WO1996021630A1

WO1996021630A1 - Coatings on glass

Info

Publication number: WO1996021630A1
Application number: PCT/GB1996/000015
Authority: WO
Inventors: David William Shell; Joseph Earle Lewis
Original assignee: Pilkington Plc
Priority date: 1995-01-09
Filing date: 1996-01-05
Publication date: 1996-07-18
Also published as: GB9500333D0; AU4313096A

Abstract

A method of producing mirrors comprising depositing a reflecting metal layer by pyrolysis on a ribbon of hot glass during the glass production process characterised by applying a metal precursor to the glass ribbon at a location where the glass temperature is above 200 °C. The invention also provides a method of producing mirrors during the glass production process, the method comprising pyrolytically depositing a durable reflecting metal layer over the glass surface at temperature of from 200 to 550 °C.

Description

COATINGS ON GLASS

The invention relates to a method of producing mirrors, and to coated glass substrates incorporating highly reflecting "mirror" coatings.

The light reflecting properties of mirrors are generally provided by a layer of highly reflecting metal, especially silver, aluminium or chromium, applied to a glass or plastics substrate; copper layers are sometimes used as an alternative, but are generally less acceptable because of the strong red tint of the reflected light.

Silver coatings are generally applied to preformed glass plates, in the cold, by wet chemical methods in which a solution of silver salt is applied to the glass surface and reacted with a reducing agent which reduces silver ions present to silver metal which deposits on the glass surface. The silver used is not very durable in use and in practice requires protection by other layers, and these methods are generally unsuitable for application to glass on the production line on which it is formed so that a separate "silvering" line is required to produce the silvered glass.

Aluminium coatings are difficult to apply by chemical methods because of the strongly reducing nature of aluminium metal, and aluminium mirrors are generally produced by deposition methods carried out at low pressure e.g. by sputtering. Such low pressure methods are essentially batch processes and, like the wet chemical methods used for deposition of silver mirrors, are generally unsuitable for on¬ line application on the production line on which the glass is made.

US-A-3656926 and US-A-3681042 each propose a process for the production of mirror coatings by condensing a metal vapour on the hot glass surface on the production line on which the glass is made. According to US-A-3656926, a body of molten metal, for example silver, aluminium copper or gold, is located adjacent the upper surface of the glass by a beam of refractory material extending transversely across the glass width, and an electrical current is passed through the beam of refractory material extending transversely across the glass width, and an electrical current is passed through the beam to heat the beam, for example to a temperature of 2000°C, and evaporate the metal for condensation on the adjacent glass. According to US-A- 3681042 a body of molten metal, for example silver, aluminium, gold, copper or tin, is contained in a trough in a duct supported over the glass ribbon and heated to a high temperature, for example 2000°C, and a carrier gas passed over the molten metal body in the trough to entrain metal vapour evaporating therefrom. The carrier gas containing the metal vapour is directed towards the glass surface where the metal vapour condenses on the glass. To improve the adhesion of the reflecting metal to the glass, it is further proposed to first deposit, in a similar manner, a keying layer of tungsten palladium, nickel or a palladium nickel alloy on the glass before deposition of the reflecting layer. Both these techniques require use of extremely high temperatures and neither has found commercial application.

Silicon has been deposited on hot glass during the glass production process to produce reflecting layers (which, like silver and aluminium layers, are substantially neutral in reflection colour) for use on architectural glazing for aesthetic and solar control purposes. GB-A-1507465, GB-A- 1507996 and GB-A-1573154 relate to a continuous chemical vapour deposition method for producing float glass having such a silicon layer, and US-A-4661381 describes a development of that method. However, such silicon layers do not provide the high reflections commonly required in mirrors. Thus REFLECTAFLOAT (trade mark) glass, commercially available from Pilkington Glass Limited of St Helens, England, has a reflection of about 50%, and MIRROPANE EP (trade mark) commercially available from Libbey-Owens-Ford Co, has a reflection of about 60%. EP-A-0583871 discloses mirrors and their production in which a mirror coating comprises a stack of layers. The mirrors may be produced on-line during manufacture of the glass, for example during the float glass production process. The layers comprise materials which can be deposited on-line, for example non-metallic materials such as silicon, silicon dioxide, titanium dioxide, etc.. It is disclosed that reflective metals, for example, aluminium, chromium, cobalt or titanium may be used as an alternative to silicon and that a metal may be deposited by condensation of a metal vapour or by chemical vapour deposition using a suitable organometallic vapour. However, no specific conditions for metal deposition onto the glass substrate are disclosed.

