Conductive coating treatment of glass sheet bending process

The specification discloses a method for producing a curved glass member having a transparent, electroconductive coating by first coating the glass flat with indium-tin oxide, followed by coating that film with a layer of carbonaceous material and covering that layer with a mating piece of glass while the two pieces of glass are bent in a bending oven.

BACKGROUND OF THE INVENTION 
The ultimate object of the present invention is to produce or make possible 
the production of a curved glass member having a transparent, 
electroconductive coating. The invention is particularly adapted to 
producing such a member for use in heating applications at low voltages 
relative to prior art. Such glass might be used for CRT screens which you 
can input by touching, architectural glass (such coatings are infrared 
reflective), curved, heated mirrors, and heated windows. 
One possible application for such a product is automotive windshields. By 
applying a current to such windshields, accumulated frost, ice or fog can 
be removed by heating. It has been found that one should be able to 
generate 0.6 watts per square inch within a windshield of typical shape, 
with a space of 25" between the electrical bus bars, in order to clear 
rime ice 0.1" thick in about five minutes. Using a 60 volt auxiliary power 
source, which systems are readily available for use in automobiles, this 
means that the sheet resistivity of any electroconductive coating on the 
glass must only be about 10 ohms per square. 
Further, federal light transmissibility standard (FMVSS No. 205) requires a 
70% minimum visible light transmission at an angle of 90 degrees to the 
glass surface. Further, the electroconductive coating used, in order to be 
suitable in appearance, must be very thin, uniform, low in absorption, and 
low in reflection. These constraints make it particularly difficult to 
achieve such low resistivity. Assuming a coating with a range of index of 
refraction of 1.6 to 2.1, as is the case with indium-tin oxide coatings, a 
nominal full wave coating would have a physical thickness of approximately 
3,400 to 2,600 angstroms (i.e., the median wavelength for visible light, 
5,500 A, divided by the index of refraction). 
Prior to my work, there were no known commercially viable techniques 
available for providing satisfactory curved glass uniformly coated with 
such a thin film having such a low resistivity. Of course it is known to 
put transparent conductive coatings on aircraft windows, display cases and 
the like. Generally, however, flat glass is uscd in such applications. 
Also, high voltage power sources are available in such environments so 
that a high resistance in the film coating can be tolerated. 
A common technique for preparing aircraft windows with electrically 
conductive coatings is to coat the hot surface of the glass with a tin 
oxide deposited pyrolytically. U.S. Pat. No. 2,954,454 discloses such a 
method for creating a coated, bent glass windshield. The problem with such 
a system is that in order to achieve coatings which make it possible to 
deliver 0.6 watts per square inch at low voltages, i.e., about 60 volts, 
one must provide a relatively thick film of between 5,000 and 10,000 
angstroms. This results in a windshield or curved glass article which 
yields a "rainbow" of reflected color when exposed to light. This is 
partially due to the thickness of the coating, partially to the high index 
of refraction of tin oxide, and partially to the inherent nonuniform 
thickness of pyrolytically deposited tin oxide. Also it is suspected that 
such a process will yield a coating which will craze when the glass is 
bent, at least if produced on an economical commercial basis and using 
conventional bending procedures. 
Indium-tin oxide has heretofore been sputter coated onto flat glass for use 
in liquid crystal display electrodes. However, when processed in 
accordance with conventional procedures, such indium-tin oxide, sputter 
coated flat glass cannot be bent on a viable production basis without 
crazing the indium-tin oxide coating. Such crazing of course interrupts 
the conduction of electricity and leads to serious imperfections in the 
heating pattern in the product. 
United Kingdom Pat. No. 1,446,849, published Aug. 18, 1976, discloses the 
sputter coating of an already curved sheet of glass. It is believed that 
such a process would be slow, and uniformity is believed to be a serious 
problem. It is not believed that such a process would be acceptable in 
commercial production. 
As a result of such drawbacks, it is believed that prior artisans have not 
heretofore commercially produced electrically conductive curved parts for 
low voltage by glass coating processes, though a concerted effort is now 
being made to introduce a heated windshield coated curved with zinc oxide 
and silver layers. Further, even alternatives other than coating the 
glass, such as embedded wires or embedded conductively coated plastic 
films, have not proven commercially acceptable for heating applications 
where visibility is primary. 
I have heretofore found that a key to making an indium-tin oxide coated 
substrate bendable is to provide for a degree of oxygen substoichiometry 
in the oxide coating at the time of bend. The extent of substoichiometry 
must be sufficient to avoid crazing during bending, but not so great as to 
cause the final product to have less than 70% light transmission or be 
hazy. This method is disclosed and claimed in my U.S. Pat. No. 4,490,227. 
