Improved mercury source for dosing small amounts of mercury, method of manufacturing and use of said source for the production of mercury requiring devices

Abstract

 Improved mercury dosing source with a core-shell structure, wherein the core is essentially made by liquid mercury and the shell is made essentially by a binder containing a filler in form of a plurality of solid discrete elements.

Description

[0001] The present invention is inherent to a novel type of mercury source to precisely dose small amounts of mercury, to its use for lamp manufacturing and to production methods for said mercury source.

[0002] One of the problems with some types of illumination devices, with particular reference to the so-called low pressure mercury lamps, is linked to the presence of mercury in their internal volume. Among these lamps the so called fluorescent lamps, also known in the field with the acronym of FL lamps, are widely diffused: the three main subcategories of these devices are Compact Fluorescent Lamp (CFL), Linear Fluorescent Lamps and Circular Fluorescent Lamps. Typically during manufacturing of these lamps, a small quantity of mercury (a few milligrams) is dosed by introducing a mercury source in the lamp body.

[0003] Other illumination devices requiring mercury are the Cold Cathode Fluorescent Lamps (CCFL), in this case the mercury source may not be present in the final device, since in the most diffused production process, after mercury release, the source, connected by means of a glass chamber to the main body of the lamp, is discarded and the lamp sealed. This production technique is known in the field as double tip-off or double pinch-off, more details may be found in the international patent WO 98/53479 in the applicant's name.

[0004] Evolutions in the environmental regulation constantly decrease the amount of mercury tolerated for these devices, also in view of the high number of lamps containing mercury produced each year, such number being actually in the order of several billions. This leads to the problem to precisely introduce in a reproducible manner the required quantity of mercury. In particular current regulation limits the maximum amount of mercury to some milligrams and this limit will be set at few milligrams for the next generation of such lamps.

[0005] Another relevant aspect to be considered when dealing with mercury sources is inherent to safety due to the mercury vapor pressure; in this regard it is desirable to have a dispensing solution capable to avoid the mercury release until the required stage in the device manufacturing process. In many cases the source of mercury is introduced in one of the latest process stages of lamps manufacturing and an ideal source should avoid premature mercury release at low temperatures, up to 70°C and also having an optimized coupling with the lamp production process, meaning that mercury may be fully released at temperatures above 100°C, with times for the mercury release typically ranging from several minutes at temperatures proximate to 150°C, down to tens of seconds for temperature close to 350°C. This mercury dispensing solution, being more versatile, provides advantages in the manufacturing of the light devices with respect to high temperature sources since the heating phase and the required heating equipments can be simplified and more easily matched with the lamp manufacturing process constraints resulting in a more efficient and leaner process phase.

[0006] The third important feature that a mercury source should possess is linked to the miniaturization of devices that poses a constraint on its maximum size, this aspect is of particular relevance in case of illumination devices where the dispenser remains within the device, such as the above mentioned Fluorescent Lamps, since the mercury dispenser may create shadow effects for the generated light.

[0007] These aspects, i.e. relating to environment, safety, smallest dimensions, efficient and versatile temperature releasing, are not concurrently addressed in an efficient manner by the solution known in the art.

[0008] For example one of the most successful solutions for mercury dosing, described in the published International patent application WO 98/53479 provides for the use of an elongated dispenser containing a mercury releasing alloy stable up to high temperatures. Low levels of mercury loading may be reached by "diluting" the mercury releasing material by mixing it with a mercury free material. One of the limits of this solution is linked to the need of a specific activation process to temperatures higher than 850°C to have an efficient mercury release; in addition its size is relatively big due to the elongated shape; this may pose a problem for its use in illumination devices, especially for the small size lamps.

[0009] Another solutions adopted in the technical field, especially for "low end" linear fluorescent lamps and compact fluorescent lamps, envisages the use of spheres of a mercury amalgam, as described in the published International patent application WO 2007/146196. One of the major problem linked to their use is related to safety, since mercury is emitted at a significant level at temperatures below 70°C. This premature and hardly controllable Hg release is among the causes of the low reproducibility of the dispensed mercury quantity per spherical unit.

