Process for applying ceramic paint to a surface of a glass sheet

Layers of ceramic paint are applied to a surface of a glass sheet in a process wherein each layer of paint is dried by microwave energy before a successive layer of ceramic paint is applied thereover.

FIELD OF THE INVENTION 
The present invention is directed to a process for applying ceramic paint 
to a surface of a glass sheet. More particularly, the invention 
contemplates a process for applying successive layers of ceramic paint to 
the glass sheet, including a method for quickly and efficiently drying 
each layer before the subsequent layer is deposited, without heating the 
associated glass sheet. 
BACKGROUND OF THE INVENTION 
Ceramic paint compositions are generally known to those ordinarily skilled 
in the art of applying ceramic paints to automotive and architectural 
glazings. Such ceramic paints are used, for example, to form opaque 
borders around the peripheral marginal surfaces of glazings used as 
windshields, sidelites, and backlites of motor vehicles. Frequently such 
paints are applied in successive layers in order to increase the thickness 
of the ultimately fired ceramic paint coating; provide an outer, exposed 
layer which will not adhere to the forming surface of a press bending 
mold; provide multi-colored decorative surface treatments; etc. 
Ceramic paints or pastes generally contain a mixture of metal oxides which 
together act as a coloring agent. The metal oxides are non-reactive with 
one another, as well as non-reactive with other compounds contained in the 
ceramic paint or the glass to which the ceramic paint is applied, at 
temperatures up to about 1,300.degree. F. The mixture of metal oxides may 
be adjusted so as to achieve a specific color for the ultimately produced 
fired ceramic paint. For example, the opaque peripheral marginal paint 
bands fired onto automotive glazings are generally black. 
Additionally, the ceramic paints generally known in the art contain one or 
more low melting glass frits, which melt at temperatures below 
1,300.degree. F. These low melting glass frits are the materials which 
ultimately fuse the ceramic paint together, and to the glass sheet, to 
insure that the ceramic paint remains affixed to the glass sheet after 
being cooled to room temperature. 
A vehicle is also generally mixed with the metal oxides and glass frit, to 
allow the ceramic paint to be applied to the surface of the glass sheet by 
conventional paint application processes. Generally, organic vehicles, 
e.g., pine oil, mineral oils, low molecular weight petroleum fractions, 
and the like, are used to allow the ceramic paint to be applied to the 
glass surface by brushing, spreading, or screen printing. 
When multiple layers of ceramic paint are to be applied to the surface of a 
glass sheet, the process for doing so is known to be time consuming and 
wasteful of energy. A previously applied layer of ceramic paint must be 
dried before a successive layer of ceramic paint may be deposited 
thereover. Generally this drying process is accomplished by heating the 
glass sheet having the previously applied layer of ceramic paint thereon. 
The thermal energy, which is generally supplied by infrared radiant 
sources or direct fired heaters, is effective to both heat the glass sheet 
and volatize the organic vehicle from the paint, causing it to dry. 
Thereafter, the glass sheet having the dried layer of ceramic paint 
thereon must be cooled before a successive layer of ceramic paint may be 
deposited thereover. This cooling process generally involves the use of 
air blowers to remove heat from both the ceramic paint and the glass sheet 
by convective heat transfer. The finally applied layer of ceramic paint is 
generally not subjected to the known heating-then-cooling process. 
Instead, the glass sheet having the layer or layers of dried ceramic paint 
and the outer, exposed layer of undried ceramic paint thereon is conveyed 
through a heating chamber where the layers of ceramic paint are 
devitrified and "fired" onto the surface of the glass sheet. 
Clearly, the steps of heating-then-cooling the glass sheet between 
successive applications of ceramic paint, in order to dry the previously 
applied layer of ceramic paint, add significant costs and time to the 
manufacture of glass sheets having multiple layers of ceramic paint 
thereon. 
By way of example, U.S. Pat. No. 4,770,685 to Boaz discloses a method for 
manufacturing a formed glass sheet with paint thereon, wherein a layer of 
ceramic paint having a non-stick characteristic is deposited over an 
initial layer of ceramic paint which does not possess such a quality. It 
is disclosed that the initial layer of ceramic paint is dried by heating 
the glass sheet before the second non-stick layer of ceramic paint is 
applied thereover. This procedure, of course, requires energy and time for 
heating-then-cooling the glass sheet between the applications of the first 
and second layers of ceramic paint. 
The need to dry ceramic paint layers exists, of course, only when dealing 
with ceramic paints having liquid vehicles which must be volatilized to 
solidify the paint. In an effort to eliminate the heating-then-cooling 
process required for the application of successive ceramic paint layers, 
other more expensive ceramic paints have been developed. For example, U.S. 
Pat. No. 4,684,389 to Boaz discloses a UV-based ceramic paint composition. 