In the foregoing patent, I disclose two specific ways to achieve a degree 
of substoichiometry at the moment of bend. The first involves initially 
coating the substrate to a low initial light transmittance (T.sub.o), 
i.e., between about 10 and about 40%. This is sufficiently low to insure a 
degree of substoichiometry even though the film is subsequently fired at 
atmosphere. The bending is also conducted in atmosphere. 
The second method disclosed involves coating the substrate normally, i.e., 
to an initial light transmittance which may be as high as about 75%, 
followed by bending the coated glass in a reducing environment. 
Both of these techniques for achieving substoichiometry at the moment of 
bend are acceptable commercial procedures. However, experience in 
practicing these previously disclosed techniques has taught that a degree 
of care has to be taken to avoid faster bends, i.e., bends at higher 
temperatures, deeper bends and thicker indium-tin oxide films. 
SUMMARY OF THE INVENTION 
In the present invention, I have found that I can increase processing 
latitude and make a less expensive product by achieving substoichiometry 
at the moment of bend by applying a reducing layer of carbonaceous 
material to an indium-tin oxide coating on a flat glass substrate. This 
substrate can then be bent under normal bending conditions without causing 
crazing of the indium-tin oxide film. The presence of the reducing layer 
of carbonaceous material insures the degree of oxygen substoichiometry at 
the moment of bend such that crazing the indium-tin oxide layer does not 
occur. 
The present invention lends itself exceptionally well to commercial use in 
that one does not have to be as careful about the bending temperature, 
deepness of bend or thickness of the indium-tin oxide coating. Further, 
one can avoid the so-called "gas forming" step or "reduction cure" which 
is used in one of the two processes disclosed in my prior '227 patent. 
Similarly, it is unnecessary to bend the coated glass in a reducing 
environment as disclosed in the other of the two processes described in 
the '227 patent.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In the preferred embodiment, a flat glass substrate is seamed and washed 
(step 1, FIG. 1), silk screened with bus bars (step 2), fired in order to 
harden the bus bars (step 3), washed (step 4), silk screened to provide a 
peripheral mask (step 5), coated with indium-tin oxide in a magnetron 
sputterer (step 6), fired to remove the peripheral mask (step 7), washed 
(step 8), coated with a layer of carbonaceous material (step 9), covered 
with a glass mate and bent (step 10), labeled and cleaned (step 11), the 
coated glass tested for resistance and transmittance (step 12), leads 
soldered to the bus bars (step 13), the coated glass and its mate 
laminated (step 14) and the laminated windshield subjected to a final test 
(step 15). All of these steps except the coating of the indium-tin oxide 
film with a layer of carbonaceous material are conducted in accordance 
with conventional techniques known to those skilled in the art. An 
acceptable part is obtained without reduction curing and without bending 
the coated part in a reducing environment. 
Examples of carbonaceous material which can be used as the reducing layer 
include powdered activated charcoal, powdered charcoal (not activated) and 
carbon black. Carbon black is particularly effective. One such material is 
known as "Thermax Ultrapure.TM.", available from R. T. VanderBilt Company, 
Inc. of Norwalk, Conn. It is 99.6% carbon, has a mean particle diameter of 
270 nanometers and a particle diameter range of from 80 to 500 nanometers. 
Its nitrogen surface area is 8.5 square meters per gram. 
In some of the experimental work reported herein, the carbonaceous material 
was applied by dusting powdered activated charcoal onto the indium-tin 
oxide coating. However, it is preferable to prepare and apply a 
carbonaceous paint. This renders it easier to apply the material. It goes 
on smoothly and dries in a few seconds. 
Preferably, the paint comprises the carbonaceous material, most preferably 
carbon black, dispersed in a film forming material, e.g. a resinous 
vehicle such an alkyd resin. Possible alternatives to alkyd resins include 
ethylcellulose, polyvinyl acetate and polyvinyl buterate resins. 
The film former must be a good vehicle for dispersing the carbon black and 
keeping it in suspension. It must also burn off at temperatures at which 
the glass is bent. 
Volatile solvents are also used to make it easier to apply the paint. Yet, 
the volatile solvents readily evaporate from the painted surface. A 
preferred carbon black paint formulation is as follows: 
25 grams of carbon black 
100 milliliters of alkyd resin solution (45% solids) 
100 milliliters toluene 
50 milliliters methylethylketone 
The most preferred alkyd resin is a methyl methacrylate copolymer. It is 
itself dissolved in solvents such as toluene and methylethylketone to a 
solids level of around 45%. At that concentration, it has a viscosity of 
6,000 to 10,000 cps. Its glass transition temperature T.sub.g is about 50 
degrees C. and when dried, has a Tukon hardness of 11-12. One such 
material commercially available is Acryloid B485.TM. available from Rhom 
and Haas. 
The foregoing paint is applied by brushing, or spraying. It is applied to a 
density of about 0.6 grams per square foot, plus or minus about 30%. 
