Coating processes – Optical element produced – Transparent base
Reexamination Certificate
1998-11-03
2001-06-19
Beck, Shrive (Department: 1762)
Coating processes
Optical element produced
Transparent base
C427S255180, C427S255270, C427S255370, C065S060500, C065S060800
Reexamination Certificate
active
06248397
ABSTRACT:
The invention relates to coating glass, and especially to coating flat glass with silicon oxide at temperatures below float bath temperatures.
It has been known to deposit silicon oxide coatings on glass at float bath temperatures (around 600° C. and above) using a gaseous mixture of silane, ethylene and usually an oxygen containing gas such as CO
2
(see, for example, GB 1 573 154, EP 0 275 662B and EP 0 348 185B), and more recently it has been proposed, in EP 0 611 733A2, to deposit mixed coating layers containing both tin oxide and silicon oxide using, inter alia, alkoxysilane compounds as the source of the silicon oxide, with an accelerant to increase the rate of growth of the coating. EP 0 611 733A2 discloses accelerants for coating systems operating above 1000° F. (typically at float bath temperatures), and suggests the use of a wide range of accelerants including Lewis acids, Lewis bases, water, certain compounds of nitrogen, phosphorus, boron and sulphur of specified structural formula, certain compounds of aluminum of specified structural formula, and ozone. The only accelerants used in the Examples, which are all carried out at glass temperature of 1200° F. (650° C.), are trialkylphosphites.
Unfortunately, the processes known for depositing silicon oxide (especially silicon oxides containing a high proportion of oxygen resulting in a refractive index of 1.5 or less) at substantially atmospheric pressure suffer from one or more of the following disadvantages when used to deposit coatings on hot glass at temperatures at or below float bath temperatures, especially temperatures below 550° C.
(a) low coating growth rate,
(b) expensive reactants,
(c) tendency for gas phase deposition of the reactants resulting in unacceptable particulate formation,
(d) explosion hazards in handling the reactant gas mixture used.
We have now found that one or more of these disadvantages may be overcome, or at least strongly alleviated, by using a mixture of a source of silicon and oxygen enriched with ozone to deposit the coatings.
According to an aspect of the present invention, there is provided a method of depositing a silicon oxide coating on hot glass at a temperature below 600° C. comprising contacting the hot glass with a gaseous mixture of a source of silicon and oxygen enriched with ozone.
The silicon oxide coating may be stoichiometric or non-stoichiometric and may include components, for example nitrogen, carbon, and organic moeities. The method will usually be performed by contacting the hot glass in the form of a hot glass ribbon with the gaseous mixture during the float glass production process downstream of the float bath in the annealing lehr or in the gap between the float bath and the annealing lehr. The hot glass will usually be at a temperature in the range 200-600° C., preferably 200° C. to 575° C. and more preferably 225° C. to 500° C.
The deposition process of the present invention is most useful at temperatures of at least 350° C., and preferably at least 375° C. in order to ensure a high deposition rate. Because it is applicable at relatively low temperatures, it is especially suitable for use when a coating of silicon oxide is required to be applied to a ribbon of float glass outside the float bath, for example, in the annealing lehr, or in the gap between the float bath and the annealing lehr.
It is also especially useful when the temperature of the substrate is below about 525° C., when the rate of deposition achieved with alternative silane/oxygen systems begins to fall off significantly. Thus according to an especially preferred aspect of the invention, the glass is contacted with the silicon source/oxygen/ozone mixture with the glass at a temperature in the range 375° C. to 525° C.
The hot glass will normally be contacted with the gaseous mixture at substantially atmospheric pressure.
A wide variety of silicon compounds have been used or been proposed for use as a source of silicon in vapor deposition processes, including in particular silanes and siloxanes. The suitability of any particular silicon compound for use in the processes of the present invention may be determined by routine experiment. Those compounds which do not form coatings rapidly at the relatively low temperatures used in the processes of this invention are less preferred for use in float glass production processes although they may be useful in “off-line” coating processes where the coating time can be extended. Examples of suitable silanes include silane (SiH
4
), disilane, alkyl silanes (for example, tri or tetramethylsilane, hexamethyldisilane, and other alkylsilanes having straight or branched chain substituted or unsubstituted alkyl groups with between 1 and 12 carbon atoms), especially dialkylsilanes preferably dimethylsilane; alkoxysilanes (for example methyltrimethoxysilane, dimethyldimethoxysilane and other alkyl alkoxysilanes having substituted or unsubstituted straight or branched chain alkyl groups with between 1 and 12 carbon atoms) especially tetra(alkoxy)silanes such as tetraethoxy silane (TEOS); di(alkoxy)silanes such as diacetoxyditertiary butoxy silane and oligomeric silanes especially oligomeric alkoxysilanes such as ethylsilicate 40. Examples of suitable siloxanes include hexa (alkyl) disiloxanes such as hexamethyldisiloxane and cyclic siloxanes especially the tetra (alkyl) cyclotetrasiloxanes such as tetramethylcyclotetrasiloxane and the octa (alkyl) cyclotetrasiloxanes such as octamethylcyclotetrasiloxane (OMCTS). A silicon halide, for example silicon tetrachloride, may be used as the source of silicon. The source of silicon may comprise a mixture of two or more silicon compounds.
The preferred alkoxysilane, tetraethoxysilane, undergoes pyrolysis with oxygen to produce silane at decomposition rates practically useful for on-line coating of glass only at temperatures of above 650° C. The deposition rate can be increased by using a plasma enhanced or low pressure CVD technique, but neither is suitable for commercial use on a continuous glass ribbon. Surprisingly, the enrichment of the oxygen with only a small proportion of ozone enables silicon oxide coatings with a high ratio of oxygen to silicon (about 2, providing a coating with a refractive index of 1.5 or less) to be deposited on hot glass at temperatures at least as low as 375° C. at a rate sufficient for practical use in the on-line coating of glass at substantially atmospheric pressure i.e. without the need to use a vacuum or low pressure method such as sputtering which would be impractical for commercial on-line application.
A gas stream comprising oxygen enriched with ozone suitable for use in the processes of this invention may conveniently be prepared by passing an oxygen stream through an ozone generator. Ozone generators of this type are available as articles of commerce and are able to produce a range of concentrations of ozone in the oxygen stream. In a preferred embodiment of the present invention, an oxygen stream enriched with ozone produced from a conventional ozone generator is mixed with a second gas stream containing at least one source of silicon in a carrier gas to form a gaseous mixture which is contacted with the hot glass. The precise concentration of ozone in the gaseous mixture is not normally critical, and depending on the concentration of silicon compound used, increasing the ozone concentration in the oxygen stream beyond 1% by weight, based on the total weight of oxygen and ozone in the oxygen stream, may give little or no increase in the rate of deposition. Indeed, we have found that, beyond a certain level, increasing the ozone concentration in the gaseous mixture may lead to a reduced deposition rate, presumably because competing reactions and especially competing reactions in the gas phase resulting in powder formation, become more significant at high ozone concentrations. Thus, for any particular conditions used, the ozone concentration in the gaseous mixture will be adjusted to optimise the deposition rate. Typically this will involve using an ozone concentration (in the oxygen st
Beck Shrive
Chen Bret
Marshall & Melhorn LLC
Pilkington plc
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