WO2006054212A2

WO2006054212A2 - Coating for lamps and method for the preparation of such a coating

Info

Publication number: WO2006054212A2
Application number: PCT/IB2005/053707
Authority: WO
Inventors: Maarten De Leuw; Anna L. Bouwkamp-Wijnoltz
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2004-11-16
Filing date: 2005-11-10
Publication date: 2006-05-26
Also published as: WO2006054212A3

Abstract

The invention relates to coating for lamps, comprising a network obtainable by conversion of an organically modified silane by means of a sol-gel process, charactered in that silicon particles obtainable from an acid-stabilized colloidal silicon dispersion are substantially incorporated in said network. The invention also relates to a lamp comprising a vessel or a bulb, wherein said vessel or said bulb is at least partly provided with such a coating. The invention further relates to an electronic device comprising at least one such lamp. Moreover, the invention relates to a method for the preparation of a coating according to the invention.

Description

Coating for lamps and method for the preparation of such a coating

The invention relates to a coating for lamps, comprising a network obtainable by conversion of an organically modified silane by means of a sol-gel process. The invention also relates to a lamp comprising a vessel or a bulb, which vessel or bulb is at least partially provided with such a coating. The invention further relates to an electronic device comprising at least one such lamp. Moreover, the invention relates to a method for the preparation of a coating according to the invention.

It is commonly known to apply coatings onto bulbs or vessels of various kinds of lamps to achieve a desired optical effect, for example to absorb, to reflect, and/or to convert incoming light with (specific) wavelengths. The international application WO2004/0377838 discloses an improved coating for low-pressure mercury vapor discharge lamps capable of converting UV light into other wavelengths, for example into UV-B light and UV-A light for tanning purposes (sun panel lamps) or into visible radiation for general illumination purposes. Such discharge lamps are also referred to as fluorescent lamps. The improved known coating comprises a network obtainable by conversion of an organically modified silane by means of a sol-gel process, resulting in an improved adhesion to a vessel of the low-pressure mercury vapor discharge lamp at higher temperatures with respect to the earlier used and commonly known coatings based on organic lacquers. When a relatively thick coating layer (with a layer thickness of about 30 microns instead of a conventional layer thickness of between 5 and 20 microns) needs to be applied, a stabilizing agent should usually be added to the network obtainable by conversion of an organically modified silane by means of a sol-gel process in order to prevent shrinking and slumping of the coating layer. This addition serves to prevent an irregular coating thickness, which would have a negative effect on both the optical and the aesthetical properties of the coating. The stabilizing agent comprises sodium- or ammonium-stabilized silica nanoparticles in water, in particular an aqueous silica sol marketed under the trade name Ludox^® TM50 (obtainable from E.I.

Dupont de Nemours and Co., Wilmington, Del. under the trade designation Ludox^®) with a pH of between 8 and 10. A major drawback of this aqueous alkaline stabilizing agent is that the coating solution obtained becomes unstable when the stabilizing agent is added to the acid precursor solution containing alkoxysilane. This instability leads to irreversible flocculation and sedimentation that renders the coating unusable.

It is an object of the invention to provide an improved coating for lamps that can be prepared in a relatively easy and stable manner. This object can be achieved by the coating according to the preamble, characterized in that silica particles obtainable from an acid-stabilized colloidal silica dispersion are substantially incorporated in said network (OK?, cf. claim 1). Surprisingly, it has been found that instability of the coating solution occurs in particular in a critical acidity range of the coating solution with a pH of between about 6 and 8. It is therefore advantageous to apply a colloidal silica dispersion, wherein the stability of the silica sol increases continuously with decreasing pH. The use of a stabilizing silica sol based on an acidic stabilization mechanism renders it no longer necessary to pass through said critical pH range, thereby preventing flocculation and sedimentation. The addition of the acid-stabilized colloidal silica dispersion thus causes a relatively stable, solid, and reliable coating to be obtained in a relatively easy manner, thereby minimizing the risk of failure of the coating due to flocculation during preparation. With the coating according to the invention, a relatively thick coating layer (with a thickness of about 30 micron) can be applied in a relatively reliable, solid, and sustained way, in particular during exposure of the coating layer to higher temperatures. The coating according to the invention can thus be prepared relatively easily, while the risks of flocculation and sedimentation of the coating, and therefore of rendering the coating unfit for use, can be eliminated or at least reduced to a minimum. Silica sols typically comprise silica particles. As used herein, the term "sol" means a colloidal dispersion of substantially non-aggregated, inorganic oxide particles (silica particles in this case) in a liquid medium (e.g., aqueous or non-aqueous). The coating according to the invention is in particular adapted to be applied to lamps, with a wide variety as regards both the purpose of the application of the coating and of the lamp on the one hand and the kind of lamp on the other. However, the coating according to the invention may be applied to surfaces other than glass, for example metal, to provide e.g. a corrosion protective and/or electrically conductive coating. It is also conceivable to apply the coating according to the invention as a temperature- and UV-resistant coating for colored glass filters (used for luminaries, sun glasses, cameras, etc.), car windscreens, window glass, etc.