Another way to measure the density is to determine the extent to which it 
absorbs light. A layer of the above described carbon black paint applied 
to a proper thickness on a piece of clear glass leaves the coated glass 
with a light transmission of only 20% with reference to light striking the 
coated glass at a 90 degree angle. 
The quantity of carbonaceous material is not overly critical. However, if 
insufficient carbonaceous material is used, it fails to prevent crazing. 
If too much is used, the film becomes excessively substoichiometric, and 
not sufficiently light transmissive. Also, resistance in the final product 
actually starts coming back up. 
Artisans may find that the necessary amount will vary depending on the 
specific parameters used in other steps, and especially the application of 
the indium-tin oxide layer. Even so, those skilled in the art will be able 
to determine the desired carbonaceous material density empirically for any 
given set of process variations. 
The experiments reported in FIGS. 6 and 7 illustrate that about 0.6 grams 
per square foot, one minimizes the resistivity of the end product 
indium-tin oxide layer, i.e., to about 5 ohms per square, while 
maintaining an acceptable layer of light absorption in the final product, 
i.e., about 6% (FIG. 6). FIG. 6 shows that at 0.6 grams per square foot, 
an index of refraction of about 1.8 is achieved. This is also acceptable. 
The user has broad latitude in particle size of the carbonaceous material. 
I have successfully used powdered activated charcoal at about 8 mil 
diameter, with a range of about 3 to 8 mils. Particles larger than about 
20 mils are undesirable in that they create spots on the ITO film. At the 
other extreme, the very fine carbon black particles worked very well. 
These had a mean diameter of 270 nanometers with a range from 80 to 500 
nanometers. 
Now turning to the remaining steps of the process, step 1 (FIG. 1) of the 
process involves seaming and washing the long, cut part. A slightly 
shorter part is provided as the mate for the longer part and is similarly 
seamed and washed. 
Step 2 involves silk screening electrically conductive bus bars along the 
top and bottom edges of the long flat piece of seamed glass. This is 
followed by the conventional step of firing the bus bars in a conventional 
firing oven (step 3). The part is again washed (step 4) and is ready for 
step 5 wherein a peripheral zone mask is silk screened onto the part. This 
is a conventional paint mask, and is intended to prevent the peripheral 
edges of the glass from becoming coated with a conductive film. This 
prevents the conductive element from being accidentally grounded onto any 
contiguous conductive material. 
Step 6 involves sputter coating the unheated long glass part to the desired 
thickness in a partial oxygen atmosphere to a desired initial light 
transmittance T.sub.o. Planar magnetron sputtering is a well-known coating 
method. The subject is discussed in prior publications such as "Planar 
Magnetron Cathodes-Past, Present, and Future", Theodore Van Vorous, 
published by Vac-Tec Systems, Inc., Boulder, Colo.; U.S. Pat. No. 
4,046,659 issued Sept. 6, 1977 to Cormia et al.; U.S. Pat. No. 4,022,947 
issued to Grubb et al. on May 10, 1977; and Canadian Pat. No. 566,773 
issued to Preston on Dec. 2, 1958. 
Basically, in typical cathodic sputtering processes, a high negative 
potential is applied to a cathode assembly which is comprised of the 
metal(s) sought to be deposited on a substrate, typically glass. The 
substrate is located in the vicinity of and substantially parallel to the 
cathode assembly in a vacuum chamber with a low pressure (about 
5.times.10.sup.-3 torr) atmosphere of oxygen and inert gas such as neon, 
argon, krypton or xenon. The high potential and reduced pressure cause a 
glow discharge between the cathode and the anode. Under such conditions, 
the gas is energized and the gaseous ions strike the cathode causing metal 
ions to be ejected from the cathode and deposited on the surface of the 
substrate. This process is enhanced where a magnetic field is present 
which magnetically confines the glow discharge plasma, i.e., a planar 
magnetron is employed. 
FIG. 2 shows schematically a typical planar magnetron sputtering apparatus 
of the type which can be used in practicing the present invention. The 
part to be coated is placed on a tray 10, located initially on feed table 
20, and is passed through an opened gate valve 30 into a load lock 40 
having a second gate valve 31 at its other end. A vacuum booster 50 and 
backing pump 51 are used to evacuate load lock 40 with both gate valves 30 
and 31 closed. When the desired vacuum is achieved, load lock 31 is opened 
and the tray 10 carrying the part to be coated is transported into the 
evacuated sputtering chamber 60, from motor driven rollers 41 onto motor 
driven rollers 61. 
Vacuum is maintained within sputtering chamber 60 at all times during 
operation by means of diffusion pump 70 and backing pump 71. A throttle 72 
is typically used to help control the vacuum. Tray 10 and the part it 
carries are passed back and forth on motorized rollers 61 beneath the 
sputtering target or cathode assembly 62 for as many times as is necessary 
to coat the part to a desired thickness. 