GB-A-2248853 discloses a pyrolytic metal of coating glass with aluminium to form a mirror. A solution of an alane amine adduct is formed and the liquid is deposited onto heated glass. The adduct decomposes to form an aluminium coating. It is envisaged that the invention may be used in conjunction with float glass production, and suggested that the aluminium deposition may be carried out on hot glass, typically at 180°C, emerging from the float glass process. Unfortunately, it has been found that coatings produced in the manner described are insufficiently durable for commercial application as mirrors.

There is a need in the art for a method for reliably depositing a reflective metal onto a glass substrate during the glass production process to yield a reflecting coating having good optical and mechanical properties to enable the coated glass substrate to be used as a mirror.

According to the present invention there is provided a method of producing mirrors comprising depositing a reflecting metal layer by pyrolysis on a ribbon of hot glass during the glass production process characterised by applying a metal precursor to the glass ribbon at a location where the glass temperature is above 200°C. The expression "mirror" is used in the present specification and claims to refer to a coated glass substrate having a visible light reflection (when viewed from the coated side or the glass side, whichever gives the higher reflection) of at least 70%. The expression "visible light reflection" refers to the percentage of light reflected under Illuminant D65 source, 1931 Observer Conditions. The term "pyrolysis" is used to refer to a process of decomposition of precursor material with or without involvement of an additional reactant, for example oxygen or water, under the effect of heat.

It has been found that, in practice, the durability of the product increases with increasing deposition temperature, while a tendency has been noted for the reflectivity to decrease with increasing deposition temperature. It is preferred to operate at a glass temperature in the range 300°C to 700°C, especially a temperature in the range 350°C to 600°C, more preferably from 400 to 500°C.

The present invention also provides a method of producing mirrors during the glass production process, the method comprising pyrolytically depositing a durable reflecting metal layer over the glass surface at a temperature of from 200 to 550°C.

The metal deposition may be carried out on the glass surface directly or on a barrier layer which has previously been applied to the glass surface, for example by a pyrolytic deposition process. The barrier layer acts to reduce or prevent ions in the glass most particularly alkali metal ions, e.g. sodium, from interfering with the nucleation and growth of the reflecting metal layer. This has been found by the inventors to reduce the haze of subsequently deposited aluminium coatings. The barrier may comprise SiC_xO_y, i.e. a silicon oxide with a significant proportion, typically around 25 to 30 at%, of carbon therein; silicon oxide or aluminium oxide. In accordance with a particularly preferred aspect of the present invention, a surface pre-treatment of the glass or a barrier layer overlying the glass may be carried out prior to metal deposition. The pre-treatment comprises surface activation of a surface to be coated by the reflecting metal layer by the use of a metal halide, most preferably titanium tetrachloride, which is introduced into the atmosphere above the surface to be coated. The inventors have found that the use of such a surface activation with titanium tetrachloride greatly increases the degree of reflectivity of the metal coating and also the uniformity of the metal coating obtained. The adhesion, density and durability of the metal layer can be correspondingly increased. The precise mechanism by which the surface is activated by the titanium tetrachloride prior to deposition of the metal is not fully comprehended. Without being bound by theory, the titanium tetrachloride is believed to react with the underlying surface to form a titanium compound, which may include a Ti-0 bond and which acts as a low energy site for nucleation of the metal layer. The present inventors have carried out Auger profiling of reflecting coatings of aluminium deposited using a titanium tetrachloride pre-treatment during on-line manufacture of the glass. The inventors found titanium present in only a very small amount of from 0.5 to 1 at% underneath the reflecting aluminium layer. This suggests that the titanium tetrachloride may be acting primarily as a catalyst activating the nucleation of the aluminium metal on the glass or barrier layer surface.