In a preferred embodiment, a mono-dispersed silica sol is applied, which means that the sol contains silica particles with a very narrow particle size distribution. Said particles are preferably formed by nanoparticles with an average diameter of between 16 and 28 nm, more preferably with an average diameter of 22 nm so as to be able to achieve a solid structure of the coating according to the invention. This solid structure makes the coating suitable for being applied in relatively thick layers on a lamp surface. For this purpose, for example, aqueous colloidal silica sol dispersions marketed under the trade names Ludox^® TMA and Ludox^® CL are commonly very sμitable for use in the coating according to the invention. Ludox^® CL comprises colloidal silica coated with a layer of aluminum, wherein the stabilizing counter-ions are formed by chloride ions.

In another preferred embodiment, said acid-stabilized colloidal silica dispersion is formed by an amorphous silica sol. Said colloidal silica dispersion preferably comprises between 30% and 38%, more preferably 34% of negatively charged silica particles, wherein the negative charge may be determined substantially by the presence of chloride ions. The stabilization of the silica particles of the silica sol may otherwise be obtained by the use of silica particles forming (?) part of a deionized sol.

It is conceivable to apply a colloidal silica dispersion, wherein the stability of the silica sol increases continuously with a decreasing pH to prevent flocculation. Preferably, the pH of said colloidal silica dispersion is substantially situated between 4 and 6, more preferably between 4 and 5. The use of an acid-based stabilizing agent prevents the coating solution from passing through a usually relatively critical acidity range with a pH of between 6 and 8. The silica particles forming part of the colloidal silica dispersion are preferably substantially formed by discrete uniform spheres of silica. In this way a coating with a relatively uniform structure and therefore with a relatively homogeneous solidity can be obtained.