Power to the cathode or target 62 is provided by power supply 80. Target 62 
is cooled through a regenerative water cooling system 90. Argon and oxygen 
are supplied to sputtering chamber 60 by assemblies 100 and 110 
respectively. 
The composition of the indium-tin cathode 62 is conventional. A tin level 
of from 1 to 25% is typical, with 9% or 10% optimum. I use a 10% tin, 90% 
indium target. 
It is well-known that the degree of light transmission of the coating, 
after the sputtering process is complete, is a function of the extent to 
which the metal has been oxidized as it coats the glass substrate surface. 
This degree of oxidation is in turn controlled by the relative amount of 
oxidizer and reaction activating influences (such as substrate biasing, 
e.g., positive, negative, R.F.) available in the vacuum chamber. 
To produce acceptable parts with respect to heating uniformity and 
appearance, it is also important that the indium-tin oxide film be 
relatively uniform in thickness across the surface of the parts. 
Preferably, the thickness should not vary more than about .+-.7.5%. Such 
uniformity can be achieved using conventional sputtering techniques, 
particularly in view of the fact that the present invention contemplates 
sputter coating a flat part, rather than a curved one. 
In the present invention, the initial light transmittance T.sub.o is not 
particularly critical. Conventionally, the level of oxygen in the planar 
magnetron sputter is controlled so that T.sub.o of the indium-tin oxide 
layer is typically around 60 to 70%. This is acceptable for purposes of 
the present invention. 
Indeed, the present invention allows great latitude in T.sub.o. I have 
found that satisfactory product can be made using T.sub.o of anywhere from 
1 to 75%. Above about 75%, sputtering efficiency drops off. One tends to 
get thinner films at 75% or greater T.sub.o, thus making it more difficult 
to obtain lower sheet resistance in the final product. This has no impact 
per se on the present invention, but there is no advantage in terms of the 
present invention to employ a T.sub.o above 75%. Hence, there is no reason 
to accept the lower sputtering efficiency one would obtain at such T.sub.o 
levels. 
For purposes of clarity of the final product, the coating should be as thin 
as possible while still sufficiently thick to achieve low sheet 
resistance. Maximum transmission is achieved when the optical film 
thickness is an integral multiple of one-half of the nominal wavelength of 
visible light. Variations at other than integral multiples of half wave 
films will tend to reduce transmissibn and change the reflected color. 
Going to a 1.5 wavelength coating, one can achieve the desired film 
conductivity more easily since, for a given bulk resistivity, a thicker 
film will be more conductive than a thinner film. However, one undoubtedly 
sacrifices some light transmission in the final product. I have produced 
two wave coatings, which can be bent without crazing and believe that even 
thicker coatings, e.g., three wave, could be produced without crazing. 
Going to a half wavelength coating affords some improvement in 
transmission, but results in a more resistive film because it is thinner 
(assuming everything else is constant). This makes it more difficult to 
achieve the ultimate desired power density. Hence a full wave coating has 
been found quite desirable in the present invention. 
As is known to those skilled in the art, the part is passed back and forth 
beneath the sputtering target until the desired film thickness has been 
achieved. The number of passes to yield a desired film thickness will of 
course vary from specific apparatus to specific apparatus, and in 
accordance with other variations in process parameters. 
As will be appreciated by those skilled in the art, the various settings 
for argon pressure, oxygen pressure, power density to the cathode 
sputtering target, and other such settings may vary from machine to 
machine and from procedure to procedure used in order to achieve the 
desired T.sub.o. Of course, it is appreciated by those skilled in the art 
that the primary factor to be controlled is the quantity of oxygen 
present. Further, it is perhaps significant to other operational 
parameters that the object of this invention is achieved primarily as a 
function of T.sub.o, regardless of other process variations such as power 
density to the cathode or anode, argon pressure, or the like. Naturally, 
variation of these factors may have other process ramifications. 
Other problems, unrelated to the essential object of the present invention 
may show up by varying these factors. I typically deposit the coating 
without auxiliary substrate heating (the sputtering process itself may 
warp the glass somewhat). Others may wish to use such auxiliary heat for 
various reasons. Such heating is one example of a reaction activating 
influence which will modify the transmission and absorption of the 
as-deposited film. 
Power density to the cathode or anode can be varied as desired by the 
operator. Some may wish to use various power densities in order to improve 
the rate of production. 
It should be noted that the T.sub.o selected by the operator will have an 
impact on the subsequent firing step, step 7 (FIG. 1). The firing step is 
especially important when the indium-tin oxide coating has been applied to 
a low T.sub.o. If a higher T.sub.o is used, e.g. in excess of about 40%, 
it is not essential for purposes of final light transmissibility of the 
product to fire. Of course, since firing is necessary to burn off the silk 
screened peripheral paint mask, an alternative type of mask has to be used 
if the firing step is eliminated. For example, a metal mask might be 
clamped or otherwise secured in place for removal later prior to bend. Of 
course, one advantage to firing is that it helps toughen the indium-tin 
oxide coating, making it easier to ship the product. 
