U.S. patent number RE37,328 [Application Number 07/567,461] was granted by the patent office on 2001-08-14 for transparent infrared absorbing glass and method of making.
This patent grant is currently assigned to PPG Industries Ohio, Inc.. Invention is credited to George A. Pecoraro, Larry J. Shelestak.
United States Patent |
RE37,328 |
Pecoraro , et al. |
August 14, 2001 |
Transparent infrared absorbing glass and method of making
Abstract
A glass product having high visible transmittance, low infrared
transmittance, and, optionally, reduced ultraviolet transmittance
is produced in a manner compatible with continuous, commercial
manufacture of flat glass by employing a moderate amount of iron in
the glass composition and controlling reducing conditions to
maintain a relatively large portion of the iron in the ferrous
state.
Inventors: |
Pecoraro; George A. (Lower
Burrell, PA), Shelestak; Larry J. (West Deer Township,
PA) |
Assignee: |
PPG Industries Ohio, Inc.
(Cleveland, OH)
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Family
ID: |
22076423 |
Appl.
No.: |
07/567,461 |
Filed: |
July 27, 1990 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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933800 |
Nov 23, 1992 |
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Reissue of: |
067504 |
Jun 29, 1987 |
04792536 |
Dec 20, 1988 |
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Current U.S.
Class: |
501/70; 501/64;
65/135.9; 65/32.1 |
Current CPC
Class: |
C03B
5/04 (20130101); C03C 4/082 (20130101); C03C
4/02 (20130101); C03C 3/087 (20130101); C03B
5/2252 (20130101); C03B 5/225 (20130101); C03B
5/1875 (20130101); C03B 5/12 (20130101); C03B
3/02 (20130101); Y02P 40/57 (20151101); Y02P
40/50 (20151101) |
Current International
Class: |
C03B
5/225 (20060101); C03B 5/04 (20060101); C03B
5/00 (20060101); C03B 3/02 (20060101); C03B
5/12 (20060101); C03C 3/076 (20060101); C03C
3/087 (20060101); C03B 3/00 (20060101); C03B
5/187 (20060101); C03C 003/087 () |
Field of
Search: |
;501/70,71
;65/32.1,135.9 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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602115546 |
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Mar 1985 |
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JP |
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611336936 |
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Jun 1986 |
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JP |
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734152 |
|
May 1980 |
|
SU |
|
948912 |
|
Aug 1982 |
|
SU |
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Other References
W A. Weyl, "Colored Glasses," pp. 89-120 and 238-281.* .
C. R. Bamford, "Color Generation and Control in Glass," 1977, pp.
35, 36, 78, 79, 106-109, 142-146.* .
G. F. Brewster and N. J. Kreidl, "The Color of Iron--Containing
Glasses of Varying Composition," J. Soc. Glass Techn., 1950, pp.
332-406.* .
J. W. Forrest, N. J. Kreidl, Tyler G. Pett, "Color Variations in
Glasses Containing Iron," J. Optical Soc. of Amer., vol. 38, No. 6,
Jun. 1948, pp. 554-560.* .
N. E. Densem and W. E. S. Turner, "The Equilibrium between Ferrous
and Ferric Oxides in Glasses," J. Soc. Glass Techn., 1937, pp.
372-389..
|
Primary Examiner: Group; Karl
Attorney, Agent or Firm: Lepiane; Donald C.
Parent Case Text
.Iadd.This application is a RE of Ser. No. 07/067,504 filed Jun.
29, 1987, U.S. Pat. No. 4,792,536, and a Div. of Ser. No.
07/933,800 filed Nov. 23, 1992, abandoned..Iaddend.
Claims
We claim:
1. A method of manufacturing soda-lime-silica flat glass in a
continuous process including feeding raw materials to a melting
operation that includes separate liquefying and refining stages,
passing from the melting operation to a flat glass forming
operation a continuous stream of molten glass having at least 0.45
percent by weight iron expressed as Fe.sub.2 O.sub.3, forming the
glass into a flat glass product in the forming operation, melting
the raw materials in a thin layer in the liquefying stage while
controlling oxidation-reduction conditions in that stage and in
subsequent stages so as to yield a glass having at least 35 percent
of the iron in the ferrous state expressed as FeO and which when
formed into a flat glass product of suitable thickness exhibits the
combination of luminous transmittance of at least 65 percent and
infrared transmittance of no more than 15 percent.
2. The method of claim 1 wherein the glass is provided with a total
iron content less than 0.65 percent expressed as Fe.sub.2
O.sub.3.
3. The method of claim 1 wherein the oxidation--reduction
conditions are controlled so as to provide at least 40 percent of
the iron in the ferrous state.
4. The method of claim 1 wherein the sulfur content of the product
glass is less than 0.02 percent by weight expressed as
SO.sub.3.
5. The method of claim 4 wherein the ratio of iron in the ferrous
state to total iron in the product glass is at least 50
percent.
6. The method of claim 5 wherein the sulfur content of the product
glass is less than 0.01 percent by weight expressed as
SO.sub.3.
7. The method of claim 1 wherein the oxidation-reduction conditions
are controlled in the melting operation to yield a glass product
having at least 0.23 percent by weight ferrous iron expressed as
FeO.
8. The method of claim 7 wherein fuel-rich combustion is provided
in the melting operation to provide reducing conditions.
9. The method of claim 7 wherein reducing conditions are provided
in the melting operation by including carbonaceous material in the
raw materials being fed to the melting operation.
10. The method of claim 1 wherein the raw materials being fed to
the melting operation include an iron source in an amount
sufficient to yield the desired iron content of the glass
product.
11. The method of claim 10 wherein the iron source included in the
raw materials includes a majority of the iron in the ferrous
stage.
12. The method of claim 1 wherein a source of iron is added to
molten glass downstream from the melting operation.
13. The method of claim 12 wherein a substantial portion of the
added iron is in a reduced state relative to Fe.sub.2 O.sub.3.
14. The method of claim 12 wherein a portion of the product glass
iron content is provided by an iron source included in the raw
materials being fed to the melting operation.
15. The method of claim 1 wherein the molten glass is brought into
contact with molten metal prior to the forming operation.
16. The method of claim 15 wherein the molten glass is stirred
while in contact with the molten metal.
17. The method of claim 1 wherein the glass is provided with a
total iron content less than 0.60 percent expressed as Fe.sub.2
O.sub.3.
18. The method of claim 15 wherein the forming operation includes
supporting molten glass on a pool of molten metal.
19. The method of claim 16 wherein the refining stage includes
subjecting molten glass to subatmospheric pressure.
20. The method of claim 17 wherein the sulfur content of the glass
is reduced below 0.02 percent expressed as SO.sub.3 by the refining
stage.
21. The method of claim 7 wherein the oxidation-reduction
conditions are controlled in the melting operation to yield a glass
product having at least 0.25 percent by weight ferrous iron
expressed as FeO.
22. The method of claim 1 wherein the glass product has a
composition consisting essentially of, on a weight basis: 66 to 75%
SiO.sub.2, 12-20% Na.sub.2 O, 7-12% CaO, 0-5% MgO, 0-4% Al.sub.2
O.sub.3, 0-3% K.sub.2 O, 0-1% Fe.sub.2 O.sub.3, and 0-1.5% total of
CeO.sub.2, TiO.sub.2, V.sub.2 O.sub.5 or MoO.sub.3.