[0010] European patent application EP 1434249 and Japanese patent application JP 2007273346 disclose the use of composite spheres having an inner metallic core with a mercury coating or a coating with an alloy containing mercury. These solutions pose a series of safety problems since mercury or the mercury containing alloy is on the external part of the dispenser and therefore in direct contact with the environment; furthermore also the metallic core of the sphere, having only a support function, increases the size of the spherical mercury dispenser.

[0011] Chinese patent application CN 101000848 discloses spherical dispensers, where the core is made of a mercury amalgam covered by a membrane of fluorescent powder and a metallic oxide. The problem entrained by this solution relates to the low precision in the amount of mercury contained in the amalgam, intrinsic in the amalgam formation process. In addition its use in devices where the dispenser remains sealed at the inside, presents the drawback that the amalgam may partly re-absorb the mercury after the evaporation.

[0012] International patent application WO 2009/043728 describes a solution where a mercury containing core is coated with ceramic/glass composition, and the mercury is released after heating thanks to the melting of the coating. The main disadvantage of this solution is linked to the high temperatures required for the mercury release (400° C and more), necessary for the melting of the glass or ceramic shell of the liquid mercury core.

[0013] Object of the present invention is to provide an improved source for mercury dispensing and precise dosing capable to overcome the problems and drawbacks still present in the prior art with reference to aspects relating to environment, safety, size and releasing temperature and in a first aspect thereof consists in a core-shell mercury source, wherein the core is made essentially by liquid mercury, characterized in that the shell enclosing said mercury core is a composite structure, comprising an organic binder and a filler made by a plurality of solid discrete elements, wherein:

• the wt% of the organic binder in the shell is comprised between 3% and 70% and the wt% of the filler is comprised between 30% and 97% when the filler is organic or the filler is inorganic and the true density of the material of the filler is equal or less than 13.6 g/cm³,
• the wt% of the organic binder in the shell is comprised between 0,5% and 50% and the wt% of the filler is comprised between 50% and 99.5% when the filler is inorganic and the true density of the material of the filler is greater than 13.6 g/cm³.

[0014] With the expression "made essentially by liquid mercury" it is intended that the core contains at least 95% wt of mercury, since it is tolerable to have impurities, such as traces of other elements not imparting additional features and properties to the mercury liquid source.

[0015] According to a preferred embodiment the composite is a composition of the shell from 4 wt% to 35 wt% of an organic binder and between 65 wt% and 96 wt% of a filler made by a plurality of solid discrete elements independently from the true density or nature of the filler.

[0016] The ranges for the wt% of the filler depend on an intrinsic physical property of the filler, its true density that is defined as the density tabulated and characteristic of the element constituting the filler, not including voids or porosities.

[0017] In case of a plurality of filler materials, it is the density averaged over the amount of the fillers that is relevant for the limit determination. For example in the case of two fillers F1 and F2 with true densities D1 and D2 and present in amounts, by weight, given by wt1 and wt2, respectively, the average density D will be expressed as follows:

[0018] In case of a mix between a plurality of solid discrete elements of an organic filler and an inorganic filler, in case wt ratio between the organic filler and the inorganic one is at least 0.1, then the wt% of the organic binder in the shell is comprised between 3% and 70% and the wt% of the filler is comprised between 30% and 97%.

[0019] The thickness of the shell can span from 100 to 1000 micron, and preferably is comprised between 140 and 500 µm, while the liquid core may contain up to 12 mg of Hg, whereas the lower limit is at 0.1 mg Hg.

[0020] With regard to the filler made by a plurality of solid discrete elements, it is intended a filler made by discrete particles with no particular restriction on the shape that may be for example spherical, cylindrical, elongated or exhibiting an irregular (corrugated) surface. What is relevant for the definition of the filler is the volume of the discrete elements that shall be comprised between 1*10^-3 and 2*10⁶ µm³ and preferably is comprised between 4 and 2*10⁵ µm³.