The firing step (step 7) is conducted at approximately 540 degrees C. (for 
soda lime glass). This oxides the indium-tin oxide film to some extent and 
it also oxides the peripheral silk screened paint mask. The firing is done 
in the presence of an oxidizing atmosphere and takes about 3-6 minutes 
above 500 degrees C. For temperatures less than 500 degrees C., longer 
times are needed. 
In step 8, the oxidized paint mask is removed by washing. The indium-tin 
oxide coated glass is then ready for step 9, the coating with the 
carbonaceous layer. This coating is applied as described above. The glass 
is then ready for step 10, bending. 
As is conventional in the bending process, the two pieces which are to be 
later assembled into a single part are bent together. The longer piece is 
typically placed on the bottom. The shorter piece is placed on top. The 
two are placed in a bending fixture 120 (FIG. 3). Such fixtures are 
conventional and generally comprise several hinged segments, 121, 122, and 
123 as shown, which in turn are hingedly mounted at some point to a base 
support 124. The particular shape of the hinged segments will vary 
depending on the final shape desired for the part. The longer flat piece 1 
is placed in the fixture such that it extends between the ends thereof as 
generally shown in solid lines in FIG. 3, with the shorter piece 2 on top. 
The entire fixture 120 and parts 1 and 2 are then passed on a conveyor 131 
into oven 130. Heat is supplied by heating elements 132 as is 
conventional. The glass is heated in the oven to a temperature just 
sufficiently near its softening point to cause it to sag down into the 
hinged segments 121, 122, and 123 of the bending fixture until it comes to 
rest against the edges of hinged segments 121, 122 and 123. The sagged or 
bent condition for the parts and the hinged segments 121, 122, and 123 is 
indicated in phantom lines in FIG. 3. 
This procedure is entirely conventional and can be carried on in a 
conventional way without crazing the indium-tin oxide coating. 
It is necessary that the carbonaceous covered indium-tin oxide film be 
covered with the mating glass part during bending. This is typically done 
anyway. The indium-tin oxide film should be located between the lights. 
Failure to do so will lead to crazing. 
The bending is conducted at normal bending temperatures. In the preferred 
embodiment procedures described in my U.S. Pat. No. 4,490,227, I cautioned 
against unusual bending procedures. Using the present invention, the 
operator can be less concerned about bending temperatures and the rate of 
bend. When the present invention is followed, I have found that the 
bending temperature can vary between 1100 and 1180 degrees F. without 
excessive crazing. This may be particularly important in a commercial 
setting, where it may be more difficult to maintain narrower temperature 
ranges. 
One problem encountered in the present invention is the leakage of oxygen 
at the edges of the mating sheets of glass. After they are bent together, 
as the temperature drops, there appears to be excessive oxidation of the 
indium-tin oxide film at the edges of the indium-tin oxide coated sheet. 
Resistance at the edges tends to be too high. 
This will not be a problem if the product can be designed so that the bus 
bars are located one inch in from the edge. If that is not possible, 
another way to solve the problem is to scribe the glass a half inch to an 
inch from the edge and break off the outside edge after bending has been 
completed. Another possible solution might be to seal the edges prior to 
the bending process. This a minor drawback which has to be dealt with in 
specific overall processing. Nevertheless, it does not detract from the 
fundamental efficacy of the present invention. 
After bending, the two pieces of glass, one coated and one not coated, are 
separated, labeled for later reassembly, and cleaned (step 11, FIG. 1). 
The layer of carbonaceous paint can be readily wiped off or dusted off. 
The paint film forming vehicle has been oxidized during the bending step. 
An adherence which the paint initially had has been eliminated by 
decomposition of the resin binder. 
After washing, the indium-tin oxide coated part is tested for resistance 
and transmittance, step 12, FIG. 1. Leads are then soldered onto the 
previously applied bus bars (step 13). The coated part and its similarly 
curved mate are then laminated together in a conventional manner using an 
intermediate polybuterate plastic sheet (step 14). After final testing, 
the product is ready for shipment to the customer. 