23. A soda-lime-silica glass article having a composition
consisting essentially of, on a weight basis: 66 to 75% SiO.sub.2,
12-20% Na.sub.2 O, 7-12% CaO, 0-5% MgO, 0-4% Al.sub.2 O.sub.3, 0-3%
K.sub.2 O, 0.45-1% Fe.sub.2 O.sub.3, 0-1.5% total of CeO.sub.2,
TiO.sub.2, V.sub.2 O.sub.5 or MoO.sub.3, at least 50 percent of the
iron being in the ferrous state expressed as FeO, less than 0.02
percent by weight sulfur expressed as SO.sub.3, and exhibiting
luminous transmittance of at least 65 percent and total solar
infrared transmittance of no more than 15 percent at a selected
thickness.
24. The article of claim 23 wherein the ferrous iron content
expressed as FeO, is greater than 0.270 percent by weight of the
total glass composition.
25. The article of claim 24 wherein the ferrous iron content,
expressed as FeO, is less than 0.300 percent by weight of the total
glass composition.
26. The article of claim 23 wherein the sulfur content, expressed
as SO.sub.3, is less than 0.01% of the total glass composition.
27. The article of claim 23 wherein the total iron content,
expressed as Fe.sub.2 O.sub.3, is less than 0.65 percent by weight
of the total glass composition.
28. The article of claim 23 comprising a flat glass sheet.
29. The article of claim 28 wherein the sheet is 2 to 6 millimeters
thick.
30. The article of claim 28 wherein the sheet has traces of tin
oxide in a surface portion.
31. The method of claim 1 wherein the glass is formed to a
thickness of 2 to 6 millimeters.
32. The method of claim 31 wherein the transmittance properties are
exhibited at a product thickness of 5 millimeters..Iadd.
33. A soda-lime-silica glass article having a composition
consisting essentially of, on a weight basis: 66-75% SiO.sub.2,
12-20% Na.sub.2 O, 7-12% CaO, 0-5% MgO, 0-4% Al.sub.2 O.sub.3, 0-3%
K.sub.2 O, 0-1.5% total of CeO.sub.2, TiO.sub.2, V.sub.2 O.sub.5,
or MoO.sub.3, optionally residual amounts of refining aids, and
less than about 0.55% total iron oxide expressed as Fe.sub.2
O.sub.3, at least 50 percent of the iron being in the ferrous state
expressed as FeO, the iron being present in sufficient quantities
to limit total solar infrared transmittance to no more than 15
percent while luminous transmittance is at least 65 percent at a
selected thickness..Iaddend..Iadd.
34. A soda-lime-silica glass article having a composition
consisting essentially of, on a weight basis: 66-75% SiO.sub.2,
12-20% Na.sub.2 O, 7-12% CaO, 0-5% MgO, 0-4% Al.sub.2 O.sub.3, 0-3%
K.sub.2 O, 0-1.5% total CeO.sub.2, TiO.sub.2, V.sub.2 O.sub.5, or
MoO.sub.3 and less than about 0.55% total iron expressed as
Fe.sub.2 O.sub.3, at least 50 percent of the iron being in the
ferrous state expressed as FeO, the iron being present in
sufficient quantities to limit total solar infrared transmittance
to no more than 15 percent while the luminous transmittance is at
least 65 percent at a selected thickness..Iaddend.
Description
BACKGROUND OF THE INVENTION
This invention relates to the production of glass that has low
infrared energy transmittance and relatively high visible light
transmittance. Such a glass is useful in glazing vision openings
for the sake of reducing air conditioning requirements without
unduly impairing vision, and is particularly applicable for vehicle
windows.
The passage of infrared radiation through glass windows is a major
cause of heat buildup in enclosed spaces such as automobiles. The
accumulation of heat is, in turn, undesirable because of the burden
placed on the air conditioning system, or because of the discomfort
caused in occupants with or without air conditioning. The
conventional approach has been to use "tinted" glass in such
applications, which is usually darker green in color than ordinary
"clear" glass due to a larger amount of iron included in the glass
during melting. The iron renders the glass more absorptive of
radiation in the infrared range of wavelengths (greater than 700
nanometers) and also reduces the visible light (luminous)
transmittance. Conventional soda-lime-silica flat glass products
tinted with iron typically transmit about 25 to 30 percent of the
infrared radiation incident on a 5 millimeter thick sheet, and
recently some products adapted to reduce infrared transmittance
transmit less, approaching 15 percent transmittance levels. It
would be desirable to reduce infrared transmittance levels even
further, below the 15 percent level, without unduly decreasing
luminous transmittance.
It is known in the art that infrared transmittance can be further
reduced by including larger amounts of iron in the glass, but
luminous transmittance is also reduced below levels considered
desirable for adequate vision or for aesthetic purposes. It would
be preferred to maintain luminous transmittance about 65 percent,
preferably at least 70 percent. It is known that iron in the
ferrous state (Fe.sup.+2) is largely responsible for absorption of
infrared energy in glass (W. A. Weyl, "Coloured Glass," page 91).
Therefore, attaining lower infrared transmittance without
substantially reducing luminous transmittance would theoretically
be possible by maintaining reducing conditions during the
glassmaking process so as to increase the amount of iron in the
ferrous state for a given total iron concentration. Unfortunately,
such an approach has significant drawbacks for commercial
production of glass.
The automotive and architectural glass markets, to which infrared
absorbing glass is directed, require mass production on a large
scale, with the necessity of melting, refining, and forming the
glass on a continuous basis. Most large scale production of glass
is carried out in overhead fired, tank type, continuous melting
furnaces. When the glass is a reduced condition so as to enhance
the proportion of iron in the ferrous state, the glass becomes so
absorptive that penetration of heat into the body of molten glass
is rendered very difficult. The result is substantially reduced
thermal efficiency, and at higher ferrous levels adequate melting
and refining becomes impractical or impossible in a conventional
furnace. A typical tinted glass with a ferrous to total iron ratio
of about 25 percent (ferrous iron expressed as FeO and total iron
expressed as Fe.sub.2 O.sub.3) strains the ability of a commercial
furnace to produce adequately melted and refined glass. Ferrous to
total iron ratios in excess of 35 percent would heretofore have
been considered unfeasible for continuous commercial flat glass
production.
Another drawback for producing reduced glass on a continuous
commercial basis is the conventional presence of substantial
amounts of sulfur in soda-lime-silica glass, especially flat glass.
Sulfur, typically included in the batch materials as a sulfate and
analyzed in the glass as SO.sub.3, is present as a melting and
refining aid. Although much of the sulfur volatilizes during
melting and refining, conventional commercially produced flat glass
has a residual SO.sub.3 content greater than 0.1 percent by weight
of the glass, usually about 0.2 percent. In a glass composition
that includes iron and sulfur, providing reducing conditions is
known to create amber coloration which substantially lowers the
luminous transmittance of the glass. In "Colour Generation and
Control in Glass" by C. R. Bamford (Elsevier, 1977), on page 106,
it is stated that "A rich golden-brown or amber colour is produced
by the combination of sulphur and iron oxide in a soda-lime-silica
glass melted under strongly reducing conditions." It is further
stated on page 107 that "Onset of the amber colouration occurs at a
ferrous value of 50 percent . . . " Therefore, in commercial flat
glass manufacturing operations, the reliance on sulfur as a melting
and refining aid has limited the degree to which the ferrous
concentration of the glass could be increased to lower the infrared
transmittance without unacceptably reducing the luminous
transmittance. It would be desirable to be able to produce flat
glass commercially with a ferrous content greater than 50 percent
of the total iron content so as to minimize the total amount of
iron needed to yield the desired infrared absorption.