The use of a pre-treatment in accordance with this preferred aspect of the present invention is most beneficial when the reflecting metal layer is applied to the glass ribbon at a location where the glass temperature is below 400°C, and becomes more beneficial as that application temperature is reduced.

It is preferred, for reasons of operational convenience and control, to deposit the reflecting metal layer by pyrolysis of a metal precursor supplied in the vapour phase, and the ability to achieve a durable product at a low deposition temperature facilitates the use of metal precursors which are volatile at low temperature and/or tend to decompose in the gas phase at higher temperatures .

The reflecting metal is conveniently aluminium, which has both a high light reflection and a neutral colour in reflection, although other metals with sufficiently high light reflection, for example copper, silver, gold, palladium, rhodium, or platinum may be used provided suitable precursor materials are available. A combination of metals, for example, a metal alloy may be used if desired. It is not essential for the metal layer to be of pure metal, and small amounts of other elements, for example up to about 20at%, preferably up to about 10at%, e.g. one or two atomic percent of oxygen or carbon, may be present provided the required high reflection is achieved.

Suitable precursor materials include metal hydrides and organometallic compounds, for example metal alkyls, alkyl metal hydrides and metal acetylacetonates, which may be used in solution or in the vapour phase. The vapour may be produced by vaporisation in conventional manner, or by "nebulising", that is, forming a solution of the metal precursor (or liquid metal precursor) into very fine droplets (commonly described as an aerosol) in a heated carrier gas so that the metal precursor vaporises in the carrier gas. Typically, the vapour is formed by bubbling nitrogen gas through the precursor in liquid form. When depositing aluminium as the reflecting metal, we prefer to use aluminium alkyls such as triisobutyl aluminium and tritertiary butyl aluminium and alkyl metal hydrides such as dimethyl aluminium hydride.

Whatever precursor material is employed, it is important to ensure that the precursor material is delivered to the glass at the correct temperature so as to ensure that deposition of aluminium is achieved by decomposition or reaction of the precursor and it is also important that the glass substrate is at the appropriate temperature for successful deposition. For example, for successful deposition of aluminium from triisobutyl aluminium or tritertiary butyl aluminium the glass substrate is typically at least around 400°C. In addition, the precursor should preferably be delivered from the bubbler and delivery lines held at at least approximately 50°C and once at this temperature preferably the precursor should be utilised for coating as soon as possible in order to avoid degradation of the coating. If the precursor is delivered at lower temperatures, then decomposition of the precursor may not be reliably achieved. If the precursor is delivered at higher temperatures, then it is possible that the precursor decomposes on surfaces other than the glass substrate, for example parts of the coating apparatus which can reduce or even prevent successful deposition of the aluminium on the glass substrate. The temperature of the coating apparatus may be increased as compared to the temperature of the bubbler in order to obtain reliable deposition of the aluminium from the precursor.

The durability of the reflecting metal layer may be further enhanced by depositing a protective overlayer over the reflecting metal layer. The overlayer may be of silicon oxide or a metal oxide, preferably aluminium oxide, tin oxide or titanium oxide. The protective overlayer may be deposited, like the reflecting metal layer, by pyrolysis on the hot glass ribbon during the glass production process, and is preferably deposited by pyrolysis of a precursor compound, for example a silane or organometallic compound, applied to the hot glass ribbon in the vapour phase.

Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which: -

Figure 1 is a section (not to scale) through a mirror produced in accordance with one embodiment of the present invention;

Figure 2 is a scanning election micrograph of an edge of a mirror having the structure shown in Figure 2; and

Figure 3 is a diagrammatic representation of an arrangement of coating stations on a float glass production line for production of mirrors in accordance with the method of the present invention.

Referring to Figure 1, a mirror, designated generally as 10 and acting a rear surface mirror, comprises a glass substrate 11 carrying a barrier layer 12 and a reflective metal layer 13. The barrier layer comprises a silicon oxycarbide, i.e. SiC_xO_y, containing around 25-30 at% carbon. The barrier layer has a thickness of typically from 300 to 700 Angstroms. The reflective metal layer 13 comprises a layer of aluminium at least around 200 Angstroms thick, typically around 500 to 700 Angstroms thick. The barrier layer may alternatively comprise silica or alumina. In this embodiment, the surface of the barrier layer 12 remote from the glass substrate 11 has been subjected, prior to the deposition of the reflective aluminium layer 13, to a surface pre-treatment comprising activation in an atmosphere of titanium tetrachloride. Figure 2 is a scanning electron micrograph of a mechanically broken edge of the mirror illustrated in Figure 1 which shows the glass substrate 11, the SiC_xO_y barrier layer 12, (whcih is 400A thick, and the overlying aluminium layer 13, which is 600 A thick. The micrograph shows that no separately formed primer layer of any significant thickness can be seen. In the scale at the bottom of the micrograph, each graduation represents 600 Angstroms .

In the practical application of the invention, the pre- treatment and the reflecting metal layer will be applied to the hot ribbon of glass, generally but not necessarily a ribbon of float glass, from coating stations located at appropriate positions (to provide the required glass temperatures) on the glass production line.

In applying the invention to a ribbon of float glass, the metal precursor will be applied to the glass ribbon either in the float bath (generally at a position where the glass ribbon has reached substantially its final thickness - usually a glass temperature of around 750°C - so it is not subject to further stretching which might crack any coating applied) or preferably (in view of the increased reflectivity achieved at lower application temperatures) between the float bath and the lehr or in the lehr.

Figure 3 illustrates, diagrammatically, a float glass production line comprising a glass melting section 21, a float batch section 22 for forming the molten glass into a continuous ribbon, a lehr section 23 for annealing the said glass ribbon and a warehouse section 24 for cutting glass from the ribbon for storage and/or distribution and use. A first coating station for performing the surface pre-treatment will normally be located in or between the float batch section 22 and lehr section 23, at a position where the glass ribbon has substantially reached its final thickness (usually at a glass temperature of around 750°C) so that it is not subject to further stretching which might crack any coating applied, but where its temperature remains sufficiently high for the pre- treatment. That temperature is dependent upon the nature of the pre-treatment. The first coating station may additionally or alternatively be employed to deposit a barrier layer on the glass surface. In the embodiment illustrated this first coating station 25 is shown located toward the downstream (cooler) end of the float bath section 22.

The coating station for applying the reflecting metal layer is located downstream of the first coating station and will usually, but not necessarily, be in the lehr section 23 where the glass temperature is above 200°C but has fallen below 700°C and preferably below 500°C or even below 400°C; in the drawing, this coating station 26 is shown located toward the downstream (cooler) end of the lehr section 23.

When a pre-treatment is employed, titanium tetrachloride may be delivered in vapour form in oxygen-free nitrogen gas which is passed over the heated glass surface. The hot ribbon of glass, bearing the reflecting metal layer, is cut into sections in generally known manner to provide large mirror sheets for cutting to the required size. The reflecting metal layers will generally have a sufficiently low light transmission for use as either front or back surface mirrors without the need for an opacifying layer. However, it may be desirable to apply a protective layer over the reflecting metal layer to enhance the durability of the mirror still further although, if the mirror is to be used as front surface mirror, such protective layer will obviously be chosen to have a high light transmission.

