METALLIZATION OF PHOSPHOR SCREENS

Abstract: A process for the metallization of a phosphor screen comprises the steps of applying to a phosphor screen a preformed film of a polymeric material which decomposes and/or volatilizes at a temperature of below 450 DEG C, the film either having a film of a metal deposited on at least one side thereof, or being metallized in situ, and heating the composite so formed to a volatilization temperature above the decomposition temperature of the polymeric film in order to decompose and/or volatilise the polymeric material.

Description: METΆLLIZΆTION OF PHOSPHOR SCREENS The present invention relates to a process of metallizing phosphor screens, in particular for cathode ray tubes (CRTs) . The electron permeable, light reflecting aluminium film on the target side of the phosphor screen of a CRT is formed by the evaporation of aluminium onto a smooth film of an organic material formed over the surface of a phosphor screen. This smooth film is subsequently burnt out to leave a mirror-like film of aluminium "tenting" across the top of the phosphor screen- Various processes for metallizing phosphor screens have been proposed in the prior art and these can generally be classified as solvent based systems and aqueous based systems. In the solvent based system, the phosphor layer is first wetted with an aqueous based pre-wet and a solvent based lacquer, comprising an approximately 2% solution of a polymer such as poly(iso-butylmeth- acrylate) in a solvent such as toluene, is floated on the top of the pre-wet. The water and solvent are removed by heating leaving a film of the polymeric material approximately l in thickness. The major disadvantage of this approach is that it involves the use of large quantities of volatile organic solvents, such as toluene, with all the associated environmental concerns. Aqueous based systems are described in US-A-3067055, US-A-3582389 and US-A-3582390 in which a water-based emulsion of a water insoluble, film forming resin such as an acrylate resin copolymer is coated onto the phosphor screen, the coating is dried, the coated layer is metallized, and the coating of the resin film volatilized by heating at a temperature of up to about 450°C. The emulsion contains about 5 to 20 weight percent of the resin. In the latter patent specification hydrogen peroxide is added to the emulsion, whereby it is stated that the tendency of the metal layer to blister over the phosphor screen area during the baking-out step is reduced. One disadvantage of the aqueous based system is that the dispersion fills all of the spaces between the phosphor strips or dots and is thus a thicker layer than in the solvent based system. Accordingly, the amount of polymer left on the screen is greater than utilized in solvent based processes and is therefore more difficult to remove. Consequently, increased energy requirements may result from the application of extended or even multiple burn out steps. In the solvent based system as described generally above the polymer film solution and the aluminium are applied to the phosphor screen and then the funnel of a CRT is attached to the screen with a glass frit in an organic binder. It is possible to remove both the polymer film and the organic binder in one heating cycle. In the aqueous based systems the quantities of polymer to be removed are such as to generally necessitate the bake out of the polymer film before the addition of the funnel of a CRT. Therefore two heating cycles are required with increased energy costs, and greater investment required in the number of ovens and thus also the space required on the manufacturing site. Alternatively, an oxygen rich environment may be used to bake out the polymer in one heating cycle. This approach involves increased equipment costs and the requirement of oxygen. Another approach described in the prior art for metallizing phosphor screens is described in US-A-3910806. In the method as disclosed a thin layer of a metal such as aluminium is applied to the phosphor screen of a cathode ray tube by the transfer of a metal layer formed on a substrate directly to the phosphor layer on the phosphor screen. The metal layer is adhered to the phosphor layer by means of a pressure sensitive adhesive and the substrate which is releasabiy attached to the metal layer is then remove . We have now developed a process of metallizing phosphor screens which is more energy efficient than the aqueous based systems of the prior art which does not involve the use of organic solvents, as in the solvent lacquers, and which does not involve the use of a pressure sensitive adhesive and the complicated transfer mechanism described in the prior art. Accordingly, the present invention provides a process for the metallization of a phosphor screen which process comprises the steps of: either a(i) applying to a phosphor screen a preformed film of a polymeric material which volatilizes at a temperature of below 450°C; a(ii) depositing a layer of