Liquid preparation for the production of electrically conductive and infrared-reflecting fluorine-doped tin oxide layers on glass or glass-ceramic surfaces, as well as a method for the production of such layer

A liquid preparation for the production of electrically conductive and infrared-reflecting, fluorine-doped tin oxide layers on glass or glass-ceramic surfaces is disclosed. The preparation has a content of tin-IV-chlorides, which optionally comprise organic groups. The preparation contains also tin-II-fluoroborate as the sole doping agent. A preferred preparation comprises PA1 30 to 89% by weight of tin-IV chloride, which optionally comprise organic groups, PA1 0.5 to 7.5% by weight of tin-II fluoroborate in the form of a 30 to 60% by weight aqueous solution and PA1 10 to 62.5% by weight of a polar organic solvent. The inventive preparation is procedurally simple to handle and contains no chemically aggressive or toxic compounds. The components of the preparation are easily accessible, cognate and inexpensive chemicals, yet permit high-grade surface layers to be produced on glass or glass-ceramic. A method for forming the layers is also disclosed.

Field of the Invention 
The invention generally relates to the coating of glass and glass-ceramic 
surfaces and is particularly directed to a liquid preparation suitable for 
the formation of electrically conductive and infrared-reflecting 
fluorine-doped tin oxide layers on such surfaces with a content of tin-IV 
chlorides, which optionally contain organic groups. The invention is 
furthermore concerned with a method for the production of such layers with 
the preparation. 
BACKGROUND INFORMATION AND PRIOR ART 
It is well known that fluorine-doped tin oxide layers on glass surfaces 
decrease the electrical resistance of the thusly coated surfaces and 
increase the infrared reflection. To produce these tin oxide surface 
layers, a suitable tin compound and a fluorine-emitting compound are 
brought simultaneously into contact with the surface which is heated to 
temperatures of 400.degree. to 800.degree. C. The tin compound (base 
compound) forms then a coherent layer of tin oxide on the surface of the 
glass or the glass-ceramic. The fluorine-emitting compound (doping agent) 
causes the tin oxide layer which is being or has been formed to be doped 
with fluorine. This doping is of decisive importance for the development 
of the desired properties, such as conductivity and infrared reflection. 
The method for producing these fluorine-doped tin oxide layers on suitable 
surfaces can be carried out in various ways. The tin- and 
fluorine-containing compounds may be allowed to act from the gas phase on 
the surface to be improved or the surface may be subjected to a stream of 
gas, which contains the compounds in the form of a fine powder. From a 
process technology point of view, the spraying of solutions of these 
compounds in a suitable solvent is particularly easy to carry out. 
This art is dealt with extensively in the patent literature, from which the 
following Offenlegungsschriften or patents are named as representative. 
German Patent 3,010,077 discloses a method for the application, by 
pyrolysis, on a glass surface of an infrared-reflecting layer of tin 
oxide, doped with a halogen, preferably fluorine. In this method, an 
organic tin compound (base compound) in powder form with a particle size 
of less than 20 .mu.m is applied as a suspension in a gaseous carrier 
stream in the presence of a halogen compound (doping agent) on the glass 
surface having a temperature of 400.degree. to 650.degree. C. As halogen 
compound (doping agent), a pulverulent compound is used, which is 
suspended in the gaseous carrier for the tin compound (base compound). An 
example of a base compound is dibutyl tin oxide, while dibutyl tin 
difluoride is named as doping agent. Base compound and doping agent may 
also be realized in a single compound, such as ammonium dibutyl tin 
tetrafluoride. However, it has been ascertained that it is extremely 
difficult to achieve a reproducible formation of a uniform flow of carrier 
gas and pulverulent compounds, so that surface layers with constantly 
changing properties are obtained. 
German patent 2,806,468 teaches a method for which dibutyl tin oxide, in 
the form of a powder dispersed in an anhydrous stream of air, is used as 
base compound, while the doping agent is a gaseous fluorine compound, 
which is mixed in with the dispersion shortly before it reaches the 
discharge nozzle. Anhydrous hydrogen fluoride, BF.sub.2, BrF.sub.5, 
ClF.sub.3, SiF.sub.4, WF.sub.6 and MoF.sub.6 are named as gaseous fluorine 
compounds. These gaseous fluorine compounds have the tendency of etching 
the surface of the glass or glass-ceramic that is to be improved. 