The content of the silica particles in the coating may vary in dependence on the envisaged properties of the coating. However, said coating preferably has a silica particle content of between 12 and 20% by volume. The use of a silica particle content within this range renders it possible to obtain a relatively stable coating structure . It has been found that a particularly reliable coating can be obtained with a silica particle content of about 17% by volume. In a preferred embodiment, said coating comprises a pigment for absorbing and/or reflecting part of the visible or UV light. To manufacture coatings having the desired optical properties, said coatings having the desired thermal stability during the service life of the lamp, use is preferably made of inorganic pigments. In a favorable embodiment of the coating in accordance with the invention, the pigment is selected from the group formed by iron oxide, iron oxide doped with phosphorus, zinc-iron oxide, cobalt aluminate, neodymium oxide, bismuth vanadate, zirconium-praseodymium silicate, and mixtures thereof. Iron oxide (Fe₂O₃) is an orange pigment and P-doped Fe₂O₃ is an orange-red pigment. Zinc- iron oxide, for example ZnFe₂O₄ or ZnO-ZnFe₂O₄, is a yellow pigment. Mixing (P-doped) Fe₂O₃ with ZnFe₂O₄ yields a pigment of a deep orange color. Cobalt aluminate (CoAl₂O₄), neodymium oxide (Nd₂Os) and Hostaperm Blue are blue pigments. Bismuth vanadate (BiVO₄), also referred to as pucherite, is a yellow-green pigment. Zirconium-praseodymium silicate is a yellow pigment. Experiments have shown that a network including said inorganic pigments does not appreciably degrade during lamp life and at the relatively high temperature load on the coating. In an alternative preferred embodiment of the coating according to the invention, coatings are obtained wherein organic pigments are used. Particularly suitable pigments are the so-called Red 177 (anthraquinone), chromium phthalic yellow, and chromium phthalic red from "Ciba". Further suitable pigments are Red 149 (perylene), Red 122 (quinacridone), Red 257 (Ni-isoindoline), Violet 19 (quinacridone), Blue 15:1 (Cu-phthalocyanine), Green 7 (hal.Cu-phthalocyanine) and Yellow 83 (dyaryl) from "Clariant". Amber-colored chromophtal yellow, chemical formula C₂₂HeCi₈N₄O₂ and C.I. (constitution number) 56280, is an organic dye and is also referred to as "C.I.-l 10 yellow pigment", "C.I. pigment yellow 137" or Bis[4,5,6,7-tetrachloro-3-oxoisoindoline-l-ylidene)-l,4-phenylenediamine. Amber- colored anthraquinone, chemical formula C₃₇H_2INsO₄ and C.I. 60645, is an organic dye and is also referred to as "Filester yellow 2648 A" or "Filester yellow RN", chemical formula 1,1'- [(6-phenyl-l,3,5-triazine-2,4diyl)diimino]bis-. Red-colored "chromophtal red A2B" with C.I. 65300 is an organic dye and is alternatively referred to as "pigment red 177", dianthraquinonyl red or as [l,r-Bianthracene]-9,9',10,10'-tetrone, 4,4'- diamino-(TSCA, DSL). Mixtures of inorganic and organic pigments are also suitable, for example a mixture of chromium phthalic yellow and (zinc)iron oxide. An alternative embodiment of the coating in accordance with the invention is characterized in that the pigment causes a change in the color temperature of the lamp. The application of a coating of the blue pigments cobalt aluminate (CoAl₂O₄) or neodymium oxide (Nd₂Os), for example, increases the color temperature of the lamp. A preferred embodiment of the coating according to the invention is characterized in that the reflecting particles are selected from the group formed by aluminum, silver, aluminum oxide, titanium (di)oxide, calcium halophosphate, zinc oxide, barium sulphate, and calcium carbonate. Preferably, an average diameter d_p of the pigment particles complies with d_p < 100 nm. Optically transparent coatings are obtained which exhibit relatively little light scattering when pigments of such relatively small dimensions are used. Since the coating according to the invention is often applied in specially designed reflectors, wherein the light source is embodied so as to be punctiform, light scattering by the coatings is an undesirable property. The effect of light scattering is at least substantially precluded if the average diameter of the pigment particles d_p < 50 nm. Particularly suitable coatings are obtained with the use of a pigment composed of a mixture of iron oxide and bismuth vanadate or of iron oxide doped with phosphorus and bismuth vanadate in the coating.

Said organically modified silane is preferably selected from a group formed by compounds of the structural formula R¹Si(OR¹¹^, wherein R¹ comprises an organic group, preferably an alkyl group or an aryl group, and wherein R¹¹ comprises an alkyl group. Replacement of the conventional organic lacquer in the coating by a network comprising an organically modified silane as the starting material leads to an optically transparent, non- scattering coating which can resist high temperatures (up to 400 ⁰C). Part of the R¹ groups, i.e. the alkyl or aryl groups, remains present as an end group in the network when an organically modified silane is used in the manufacture of the network. As a result, the network in accordance with the invention does not have four network bonds per Si atom, but fewer than four network bonds per Si atom. Unlike the customarily used silica network, a network which is partly composed of said alkyl or aryl groups has a greater elasticity and flexibility. This enables relatively thick coatings to be manufactured.