EXPERIMENTAL RESULTS 
Coating Density 
Coating density determinations were made by applying powdered charcoal with 
a charcoal duster 100 operating on the principle of a sifter (FIGS. 4 and 
5). The apparatus comprises a dispensing assembly 110 at the top of 
vertical tunnel 130 (FIG. 4). Dispensing assembly 110 comprises powder 
tubes 114 which sit atop an assembly of three sifter plates 111, 112 and 
113 (FIG. 5). Sifter plates 111, 112 and 113 are adapted to sift powdered 
charcoal out of tubes 114 and allow it to fall downwardly through a 
vertical tunnel 130 and onto an indium-tin oxide glass plate located at 
the bottom of tunnel 130 (FIG. 4). The first sifter plate 111 and the 
third sifter plate 113 are both fixed. The intermediate sifter plate 112 
is movable. Nine 5/64" diameter circular apertures 122 are arrayed about 
the area of plate 112 on 3" centers. Plate 111 has the same array of the 
same sized apertures 121 such that the apertures 122 in plate 112 can be 
aligned with the apertures 121 in plate 111. Nine apertures 123 in plate 
113 are similarly arrayed, except that each aperture 123 is slightly 
larger in diameter than apertures 122 and 121. Further, plates 111 and 113 
are offset from one another so that when the apertures 122 in plate 112 
are aligned with those (121) in plate 111, they will not be aligned with 
apertures 123 in plate 113 and vice versa. In this way, a precisely 
measured amount of powdered charcoal can be metered out of tubes 114 on 
each double shift of movable plate 112, i.e., first to alignment with the 
apertures 121 in plate 111 and then to alignment with apertures 123 in 
plate 113. Shifting plate 112 is provided with a handle 115 to facilitate 
its manipulation. 
Tunnel 130 has an opening 131 at the bottom of one of its walls so that a 
piece of glass can be inserted at the base of tunnel 130. Tunnel 130 is 
approximately four feet in height and includes a side window 132 through 
which the fall of charcoal can be observed. 
This apparatus was constructed so that each double shift of plate 112 
deposited approximately 0.2 of a gram of powdered charcoal onto the glass 
plate per square foot. Samples of glass were coated to 0.2, 0.6, 1.0 and 
1.8 grams of powdered charcoal per square foot. A control sample of 
indium-tin oxide coated glass having no layer of powdered charcoal was 
also tested. The powdered charcoal was of a 50-200 mesh size, thus having 
a particle size range of from about 3 to about 12 mils. The bulk of the 
particles were around 8 mils. 
Each of the samples was covered with a piece of mating glass and bent at 
approximately 1120 degrees F. on the test bending fixture shown in FIG. 8 
and discussed below. The powdered charcoal was then washed away and the 
parts tested for resistivity, light absorption and index of refraction. In 
FIGS. 6 and 7, grams of powdered charcoal per square foot are charted on 
the abscissa. Resistivity is charted on the left hand ordinate of FIG. 6 
and the resistivity results are shown by the solid line of FIG. 6. Percent 
absorption of the final product is charted on the dashed right hand 
ordinate and is shown by the dashed line in FIG. 6. Index of infraction is 
shown on the ordinate of FIG. 7. 
As expected, the resistivity in the untreated control is far too high, in 
excess of 30 ohms per square. At about 0.6 grams per square foot, 
resistivity is brought down to approximately 6 ohms per square. This is 
acceptable, and very little improvement is seen by adding more charcoal. 
At 0.6 grams per square foot, the percent light absorption is still 
acceptable, i.e. around 6%. This refers to the percent of light absorbed 
which strikes the glass at a 90 degree angle. Adding more carbonaceous 
material merely tends to increase the percent absorption without giving a 
concurrent improvement in resistivity lowering. 
FIG. 7 illustrates that the index of refraction does vary in accordance 
with the amount of charcoal used. At no charcoal, the index is in excess 
of 1.9. At 0.6 grams per square foot, the index of refraction is 
approximately 1.8. Lowering the index of refraction cuts down on 
reflection and increases transmission. It also makes the glass appear more 
clear and colorless. 
Comparative Testing 
In Examples 1-9, the following parameters were varied: In process step 6, 
the light transmittance (T.sub.o) to which the indium-tin oxide coating 
was sputtered; in process step 7, firing or eliminating firing of the 
film; in process step 9, coating the film with a carbonaceous reducing 
layer or not coating it; in process step 10, the temperature of bend, 
e.g., 1100 degrees F., 1140 degrees F., 1180 degrees F., or 1220 degrees 
F. For each example, four different test samples were prepared, and each 
was bent at one of the different temperatures. The carbonaceous layer was 
applied in the form of the preferred embodiment paint described above. Ten 
by fourteen inch samples were tested. The samples were bent on the bending 
fixture shown in perspective in FIG. 8. Bending test fixture 50 comprises 
three parallel, curved ribs 51 joined at end members 52 and 53. Each rib 
has an identical curved configuration such that the ten by fourteen inch 
piece of glass bent on test fixture 50 has the longitudinal cross section 
illustrated in FIG. 9, but a wavy lateral cross section (FIG. 10) with a 
raised central rib 61. 