Much of the published information on infrared absorbing glass is
based on small scale, discontinuous, laboratory melts in which the
commercial scale problems of achieving adequate melting and
refining are usually not addressed. Small scale melts usually do
not entail problems such as penetration of heat into a substantial
depth of melt, limited residence time, homogenization of impurities
from mineral batch materials or vessel erosion, and the presence of
refining aids. This is because a batch-wise melting of a crucible
or not of glass may be provided with indefinite melting times, may
involve non-contaminating vessels of a material such as platinum,
and may utilize purified grades of chemical compounds as raw
materials. In the past, pot melts of glass having a desirable
combination of infrared and luminous transmittance properties were
produced in sufficient quantities to be cast, rolled, ground, and
polished to produce flat glass plates that were marketed. Some of
these melts had ferrous to total iron ratios between 40 percent and
50 percent. These pot melted glass compositions required long
melting and refining times, were difficult to refine in spite of
the user of sulfur refining aid, and were considered unsuitable for
continuous flat glass production.
Japanese patent publication No. 60215546 (1985) has as its object a
transparent, infrared absorbing glass wherein substantial amounts
of barium oxide are included in the glass to shift the absorption
peak toward the infrared wavelengths. However, barium oxide is a
costly batch material, and it would be desirable to avoid the
inconvenience of handling an additional batch constituent.
Furthermore, it is taught that in glass in which sulfur is present
as a refining aid, as would be the case with most commercially
produced flat glass, substantial amount of zinc oxide should be
included to prevent the formation of amber coloration when reducing
conditions are imposed. But glass containing zinc oxide has been
found to be incompatible with the float process, by which most flat
glass is produced. This is due to the volatility of zinc oxide in
the float forming chamber, which not only contaminates the interior
of the chamber, but also leads to amber streaks in the glass where
the zinc oxide content has been depleted.
Incompatibility with the float process also prevents the use of
alternative refining aids such as antimony oxide or arsenic oxide
instead of sulfur. Glass containing those constituents tend to
discolor when brought into contact with molten tin in the float
process. Fluorine and chlorine are also sometimes considered as
alternatives to sulfur, but their volatility and associated
environmental problems discourage their use.
U.S. Pat. No. 3,652,303 (Janakirama Rao) discloses the production
of a reduced, heat absorbing glass by inclusion of tin oxide and
chlorine in the glass. Providing tin as a substantial batch
ingredient significantly increases the cost of the glass, and the
volatility problems of chlorine are a drawback. It would be
desirable if the combination of high visible light transmittance
and low infrared transmittance could be attained with glass
compositions not significantly different from stand, commercial,
soda-lime-silica glass. It also appears that the Janakirama Rao
glass compositions would not lend themselves to manufacture in a
conventional continuous melting furnace.
Reducing the amount of transmitted ultraviolet radiation is also a
desirable feature for the sake of reducing the fading of fabrics
and other interior components. Japanese patent publication No.
61136936 (Asahi Glass) provides titanium dioxide to improve the
ultraviolet blocking properties of glass and asserts that reduction
in infrared transmittance is also achieved. However, the effect of
titanium dioxide on infrared transmittance is less than desired as
evidenced by the total solar energy transmittance of 51 percent
reported in the Japanese patent document for five millimeter thick
glass. Since infrared transmittance is the major component of total
solar energy transmittance, the total solar energy transmittance of
a satisfactory infrared absorbing glass would be less than 50
percent and preferably less than 40 percent. The primary object of
the present invention is to provide low infrared transmittance, but
additionally providing low ultraviolet transmittance would also be
desirable.
SUMMARY OF THE INVENTION
The present invention provides a soda-lime-silica flat glass
composition and a process for its commercial manufacture whereby
infrared transmittance is less than 15 percent, preferably less
than 14 percent, and luminous transmittance is greater than 65
percent, preferably at least 70 percent. Such a glass exhibits a
total solar energy transmittance within the range of 30 to 45
percent, typically between 32 and 40 percent. This combination of
properties has been found to be yielded when glass containing a
moderate amount of iron is produced under relatively reducing
conditions so as to enhance the proportion of iron in the ferrous
state. The total amount of iron is preferably about 0.45 to 0.65
percent of the total glass composition, expressed as Fe.sub.2
O.sub.3. Greater than 35 percent, preferably at least 40 percent,
and most preferably at least 50 percent of the total iron content
(as Fe.sub.2 O.sub.3) is provided in the ferrous state (as FeO) by
maintaining reducing conditions in the melting and/or refining
processes. Effective and efficient melting and refining of such a
reduced glass on a large scale, continuous basis are provided by
employing techniques that avoid a requirement for transmittance of
radiant energy through a substantial thickness of the melt. Thus,
overhead fired, tank type melting furnaces are avoided for purposes
of this aspect of the present invention. Various non-conventional
melting and refining techniques may be suitable for this purpose,
but a preferred arrangement is one in which the melting and
refining process is separated into discrete stages, without a large
volume of melt being retained in any stage. The initial melting
stage is preferably that disclosed in U.S. Pat. No. 4,381,934
(Kunkle et al.). Refining may be carried out in a subsequent stage
by the techniques disclosed in U.S. Pat. Nos. 4,539,034 (Hanneken)
or 4,610,711 (Matesa et al.) for example. A preferred technique for
refining is by means of vacuum as disclosed in U.S. patent
application Ser. No. 894,143 filed Aug. 7, 1986, by G. E. Kunkle et
al., the disclosure of which is hereby incorporated by reference.
Another sequence of discrete melting and refining stages is shown
in U.S. Pat. No. 3,951,635 (Rough). Alternatively, it may be
feasible to adapt electric melting means to melt the reduced glass
of the present invention, either as the sole melting means or as an
adjunct to combustion melting, but electric melting for large scale
flat glass manufacturing is usually disadvantageous
economically.
In another aspect of the invention, the desired combination of low
infrared transmittance with high luminous transmittance is attained
with minimized total iron content, highly reduced glass (above 50
percent ferrous), and very low sulfur content (less than 0.02
percent SO.sub.3, preferably less than 0.01 percent SO.sub.3). The
lower sulfur levels are attained by the avoidance of
sulfur-containing refining aids, preferably avoiding all deliberate
inclusion of sulfur in the batch materials (some may be present as
impurities). To be able to continuously melt and refine without a
chemical refining aid entails selection of processing techniques
other than the conventional use of combustion fired tank type
furnaces that involve deep pools of molten glass. The use of
melting and refining processes that are based on discrete stages
with minimal volumes of molten material being retained are
preferred, as described previously. The use of vacuum to assist the
refining process is particularly desirable in this regard in that
the vacuum actively removes sulfur from the melt, thereby reducing
the sulfur content to a mere trace and further lessening the
tendency of reduced glass to form amber coloration. For the sake of
compatibility with the float process, and for environmental
purposes, alternative chemical refining aids such as arsenic and
antimony are also avoided.