Preferably, the R¹ group comprises CH₃ or C₆Hs. These substances have a relatively good thermal stability. A network comprising methyl or phenyl groups enables thicker coatings to be obtained. Experiments have further shown that coatings wherein methyl or phenyl groups are incorporated in a network are stable up to a temperature of at least 35O⁰C. Said groups are end groups in the network and remain part of the network at said higher temperatures. Given the prevailing relatively high temperature load on the coating, no appreciable degradation of the network occurs during the service life of the lamp on which the coating is applied. In an alternative embodiment, Ri comprises an organic group in the form of an epoxy-amino group. Since the operation temperature and the UV output of fluorescent lamps are relatively low, such coatings can be applied and are stable during the operational life of the discharge lamp. Preferably, the R¹¹ group comprises CH₃ or C₂H₅. Methyl and ethyl groups are particularly suitable because methanol and ethanol are formed in the hydrolysis process, are compatible with the pigment dispersion, and evaporate relatively easily. In general, the methoxy groups (-OCH₃) react more rapidly than the ethoxy groups (-OC₂H₅) which, in turn, react more rapidly than (iso)propoxy groups (-OC₃H₇). For a smooth hydrolysis process, use is advantageously made of R¹¹ groups which are not very long. Very suitable starting materials for the manufacture of the network in accordance with the invention are: methyltrimethoxy silane (MTMS), where R¹ = R¹¹ = CH₃, methyltriethoxy silane (MTES), where R¹ = CH₃ and R¹¹ = C₂H₅, phenyltrimethoxy silane (PTMS), where R¹ = C₆H₅ and R¹¹ = CH₃, and phenyltriethoxy silane (PTES), where R¹ = C₆H₅ and R¹¹ = C₂H₅.

Beside these components, it is also imaginable to apply more complex hybrid sol gels like glycidoxypropyltri(m)ehtoxysilane (GLYMO). Such starting materials are known per se and commercially available. To improve the adhesive capacity of the coating, small quantities of tetraethoxysilane may be used.

The invention also relates to a lamp comprising a vessel or a bulb, wherein said vessel or said bulb is at least partly provided with a coating according to the invention. The lamps provided with coatings according to the invention may be of various kinds, but the coating is preferably applied to a light-transmitting discharge vessel of a discharge lamp, the discharge lamp further comprising a discharge vessel that encloses, in a gastight manner, a discharge space provided with a filling of an ionizable substance, the discharge vessel comprising means for maintaining a discharge in the discharge space, at least a part of the discharge vessel being provided with said coating comprising a luminescent layer of a luminescent material, while at least a portion of the discharge vessel facing away from the discharge space is provided with the coating according to the invention. The coating may be applied to low-pressure discharge lamps such as, for example, a low-pressure mercury vapor discharge lamp as well as to high- intensity discharge (HID) lamps such as, for example, HID automotive headlights. It is, however, also conceivable to apply the coating according to the invention to conventional incandescent lamps. The invention further relates to an electronic device comprising at least one lamp according to the invention. Preferably, the device is selected from the group formed by the following electronic devices: a solarium or other sun panel for tanning, an LCD (in particular LCD backlighting), an automotive device or vehicle, and a signaling device. It must be clear that the fields of application of the lamps according to the invention are by no means limited to the enumeration given above. It will be obvious to those skilled in the art that a lamp provided with a coating according to the invention may be used for purposes other than the purposes explicitly mentioned in this paragraph.

Moreover, the invention relates to a method for the preparation of a coating according to the invention, comprising the steps of: a) converting an organically modified silane into a network by means of a sol-gel process, and b) mixing an acid-stabilized colloidal silica dispersion with said network or with said organically modified silane to incorporate silica particles in said network. Since the colloidal silica dispersion is acidic, or is at least stable in an acidic environment, no flocculation or sedimentation of the coating solution will occur, since the entire process for the preparation of the coating according to the invention can be carried out in a (substantially) acidic environment.

The preparation of the coating according to the invention will be elucidated in the non- limitative illustrative examples described hereinafter.

EXAMPLE 1

A reflecting coating is prepared from 40 g methyltrimethoxysilane, 1 g glycolic acid, 20 g ethanol, and 40 g Ludox^® TMA (Aldrich 34 wt.% silica in water, deionized sol). The solution is hydrolyzed for 45 minutes. Aluminum oxide particles should be in the order of 0.6 micron for optimal scattering properties. Alternatively, the aluminum oxide particles may be stabilized with Disperbyk 190 (0.05 g Disperbyk per gram CR6). The particle suspension is milled with a high-speed dissolver. The coating liquid is deposited on the outer surface of the discharge vessel by means of spraying. After deposition, the coating is dried at 90°C for a 5 minutes and subsequently the coating is cured for 30 minutes at 150⁰C.