metal upon the film coated screen to form a composite; or b) applying to a phosphor screen a preformed film of a polymeric material which volatilizes at a temperature of below 450°C, the preformed film having a layer of metal deposited on one side thereof, to form a composite; or c) applying to a phosphor screen a preformed sandwich of aluminium between two layers of a polymeric material; and d) heating the composite formed in step a(ii) , step (b) or step (c) to a temperature above the decomposition temperature of the polymeric film in order to decompose and/or volatilise the polymeric film coating. The films described in (b) above can be applied with the aluminium layer directly in contact with the phosphors on the TV screen, or, with the polymer layer directly in contact with the phosphors on the TV screen. The advantage of the first approach is that there is no polymer layer between the aluminium layer and the phosphors. As a result, the aluminium layer is directly in contact with the phosphors and there is a much reduced chance of the aluminium film blistering during the burn out step. Additionally, since the polymer burns out above the aluminium layer a thicker and hence mechanically stronger polymer backing layer can be used to transfer the delicate aluminium layer to the phosphor screen. This facilitates the ease of application of the pre-formed aluminised film to the phosphor screen. The advantage of using the sandwich approach (c) is that the important aluminium layer is fully protected. In step (c) the polymeric material used on either side of the aluminium layer may be the same or di ferent. The film of the polymeric material which is used in the process of the invention preferably has a thickness in the range of from 0.1 to lOμm, or a film area density of from 0.1 to 10 mg/cm 2 or more preferably 0.1 to 3.0 mg/cm 2 . A conventional solvent based lacquer will give a film in the order of one micrometre thickness. The polymeric film is a film which decomposes and/or volatilizes at a temperature of below 450°C. Suitable polymeric films comprise poly(acrylate) , poly(methacrylate) poly(hydroxyalkanoate) poly¬ carbonate) , poly(ethyleneoxide) -poly(propyleneoxide) block copolymer, poly(alpha-methylstyrene) , hydroxy- propyl cellulose, methylcellulose, hydroxypropyl methyl cellulose, alginic acid or an associative thickener, such as Rheox from Rohm and Haas and Rheovis from Allied Colloids. The preferred polymeric film material for use in the present invention comprises a film of a poly- (hydroxyalkanoate) , preferably poly(3-hydroxy- butyrate) , poly(3-hydroxyvalerate) , or a copolymer of 3-hydroxybutyric acid and 3-hydroxyvaleric acid. Suitable copolymers may comprise, for example 60 to 90% by weight of 3-hydroxybutyric acid and 40 to 10% by weight of 3-hydroxyvaleric acid. Suitable poly- (hydroxyalkanoates) for use in the present invention are supplied in granular form or in the form of a latex under the Trade Name Biopol (Zeneca Specialities) . Biopol films are available from Goodfellow Limited, or can be prepared from Biopol in granular form such as by the methods as disclosed in W094/00163, EP-A-0226 39, W091/13207 and DE-A-4040158. They can also be formed from a Biopol latex. Optionally the following materials can be formulated into the polymeric film; hydrogen peroxide, ammonium oxalate or the boric acid complex of poly(vinyl alcohol) . These materials are used to prevent blisters on the metal film caused by the evaporation of the polymer during the burn out cycle. Additionally, appropriate plasticisers such as phthalates and glycolates may be incorporated to reduce the brittleness of the polymer film. A metal layer may be deposited onto the film coated screen according to techniques known in the art. For the production of CRTs the metal layer is aluminium which is preferably deposited onto the phosphor screen by vacuum evaporation. The aluminium layer preferably has a thickness in the range of from 0.1 to 0.3μm. Alternatively, the film of the polymeric material may be metallized on one side thereof to form a laminate which is applied to the phosphor screen to form a composite, or the film of the polymeric material may be metallised on one side and an additional polymeric film applied to form a three layered laminate which is applied to the phosphor screen in the form of a composite. The application of the preformed film or the metallized polymeric film to the phosphor screen may be carried out by any suitable technique. It may be advantageous to pre-wet the phosphor screen, for example with a sodium silicate based aqueous solution, in order to assist in the even and uniform application of the film on the phosphor screen. The composite formed either in step (a) (ii) , step (b) or step (c) of the method of the invention is then heated to a temperature above the decomposition and/or volatilization temperature of the polymeric film to burn out the polymeric film. The preferred polymeric films for use in the present invention