Reflective metal films formed in accordance with the invention were found by the inventors to have improved durability as compared to conventional silver metal mirrors and mirrors having reflecting layers of evaporated aluminium. The tests performed by the inventors yielded qualitative results.

Reflective aluminium films formed in accordance with the invention were tested for adhesion by wiping and by adhering and removing pressure-sensitive adhesive tape. It was found that the reflective metal films formed in accordance with the invention exhibited enhanced durability as compared to conventional silver metal layers and evaporated aluminium layers, with the reflective aluminium layer remaining firmly adhered to the underlying glass substrate.

The "handleability" of the reflective metal layers formed in accordance with the invention was acceptable because the films survived general physical and mechanical handling during the manufacturing and testing procedures.

The chemical durability of the reflective metal films was tested by application of solvents and weak alkali solutions. The chemical durability was found to be improved as compared to the known silver metal and evaporated aluminium mirrors. It is believed by the inventors that this enhanced chemical durability may possibly result from the metal films containing carbon and oxygen contaminations therein.

The thermal durability of the reflective metal films was also tested by heat soaking the coated substrates at elevated temperatures. The reflective metal films were seen to be more stable to a heat soak test than evaporated aluminium.

The invention is still further illustrated by the following non-limiting Examples.

Example 1

In an experiment designed to simulate production of a mirror by on-line application of a reflecting metal layer to a hot glass surface, a substrate of 4mm clear float glass was placed on an electrically heated support in a tubular reactor. The glass was heated to 375°C and aluminium triisobutyl vapour in a nitrogen carrier gas was passed over the hot glass in the reactor to deposit a reflecting aluminium layer on the hot glass surface.

The resultant reflecting aluminium layer, which was about 2500 A thick, was durable.

Comparative Example 1

An aluminium layer applied to a second substrate in a similar manner to that of Example 1, but using the alane adducts of GB-A-2248853 applied to the glass at a glass temperature of 180°C, was found to have poor adhesion to the glass surface and consequent poor durability.

Example 2

In this Example, a reflecting metal layer was deposited onto a glass substrate in a dynamic laminar coater which is capable of depositing multilayer coatings onto moving glass substrates in a controllable atmosphere. The dynamic laminar coater simulates the deposition of coatings onto glass during on-line production of the glass, for example in the float glass process. For example, an aluminium coating was deposited using a precursor of tritertiary butyl aluminium. A pre-treatment of the substrate employing titanium tetrachloride was also employed.

A substrate of 4mm thick SiCO coated float glass was placed in a substrate holder which in turn was positioned inside a preheated controlled atmosphere furnace. The glass was heated up to a temperature of around 400°C within a nitrogen atmosphere. The glass was transported below a coating head, held at 57°C, at a speed of 240mm/min, during which titanium tetrachloride vapour in oxygen-free nitrogen gas was passed over the heated glass surface. In this way, the barrier layer of SiCO was "primed" or activated by the titanium tetrachloride. The glass, still at a temperature of around 400°C, was then transported below a second coating head, held at 71°C, from which a vapour of tritertiary butyl aluminium in an oxygen-free nitrogen carrier gas was passed over the substrate surface for 5 minutes. Such a vapour was achieved by bubbling nitrogen through the liquid solution of the precursor. A reflecting aluminium layer was deposited over the glass surface. This layer was durable.

Example 3

Example 3 essentially repeated Example 2 in passing a 4mm SiCO coated float glass substrate through the dynamic laminar coater. The glass temperature was however approximately 440°C. The glass was subjected to a pre-treatment with titanium tetrachloride from a coating head at 61°C at a carrier speed of 240mm/min. A different precursor of triisobutyl aluminium was held in a pressure pot at 48°C, the pot was pressured with oxygen free nitrogen gas and the liquid was injected to an evaporator held at 77°C. The vapour was delivered through a second coating head onto the stationary glass surface.