The iron colorant may included in the batch mixture and pass
through the entire melting and refining process, or it may be added
at an intermediate point. A particularly advantageous technique for
adding the colorant to the molten glass after refining and before
forming is disclosed in U.S. patent application Ser. No. 26 filed
Jan. 2, 1987, by G. A. Pecoraro and J. A. Gulotta, the disclosure
of which is hereby incorporated by reference. Adding the colorant
at a downstream location has the advantage of expediting color
changes because of the relatively low volume of residual colored
glass in the system. Whether mixed with the batch or added to the
molten glass, it is advantageous to use iron colorant sources that
include relatively high concentrations of iron in the ferrous state
or elemental iron. An ptional approach that has economic advantages
is to provide a base concentration of iron by including
conventional Fe.sub.2 O.sub.3 containing sources in the batch and
increasing the amount of iron in the ferrous state by adding a
colorant high in FeO at a downstream location.
An optional feature of the invention is the inclusion in the glass
of agents that reduce the ultraviolet transmittance of the glass.
Oxides of cerium, titanium, molybdenum or vanadjium singly or in
combination have the effect of reducing ultraviolet transmittance
through the glass. For the sake of maintaining high luminous
transmittance, cerium oxide is preferred. Cerium oxide content of
the glass at levels of about 0.25 percent to 0.5 percent have been
found to reduce the ultraviolet transmittance to less than 50
percent, preferably less than 40%, at a thickness of 5 millimeters.
Larger amounts of cerium oxide reduce the ultraviolet transmittance
even further, but cerium oxide has the effect of increasing
infrared transmittance, and additional amounts of cerium oxide may
unduly compromise the overall transmittance properties of the
glass, depending upon the requirements of a particular application.
For reducing total solar energy transmittance, the infrared
transmittance is a far more significant factor than is the
ultraviolet transmittance.
THE DRAWINGS
FIG. 1 is a cross-sectional view of a preferred embodiment of
melting and refining apparatus for carrying out the process of the
present invention.
FIG. 2 is a cross-sectional view of a preferred embodiment of a
colorant addition and homogenizing apparatus that may be employed
for carrying out the process of the present invention.
FIG. 3 shows plots of transmittance versus wavelength for several
prior art glasses in comparison with an example of the present
invention.
DETAILED DESCRIPTION
The detailed description will be set forth in conjunction with a
preferred method and apparatus specifically adapted for melting
glass in discrete stages which have been found to be advantageous
for controlling reducing conditions and for accomplishing the tasks
of melting and refining without some of the restraints of
conventional glassmaking furnaces.
Referring to FIG. 1, the overall melting process of the preferred
embodiment consists of three stages: a liquefaction stage 10, a
dissolving stage 11 and a vacuum refining stage 12. Various
arrangements could be employed to initiate the melting in the
liquefaction stage 10, but a highly effective arrangement for
isolating this stage of the process and carrying it out
economically is that disclosed in U.S. Pat. No. 4,381,934 which is
hereby incorporated by reference for details of the preferred
liquefaction stage embodiment. The basic structure of the
liquefaction vessel is a drum 15 which may be fabricated of steel
and has a generally cylindrical sidewall portion, a generally open
top, and a bottom portion that is closed except for a drain outlet.
The drum 15 is mounted for rotation about a substantially vertical
axis, for example, by means of an encircling support ring 16
rotatably carried on a plurality of support wheels 17 and held in
place by a plurality of aligning wheels 18. A substantially
enclosed cavity is formed within the drum 15 by means of a lid
structure 20 which is provided with stationary support by way of a
peripheral frame 21, for example. The lid 20 may take a variety of
forms as may be known to those of skill in the art of refractory
furnace construction. The arrangement depicted in FIG. 1 is an
upwardly domed, sprung arch construction fabricated from a
plurality of refractory blocks, but flat suspended designs could be
employed for the lid. Water-cooled, metallic lid designs may be
used to some advantage.
Heat for liquefying the batch material may be provided by one or
more burners 22 extending through the lid 20. Preferably, a
plurality of burners are arranged around the perimeter of the lid
so as to direct their flames toward a wide area of the material
within the drum. The burners are preferably water cooled to protect
them from the harsh environment within the vessel. Exhaust gases
may escape from the interior of the liquefaction vessel through an
opening 23 in the lid. Advantageously the waste heat in the exhaust
gases may be used to preheat the batch material in a preheating
stage (not shown) such as that disclosed in U.S. Pat. No.
4,519,814.
Batch materials, preferably in a pulverulent state, may be fedinto
the cavity of the liquefying vessel by means of a chute 24, which
in the embodiment depicted extends through the exhaust opening 23.
Details of the feed chute arrangement may be seen in U.S. Pat. No.
4,529,428. The batch chute 24 terminates closely adjacent to the
sidewalls of the drum 10, whereby batch material is deposited onto
the inner sidewall portions of the drum. A layer 25 of the batch
material is retained on the interior walls of the drum 10 aided by
the rotation of the drum and serves as an insulating lining. As
batch material on the surface of the lining 25 is exposed to the
heat within the cavity, it forms a liquefied layer 26 that flows
down the sloped lining to a central drain opening at the bottom of
the vessel. The outlet may be fitted with a ceramic refractory
bushing 27. A stream of liquefied material 28 falls freely from the
liquefaction vessel through an opening 29 leading to the second
stage 11.
In order to provide reducing conditions for the purposes of the
present invention the burner or burners 22 may be operated with an
excess amount of fuel relative to the amount of oxygen being
supplied to each burner. A ratio of 1.9 parts by volume oxygen to
one part by volume natural gas has been found satisfactory for
effecting the desired reduction levels in the glass. Reduction
conditions may be enhanced in the liquefaction stage 10 by
including a reducing agent in the batch mixture being fed to that
stage. The reducing agent may be a finely divided carbon-containing
material such as coal, which may be provided in an amount
constituting about 0.01 to 0.05 percent by weight of the total
batch. Coal in the amount of 0.025 percent was found to be
satisfactory in combination with reducing burner flames.
The second stage may be termed the dissolving vessel because one of
its functions is to complete the dissolution of any unmelted grains
of batch material remaining in the liquefied stream 28 leaving the
liquefaction vessel 10. The liquefied material at that point is
typically only partially melted, including unmelted and grains and
a substantial gaseous phase. In a typical soda-lime-silica melting
process using carbonate batch materials, the gaseous phase is
chiefly comprised of carbon oxides. Nitrogen may also be present
from entrapped air.
The dissolving vessel 11 serves the function of completing the
dissolution of unmelted particles in the liquefied material coming
from the first stage by providing residence time at a location
isolated from the downstream refining stage. Soda-lime-silica glass
batch typically liquefies at a temperature of about 2200.degree. F.
(1200.degree. C.) and enters the dissolving vessel 11 at a
temperature of about 2200.degree. F. (1200.degree. C.) to about
2400.degree. F. (1320.degree. C.) at which temperature residual
unmelted particles usually become dissolved when provided with
sufficient residence time. The dissolving vessel 11 shown is in the
form of a horizontally elongated refractory basin 30 with a
refractory roof 31, with the inlet and outlet at opposite ends
thereof so as to assure adequate residence time. The depth of
molten material in the dissolving vessel may be relatively shallow
in order to discourage recirculation of material.
Although the addition of substantial thermal energy is not
necessary to perform the dissolving step, heating can expedite the
process and thus reduce the size of the dissolving vessel 11. More
significantly, however, it is preferred to heat the material in the
dissolving stage so as to raise its temperature in preparation for
the refining stage to follow. Maximizing the temperature for
refining is advantageous for the sake of reducing glass viscosity
and increasing vapor pressure of included gases. Typically a
temperature of about 2800.degree. F. (1520.degree. C.) is
considered desirable for refining soda-lime-silica glass, but when
vaccum is employed to assist refining, lower peak refining
temperatures may be used without sacrificing product quality. The
amount by which temperatures can be reduced depends upon the degree
of vacuum. Therefore, when refining is to be performed under vacuum
in accordance with the preferred embodiment, the glass temperature
need be raised to no more than 2700.degree. F. (1480.degree. C.),
for example, and optionally no more than 2600.degree. F.