Such a paint composition applied to the surface of a glass sheet may be 
"cured" by ultraviolet radiation prior to the application of a second 
layer of ceramic paint. It is also known to apply a hot-melt ceramic paint 
to the surface of a glass sheet. Such a paint "freezes" to the surface of 
the glass sheet, allowing the immediate application of a successive layer 
of ceramic paint thereover. Thus, efforts to eliminate the 
heating-then-cooling steps required for the application of successive 
layers of ceramic paints to a glass sheet have been directed toward 
eliminating the use of a liquid volatile vehicle in the ceramic paint 
compositions. Despite the knowledge of methods for quickly heating liquid 
materials, a process for quickly heating ceramic paint compositions 
containing volatile liquid vehicles, which process does not require a 
subsequent cooling step to cool the hot glass sheet, has not heretofore 
been discovered. 
U.S. Pat. Nos. 3,472,200 to Gerling and 4,765,773 to Hopkins disclose 
apparatus and a method for applying and quickly drying highway-marking 
paint using microwave energy, so that motor vehicles may drive over the 
painted surface immediately after it is formed. The patents do not suggest 
the application of multiple layers of paint, nor the use of such a process 
in conjunction with ceramic paint layers applied to glass sheets. 
It must be noted that the prior art referred to hereinabove has been 
collected and examined only in light of the present invention as a guide. 
It is not to be inferred that such diverse art would otherwise be 
assembled absent the motivation provided by the present invention, nor 
that the prior art when considered in combination suggests the present 
invention absent the teachings herein. 
It would be desireable to develop a process for applying successive layers 
of ceramic paint to a surface of a glass sheet, wherein the time and 
energy required for drying the paint layers between successive 
applications thereof would be minimized. 
SUMMARY OF THE INVENTION 
Accordant with the present invention, a process for applying a fired 
ceramic paint coating to a surface of a glass sheet has surprisingly been 
discovered. The inventive process comprises the steps of applying a layer 
of ceramic paint to at least a portion of the surface of the glass sheet; 
heating the ceramic paint without heating the glass sheet, by directing 
microwave energy onto the ceramic paint, to dry the ceramic paint; cooling 
the ceramic paint by conducting heat energy from the ceramic paint to the 
glass sheet; applying a successive layer of ceramic paint to at least a 
portion of the previously dried layer of ceramic paint; and heating the 
glass sheet having the ceramic paint coating thereon, to devitrify and 
simultaneously fuse the ceramic paint together and to the glass sheet. 
Optionally, steps B, C, and D may be repeated any number of times 
sufficient to prepare a desired ceramic paint coating comprising multiple 
layers of ceramic paint. 
The ceramic paint for use according to the present invention must contain a 
vehicle component which is responsive to microwave energy. Suitable 
microwave responsive vehicle components include Polar liquids such as 
water. 
The present process is particularly useful for preparing automotive and 
architectural glazings having opaque concealment bands on the peripheral 
marginal surfaces thereof.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The present invention is directed to a process for applying ceramic paint 
to a surface of a glass sheet, wherein the ceramic paint is deposited as 
successive layers and each layer is dried by microwave energy before the 
next layer is deposited thereover. The glass sheet having the layers of 
ceramic paint thereon is thereafter heated to fire the ceramic paint onto 
the glass sheet. This process eliminates the steps of heating the glass 
sheet to drive off the ceramic paint volatile components and thereafter 
cooling the glass sheet so that the next layer of ceramic paint may be 
applied. The glass sheet possesses a high thermal inertia due to the high 
heat capacity of glass. Therefore, the conventional process of 
heating-then-cooling the glass sheet would require a significant amount of 
energy and a substantial period of time, in contrast to the drying process 
of the present invention. 
The ceramic paints suitable for use according to the present invention 
include any ceramic paints which are responsive to microwave energy. By 
the term "responsive to microwave energy" is meant that the temperature of 
the paint increases upon exposure to microwave energy. Such paints 
generally comprise glass frit, a pigment, and a microwave responsive 
vehicle. 
Suitable glass frits according to the present invention include one or more 
glass frits prepared from conventional lead borosilicate glass 
compositions that are high in lead content. Alternatively, the frit may 
comprise a combination of metal oxides such as those selected from the 
oxides of zinc, bismuth, lead, titanium, zirconium, sodium, boron, 
lithium, potassium, calcium, aluminum, tin, vanadium, molybdenum, 
magnesium, iron, manganese, and the like. The frit is prepared by melting 
the frit batch ingredients at temperatures from about 1650.degree. F. to 
about 2,900.degree. F., and then quenching the molten frit composition 
either with a stream of water or by pouring the melt between cooled metal 
rolls rotating in opposite direction. The resulting chunks of frit are 
then ground into fine particles so as to pass through a 325 U.S. Standard 
Sieve mesh screen. A lead borosilicate glass frit high in lead content is 
preferred because it is readily available and relatively inexpensive. The 
frit is generally added to the ceramic paint composition at a 
concentration from about 20% to about 60% by weight. Preferably, the 
concentration of glass frit in the ceramic paint composition is from about 
30% to about 50% by weight. 
