Electric lamp and discharge devices – Cathode ray tube – Envelope
Reexamination Certificate
1999-03-15
2002-04-02
Patel, Nimeshkumar D. (Department: 2879)
Electric lamp and discharge devices
Cathode ray tube
Envelope
C313S478000, C348S832000
Reexamination Certificate
active
06366013
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a material comprising a flexible glass substrate provided with an anti-reflective coating and to methods for manufacturing said material and using said material for reducing the reflection of information display devices.
BACKGROUND OF THE INVENTION
In order to improve the perceptibility of the information displayed by devices such as television screens and computer monitors, the outer surface of the front panel of the display elements therein, such as cathode ray tubes (CRT's), liquid crystal displays (LCD's) and plasma tubes, may be provided with an anti-reflective (AR) coating. AR coatings are characterised by a very low reflectance (ratio of reflected light intensity versus incident light intensity) over the whole visible spectrum, roughly from about 400 to about 700 nm. Their performance may be quantified by the so-called bandwidth (BW) which is defined as the ratio of the longer (&lgr;
L
) and shorter (&lgr;
s
) limits of the wavelength range in which the reflectance is not higher than 1% (BW=&lgr;
L
/&lgr;
s
).
Suitable AR coatings are multi-layer stacks which may be applied onto a display surface by various coating techniques such as spin coating or vacuum deposition methods, e.g. magnetron sputtering, and said coating may be performed either directly on the display surface or indirectly. The indirect method typically involves the lamination to the display surface of an AR sheet which comprises a transparent flexible substrate provided with an AR coating.
The first alternative mentioned above (coating directly onto the display surface) is considered less favourable than the second (lamination of an AR sheet) because of the following drawbacks : (i) if the coating for some reason does not comply with the specified quality, then the AR coating as well as the display device itself is lost; and (ii) the display surface needs to be polished before coating in order to remove surface marks originating from the die casting process used for manufacturing the display device. Surface marks on CRT tubes are cavities having typical dimensions in the range from 90 to 190 &mgr;m. In the indirect, i.e. lamination method these marks are filled up with the adhesive layer used to laminate the AR sheet to the display surface and are invisible if the refractive index of the adhesive matches the refractive index of the display panel. Therefore the latter method does not require the step of display surface polishing, thereby eliminating an expensive process.
In addition to both problems mentioned above, the direct coating of display surfaces is hard to implement if a vacuum deposition technique is chosen as coating method because of the following difficulties : (i) as each display is an individual element, the coating is a batch process which is inherently characterised by a much higher complexity and cost than continuous production methods (e.g. batch vacuum systems need load locks); and (ii) non-planar display panels such as in CRT's require geometrical modifications of the vacuum deposition equipment in order to guarantee a uniform coating on the curved surface.
The above problems associated with direct AR coating of display surfaces can be solved by the alternative method of laminating to the front surface of a display panel an AR sheet comprising a flexible plastic substrate and an AR coating. However, in spite of their many advantages, AR sheets known from the prior art still need improvement especially when used for CRT's. The substrate used in an AR sheet is typically a very thin (<0.3 mm) plastic substrate consisting of poly(ethylene terephtalate) (PET), polycarbonate or cellulose-triacetate, and is therefore characterised by a low indentation hardness, expressed herein as pencil hardness. Pencil hardness may be measured by drawing one or more lines on the surface of a material with pencils of varying hardness using a predetermined force. A material having a pencil hardness equal to e.g. 3H means that a pencil having hardness 3H is not capable of scratching the surface of the material, whereas a pencil of hardness 4H does make scratches on the surface. Pencil hardness values referred to herein were measured by drawing 5 tracks of at least 1 cm using a force of 1 kg and then establishing by visual control which pencil hardness is required to scratch the surface. More details about measuring pencil hardness may be found in ASTM specification D 3363.
Due to the low pencil hardness of the plastic substrate of an AR sheet, the AR coating which typically consists of brittle inorganic materials may be damaged easily upon a sharp local pressure. To improve its mechanical strength (indentation strength), the plastic substrate may be provided with an organic hard-coat prior to vacuum deposition of the AR coating. A suitable hard-coat consists of a UV-cured acrylate and has a thickness of 3-5 &mgr;m. This treatment may increase the pencil hardness of a PET substrate to 2-3H. However, the pencil hardness of a CRT panel itself is much higher (8-9H) and therefore the application of an AR sheet consisting of a flexible plastic substrate, a hard-coat and an AR coating to a CRT panel reduces the overall hardness of the display surface.
Still other problems are associated with the use of plastic substrates and organic hard-coats. The refractive indices of a plastic substrate and a hard-coat may differ significantly from the refractive index of glass (typical value of PET is about 1.60, whereas that of glass used in display panels ranges from 1.45 to 1.54; all values at 510 nm). As a result, the reflectance of these plastic AR sheets is higher than what would be obtainable by direct AR coating. Some additional reflectance will further arise from the substrate/adhesive interface as the refractive index of the adhesive is preferably matched to the glass display panel and thus also differs significantly from the refractive index of the plastic substrate. These characteristics of plastic AR sheets limit the lowest obtainable value of the average reflectance in the visible spectrum and also limits the bandwidth and may cause clearly visible Newton rings.
Finally, the inorganic AR coating is difficult to adhere to cured acrylates and though highly crosslinked hard-coats are preferred for better hardness, such hard-coats are incompatible with high adhesion strength even if a plasma pre-treatment is applied as a chemical surface functionalisation prior to vacuum deposition of the inorganic AR coating.
Due to the above problems associated with the use of AR sheets comprising plastic substrates and hard-coats, AR-sheets having class substrates may be preferred. Since a glass substrate has the same pencil hardness as the (glass) surface of a display panel, a hard-coat is not necessary. The continuous production of sputter-coated glass sheets is disclosed in e.g. U.S. Pat. No. 3,904,506; U.S. Pat. No. 3,945,911 and U.S. Pat. No. 4,009,090. A flexible substrate is however preferred when one prefers the use of a classical web coating process for depositing an AR coating. In addition, a flexible substrate is needed for application on CRT displays in view of the curved geometry of the display surface.
EP-A 716,339 and WO 87/06626 have each disclosed that thin glass substrates having the right physical characteristics are flexible enough to be wound up on a coil and wound off from said coil and thus may be coated with various layers in a continuous web coating process. Though the solutions disclosed by both the latter patent applications indeed make it possible to obtain coatings on a flexible glass substrate, the probability of fracturing the thin glass substrate during handling and coating is still significant. In addition to productivity loss, glass fracture may cause serious damage to the vacuum pumps when using vacuum deposition techniques. The probability of the presence of glass fragments in the vacuum chamber thus should be eliminated completely in order to make the process suitable for industrial application.
SUMMARY OF THE
Leenders Luc
Lievens Hugo
Lippens Paul
Tahon Jean-Pierre
Verlinden Bartholomeus
Agfa-Gevaert
Breiner & Breiner L.L.C.
Guharay Karabi
Patel Nimeshkumar D.
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