(1430.degree. C.) prior to refining. When the lower range of
pressures disclosed herein are used, the temperature in the
refining vessel need be no higher than 2500.degree. F.
(1370.degree. C.). Peak temperature reductions on this order result
in significantly longer life for refractory vessels as well as
energy savings. The liquefied material entering the dissolving
vessel need be heated only moderately to prepare the molten
material for refining. Combustion heat sources may be used in the
dissolving stage 11, but it has been found that this stage lends
itself well to electric heating, whereby a plurality of electrodes
32 may be provided as shown in FIG. 1 extending horizontally
through the sidewalls. Heat is generated by the resistance of the
melt itself to electric current passing between electrodes in the
technique conventionally employed to electrically melt glass. The
electrodes 32 may be carbon or molybdenum of a type well known to
those of skill in the art. A skimming member 33 may be provided in
the dissolving vessel to prevent any floating material from
approaching the outlet end.
A valve controlling the flow of material from the dissolving stage
11 to the refining stage 12 is comprised of a plunger 35 axially
aligned with a drain tube 36. The shaft 37 of the plunger extends
through the roof 31 of the dissolving vessel so as to permit
control over the gap of the plunger 35 and the tube 36 to thereby
modulate the rate of flow of material into the refining stage.
Although the valve arrangement is preferred, other means could be
provided to control the flow rate of molten material to the
refining stage as are known in the art. An example would be the use
of heating and/or cooling means associated with the drain tube so
as to modulate the viscosity, and thus the flow rate, of the molten
material passing therethrough.
The refining stage 12 preferably consists of a vertically upright
vessel that may be generally cylindrical in configuration having an
interior ceramic refractory lining 40 shrouded in a gas-tight,
water-cooled casing. The refractory may be an
alumina-zirconia-silica type well known in the art. The casing may
include a double walled, cylindrical sidewall member 41 having an
annular water passageway therebetween and circular end coolers 42
and 43. A layer of insulation (not shown) may be provided between
the refractory 40 and the sidewall 41. The valve tube 36 may be
fabricated of a refractory metal such as platinum and is sealingly
fitted into an orifice 44 at the upper end of the refining
vessel.
As the molten material passes through the tube 36 and encounters
the reduced pressure within the refining vessel, gases included in
the melt expand in volume, creating a foam layer 50 resting on a
body of liquid 51. As foam collapses it is incorporated into the
liquid body 51. Subatmospheric pressure may be established within
the refining vessel through a vacuum conduit 52 extending through
the upper portion of the vessel. As used herein, "foaming" can be
considered to be characterized by at least a doubling of the volume
of the molten material. A definition of the state of being
completely foamed is that the bubble membranes are in contact with
other each. If the material is completely foamed, the volume
increase is usually much greater than double. Distributing the
molten material as thin membranes of a foam greatly increases the
surface area exposed to the reduced pressure. Therefore, maximizing
the foaming effect is preferred. It is also preferred that the foam
be exposed to the lowest pressures in the system, which are
encountered at the top of the vessel in the headspace above the
liquid, and therefore exposure is improved by permitting newly
introduced, foamed material to fall through the headspace onto the
top of the foam layer. Also, it is more consistent with the mass
transfer in the vessel to deposit freshly foamed material onto the
top of the foam layer rather than generating foam from the surface
of the liquid pool beneath the foam layer. Depending upon the
pressure in the vacuum space and the volume flow rate of the molten
material entering the refining vessel, the entering stream may
either penetrate through the foam layer as a generally coherent
liquid stream, whereby foaming occurs from the surface of the pool
51, or the stream may foam immediately upon encountering the
reduced pressure. Either mode can be used, but for the reasons
stated above, the latter mode has been found to be more
effective.
The heat content of the molten throughput material entering the
refining vessel 12 can be sufficient to maintain suitable
temperatures within the vessel, but at lower throughput rates
energy losses through the walls may exceed the rate at which energy
is being transported into the vessel by the molten material. In
such a case, it may be desirable to provide heating within the
refining vessel for the sake of avoiding undue temperature
reduction. The amount of heating could be relatively minor since
its purpose would be merely to offset heat losses through the
walls, and may be carried out by conventional electric heating
arrangements whereby electrodes extend radially through the side
wall and electric current is passed between the electrodes through
the glass.
Regardless of the throughput rate, the space above the molten body
51 in the vessel 12 can tend to be cooler than desired because of
the absence of the molten mass and because radiation from the
molten mass is insulated by the foam layer 50. As a result, the
upper portion of the foam layer can become cooler, which in turn
increases the viscosity of the foam and slows the rate at which
gases are expelled. In that case, it has been found advantageous to
provide means for heating the headspace above the liquid and foam.
For this purpose, it has been found feasible to provide a burner 53
and to sustain combustion with the vacuum space. A conduit 54 may
be provided at the upper end of the vacuum vessel whereby a small
amount of water may be sprayed onto the foam periodically. The
water spray has been found to assist the foam to collapse.
In the embodiment depicted, refined molten material is drained from
the bottom of the refining vessel 12 by way of a drain tube 55 of a
refractory metal such as platinum. It would also be feasible to
locate the drain in a side wall of the vessel in the region of the
bottom. The drain tube 55 preferably extends above the surface of
the refractory bottom section 56 within which it is mounted to
prevent any debris from entering the output stream. Leakage around
the tube is prevented by a water cooler 57 under the bottom section
56. The flow rate of molten material from the drain tube 55 is
controlled by a conical throttle member 58 carried at the end of a
stem 59. The stem 59 is associated with mechanical means (not
shown) to adjust the elevation of the throttle member 58 and thus
adjust the gap between the throttle member and the tube 55 so as to
control the flow rate therefrom. A molten stream 60 of refined
material falls freely from the bottom of the refining vessel and
may be passed to the subsequent stage as shown in FIG. 2.
The height of molten material 51 retained in the refiner 12 is
dictated by the level of vacuum imposed in the chamber. The
pressure head due to the height of the liquid must be sufficient to
establish a pressure equal to or greater than atmospheric at the
outlet to permit the material to drain freely from the vessel. The
height will depend upon the specific gravity of the molten
material, which for soda-lime-silica glass at the temperatures in
the refining stage is about 2.3. A height in excess of the minimum
required to offset the vacuum may be desired to account for
fluctuations in atmospheric pressure, to permit variation of the
vacuum, and to assure steady flow through the outlet.
The benefits of vacuum on the refining process are attained by
degrees; the lower the pressure, the greater the benefit. Small
reductions in pressure below atmospheric may yield measurable
improvements, but to economically justify the vacuum chamber, the
use of substantially reduced pressures are preferred. Thus, a
pressure of no more than one-half atmosphere is preferred for the
appreciable refining improvements imparted to soda-lime-silica flat
glass. Significantly greater removal of gases is achieved at
pressures of one-third atmosphere or less. More specifically, a
refining pressure below 100 torr, for example 20 to 50 torr, is
preferred to yield commercial float glass quality of about one seed
per 1,000-10,000 cubic centimeters. Seeds less than 0.01 millimeter
in diameter are considered imperceptible and are not included in
the seed counts.