In each test, the resistivity of the sample in ohms per square was 
determined, the light absorption was determined and a degree of craze was 
established. Resistivity was determined at two points, at the more severe 
bend defined by rib 61 near the center of glass 60, and away from rib 61 
at point 62, still near the longitudinal center of the sample (FIG. 10). 
Light absorption was determined based on incident light at 90 degrees to 
the surface of the glass sample. The amount of light transmitted and the 
amount of light reflected were added together and subtracted from 100 to 
determine the amount of light absorbed. 
The degree of crazing was established by reference to an arbitrary scale. A 
set of previously prepared crazed samples were rated with numbers between 
0 and 10. Ratings for the samples tested in Examples 1-9 were established 
by examining them at 7.times. magnification and comparing each sheet of 
glass 60 to one of the controls at 7.times. magnification. The degree of 
crazing in the controls is illustrated in FIGS. 11-14. 
The variations in steps 6, 7 and 9 in preparing the four specimens of 
Examples 1-9 are shown in Table 1 below. 
TABLE 1 
______________________________________ 
T.sub.o Fired Coated With Carbon 
Ex (step 6) (step 7) 
Black Paint (step 9) 
______________________________________ 
1 20 Yes No 
2 5 Yes No 
3 10 Yes Yes 
4 20 Yes Yes 
5 40 Yes Yes 
6 60 Yes Yes 
7 20 No Yes 
8 40 No Yes 
9 60 No Yes 
______________________________________ 
In evaluating the results, it should be kept in mind that the objective of 
the present invention is to achieve a windshield which will transmit 70% 
visible light striking the glass surface at an angle of 90 degrees, have a 
low resistivity and have as little crazing as possible. The resistivity 
desired is less than 20 and most preferably less than 10 ohms per square. 
On the arbitrary scale of 0 to 10 which I have established, the degree of 
crazing should be less than about 4. Crazing in excess of 4 results in an 
unacceptable product. 
Table 2 below presents the results for each of the four samples made in 
accordance with each of the nine Examples. The Table is arranged with the 
example number in the left vertical column and the temperature of bend 
across the top in the next four columns. Each column is divided into three 
subcolumns for presentation of the resistivity of the sample in ohms per 
square, as measured at two different points, light absorption and the 
degree of crazing. Light absorption of less than 15% is acceptable, though 
it is more preferable that light absorption be below 10%. 
TABLE 2 
__________________________________________________________________________ 
1100.degree. F. 
1140.degree. F. 
1180.degree. F. 
1220.degree. F. 
Ex 
.OMEGA./.quadrature. 
A Craze 
.OMEGA./.quadrature. 
A Craze 
.OMEGA./.quadrature. 
A Craze 
.OMEGA./.quadrature. 
A Craze 
__________________________________________________________________________ 
1 84, 
1.4 
0 89, 
1.4 
2.2 91, 
1.6 
4.5 83, 
1.7 
5.2 
91 88 86 71 
2 14, 
7.0 
0 17, 
8.0 
0 14, 
7.8 
1.2 14, 
7.4 
3.5 
-- 18 15 14 
3 23, 
6.9 
0 17, 
7.8 
.2 13, 
7.8 
1.5 17, 
8.0 
3.2 
15 15 15 14 
4 11, 
6.4 
0 11, 
8.0 
.2 13, 
9.1 
2.5 11, 
7.6 
4.2 
11 11 10 10 
5 10, 
6.0 
0 13, 
7.4 
1.5 9, 9.1 
3.2 11, 
7.6 
5.2 
12 10 10 10 
6 12, 
6.6 
0 12, 
12.0 
2.2 10, 
11.4 
3.0 7, 11.9 
6.5 
19 8 10 12 
7 15, 
23.6 
.2 20, 
27.1 
0 20, 
20.8 
0 18, 
12.0 
1.2 
18 19 20 16 
8 16, 
7.6 
0 15, 
8.4 
1.5 15, 
7.5 
3.2 12, 
7.3 
3.2 
17 20 12 16 
9 12, 
4.8 
0 10, 
9.4 
1.5 10, 
11.0 
3.2 8, 11.2 
6.5 
11 9 10 8 
__________________________________________________________________________ 
Reviewing the Examples, it can be seen that the four samples of Example 1 
are made generally in accordance with the first alternative embodiment 
process described in my prior U.S. Pat. No. 4,490,227. The one exception 
is that the parts have not been reduction cured. Hence in each case, the 
resistivity in ohms per square is rather high, e.g., between 71 and 91. 
These numbers would be substantially reduced if the reduction curing step 
called for in alternative embodiment process 1 of my '227 patent were 
employed. 
For purposes of the present invention, it is more significant to note that 
while the samples of Example 1 showed acceptable crazing levels when bent 
at 1100 and 1140 degrees F., they showed unacceptable crazing levels when 
bent at 1180 and 1220 degrees F. 