will decompose on heating to leave no residue, preferably at a temperature of below 350°C. In putting the process of the present invention into practice, the heating of the composite in step (d) may be combined with the step of sealing the funnel of a cathode ray tube to the phosphor screen, i.e. a separate baking step to volatilise the polymeric film coating becomes unnecessary. The sealing of a cathode ray tube funnel to a metallized phosphor screen is well known in the art, the seal generally being effected by using a frit sealing process in which a glass frit in an organic binder is used to seal the components together. The oxygen which is present in the cathode ray tube is generally sufficient to assist in the burn out of the polymeric film coating, although it will be understood that additional air or oxygen-enriched air may be introduced into the cathode ray tube, as necessary. The frit sealing of the metallized phosphor screen to the cathode ray tube will generally occur at a temperature of about 450°C. The conventional temperature profile for the sealing cycle is termed a Lehr cycle. The present invention also includes within its scope a phosphor screen which has been metallized by the process of the invention and a cathode ray tube which includes at least one phosphor screen which has been metallized by the process of the invention. The present invention will be further described with reference to the following Examples in which the following definitions are used. Lehr Cycle The Lehr cycle used in the following Examples is as follows: heat from room temperature to 450°C at 10°C/min, hold at 450°C for 45 minutes and then cool to room temperature. Aluminised Alu inised refers to the vapour deposition of a 1 inch piece of 99.99% pure aluminium wire (0.58 mm diameter) at 1.5 x 10 '5 mbar using an Edwards coating system E306A to produce an aluminium coating on the substrate. EXAMPLE 1 The preparation of pre-formed aluminised and non- aluminised films from lvm mean particle size Biopol latex A 3" x 3" square piece of glass plate was spun horizontally on its axis at a rate of 160 rpm for 20 seconds. During this time, ca 10 ml of 29% solids containing Biopol latex (supplied by Zeneca Specialities, Batch Number: BPL No. 505/1001) was slurried directly onto the spinning glass slide. The glass slide was spun again for a further 20 seconds at 160 rpm to remove any excess latex and to leave a thin even thickness wet latex coat on the glass slide. The slide was dried at room temperature to leave a translucent plastic coating on the slide (film area density = 0.34 mg/cm 2 ). The plastic film was then peeled off the glass backing slide ready for further use. Alternatively, the film was aluminised then peeled off the slide ready for further use. The alu inisation of TV screens using pre-formed aluminised and non-alu inised Biopol films made from lun mean particle size Biopol latex EXAMPLE 2 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-formed aluminised film as made in Example 1 was placed onto the phosphor screen with the aluminised side of the film facing away from the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. EXAMPLE 3 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-formed aluminised film as made in Example 1 was placed onto the phosphor screen with the aluminised side of the film facing towards the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. EXAMPLE 4 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-formed non- aluminised film as made in Example 1 was placed onto the phosphor screen. The latter was aluminised and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. EXAMPLE 5 The preparation of pre-formed aluminised films from O.Smn mean particle size Biopol latex A 3" x 3" square piece of glass plate was spun horizontally on its axis at a rate of 160 rpm for 20 seconds. During this time, ca 10 ml of 41% solids containing Biopol latex (supplied by Zeneca Specialities, Batch Number: BPL No. 510/0301) was slurried directly onto the spinning glass slide. The glass slide was spun again for a further 20 seconds at 160 rpm to remove any excess latex and to give a thin even thickness wet latex coat on the glass slide. The slide was dried at room temperature to leave a translucent polymeric coating on the slide (film area density = 2.83 mg/cm 2 ) . The polymeric film was then aluminised and peeled off the glass backing slide ready for further use. The aluminisation of TV screens using pre-formed aluminised Biopol films made from 0 . 