Typically, flat glass batch includes sodium sulfate as a melting
and refining aid in the amounts of about 5 to 15 parts by weight
per 1000 parts by weight of the silica source material (sand), with
about 10 parts by weight considered desirable to assure adequate
refining. When operating in accordance with the preferred
embodiment, however, it has been found preferable to restrict the
sodium sulfate to two parts by weight, and yet it has been found
that refining is not detrimentally affected. Most preferably, the
sodium sulfate is utilized at no more than one part per 1000 parts
sand, with one-half part being a particularly advantageous example.
These water ratios have been given for sodium sulfate, but it
should be apparent that they can be converted to other sulfur
sources by molecular weight ratios. Complete elimination of
refining aids is feasible with the present invention, although
trace amounts of sulfur are typically present in other batch
materials and colorants so that small amounts of sulfur may be
present even if no deliberate inclusion of sulfur is made in the
batch. Moreover, the vacuum treatment has been found to reduce the
concentration of volatile gaseous components, particularly the
refining aids such as sulfur, to levels lower than the equilibrium
levels attained with conventional processes. Soda-lime-silica glass
products that are mass-produced by conventional continuous melting
processes are characterized by significant amounts of residual
refining aids. Such products would include glass sheets suitable
for glazing vision openings in buildings or vehicles (e.g., float
glass) and container were (e.g., bottles). In such products, the
residual sulfur content (expressed as SO.sub.3) is typically on the
order of 0.2% by weight and seldom less than 0.1%. Even when no
deliberate addition of sulfur refining aid is made to the batch, at
least 0.02% SO.sub.3 is usually detected in a soda-lime-silica
glass made in a conventional continuous melter. Flat glass for
transparent vision glazing applications normally has more than
0.05% SO.sub.3. In distinction thereto, soda-lime-silica glass can
be produced continuously by the preferred embodiment disclosed
herein with less than 0.02% residual SO.sub.3, even when relatively
small amounts of sulfur refining aid are being included in the
batch as described above, and less than 0.01% SO.sub.3 when no
deliberate inclusion of sulfur is being made. At the lowest
pressures, with no deliberate sulfur addition, SO.sub.3 contents
less than 0.005% are attainable. Commercial soda-lime-silica glass
of the type that is usually refined with sulfur compounds may be
characterized as follows:
Weight % SiO.sub.2 66-75 Na.sub.2 O 12-20 CaO 7-12 MgO 0-5 Al.sub.2
O.sub.3 0-4 K.sub.2 O 0-3 Fe.sub.2 O.sub.3 0-1
Small amounts of colorants or other refining aids may also be
present. Arsenic, antimony, fluorine, chlorine and lithium
compounds are sometimes used as refining aids, and residues may be
detected in this type of glass. A sheet of float glass or a bottle
represent common commercial embodiments of the above
composition.
A sheet of glass that has been formed by the float process (i.e.,
floated on molten tin) is characterized by measurable amount of tin
oxide that migrated into surface portions of the glass on at least
one side. Typically a piece of float glass has an SnO.sub.2
concentration of at least 0.05% by weight in the first few microns
below the surface that was in contact with the tin. Because the
float process entails a relatively large scale continuous melting
furnace of the type that conventionally employs significant amounts
of sulfur-containing refining aids, float glass is characterized by
minimum SO.sub.3 concentrations higher than those discussed above
for soda-lime-silica glass in general. Therefore, float glass
refined by the present process having less than 0.08% SO.sub.3
would be distinguished from conventional commercially available
float glass. Most float glass falls within the following
compositional ranges:
SiO.sub.2 72-74% by weight Na.sub.2 O 12-20 CaO 8-10 MgO 3-5
Al.sub.2 O.sub.3 0-2 K.sub.2 O 0-1 Fe.sub.2 O.sub.3 0-1
Colorants and traces of other substances may be present.
FIG. 2 shows a stirring arrangement that may be employed to
introduce transmittance altering additives into the glass after it
has been refined. Such an arrangement is optional, but is preferred
in that it permits more rapid changes in color because a smaller
volume of residual glass is involved. All of the additives may be
added at the stirring stage, or a base level of some or all of the
radiation absorbing elements such as iron may be provided
throughout the process with additional amounts being added at the
stirring stage. The glass entering the stirring stage is
advantageously in a reduced condition so that substantial portions
of iron being added will tend to be converted to or remain in the
ferrous state.
The particular embodiment illustrated in FIG. 2 includes a stirring
chamber 13 within which the stream of glass 60 is received from the
refining vessel 12. A preferred feature is the provision of a rod
61 extending downwardly from the valve member 58, which assures a
regular streamlined path for the vertically flowing glass so as to
avoid entrapment of air into the glass as it enters a body of glass
62 contained within the stirring chamber. The glass 62 is
preferably above 2200.degree. F. (1200.degree. C.) during stirring.
Therefore, the stream of glass 60 entering the stirring chamber is
at least at that temperature.
For purposes of the present invention the stirring chamber 13 is
not limited to any particular structure of stirrer, any of the
various mechanical devices that have been proposed for stirring
molten glass in the prior art being usable. Some arrangements may
be more effective than others in homogenizing the glass, but the
number of stirrers and their speed of rotation can be selected to
compensate for variations in efficiency. The particular stirrer
structure shown in FIG. 2 is a preferred example in that it
provides a strong mixing action and is a type that is readily
available commercially. Another embodiment that may be suitable is
that disclosed in U.S. Pat. No. 4,493,557 (Nayak et al.). Each of
the stirrers 63 as depicted in FIG. 2 is comprised of a helical
stirring portion at the bottom of a shaft, both of which may be
cast from a ceramic refractory material. In order to avoid drawing
air into the melt, it is preferred to rotate the helical stirrers
in such a direction that they draw the molten glass upwardly toward
the surface. This also serves to prevent additives that may be
deposited onto the surface of the melt in the stirring chamber from
being swept prematurely, and in concentrated streaks, into the zone
of active stirring. Drive means, (not shown), for rotating the
stirrers may be of any suitable type employed in the art for this
purpose. For the sake of convenience, the stirrers in a transverse
row, for example, may be rotated in the same direction, and to
enhance shearing forces imparted to the glass it is preferred to
rotate the adjacent transverse row in the opposite direction as
shown in the drawings. It should be understood, however, that any
pattern of rotation could be employed for the present invention as
long as adequate homogenization is achieved. In order to achieve
good homogeneity, it is considered desirable to stir substantially
the entire transverse cross-sectional area of the molten glass in
the stirring chamber, and the number and size of stirrers may be
selected accordingly. Thus, in the embodiment shown in FIG. 2, the
helical portion of each stirrer corresponds to virtually the depth
of the molten glass, and an array of closely spaced stirrers is
provided that actively affects substantially the full width of
molten material in the stirring chamber. The degree of
homogenization is also influenced by the amount of agitation
experienced by each increment of the melt and by the throughput
rate of the melt. Thus, a plurality of rows of stirrers is
preferred so that each increment of glass is repeatedly subjected
to mixing forces as it passes along the length of the stirring
chamber. The number of rows of stirrers will depend upon the degree
of homogenization desired, and the throughput rate of glass. As a
general guideline, one stirrer may be provided for each 10 tons per
day of glass produced for average quality flat glass. Obviously,
for some applications lower quality requirements may permit the use
of fewer stirrers. On the other hand, the use of a larger number of
stirrers will usually produce improved results.