[0021] Also with regard to the materials of the discrete elements constituting the filler, it is not required to use only one type of material, even though the preferred solution envisages the use of solid discrete elements equal to each other in composition. Particularly preferred is the use of inorganic discrete elements, and even more preferably only one type of inorganic substance is used.

[0022] Examples of materials suitable to be used for the organic discrete fillers are polytetrafluoroethylene (PTFE), polyolefines such as polyethylene, acrylic polymers such as polymethylmethacrylate (PMMA), polyetheretherketone (PEEK), polyetherimide (PEI), polybenzimidazole (PBI), polyphenylensulfide (PPS).

[0023] Examples of materials suitable to be used as inorganic discrete fillers are silicon, silica, stainless steel, brass, tungsten, molybdenum, bismuth, tin, zinc, their alloys or combinations. Also getter materials, such as zirconium and its alloys, yttrium and its alloys, titanium and its alloys, nickel and its alloys can be adopted.

[0024] Preferred is the use of silicon, silica, titanium and steel fillers.

[0025] Preferred organic compounds suitable for being used as binder components are hydrocolloid materials (also sometimes referred in the field as hydrogels), such as those based on polysaccharides, with particular reference to alginates or celluloses, such as hydroxypropyl-, methyl-, ethyl-, carboxmethylcellulose. Also binders based on crosslinkable gelatins may be employed.

[0026] The binder may contain also additional organic compounds, to impart better properties to the shell, such as an improved strength; in this regard example of suitable additives are Shellac, Gellan gum, Xanthan gum, Guar Gum, special polysaccharides, polyvinyl alcohol and their combination. The amount of these additional organic compounds is usually not higher than 10% wt, calculated over the overall shell weight.

[0027] In some specific cases, especially in order to obtain a higher mechanical resistance of the structure, it is useful to have a structure made by a plurality of shells; particularly advantageous is a two shells enclosing structure, with the outer shell having a different composition with respect to the composition of the shell in direct contact with liquid mercury. This solution enables to split the required property of the shell into two different composite structures. Particularly advantageous is providing for the composite material in direct contact with mercury a good wettability and adhesion, thereby a good mercury enclosure, while the outer shell is chosen to provide mechanical resistance to the shell, for example by modulating its breaking properties. In particular it is important to have a more robust outer shell in the case of a fragile inner one to prevent leakage of liquid and vapors of mercury. In this specific embodiment the outer shell may be without filler, while the inner shell is the structure containing the filler made by a plurality of solid discrete elements. In other specific embodiments, the filler might be in the inner shell and the outer shell shall be of a lower filler content, preferably 0-50% of the filler content of the inner shell, and providing diffusion strength or elasticity.

[0028] One of the advantages of the mercury sources according to the present invention is their stability, i.e. the lack of mercury release, for temperatures up to 70° C, and the full release of mercury at a temperature such as 150° C. In this case, typically the mercury releasing mechanism is evaporation from cracks and fissures of the shell structure, whereas for a higher temperature, such as 300° C, the shell breakage (fragmentation) and a more sudden mercury release process takes place. Shell thickness influences the releasing mechanism, with thinner shell being more subject to breakage.

[0029] Thus the mercury sources according to the present invention satisfy the safety requirement in lamp manufacturing with regard to the handling without mercury losses at temperatures up to 70° C and also show a high versatility in the releasing mechanism enabling the adoption of the very same solution for a wide range of production processes, each one with its own temperature limit and characteristics.

[0030] The above mercury sources using liquid mercury in the core have the advantages of more precise dosing and smallest dimensions with respect to mercury sources using amalgams as core, such as the one described in the above mentioned Chinese patent application CN 101000848, while with respect to the structure described in the published International patent application WO 2009/043728 they present a much lower mercury releasing temperature.