5um mean particle size Biopol latex EXAMPLE 6 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-formed aluminised film as made in Example 5 was placed onto the phosphor screen with the aluminised side of the film facing towards the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. EXAMPLE 7 The preparation of pre-formed aluminised films from poly(propylene carbonate) A 3" x 3" square piece of glass plate was spun horizontally on its axis at a rate of 160 rpm for 20 seconds. During this time, ca 10 ml of a 5% w/w solution of poly(propylene carbonate) (supplied by PAC Polymers Inc. , grade 40 M, lot number 20507-72-21) in dichloromethane was slurried directly onto the spinning glass slide. The glass slide was spun again for a further 20 seconds at 160 rpm to remove any excess liquid and to leave a thin even thickness wet polymeric coat on the glass slide. The slide was dried in an oven at 55°C for 1 hour, aluminised and peeled off the glass backing slide ready for further use. (Film area density = 0.65mg/cm 2 ) . The aluminisation of TV screens using pre-formed aluminised poly(propylene carbonate) films EXAMPLE 8 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-aluminised poly(propylene carbonate) film as made in Example 7 was placed onto the phosphor screen with the aluminised side of the film facing towards the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. The preparation of pre-formed aluminised films made from poly(isobutyl methacrylate) (PIBMA) EXAMPLE 9 A stock solution was made by dissolving 5g of poly(isobutyl methacrylate) [PIBMA] (supplied by ICI Acrylics, under the trade name Elvacite, grade 2045) in 45g of toluene. To this stock solution was added 10 drops of di-n-butylphthalate as plasticizer and 10 drops of Disperbyk 164 as wetting agent (supplied by BYK) . 2ml of this stock solution was coated onto a Teflon tile covering ca. 40 cm 2 of the Teflon. The tile was left to dry in an oven at 45°C for 3 hours, then, ca. 30 cm 2 of the resulting plastic film was peeled off the Teflon backing tile, placed flat onto a glass slide and aluminised. (Film area density = 5.2mg/cm 2 ) . The preparation of pre-formed aluminised films made from hydroxypropylcelluose EXAMPLE 10 A 3" x 3" piece of Mylar sheet was spin coated with 10 ml of the following aqueous stock solution: 40g of 5wt% hydroxypropylcellulose (supplied by Hercules Ltd, Aqualon division under the trade name Klucel (viscosity type L) in water containing 5 drops of BYK024 wetting agent, supplied by BYK) . The sheet was dried in an oven for 2 days at 45°C, aluminised and peeled off its Mylar backing sheet ready for further use. (Film area density = 1.7mg/cm 2 ) . EXAMPLE 11 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-aluminised cellulose film as made in Example 10 was placed onto the phosphor screen with the aluminium side of the film facing away from the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave in the main part an aluminium film adhering to the phosphors. EXAMPLE 12 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of pre-aluminised cellulose film as made in Example 10 was placed onto the phosphor screen with the aluminium side of the film facing towards the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave in the main part an aluminium film adhering to the phosphors. EXAMPLE 13 The formation of pre-formed Biopol/Aluminiu /PIBMA film sandwiches A piece of pre-aluminised Biopol film as made in Example 5 whilst adhered to its glass backing slide, was spin coated (160 rpm, 20 seconds) with 10ml of a 5% w/w solution of Elvacite (PIBMA) in toluene (slurried on) . The slide was re-spun (160 rpm, 20 seconds) . The resulting film was dried in an oven at 55°C for 1 hour, then the whole film sandwich was peeled off the glass backing slide ready for further use. (Film area density = 3.1mg/cm 2 ) . The aluminisation of TV screens using pre-formed Biopol/Aluminium/PIBMA film sandwiches EXAMPLE 14 A piece of TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whilst the screen was still wet, a piece of Biopol/Aluminium/ PIBMA sandwich as made in Example 13 was placed onto the phosphors with the PIBMA side of the film facing towards the phosphors. The screen was dried at room temperature and heated under the conditions of the Lehr cycle to leave an aluminium film adhering to the phosphors. EXAMPLE 15 The preparation of pre-formed poly (propylene carbonate) aluminiu /PIBMA film sandwiches A pre-aluminised poly(propylene carbonate) film as made in Example 7 whilst still on its glass backing side was spin coated (160 rpm, 20 seconds) with 10ml of a 5% w/w solution of Elvacite (PIBMA) in toluene (slurried on). The slide was re-spun (160 rpm, 20 seconds) . The resulting film was dried in an oven at 55°C for 1 hour, then the film sandwich was peeled off the glass backing slide ready for further use. (Film area density = 1.08mg/cm 2 ) . The aluminisation of TV screens using pre-formed poly(propylene carbonate) /Aluminium/PIBMA film sandwiches EXAMPLE 16 A piece of colour TV screen was pre-wetted using an aqueous sodium silicate based pre-wet. Whist the screen was still wet, a piece of poly(propylene carbonate) /aluminium/PIBMA film as made in Example 15 was placed onto the phosphor screen with the PIBMA side of the film facing towards the phosphors. The screen was dried in air at room temperature and heated under the conditions of the Lehr cycle to leave in the main an aluminium film adhered to the phosphors.