As optional feature, preferred for making higher quality flat
glass, of the arrangement shown in FIG. 2 is that the stirring
chamber 13 is integrated with a float forming chamber 14, whereby
the glass 62 in the stirring chamber rests on a layer of molten
metal 64. The molten metal may be continuous with the molten metal
constituting the support in the forming chamber, and is usually
comprised essentially of tin. Such an arrangement avoids
contaminating refractory content on the bottom and permits delivery
of the glass immediately after being stirred so as to minimize the
area of refractory subsequently contacted by the glass. It has also
been found that the contact with molten metal in the stirring
chamber tends to have a reducing effect on the glass, which is
advantageous for attaining the transmittance properties of the
present invention. In FIG. 2, a vertically adjustable tweel 65
regulates the flow of molten glass from the stirring chamber onto
the molten metal 64 within the forming chamber 14. The glass forms
a ribbon 66, which is reduced in thickness and cools as it is drawn
along the molten metal pool until it cools to a temperature
sufficient to be withdrawn from the molten metal without marring
the surface of the glass ribbon.
Because the molten glass is stirred at relatively high
temperatures, and is immediately thereafter delivered to the
forming chamber in this embodiment, the glass enters the forming
chamber at a temperature higher than is conventional for a float
type forming process. The temperature of the glass may fall
somewhat from the stirring temperature, which is above 2200.degree.
F. (1200.degree. C.). but will typically enter the forming chamber
before the glass has cooled to a conventional float process
delivery temperature of about 1900.degree. F. to 2000.degree. F.
(1040.degree. C. to 1090.degree. C.). Typically the glass entering
the forming chamber in the FIG. 2 embodiment of the present
invention will be at a temperature of at least about 2100.degree.
F. (1150.degree. C.), at which temperature the viscosity of the
glass does not lead itself to engagement by mechanical means for
attenuating the glass ribbon to the desired thickness in the
forming chamber. Therefore, a forming process that employs elevated
pressure within the forming chamber, preferably the process
disclosed in U.S. Pat. No. 4,395,272 (Kunkle et al.), lends itself
to use with those embodiments of the present invention in which the
stirred glass is delivered at relatively high temperature to the
forming chamber.
For adding coloring agents or additives to the molten glass in the
stirring chamber, a screw feeder 67 may be provided, which may, for
example, extend horizontally from the side wall near the location
at which the stream of glass 61 enters the stirring chamber.
Coloring agents are readily available commercially and are usually
in the form of dry, pulverized concentrates, which may include a
coloring compound such as a metal oxide mixed with a fluxing powder
and bound with sodium silicate or some other binder. Alternatively,
the additives may be melted separately and fed to the stirring
chamber in a molten form as disclosed in U.S. Pat. Nos. 3,343,935
(Keefer et al.) and 3,486,874 (Rough).
The total amount of iron present in the glass is expressed herein
in terms of Fe.sub.2 O.sub.3 in accordance with standard analytical
practice, but that does not imply that all of the iron is actually
in the form of Fe.sub.2 O.sub.3. Likewise, the amount of iron in
the ferrous state is reported as FeO, even though it may not
actually be present in the glass as FeO. The proportion of the
total iron in the ferrous state is expressed as the ratio
FeO/Fe.sub.2 O.sub.3.
The radiation transmittance data herein are based on the following
wavelength ranges:
Ultraviolet 300-400 nanometers Visible (luminous) 400-770
nanometers Infrared 800-2100 nanometers
Luminous transmittance (LT.sub.A) is measured using CIE standard
illuminant A. Total solar energy transmittance relates to the
separate transmittance as follows:
where TSIR is total solar infrared transmittance and TSUV is total
solar ultraviolet transmittance.
EXAMPLE I
In a glass melting and refining operation essentially as shown in
FIGS. 1 and 2, color additive was stirred into the refined glass at
a rate of about 0.85% to 1.0% by weight of the glass. The additive
was a color concentrate identified as KG-947-B by its manufacturer,
Ferro Corporation, Orrville, Ohio, and contained about 40% iron in
the form of magnetite (Fe.sub.3 O.sub.4). The glass had 0.118%
total iron before the addition and 0.479% to 0.495% total iron
after the addition. The ratio of FeO to total iron after addition
ranged from 0.47 to 0.55, and SO.sub.3 content was 0.003% to
0.005%. The resulting glass in a 5 millimeter thickness exhibited
LT.sub.A of 68.4% to 69.3% and infrared transmittance of 11.2% to
13.9%.
EXAMPLE II
In a glass melting and refining operation essentially as shown in
FIGS. 1 and 2, an iron-containing additive was included in the
batch mixture in the amount of 1.9% by weight of the batch. The
additive was "Melite 40" a nearly sulfur-free CaO--Al.sub.2 O.sub.3
--SiO.sub.2 slag containing about 20% by weight total iron, with
about 80% of the iron in the form of FeO sold by the Calumite
Company, Boca Raton, Fla. The batch mixture also included 0.025% by
weight powdered coal to enhance reducing conditions during melting.
Combustion burners in the liquefying stage were operated with
reducing flames at a volume ratio of 1.9 parts oxygen to one part
methane. The resulting glass had a total iron content of 0.449% to
0.473%, with a ratio of FeO to total iron of 55.6% to 60.6%. At a
thickness of five millimeters, the glass exhibited LT.sub.A of
68.6% to 69.9% and infrared transmittance of 10.9% to 12.9%.
The above two examples disclose two additives that serve as iron
sources with a relatively high ferrous content. Other sources of
iron in a reduced state include metallic iron powder, iron silicide
(FeSi) and iron oxalates (Fe.sub.2 (C.sub.2 O.sub.4).sub.3.6H.sub.2
O or FeC.sub.2 O.sub.4.2H.sub.2 O).
EXAMPLE III
In a glass melting and refining operation as shown in FIGS. 1 and
2, color concentrates were melted in a small furnace and fed in
molten form into the stirring chamber at about 2400.degree. F.
(1315.degree. C.). The concentrates were KG-947-I containing about
40% by weight total iron, about 60% of that iron being in the form
of FeO, and MI-380-B containing about 25% by weight CeO.sub.2, both
sold by the Ferro Corporation. The iron color concentrates was
added at the rate of 12 parts by weight per thousand parts by
weight of base glass, and the cerium color concentrate was added at
the rate of 20 parts to one thousand parts by weight. The total
iron content of the glass increased from 0.082% by weight Fe.sub.2
O.sub.3 to 0.533% Fe.sub.2 O.sub.3 in the final glass composition
with a ratio of FeO to total iron of 0.522. The final glass
composition had 0.44% by weight CeO.sub.2 and less than 0.001% by
weight SO.sub.3. The transmittance properties of a five millimeter
thick sample of the glass produced were:
LT.sub.A 70.1% TSIR 12.3% TSET 39.4% TSUV 43.7%
In Table I, several prior art glass compositions (Composition Nos.
1-4) and their transmittance properties are compared to an example
of the present invention (Composition No. 5), all at five
millimeters thickness. Composition No. 1 is a standard green tinted
float glass commonly sold for automotive use. Luminous
transmittance is high, but infrared transmittance is also high.
Composition Nos. 2 and 3 are commercial attempts to reduce the
infrared transmittance in float glass by increasing the total iron
content and represent the approximate upper limits of such an
approach using standard glass melting technology. Infrared
transmittance is reduced in the compositions, but with a
disproportionate reduction in luminous transmittance, and further
reductions of infrared transmittance would be desirable.