[0031] In a second aspect thereof the invention is related to the use of a core-shell mercury source, wherein the core is made essentially by liquid mercury, characterized in that the shell enclosing said mercury core is a composite structure comprising an organic binder and a filler made by a plurality of solid discrete elements, wherein:

• the wt% of the organic binder in the shell is comprised between 3% and 70% and the wt% of the filler is comprised between 30% and 97% when the filler is organic or the filler is inorganic and the true density of the material of the filler is equal or less than 13.6 g/cm³,
• the wt% of the organic binder in the shell is comprised between 0,5% and 50% and the wt% of the filler is comprised between 50% and 99,5% when the filler is inorganic and the true density of the material of the filler is greater than 13.6 g/cm³,

to produce devices requiring a small amount of mercury in the internal environment.

[0032] In some of these devices such as fluorescent lamps with particular reference to compact fluorescent lamps, linear fluorescent lamps and circular fluorescent lamps, and some types of cold cathode fluorescent lamps the mercury source is sealed within the device; in this case after the sealing process and opening of the core-shell structure only the shell may be found within the lamp, while mercury is present in the lamp in form of vapors or deposits being spread on the lamp components.

[0033] In some other devices, as in the most common types of cold cathode fluorescent lamps produced by means of the double tip-off process, the dispenser is just used in an intermediate manufacturing production step and not present within the sealed lamp.

[0034] In a third aspect thereof the invention relates to a production process for core-shell mercury source, wherein the core is made essentially by liquid mercury, characterized in that the shell enclosing said mercury core is a composite structure comprising an organic binder and a filler made by a plurality of solid discrete elements, wherein:

• the wt% of the organic binder in the shell is comprised between 3% and 70% and the wt% of the filler is comprised between 30% and 97% when the filler is organic or the filler is inorganic and the true density of the material of the filler is equal or less than 13.6 g/cm³,
• the wt% of the organic binder in the shell is comprised between 0,5% and 50% and the wt% of the filler is comprised between 50% and 99,5% when the filler is inorganic and the true density of the material of the filler is greater than 13.6 g/cm³ %,

and said core-shell structure is produced by means of a technique chosen from Rotary Die process, Drip Casting process, Spraying process.

[0035] The Rotary Die process is based on the formation of an organic band used to create the capsules: a warm liquid cellulose (e.g. hydroxypropyl methylcellulose) or gelatin is spread over a slowly revolving stainless steel drum. It is solidified on the rotating drum in such a way to form an elastic band rolling off of the other end. This thin band is then filled with the core material and automatically formed into capsules.

[0036] A Drip Casting process allows the production of spherical core-shell structures characterized by an envelope covering the core by using a microcapsule technology, which can be based e.g. on a vibrating nozzle suitable for droplets formation.

[0037] The Spraying process can be exploited to create an envelope or shell around already formed droplets covering them with a coating applied starting from a suitable suspension based on a combination of binder and solid filler.

[0038] For techniques of Drip Casting and Spraying the shell is produced by coating mercury with a slurry containing the organic binder and the filler preferably in form of powders. In a preferred embodiment the slurry is an aqueous suspension containing at least 50% of water. Also non aqueous slurries, such as alcohol or solvent based ones may be employed.

[0039] The invention will be also illustrated by means of the following non limiting examples.

[0040] Different batches of core-shell mercury spheres prepared by a micro-encapsulation technique, containing about 2 mg of mercury and having a diameter in the range of 0.9 - 1.4 mm were produced by alginates as organic binder in the shell, such as Alginate BR-R, Alginate BR-F5 or Alginate BR-F, sold by Brace GMBH, or Hydroxy Propyl Methyl Cellulose, named HPMC.

[0041] The composition of the different batches/groups is shown in Table 1, showing for each of them the binder, the filler type, the wt% composition of the shell, its thickness, and the size of the filler powders.

