Method of producing a wavelength-converting casting composition

Details Method of producing a wavelength-converting casting composition Method of producing a wavelength-converting casting composition

2000-03-28

2001-08-21

Koslow, C. Melissa (Department: 1755)

Compositions

Inorganic luminescent compositions with organic...

C252S30140R

Reexamination Certificate

active

06277301

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a wavelength-converting casting composition based on a transparent epoxy casting resin which is mixed with a luminous substance, for an electroluminescent component having a body that emits ultraviolet, blue or green light.
2. Description of the Related Art
A component of that type has become known, for instance, from German published, non-prosecuted patent application DE 38 04 293. The reference describes an arrangement with an electroluminescent diode or laser diode, in which the emissions spectrum emitted by the diode is shifted toward longer wavelengths, by means of a plastic element mixed with a fluorescing, light-converting, organic colorant. The light emitted by the arrangement as a result has a different color from what the light emitting diode emitted. Depending on the type of colorant added to the plastic, it is possible to produce LED arrays that light up in different colors with one and the same type of light-emitting diode (LED).
In many potential applications for LEDs, such as in display elements in motor vehicle dashboards, illumination in aircraft and automobiles, and in LED displays capable of showing full color, there is an increasing demand for LED arrays with which mixed color light and in particular white light can be generated.
However, the prior art casting compositions of the type referred to at the outset with organic luminous substances exhibit a shift in the color location, that is, the color of the light emitted by the electroluminescent component, under temperature and temperature/humidity stresses.
Japanese patent disclosure JP-07 176 794-A describes a white-light-emitting planar light source, in which two diodes that emit blue light are disposed on one face end of a transparent plate and emit light into the transparent plate. The transparent plate is coated on one of the two opposed main sides with a fluorescing substance that emits light when it is excited with the blue light of the diodes. The light emitted by the fluorescing substance has a different wavelength from the blue light emitted by the diodes. In this known component, it is especially difficult to apply the fluorescing substance in such a way that the light source emits homogeneous white light. Moreover, replicability and mass production presents major problems, because even slight fluctuations in the layer thickness of the fluorescing layer, for instance from irregularities of the surface of the transparent plate, cause a change in the white of the light emitted.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a wavelength-converting casting mass, which overcomes the above-mentioned disadvantages of the prior art devices and methods of this general type and with which electroluminescent components can be produced that emit homogeneous mixed-colored light, and which enables mass production at reasonable engineering effort and expense and with maximally replicable component characteristics. The emitted light should be color-stable even under temperature and temperature/humidity stresses. It is a further object to specify a use for the casting mass and a method for producing the composition.
With the foregoing and other objects in view there is provided, in accordance with the invention, a wavelength-converting casting composition, for converting a wavelength of ultraviolet, blue or green light emitted by an electro-luminescent component, comprising:
a transparent epoxy casting resin;
an inorganic luminous substance pigment powder dispersed in the transparent epoxy resin, the pigment powder comprising luminous substance pigments from a phosphorous group having the general formula A
3
B
5
X
12
:M;
the luminous substance pigments having grain sizes ≦20 &mgr;m and a mean grain diameter d
50
≦5 &mgr;m.
In accordance with an added feature of the invention, the mean grain diameter d
50
of the luminous substance pigments is between one and two micrometers.
Inorganic/mineral luminous substances are extremely stable with regard to temperature and temperature/humidity stresses.
In accordance with an additional feature of the invention, the composition includes the following parts:
a) epoxy casting resin ≧60% by weight;
b) luminous substance pigments >0 and ≦25% by weight;
c) thixotropic agent >0 and ≦10% by weight;
d) mineral diffusor >0 and ≦10% by weight;
e) processing adjuvant >0 and ≦3% by weight;
f) hydrophobic agent >0 and ≦3% by weight; and
g) adhesion promoters >0 and <2% by weight.
Suitable epoxy casting resins are described for instance in German published, non-prosecuted patent application 26 42 465 (pp. 4-9, in particular examples 1-4), and in European patent disclosure EP 0 039 017 and U.S. Pat. No. 4,365,052 (pp. 2-5, in particular examples 1-8 appearing in both the EP and U.S. patents). The disclosures of those documents are hereby expressly incorporated by reference.
Pyrogenic silicic acid is for instance used as the thixotropic agent. The thixotropic agent is used to thicken the epoxy casting resin, so as to reduce the sedimentation of the luminous substance pigment powder. The flow and wetting properties are also adjusted for processing the casting resin
CaF
2
is preferably used as a mineral diffusor for optimizing the luminous pattern of the component.
Glycol ether is for instance suitable as a processing adjuvant. It improves the compatibility between the epoxy casting resin and the luminous substance pigment powder and is thus used to stabilize the dispersion of luminous substance pigment powder and epoxy casting resin. To that end, surface modifiers based on silicone can also be employed.
The hydrophobic agent, such as liquid silicone wax, is also used to modify the pigment surface; in particular, the compatibility and wettability of the inorganic pigment surface is improved with the organic resin.
The adhesion promoter, such as functional alkoxysiloxane, improves the adhesion between the pigments and the epoxy resin in the cured state of the casting composition. As a result it is attained that the boundary face between the epoxy resin and the pigments will not rupture, for instance in response to temperature fluctuations. Gaps between the epoxy resin and the pigments would cause light losses in the component.
The epoxy casting resin, preferably with a reactive triple oxiran ring, preferably includes a monofunctional and/or multifunctional epoxy casting resin system (≧80% by weight, such as bisphenol-A-diglycidyl ether), a reactive diluent (≦10% by weight, such as aromatic monoglycidyl ether), a multifunctional alcohol (≦5% by weight), a degassing agent based on silicone (≦1% by weight), and a decolorizing component to adjust the color number (≦1% by weight).
In accordance with another feature of the invention, the luminous substance pigments are substantially spherical particles or flakelike particles. The tendency to clumping of such pigments is advantageously very slight. The H
2
O content is below 2%.
In the production and processing of epoxy casting resin components with inorganic luminous substance pigment powders, in general not only wetting but also sedimentation problems occur. Especially luminous substance pigment powders with d
50
≦5 &mgr;m have a strong tendency to clumping. In the last-named composition of the casting composition, the luminous substance pigments, with the above-indicated particle size, can advantageously be substantially free of clumps and can be dispersed homogeneously in the epoxy casting resin. This dispersion is stable even under long-term storage of the casting composition. Essentially no problems of wetting and/or sedimentation occur.
In accordance with a further feature of the invention, the luminous substance pigments are particles of Ce-doped garnets, such as, particularly, YAG:Ce particles. An advantageous dopant concentration is 1%, for example, and an advantageous luminous substance concentration is 12%, for example. The preferred high-purity luminous substa

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