Composition No. 4 has a good combination of relatively high
luminous transmittance and low infrared transmittance attained by
reducing conditions as evidenced by the relatively high ferrous to
total iron ratio. Consistent with the difficulty of continuously
melting such a reduced, absorptive glass. Composition No. 4 was
available in the past only by melting in pots, and then casting,
grinding, and polishing individual plates. Today, such a labor
intensive process would virtually preclude offering such a product
on a large scale, commercial basis. Composition No. 5, however,
closely matches the transmittance properties of Composition 4, but
is a continuously produced float glass product in accordance with
the present invention. In keeping with one aspect of the invention,
Composition 5 is distinguished from the other examples in Table I
by a relatively small amount of total iron, high ratio of ferrous
to total iron, and low SO.sub.3 content.
FIG. 3 shows plots of transmittance versus wavelength for
Compositions 1, 2, 3 and 5 of Table I. In comparison with the prior
art Compositions 1, 2 and 3, the present invention represented by
Composition 5 can be seen to have a relatively high peak in the
visible wavelength region, approaching that of the lightly tinted
glass of Composition 1, and a lower curve in the infrared region
than even the darkly tinted products of Compositions 2 and 3.
Table II and Table III show variations in the constituents that
affect transmittance and at the margins of or within the scope of
the invention. All of the compositions in Table II and III have
essentially the same base glass composition as Composition 5 in
Table I. In addition to variations in the iron which affect
luminous transmittance and infrared transmittance, the compositions
in Table II show the ability of CeO.sub.2, TiO.sub.2, V.sub.2
O.sub.5 and MoO.sub.3 to reduce ultraviolet transmittance.
Composition 11 is not as reduced as would be desired due to an
excess of CeO.sub.2. Composition 12 has a relatively large amount
of total iron and is only slightly above the minimum reduction
level, and as a result has good infrared absorption but marginal
luminous transmittance at the five millimeter thickness of the
example. It should be understood that at smaller thicknesses the
luminous transmittance would be increased without unduly increasing
the infrared transmittance, so that Composition 12 could be
satisfactory for some applications at thicknesses less than five
millimeters. Composition 13 illustrates the detrimental effect on
luminous transmittance of attempting to lower the infrared
transmittance by increasing the total iron content to high levels.
Composition 13 would be useful for the purpose of the present
invention only at very small thicknesses and therefore would not be
considered a desirable example. Composition 14 through 19 in Table
III were melted using magnetite (Fe.sub.3 O.sub.4) as the iron
source and with the inclusion of one half part by weight powdered
coal per thousand parts by weight sand to enhance the reducing
conditions.
The transmittance at different thicknesses may be calculated from
the following relationships: ##EQU1##
where:
D.sub.1 =original optical density
D.sub.2 =new optical density
h.sub.1 =original thickness
h.sub.2 =new thickness
T.sub.1 =original transmittance (percent)
T.sub.2 =new transmittance (percent)
Examples of the present invention and the prior art have been
presented herein with a thickness of five millimeters for the sake
of comparison on an equal basis. It should be understood that the
thickness may be varied within the usual range of flat glass
manufacture (e.g., 2 millimeters to 6 millimeters) to attain
desired combination of transmittance properties of the present
invention. In general, a composition having a difference of at
least fifty between its percent luminous transmittance and its
percent infrared transmittance at a given thickness will lend
itself to being tailored to the desired combination of
transmittance properties of altering the thickness. Larger
differences are preferred in that greater versatility is provided
in designing the product, and thus a difference of 55 or greater is
preferred.
Other variations and modifications as are known in the art may be
resorted to within the scope of the invention as defined by the
claims that follow.
TABLE I Composition No. 1 2 3 4 (Prior (Prior (Prior (Prior Art)
Art) Art) Art) 5 Composition (Weight %) SiO.sub.2 72.70 70.26 72.23
71.56 73.07 Na.sub.2 O 13.70 13.10 13.11 14.19 13.26 K.sub.2 O 0.02
0.99 0.22 0.05 0.06 CaO 8.80 8.87 8.65 12.85 8.82 MgO 3.85 3.90
3.89 0.16 3.86 Al.sub.2 O.sub.3 0.10 1.75 0.70 0.25 0.23 SO.sub.3
0.24 0.22 0.13 0.17 0.003 Fe.sub.2 O.sub.3 * 0.539 0.739 0.800
0.606 0.514 FeO** 0.137 0.196 0.229 0.270 0.280 FeO/Fe.sub.2
O.sub.3 total 0.254 0.265 0.286 0.446 0.545 Transmittance - 5
millimeter thickness LT.sub.A (%) 76.9 64.8 65.1 68.8 67.8 TSIR (%)
30.2 20.7 15.2 10.8 10.2 TSET (%) 51.6 40.9 37.5 37.7 36.8 TSUV (%)
43.6 28.5 31.3 43.8 53.0 *Total iron. **Total ferrous iron.
TABLE I Composition No. 1 2 3 4 (Prior (Prior (Prior (Prior Art)
Art) Art) Art) 5 Composition (Weight %) SiO.sub.2 72.70 70.26 72.23
71.56 73.07 Na.sub.2 O 13.70 13.10 13.11 14.19 13.26 K.sub.2 O 0.02
0.99 0.22 0.05 0.06 CaO 8.80 8.87 8.65 12.85 8.82 MgO 3.85 3.90
3.89 0.16 3.86 Al.sub.2 O.sub.3 0.10 1.75 0.70 0.25 0.23 SO.sub.3
0.24 0.22 0.13 0.17 0.003 Fe.sub.2 O.sub.3 * 0.539 0.739 0.800
0.606 0.514 FeO** 0.137 0.196 0.229 0.270 0.280 FeO/Fe.sub.2
O.sub.3 total 0.254 0.265 0.286 0.446 0.545 Transmittance - 5
millimeter thickness LT.sub.A (%) 76.9 64.8 65.1 68.8 67.8 TSIR (%)
30.2 20.7 15.2 10.8 10.2 TSET (%) 51.6 40.9 37.5 37.7 36.8 TSUV (%)
43.6 28.5 31.3 43.8 53.0 *Total iron. **Total ferrous iron.
TABLE I Composition No. 1 2 3 4 (Prior (Prior (Prior (Prior Art)
Art) Art) Art) 5 Composition (Weight %) SiO.sub.2 72.70 70.26 72.23
71.56 73.07 Na.sub.2 O 13.70 13.10 13.11 14.19 13.26 K.sub.2 O 0.02
0.99 0.22 0.05 0.06 CaO 8.80 8.87 8.65 12.85 8.82 MgO 3.85 3.90
3.89 0.16 3.86 Al.sub.2 O.sub.3 0.10 1.75 0.70 0.25 0.23 SO.sub.3
0.24 0.22 0.13 0.17 0.003 Fe.sub.2 O.sub.3 * 0.539 0.739 0.800
0.606 0.514 FeO** 0.137 0.196 0.229 0.270 0.280 FeO/Fe.sub.2
O.sub.3 total 0.254 0.265 0.286 0.446 0.545 Transmittance - 5
millimeter thickness LT.sub.A (%) 76.9 64.8 65.1 68.8 67.8 TSIR (%)
30.2 20.7 15.2 10.8 10.2 TSET (%) 51.6 40.9 37.5 37.7 36.8 TSUV (%)
43.6 28.5 31.3 43.8 53.0 *Total iron. **Total ferrous iron.
* * * * *