Pigments for use according to the present invention generally comprises 
mixtures of metal oxides which together act as a coloring agent for the 
ceramic paint. These metal oxides include, but are not necessarily limited 
to, oxides of chromium, cobalt, nickel, manganese, iron, or copper. 
Mixtures of these metal oxides form various colors, as is well known in 
the art of glass making. A particularly useful mixture of metal oxides for 
applying a ceramic paint concealment band to a peripheral marginal surface 
of an automotive glazing is conventionally known in the art as black oxide 
powder. The metal oxide pigments are non-reactive with one another, as 
well as non-reactive with other compounds contained in the ceramic paint 
or the glass to which the ceramic paint is applied, up to temperatures of 
about 1,300.degree. F. The concentration of pigment in the ceramic paint 
composition generally may be from about 5% to about 30% by weight. 
Preferably, the concentration is from about 10% to about 20% by weight of 
the ceramic paint composition. 
Vehicles suitable for use in the ceramic paint compositions of the present 
invention generally comprise organic materials which allow the ceramic 
paint composition to flow at application temperatures. Thus, the ceramic 
paint composition may be applied to the surface of the glass sheet by any 
conventional paint application process such as, for example, brushing, 
spreading, or screen printing. Examples of useful vehicles include pine 
oil, vegetable oils, mineral oils, low molecular weight petroleum 
fractions, vinyl resins, thermoplastic resins, polyolefins, solvents, and 
the like. Additionally, the vehicle must contain a material which is 
responsive to microwave energy. These include liquids containing polar 
molecules, wherein the positive and negative electrical charges are 
permanently separated, as opposed to nonpolar molecules in which the 
charges coincide. Such polar liquids include, but are not necessarily 
limited to water, alcohols such as, for example, methanol, ethanol, 
n-propanol, isopropanol, and n-butanol, isophorone, benzophenone, 
x-chloronaphthalene, diphenylsulfone, 2-phenylphenol, p-methoxyphenol, 
2-methoxyhaphthalene, ethyl-4-hydroxybenzoate, N-cyclohexyl-2-pyrrolidone, 
pentafluorophenol, dimethylphthalate, and phenylbenzoate, as well as 
mixtures thereof. A preferred polar liquid is water. The polar liquid may 
be included in the paint composition at a concentration from about 0.1% to 
about 10% by weight. Preferably, the polar liquid comprises from about 1% 
to about 5% by weight of the ceramic paint composition. 
Conventional adjuvants such as, for example, fillers, thermal stabilizers, 
ultraviolet energy absorbers, and the like, may be included in the ceramic 
paint composition at a concentration generally not exceeding about 50% by 
weight. Also, a metal powder may be added to the ceramic paint composition 
to prevent adhesion of the paint to the surface of a glass sheet bending 
mold in a subsequent forming operation or to provide an electrically 
conductive coating on the glass sheet. Such metal powders include, but are 
not necessarily limited to zinc, silver, aluminum, tin, copper, iron, 
lead, titanium, zirconium, and the like, as well as mixtures and alloys 
thereof. The use of metal powders in ceramic paint compositions is more 
fully set forth in U.S. Pat. Nos. 4,684,389 to Boaz, which is incorporated 
herein in its entirety by reference thereto. 
The glass sheets upon which the layers of ceramic paint are to be deposited 
may be prepared from any of the types of glass generally known in the art 
of glass making. Typical glass sheets contemplated for use according to 
the present invention are soda-lime-silica automotive and architectural 
glazings, generally produced by the well-known float glass process. 
However, ceramic paint may be applied to the surface of any type of glass 
sheet by the process of the present invention. 
In operation, a first layer of ceramic paint is applied to at least a 
portion of the surface of the glass sheet by a conventional paint 
application method, e.g., screen printing wherein the ceramic paint is 
spread across the screen by a squeegee to force the paint through the 
screen pattern onto the glass sheet. 