[0042] Spheres from groups 1 to 9 have a composition according to the present invention, made with silicon, that has a true density of 2.33 g/cm³, as inorganic filler; group 10 core-shell mercury spheres have a composition according to the present invention but in this case the inorganic filler is tungsten, that has a true density of 19.3 g/cm³.

[0043] Spheres from groups C1-C3 are comparative examples of compositions not covered by the present invention.

TABLE 1

Group	Organic Binder	Filler	Shell Thickness	Grain Volume
			(µm)	(µm3)
1	Alginate BR_F	Si powder	150	4-1000
	6%	94%
2	Alginate BR_F5	Si powder	170	4-1000
	20%	80%
3	AlginateBR_F5	Si powder	100	4-1000
	20%	80%
4	Alginate BR_F5	Si powder	350	4-1000
	20%	80%
5	AlginateBR_R	Si powder	150	1*10-3-3,5
	29%	71%
6	Alginate BR_R	Si powder	150	4-1000
	29%	71%
7	Alginate BR_R	Si powder	150	1*104-2*106
	29%	71%
8	HPMC	Si powder	150	4-1000
	23%	77%
9	Alginate BR-F	W powder	150	4-1000
	4,8%	95,2%
C1	Alginate BR-F	Si powder	150	4-1000
	1%	99%
C2	Alginate BR-F	Si powder	150	4-1000
	2%	98%
C3	Alginate BR-F	No filler	100	-
	100%

[0044] The core-shell spheres from the different groups were tested in order to determine their mechanical strength according to the following procedure: for each group, 20 spheres were closed in a plastic cylindrical container having height of 30 mm and base diameter of 20 mm; then the container was placed in rotation on an axis perpendicular to the cylinder axis at a speed of 30 rounds/min for 15 minutes to simulate severe stresses during handling. At the end of the tests visual inspections were carried out under a microscope with a 50X magnification and the percentage of cracked or broken capsules was determined per each batch. The groups were divided in the following three categories according to the induced defective rate: "bad" for a defective rate induced by the rotation of more than 60%, "acceptable" for an induced defective rate between 40% and 10%, "good" for a defective rate lower than 10%.

[0045] The results obtained are reported in Table 2 in the mechanical strength column.

[0046] Samples of the same groups were also submitted to heating inside a glass bulb under vacuum (inner pressure of about 1*10^-4 mbar) to evaluate the Breaking Temperature of the Core-Shell structures, i.e. temperature needed to have a rupture of the shell so as to have a fast, complete mercury release. The rupture is checked by visual inspection after the test and these results are reported in Table 2 in the Breaking Temperature column. It is to be underlined that that condition corresponds and is useful to processes requiring fast mercury releasing, while, as previously described, mercury may also be released via fissures at lower temperatures, but in this case longer times are required.

TABLE 2

Group	Mechanical Strength	Breaking Temperature (°C)
1	Good	250
2	Good	250
3	Acceptable	200
4	Good	340
5	Acceptable	240
6	Good	260
7	Acceptable	230
8	Acceptable	200
9	Acceptable	170°C
C1	Bad	N.A.
C2	Bad	N.A.
C3	Bad	N.A.

[0047] From the experimental data shown in Table 2 it is possible to observe that all the core-shell structures prepared with compositions according to the present invention, taken from groups 1 to 9, have acceptable mechanical characteristics, while spheres taken from comparative groups C1-C3 have "bad" and unacceptable characteristics. Breaking temperatures depend on the shell composition, but also on the shell thickness, as it is possible to observe from the breaking temperature of spheres from groups 2-4, all having the same shell composition but a different thickness.

[0048] It is also possible to observe that the results with a filler having a density less than 13.6 g/cm³, such as silicon (samples 1-8), are better, in terms of mechanical strength, with respect to a much denser filler, such as Tungsten (sample 9).

[0049] It was impossible to carry out tests to determine the breaking temperatures of samples of groups 10, 11 and 12 because of their poor mechanical stability.