Claims: CLAIMS : 1. A process for the metallization of a phosphor screen which process comprises the steps of: either a(i) applying to a phosphor screen a preformed film of a polymeric material which volatilizes at a temperature of below 450°C; a(ii) depositing a layer of metal upon the film coated screen to form a composite; or b) applying to a phosphor screen a preformed film of a polymeric material which volatilizes at a temperature of below 450°C, the preformed film having a layer of metal deposited on one side thereof, to form a composite; or c) applying to a phosphor screen a preformed sandwich of aluminium between two layers of a polymeric material; and d) heating the composite formed in step a(ii) , step (b) or step (c) to a temperature above the decomposition temperature of the polymeric film in order to decompose and/or volatilise the polymeric film coating. 2. A process as claimed in claim 1 wherein the film of polymeric material has a thickness in the range of from 0.1 to lOμm. 3. A process as claimed in claim 1 or claim 2 wherein the film of polymeric material has a film area density of from 0.1 to 10 mg/cm 2 . 4. A process as claimed in any one of the preceding claims wherein the polymeric film comprises a film of a poly(acrylate) , poly(methacrylate) , poly(hydroxy-alkanoate) , poly(carbonate) , poly- (ethyleneoxide) , poly(propyleneoxide) block copolymer, poly(alpha-methylstyrene)hydroxypropyl cellulose, methylcellulose, hydroxypropyl methylcellulose, alginic acid or an associative thickener. 5. A process as claimed in claim 4 wherein the poly(hydroxyalkanoate) is a poly (3-hydroxybutyrate) , a poly(3-hydroxyvalerate) or a copolymer of 3-hydroxy- butyric acid and 3-hydroxyvaleric acid. 6. A process as claimed in any one of the preceding claims wherein the phosphor screen is pre- wetted before step a(i) or (b) or (c) . 7. A process as claimed in any one of the preceding claims wherein the metal which is deposited upon the film coated screen or upon the polymeric film is aluminium. 8. A process as claimed in claim 7 wherein the layer of aluminium is deposited by vacuum evaporation. 9. A process as claimed in claim 8 wherein the layer of aluminium has a thickness in the range of from 0.1 to 0.3μm. 10. A process as claimed in any one of the preceding claims wherein the decomposition and/or volatilisation of the polymeric film is effected at a temperature of below 350°C. 11. A process claimed in any one of the preceding claims wherein the heating of the composite in step (d) is effected during the sealing of the funnel of a cathode ray tube to the phosphor screen. 12. A process as claimed in claim 11 wherein the maximum temperature reached during the sealing step is about 450°C. 13. A phosphor screen which has been metallized by a process as claimed in any one of the preceding claims. 14. A cathode ray tube which includes at least one phosphor screen which has been metallized by a process as claimed in any one of claims 1 to 12.

Inventors: ADEBAYO ADELAIDE TITILAYO OLUR, LI WARREN, WAGLAND ALISON MARY, ADEBAYO, ADELAIDE, TITILAYO, OLURONKE, MORENIKE, LI, WARREN, WAGLAND, ALISON, MARY
Applicants: ADEBAYO ADELAIDE TITILAYO OLUR, COOKSON GROUP PLC, LI WARREN, WAGLAND ALISON MARY, ADEBAYO, ADELAIDE, TITILAYO, OLURONKE, MORENIKE, LI, WARREN, WAGLAND, ALISON, MARY
Application year: 1995
Main classification: H01J29
IPC codes: H01J9, H01J29