Moreover, extensive safety precautions have to be taken when using these 
chemically aggressive and toxic gases. This renders the procedure 
cumbersome and expensive. 
The application of suitable tin compounds and doping agents from the gas 
phase can be deduced from the European Offenlegungsschrift 0 112 780. In 
this method, a gaseous mixture of butyl tin trichloride (base compound) 
and dichlorodifluoromethane (doping agent) is used. Here also, metering 
problems arise, which lead to surfaces of uneven and inadequate 
properties. Moreover, the use of dichlorodifluoromethane is undesirable 
for environmental reasons. 
A method is disclosed in US Patent 4,293,594, wherein a gaseous mixture of 
dimethyl tin dichloride and dimethyl tin difluoride is applied in a 
carrier gas on the surfaces to be improved. However, these compounds are 
solids and can be converted into the gas phase at high temperatures only. 
A liquid preparation for the production of high-grade, fluorine-doped tin 
oxide layers on glass surfaces is disclosed in the European 
Offenlegungsschrift 0 158 399. The preparation comprises 
(a) 1 to 30% by weight of a doping agent based on an organic fluorine 
compound, selected from trifluoroacetic acid or its anhydride, ethyl 
trifluoroacetate, trifluoroethanol and pentafluoro-propionic acid and 
(b) 70 to 99% by weight of an organotin compound, selected from alkyl tin 
trichloride, dialkyl tin dichloride, alkyldi-chloro tin acetate, 
alkylchloro tin diacetate, an ester of tin chloride or tin tetrachloride. 
This method also does not yet satisfy, since the majority of the 
fluorine-containing doping agents represent low boiling liquids and 
therefore largely evaporate at the hot glass surface and, for this reason, 
are not incorporated in the tin oxide layer. Due to the heat of 
evaporation of these doping agents, the hot substrate surface is cooled 
relatively strongly. This leads to a worsening of the properties of the 
applied layer. 
If pentafluoropropionic acid is used, the decomposition at the hot glass 
surface does not occur quickly enough, so that the tin oxide layer also in 
this case does not have sufficient functional values. In addition, these 
compounds are very expensive and injurious to health. 
OBJECT OF THE INVENTION 
It is therefore the primary object of the present invention to provide 
solutions of tin compounds as base compounds and fluorine-containing 
doping agents for the indicated purpose, which are easy to handle from a 
process-technical point of view, do not contain chemically aggressive or 
toxic compounds and comprise components made up from easily accessible, 
cognate, inexpensive chemicals, which nevertheless permit high-grade 
surface layers to be produced on glass or glass-ceramic. 
It is also an object of the invention to provide a method of forming 
superior electrically conductive, infrared reflecting layers on glass or 
glass-ceramic substrates which is readily performed. 
Generally, it is an object to improve on the art of forming such layers and 
to overcome the disadvantages and drawbacks of the prior art. 
SUMMARY OF THE INVENTION 
Pursuant to the invention, it has surprisingly been ascertained that the 
desired properties are superiorly obtained with a liquid preparation, 
which contains tin-IV chlorides as base compounds and tin-II fluoroborate 
to wit Sn [BF.sub.4 ].sub.2 as sole doping agent. The tin-IV chlorides may 
optionally contain organic groups. 
The superior effect of the inventive preparation is surprising, especially 
in light of U.S. Pat. No. 2,566,346. This patent discusses the use of 
solutions containing inorganic and/or organic tin-IV chlorides as base 
compounds and antimony fluorides as doping agent. Pursuant to the patent, 
if such solutions are applied to glass substrates by a spray-atomizing 
procedure, tin oxide layers are formed which do not exhibit appreciable 
infrared reflection values or low surface resistance values. 
Moreover, aqueous or aqueous-alcoholic tin fluoroborate solutions alone, 
that is, without the additional presence of tin-IV chlorides with or 
without organic groups, do not readily decompose at the hot substrate 
surface, so that interfering decomposition residues build up on the 
substrate surfaces and, therefore, no usable functional layers are 
obtained. 
It is particularly advantageous, that the solutions are devoid of 
hydrofluoric acid, fluoroboric acid and acidic fluoride compounds and thus 
do not attack or damage materials nor are injurious to health. 
