure 1 with marked the cuts to be made

(line ll-ll, ll'-ll', ll"-ll" ...) to obtain pieces like that of figure 3b (also the piece of figure 3b and the strip of figure 3a are out of scale); and

Figure 3c shows a shield for fluorescent lamps as obtained from the piece of figure 3b. The support strip can be made in various metals, but preferred is the use of nickel-plated steel showing good mechanical features and resistance to oxidation even at the high temperatures undergone by the shields during the lamps manufacturing. Such a strip has generally a thickness of about 0,5 mm. The strip width is instead determined by the circumference of the smallest shield which is intended to be produced therefrom. In particular this width is preferably equal to or less than the circumference of shield with a length of about 2-3 mm added, corresponding to the end portions of the strip or piece which are overlapped and welded to form the ring-shaped shield. Tracks of the active materials are deposited on the support strip through various techniques. The preferred techniques is the cold rolling well known in the field of the "powders" technology. The powders to be rolled have a particle size generally comprised between 40 and 120 μm Figure 1 shows the drawing of a possible strip with powders of active materials.

The pieces of a stripe-shaped support strip on which the shields are obtained, are made by cutting the strip with a pitch D and at an angle with respect to its central axis X-X, both varying according to the desired size of the shield. As stated before, the strip width is equal to or less than the smallest circumference of shield desired, there being added thereto almost two small lateral zones to be overlapped and welded. With reference to figure 2a, in case that a shield of small size has to be manufactured, it is sufficient to obtain lengths of a strip by cutting the same in a direction perpendicular to the axis, along the parallel lines l-l- and I'-l', thus obtaining the piece of rectangular shape of figure 2b; in this case the angle α=90° and the distance between the two parallel lines l-l

and I'-l' is merely corresponding to the desired height of the shield. When on the contrary it is wished to manufacture a shield with a circumference greater than the strip width, the strip is cut at an angle with respect to its axis, along lines ll-ll and IMI'; an example of this case is given in figures 3a and 3b, wherein the support strip with marked the cuts forming the angle α with the axis and a parallelogram-shaped piece of figure 3b are respectively shown. The length of the piece thus obtained is defined through the simple trigonometric relation:

C = IJsinα wherein C and L are respectively the length of the obtained piece (equivalent to the circumference of the final shield, apart from the overlapping zone for welding) and the width of the strip.

The height of the piece, and consequently of the shield is instead defined through the relation: A = D sinα wherein A and D are respectively the height of the piece and distance (or pitch) between lines ll-ll and ll'-ll', being measured along the strip axis.

From the relations above it results that the length of the piece and its height can be varied at will, for the same strip, by controlling the angle and the pitch of the parallel cuts. For example, when cutting the strip at an angle of 30°, pieces are obtained which have lengths being about twice the strip width. In a continuous manufacturing process in which the strip is moved forward at a constant feeding speed, the pitch is instead simply adjusted by controlling the time interval between one cut and another.