A durable reflecting aluminium layer was deposited over the glass surface. Example 4

Example 4 was similar to Example 2 except that the glass temperature was 405°C. The coating conditions were the same as in Example 2. A reflective durable aluminium coating was deposited over the substrate.

Claims

CLAIMS :

1. A method of producing mirrors comprising depositing a reflecting metal layer by pyrolysis on a ribbon of hot glass during the glass production process characterised by applying a metal precursor to the glass ribbon at a location where the glass temperature is above 200°C.

2. A method according to claim 1 comprising applying the metal precursor to the glass ribbon at a location where the glass temperature is above 300°C.

3. A method according to claim 1 or claim 2 which comprises depositing the reflecting layer from a metal precursor in the vapour phase .

4. A method according to any one of the preceding claims wherein the metal precursor is a compound of copper, silver, gold, palladium, rhodium, platinum or aluminium which pyrolyses to deposit a reflecting layer of said metal on the glass ribbon.

5. A method according to any of the preceding claims wherein the metal precursor comprises a metal alkyl compound.

6. A method according to any of the preceding claims wherein the reflecting metal layer comprises two or more metals.

7. A method according to any of the preceding claims which additionally comprises depositing a protective overlayer over the reflecting metal layer.

8. A method according to claim 7 wherein the protective overlayer is of silicon oxide or a metal oxide, preferably aluminium oxide, tin oxide or titanium oxide.

9. A method according to claim 7 or claim 8 wherein the protective overlayer is deposited by pyrolysis on the hot glass ribbon during the glass production process.

10. A method according to claim 9 wherein the protective overlayer is deposited by pyrolysis of a precursor applied to the glass ribbon in the vapour phase.

11. A method according to claim 10 wherein a protective overlayer of silicon oxide is deposited by pyrolysis of a silane applied to the glass ribbon in the vapour phase.

12. A method according to any one of the preceding claims further comprising, prior the deposition of the reflecting metal layer, the step of forming a barrier layer over the glass substrate.

13. A method according to claim 12 wherein the barrier layer comprises SiC_xO_y, Si0₂ or Al₂0₃.

14. A method according to claim 12 or 13 wherein the barrier layer is from 300 to 700 Angstroms thick.

15. A method of producing mirrors during the glass production process, the method comprising pyrolytically depositing a durable reflecting metal layer over the glass surface at temperature of from 200 to 550°C.

16. A method according to claim 15 wherein the reflecting metal layer is deposited from a precursor comprising a metal alkyl or an alkyl metal hydride.

17. A method according to claim 16 wherein the metal is aluminium.

18. A method according to claim 17 wherein the precursor is triisobutyl aluminium, tritertiary butyl aluminium or dimethyl aluminium hydride.

19. A method according to any one of claims 15 to 18 wherein the temperature is around 400°C.

20. A method according to any one of claims 15 to 19 wherein the pyrolytic deposition employs a metal precursor in a non- oxidising carrier gas..

21. A method according to any one of claims 15 to 20 further comprising pre-treating the glass surface with an activating agent prior to deposition of the reflecting metal layer.

22. A method according to claim 21 wherein in the pre- treatment step the glass surface is activated by an activating agent which comprises a metal halide.

23. A method according to claim 22 wherein the metal halide comprises titanium tetrachloride in a non-oxidising carrier gas.

24. A method according to any one of claims 15 to 23 further comprising, prior to deposition of the reflecting metal layer and any optional pre-treatment step, the step of forming a barrier layer over the glass substrate .

25. A method according to claim 24 wherein the barrier layer comprises SiC_xO_y, Si0₂ or Al₂0₃.

26. A method according to claim 24 or 25 wherein the barrier layer is from 300 to 700 Angstroms thick.

27. A method according to any one of claims 15 to 26 wherein the reflecting metal layer is from 500 to 700 Angstroms thick.

28. A mirror produced by a method according to any of the preceding claims.