The first layer of ceramic paint is thereafter dried utilizing microwave 
energy directed onto the ceramic paint layer. This may be achieved by 
conveying the glass sheet having the first layer of ceramic paint thereon 
past a microwave radiation source or waveguide. The ceramic paint is 
responsive to the microwave energy, and increases in temperature to a 
point where at least a portion of the vehicle is volatilized, thereby 
causing the paint to dry. Typically, the microwave energy causes the 
ceramic paint to rise to a temperature from about 200.degree. F. to about 
350.degree. F. The paint solidifies and adheres to the surface of the 
glass sheet to give a sharp image which is resistant to smudging during 
subsequent handling and paint application steps. Microwave energy causes 
the polar molecules contained in the ceramic paint vehicle to rotate 
millions of times per second, resulting in internal friction which heats 
the ceramic paint. A particularly useful microwave energy source is the 
Macrowave Model 7500 50 kw waveguide manufactured by Radio Frequency Co. 
of Millis, Mass. Thereafter, the ceramic paint immediately cools by the 
conductive transfer of heat energy from the hot ceramic paint to the 
relatively cool glass sheet. 
This method for drying the first layer of ceramic paint is particularly 
advantageous over the prior art, because the glass sheet is unaffected by 
the microwave energy. Moreover, the glass sheet actually performs as a 
heat-sink, to cool the ceramic paint when the application of microwave 
energy to the ceramic paint ceases. Therefore, a successive layer of paint 
may immediately be applied to the dried first layer of ceramic paint, 
although a time delay between the drying of the first layer and the 
application of the successive layer is also contemplated by the present 
invention. 
Following the step of drying the first layer of ceramic paint, a subsequent 
layer of the same or a different ceramic paint may be applied over at 
least a portion of the first layer of ceramic paint by a conventional 
paint application method such as, for example, screen printing. Such a 
second layer of ceramic paint may inter alia be required to increase the 
thickness (over that achievable using a single screen printing operation) 
of the ultimately fired ceramic paint coating, or to provide an outer, 
exposed layer of ceramic paint which contains additives that prevent 
adhesion of the paint to a press bending mold surface during a subsequent 
glass sheet forming operation, or to form a busbar or other electrically 
conductive layer over a previously applied concealment band for an 
automotive glazing, or simply to produce a multi-colored decorative 
surface treatment for the glass sheet. 
Finally, the glass sheet having the ceramic paint coating comprising the 
dried first layer of ceramic paint and the undried second layer of ceramic 
paint may be heated to a temperature and for a time sufficient to cause 
the ceramic paint to devitrify and simultaneously fuse together and to the 
glass sheet. The temperature required for such a process is generally from 
about 900.degree. F. to about 1,100.degree. F. The time required for 
devitrification and fusion is generally from about 1 to about 5 minutes. 
Preferably, the time is from about 2 to about 3 minutes. During this 
process step, the ceramic paint is heated to a temperature so as to 
devitrify the glass frit, converting it to a crystalline or 
semicrystalline phase characterized by a glassy matrix. Simultaneously, 
the glass frit fuses together and to the glass sheet. The temperature 
required for devitrifying and fusing the ceramic paint is lower than the 
softening point of the glass sheet, yet is high enough to allow for 
bakeout or volitization of the vehicle contained in the ceramic paint. 
Generally, the heating process is accomplished by passing the glass sheet 
through a heating chamber, tempering furnace, or glass sheet forming lehr. 
As is readily apparent from the nature of the inventive process, any number 
of successive layers of ceramic paint may be applied then dried before a 
final layer of ceramic paint is applied, to form the desired ceramic paint 
coating. Thus, for example, a ceramic paint coating comprising three 
layers of paint may be prepared by applying then microwave drying the 
first and second layers, and finally applying the third layer of ceramic 
paint. 
The process for applying ceramic paint to the surface of the glass sheet 
according to the present invention is particularly useful for preparing 
automotive glazings having a concealment band positioned on a peripheral 
marginal surface thereof. Such concealment bands are well known in the art 
of automotive glazings as useful for preventing the degradation of 
adhesives used to mount the glazing in a vehicle opening as a result of 
exposure to solar radiation, and for concealing attachment hardware and 
structural components which lie below the edges of the glazing. The band 
generally extends to the edges of the glazing, and has a width sufficient 
to conceal the underlying adhesive and structural components, but which is 
sufficiently narrow to provide maximum vision to occupants of the vehicle. 
It is known to apply such a band to the surface of an automotive glazing 
by silk screening the layers of ceramic paint onto the glazing and 
thereafter firing the ceramic paint. Such a method is more fully set forth 
in U.S. Pat. No. 4,770,685 to Boaz, which is incorporated herein in its 
entirety by references thereto. The inventive process may likewise be used 
to apply an electrically conductive ceramic paint over a portion of a 
previously applied concealment band. Clearly, other ceramic paint patterns 
may be prepared by applying various ceramic paint coatings to glass 
surfaces, depending upon the ultimate purpose for the fired ceramic paint 
coating. 
From the foregoing description, one skilled in the art can easily ascertain 
the essential characteristics of this invention, and without departing 
from its spirit and scope, can make various changes and modifications in 
the invention to adapt it to various usages and conditions.