Claims

1. Core-shell mercury source, wherein the core is made essentially by liquid mercury, characterized in that the shell enclosing said mercury core is a composite structure, comprising an organic binder and a filler made by a plurality of solid discrete elements, wherein:

• the wt% of the organic binder in the shell is comprised between 3% and 70% and the wt% of the filler is comprised between 30% and 97% when the filler is organic or the filler is inorganic and the true density of the material of the filler is equal or less than 13.6 g/cm³,

• the wt% of the organic binder in the shell is comprised between 0,5% and 50% and the wt% of the filler is comprised between 50% and 99,5% when the filler is inorganic and the true density of the material of the filler is greater than 13.6 g/cm³.

2. Core-shell mercury source according to claim 1 wherein said shell composite composition contains from 4 wt% to 35 wt% of an organic binder and between 65 wt% and 96 wt% of a filler made by a plurality of solid discrete elements.

3. Core-shell mercury source according to claim 1 wherein the amount of the liquid mercury core is comprised between 0,1 and 12 mg.

4. Core-shell mercury source according to claim 1 wherein the thickness of said shell is comprised between 0.1 mm and 1 mm.

5. Core-shell mercury source according to claim 1 wherein the thickness of said shell is comprised between 0.14 and 0.5 mm.

6. Core-shell mercury source according to claim 1 wherein the volume of said solid discrete elements is comprised between 1*10^-3 and 2*10⁶ µm³.

7. Core-shell mercury source according to claim 1 wherein the plurality of solid discrete elements have the same composition.

8. Core-shell mercury source according to claim 1 wherein said plurality of solid discrete elements are inorganic.

9. Core-shell mercury source according to claim 8 wherein the materials used for said discrete inorganic elements comprises one or more among silicon, silica, stainless steel, brass, tungsten, molybdenum, bismuth, tin, zinc, zirconium, yttrium, titanium, nickel, and their alloys or combinations.

10. Core-shell mercury source according to claim 1 wherein said plurality of solid discrete elements are organic

11. Core-shell mercury source according to claim 1 wherein said plurality of solid discrete elements comprises organic discrete elements and inorganic discrete elements.

12. Core-shell mercury source according to claim 10 wherein the materials used for said discrete organic elements comprises one or more materials as polytetrafluoroethylene (PTFE), polyolefines such as polyethylene or polypropylene, acrylic polymers such as polymethylmethacrylate (PMMA), polyetheretherketone (PEEK), polyetherimide (PEI), polybenzimidazole (PBI), polyphenylenesulfide (PPS), montan wax, hydrogenated vegetable oils, poly(lactic-co-glycolic acid) (PLGA), polyvinyliden fluorid (PVDF).

13. Core-shell mercury source according to claim 1 wherein the organic binder is chosen from one or more hydrocolloids, or crosslinkable gelatins.

14. Core-shell mercury source according to claim 13 wherein said hydrocolloids are based on polysaccharides.

15. Core-shell mercury source according to claim 14 wherein said polysaccharides are chosen from alginates or celluloses.

16. Core-shell mercury source according to claim 1 wherein the organic binder comprises up to 10%wt of the overall shell weight of additional organic compounds.

17. Core-shell mercury source according to claim 1 wherein said shell made by a composite structure is in direct contact with said liquid mercury core.

18. Core-shell source according to claim 1 further comprising over the shell made by a composite structure enclosing the core of liquid mercury an outer shell free of filler materials.

19. Use of a core-shell mercury source according to claim 1 for lamp manufacturing.

20. Use of a core-shell mercury source according to claim 19 wherein the source remains within the lamp after its sealing.

21. Method to produce a core-shell mercury source according to claim 1 wherein the mercury core is coated with a slurry containing at least one organic compound and at least one inorganic filler.

22. Method according to claim 21 wherein said slurry is based on an aqueous suspension.

23. Method according to claim 21 wherein said coating is made by a process chosen among Rotary Die process, Drip Casting process, Spraying process.