A preferred example of an inventive preparation is a solution of between 
about 
30 to 89.5% by weight of tin-IV chlorides, which optionally contain organic 
groups, 
0.5 to 7.5% by weight of tin-II fluoroborate in the form of a 30 to 60% by 
weight aqueous solution and 
10 to 62.5% by weight of a polar organic solvent. 
Aside from tin tetrachloride, suitable base compounds are alkyl tin 
trichloride, dialkyl tin dichloride, dialkylchloro tin acetate and 
alkylchloro tin diacetate, in which the alkyl group in each case is a 
group with 1 to 6 carbon atoms and especially with 4 carbon atoms and 
preferably is n-butyl. 
Especially preferred therefore is an inventive preparation which comprises 
between about 
30 to 89.5% by weight of tin tetrachloride or alkyl tin trichloride, 
0.5 to 7.5% by weight of tin-II fluoroborate in the form of a 30 to 60% by 
weight aqueous solution and 
10 to 62.5% by weight of a polar organic solvent. 
Another particularly preferred preparation comprises between about 
30 to 89.5% by weight of tin tetrachloride or butyl tin trichloride, 
0.5% to 6% by weight of tin-II fluoroborate in the form of a 30 to 60% by 
weight aqueous solution and 
10 to 64% by weight ethanol and/or ethyl acetate. 
As polar organic solvents, lower aliphatic alcohols, especially ethanol, 
and low-boiling esters, especially ethyl acetate, or ketones, such as 
methyl isobutyl ketone, come into consideration. 
A further aspect of the present invention is to provide a method for the 
production of electrically conductive and infrared-reflecting tin oxide 
layers on glass or glass-ceramic substrates by spraying an organic 
solution of tin-IV chlorides, optionally containing organic groups, as tin 
oxide layer-forming agents and fluorine-containing doping agents on the 
substrate surfaces, which have a temperature of 400.degree. to 800.degree. 
C. The method is characterized in that tin-II fluoroborate is the sole 
doping agent contained in the solution. Advantageously, the 
above-described preferred preparations are used for carrying out the 
method. 
The inventive preparation is advantageously applied on the heated 
substrate, especially on glass, with a spray gun by means of a 
spraying-atomizing method that uses compressed air. The substrate should 
have a temperature of 400.degree. to 800.degree. C. The temperature of the 
substrate must, however, be below its respective melting or softening 
temperature. 
Through pyrolysis, a tin oxide film doped with fluorine is thus produced on 
the substrate surface. This film has the properties of a semiconductor 
layer. Depending on the amount of inventive preparation used, a film with 
a thickness of 100 to 1,000 nm is obtained. Therefore, in accordance with 
the inventive method, selectively transparent coatings can be produced on 
glass. The coatings are largely transparent to visible light (75 to 83%), 
while infrared radiation with a wavelength longer than about 2,500 nm is 
reflected to the extent of 80 to 90%, depending on the thickness of the 
layer. 
The following examples serve to explain the invention further. It is 
understood that these examples are given by way of illustration and not by 
way of limitation.

EXAMPLE 1 
To a 100 mL glass flask equipped with magnetic stirrer are added 
29.0 g ethyl alcohol, 
70.0 g butyl tin trichloride and 
1.0 g of an approximately 50% by weight aqueous tin-II fluoroborate 
solution. 
The mixture is mixed with stirring and cooling. 
5 mL of this solution is filled into the bowl of a spray gun (nozzle 
diameter 1 mm; spraying pressure 4 bar) and sprayed onto a flat glass pane 
(160.times.180.times.6 mm), which had previously been heated for 5 minutes 
at a furnace temperature of 700.degree. C. After cooling, the glass pane 
had the following values: 
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layer thickness 400 nm 
infrared reflection 84.0% 
surface resistance 19 ohm/square 
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EXAMPLE 2 
To a 100 mL glass flask equipped with magnetic stirrer are added 
52.0 g ethanol 
46.1 g SnCl and 
1.9 g of an approximately 50% by weight aqueous tin-II fluoroborate 
solution. 
The mixture is mixed for 2 hours with stirring and cooling. 
5 mL of this solution is used to improve a glass surface as in Example 1. 
After cooling, the glass pane has the following values: 
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layer thickness 350 nm 
infrared reflection 80.0% 
surface resistance 28 ohm/square 
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