EXAMPLE 2

A reflecting coating is prepared from 40 g methyltrimethoxysilane, 1 g glycolic acid, 20 g ethanol, and 45 g Ludox^® CL (Aldrich 30 wt.% aluminu coated silica in water stabilized by chloride ions). The solution is hydrolyzed for 45 minutes. Aluminum oxide particles should be in the order of 0.6 micron for optimal scattering properties. Alternatively, the aluminum oxide particles may be stabilised with Disperbyk (0.05 g Disperbyk per gram CR6) The particle suspension is milled with a high-speed dissolver. The coating liquid is deposited on the outer surface of the discharge vessel by means of spraying. After deposition, the coating is dried at 9O⁰C for 5 minutes and subsequently is cured for 30 minutes at 15O⁰C.

It will be clear that, within the scope of the invention, many variations are possible to those skilled in the art. In the sol-gel process, many alternative preparation methods are possible. For example, the acid used to hydrolyze may alternatively be maleic acid. Furthermore, it is also possible to use pigment combinations to cause the color point to shift towards red. Besides, the color temperature of the light to be emitted by the electric lamp can be increased while, for example, the color co-ordinates remain substantially positioned on the blackbody locus. The scope of protection of the invention is not limited to the examples given herein. The invention is embodied in each novel characteristic and each combination of characteristics. Reference numerals in the claims do not limit the scope of protection thereof. The use of the term "comprising" does not exclude the presence of elements other than those mentioned in the claims. The use of the word "a" or "an" before an element does not exclude the presence of a plurality of such elements.

Claims

CLAIMS:

1. Coating for lamps, comprising a network obtainable by conversion of an organically modified silane by means of a sol-gel process, characterized in that silica particles obtainable from an acid-stabilized colloidal silica dispersion are substantially incorporated in said network.

2. Coating according to claim 1, characterized in that said particles are formed by nanoparticles with an average diameter of between 16 and 28 nm, more preferably with an average diameter of 22 nm.

3. Coating according to one of claims 1 and 2, characterized in that said acid- stabilized colloidal silica dispersion is formed by an amorphous silica sol.

4. Coating according to one of the foregoing claims, characterized in that said colloidal silica dispersion comprises between 30% and 38%, more preferably 34% of negatively charged silica particles.

5. Coating according to one of the foregoing claims, characterized in that the pH of said colloidal silica dispersion is substantially situated between 4 and 6.

6. Coating according to one of the foregoing claims, characterized in that said colloidal silica dispersion is formed by an aqueous silica suspension.

7. Coating according to one of the foregoing claims, characterized in that said silica particles are substantially formed by discrete uniform spheres of silica.

8. Coating according to one of the foregoing claims, characterized in that said coating has a silica particle content of between 12 and 20% by volume.

9. Coating according to one of the foregoing claims, characterized in that said coating comprises a pigment for absorbing and/or reflecting part of the visible or UV light.

10. Coating according to one of the foregoing claims, characterized in that said organically modified silane is selected from a group formed by compounds of the following structural formula: R¹Si(OR.¹¹^, wherein RI comprises an organic group, preferably an alkyl group or an aryl group, and wherein R¹¹ comprises an alkyl group.

11. Coating according to claim 10, characterized in that the R¹ group comprises CH₃ or C₆H₅.

12. Coating according to claim 10 or 11, characterized in that the R¹¹ group comprises CH₃ or C2H5.

13. Lamp comprising a vessel or a bulb, wherein said vessel or said bulb is at least partly provided with a coating according to one of the claims 1 to 12.

14. Lamp according to claim 13, characterized in that said lamp is a discharge lamp.

15. Electronic device comprising at least one lamp according to one of the claims 13 and 14.

16. Device according to claim 15, characterized in that the device is selected from the group formed by the following electronic devices: a solarium, an LCD, an automotive device, and a signalling device.

17. Method for the preparation of a coating according to one of the claims 1 to 12, comprising the steps of: a) converting an organically modified silane into a network by means of a sol-gel process, and b) mixing an acid-stabilized colloidal silica dispersion with said network so as to incorporate silica particles into said network.