This is in contrast to the four samples each for Examples 2-9, all of which 
are made in accordance with the present invention. In all of these cases, 
the crazing levels were acceptable at bending temperatures up to 1180 
degrees F. In some of the samples, the crazing levels were acceptable even 
at 1220 degrees F., see e.g., Examples 2, 3, 7 and 8. 
The foregoing comparative samples also illustrate some of the process 
variations which have to be employed depending on the particular T.sub.o 
which one elects as a starting point. In Examples 2 and 3, the T.sub.o 's 
were 5 and 10% respectively. In each of those Examples, the sample was 
fired prior to being coated with carbon black paint. In the case of all 
four samples, the resistivity and the percentage of light absorbed were 
acceptable. 
In contrast, the samples of Example 7 were initially coated with indium-tin 
oxide to a T.sub.o of 20%, but not fired. In those samples, absorption ran 
high, i.e., in excess of 20%, for each of the samples bent at 1100, 1140 
and 1180 degrees F. Absorption became acceptable, i.e., 12%, when the 
sample was bent at 1220 degrees F. due to the inherent firing which took 
place at that more elevated temperature. Hence if one does elect to begin 
with a low T.sub.o, the firing step (step 7) described above is most 
preferably used. 
Firing was acceptably eliminated in Examples 8 and 9, where the T.sub.o 's 
were 40 and 60% respectively. In those Examples, the degree of light 
absorption was in all cases acceptable. 
Examples 4, 5 and 6 were like Examples 7, 8 and 9 respectively, except that 
the firing step was used. In all cases, the degree of light absorption was 
acceptable. 
Finally, it will be noted that the resistivity in the case of all samples 
of Examples 2-9 was less than 20 ohms per square, with the exception of 
one point on one sample, i.e., the point on rib 61 of the Example 3 sample 
which was bent at 1100 degrees F. In that case, the resistivity was 
recorded at 23 ohms per square. This can probably be viewed as an 
acceptable statistical variation. 
In one more comparative test, a 50.times. magnification photograph was made 
of a piece of glass coated flat with indium-tin oxide in a conventional 
manner and subsequently bent without using the present invention or the 
invention covered by my U.S. Pat. No. 4,490,227 (FIG. 15). For comparison 
purposes, a similar 50.times. photo was taken of a piece of glass coated 
flat with indium-tin oxide in a conventional manner, subsequently coated 
with a layer of powdered charcoal in accordance with the present invention 
and bent in accordance with the present invention (FIG. 16). A comparison 
of FIGS. 15 and 16 illustrates the significant improvement in the 
continuity of the indium-tin oxide coating when the present invention is 
employed. The film shown in FIG. 15 would have a craze rating of 
approximately 9, whereas the film shown in FIG. 16 would have a craze 
rating of about 1. 
CONCLUSION 
In conclusion, it can be seen that the process of the present invention 
surprisingly broadens the window of operability of the underlying basic 
method disclosed and claimed in my prior U.S. Pat. No. 4,490,227. In 
achieving substoichiometry at the moment of bend by applying a layer of 
carbonaceous material over the indium-tin oxide film, one greatly 
increases important process latitude, e.g. in the areas of the initial 
light transmittance (T.sub.o) to which the indium-tin oxide film is 
applied and the temperature at which the coated glass is bent. Yet just as 
with the two specific preferred embodiment processes disclosed in my U.S. 
Pat. No. 4,490,227, I achieve with the present invention a windshield 
which will meet light transmission standards (70% minimum) and which will 
have an acceptably low sheet resistivity such that it can be used in 
conjunction with a readily available 60 volt auxiliary power source. Most 
importantly, the glass can be coated flat and subsequently bent without 
causing undue crazing in the indium-tin oxide film. 
Another surprising and unexpected advantage of the present invention is 
that by coating the indium-tin oxide film with a layer of carbonaceous 
material, one substantially increases the radiation emissivity of the 
surface of the coated glass. Normal bare glass has an emissivity of about 
0.88. When the glass is coated with ITO and fired, emissivity is lowered 
to 0.3 or 0.4. When one places the mating sheet of glass against the 
indium-tin oxide coated surface of the first sheet of glass, the 
difference in emissivity could lead to some problems in bending, for 
example, (1) slower bend rates (production) and/or (2) optical problems. 
The carbonaceous layer increases the emissivity of the indium-tin oxide 
coated surface to levels comparable to that of the uncoated glass. 
Additionally, the carbon promotes thermal conduction between the plies. 
Precise use of the carbonaceous layer can restore the heat transfer 
between the plies to nearly the level that occurs in normal windshield 
bending, thus minimizing changes to production rate or windshield optical 
quality in this process. 
Of course, it is understood that the above is merely a preferred embodiment 
of the invention and that various changes and alterations can be made 
without departing from the spirit and broader aspects thereof as set forth 
in the appended claims.