U.S. patent application number 10/672026 was filed with the patent office on 2004-06-10 for apparatus and method for producing float glass having reduced defect density.
Invention is credited to Pecoraro, George A., Smith, Charlene S..
Application Number | 20040107732 10/672026 |
Document ID | / |
Family ID | 32045292 |
Filed Date | 2004-06-10 |
United States Patent
Application |
20040107732 |
Kind Code |
A1 |
Smith, Charlene S. ; et
al. |
June 10, 2004 |
Apparatus and method for producing float glass having reduced
defect density
Abstract
A float glass chamber and related methods comprising a hot
section having an atmosphere in at least the lower plenum comprises
less than 3 percent hydrogen based on volume and a cold section,
wherein the boundary line between the hot section and the cold
section is where the temperature of the glass falls below a
threshold temperature
Inventors: |
Smith, Charlene S.; (Lower
Burrell, PA) ; Pecoraro, George A.; (Lower Burrell,
PA) |
Correspondence
Address: |
PPG INDUSTRIES, INC.
INTELLECTUAL PROPERTY DEPT.
ONE PPG PLACE
PITTSBURGH
PA
15272
US
|
Family ID: |
32045292 |
Appl. No.: |
10/672026 |
Filed: |
September 26, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60414516 |
Sep 27, 2002 |
|
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Current U.S.
Class: |
65/99.3 ;
65/134.4; 65/182.3; 65/99.2 |
Current CPC
Class: |
C03B 18/20 20130101;
C03B 18/18 20130101; C03B 5/2353 20130101; Y02P 40/50 20151101;
C03C 3/087 20130101; Y02P 40/55 20151101; Y02P 40/57 20151101 |
Class at
Publication: |
065/099.3 ;
065/182.3; 065/099.2; 065/134.4 |
International
Class: |
C03B 018/22 |
Claims
What is claimed is:
1. A float glass chamber comprising: a hot section having an
atmosphere in at least the lower plenum comprises less than 3
percent hydrogen based on volume; and a cold section, wherein the
boundary line between the hot section and the cold section is where
the temperature of the glass falls below a threshold
temperature.
2. A float chamber according to claim 1 wherein the threshold
temperature of the chamber is 1600.degree. F.
3. A float chamber according to claim 1 wherein the threshold
temperature of the float chamber is 1800.degree. F.
4. A float chamber according to claim 1 wherein the threshold
temperature of the float chamber is 2100.degree. F.
5. A float chamber according to claim 1 wherein the atmosphere in
at least the lower plenum of the cold section comprises up to 10
percent of hydrogen based on volume.
6. A float chamber comprising: a hot section having an atmosphere
in at least the lower plenum comprises less than 3 percent hydrogen
based on volume; and a cold section, wherein the boundary line
between the hot section and the cold section is where the
temperature of the glass falls below a threshold temperature of
greater than 1600.degree. F.
7. A float chamber according to claim 6 wherein the atmosphere in
at least the lower plenum of the cold section comprises up to 10
percent of hydrogen based on volume.
8. A method for making float glass with reduced defect density
comprising: a. melting a glass composition to form a glass melt;
and b. pouring the glass melt in a float chamber having a hot
section and an cold section, the boundary line between the hot
section and the cold section is where the temperature of the glass
falls below a threshold temperature, wherein the hot section has an
atmosphere in at least the lower plenum comprises less than 3
percent hydrogen based on volume
9. A method according to claim 8 wherein the threshold temperature
of the float chamber is 1600.degree. F.
10. A method according to claim 8 wherein the threshold temperature
of the float chamber is 1800.degree. F.
11. A method according to claim 8 wherein the threshold temperature
of the float chamber is 2100.degree. F.
12. A method according to claim 8 wherein the atmosphere in at
least the lower plenum of the cold section comprises up to 10
percent of hydrogen based on volume
13. A method according to claim 8 wherein the glass melt has a
water content equal to or greater than 0.035 weight percent based
on the total weight percent of the composition.
14. A method according to claim 8 wherein the float glass produced
comprises at least one piece of glass in a laminated product.
15. A method according to claim 14 wherein the laminated product is
a windshield.
16. A method for making float glass with reduced defect density
comprising: a. melting a glass composition to form a glass melt;
and b. pouring the glass melt into a float chamber having a hot
section and an cold section, the boundary line between the hot
section and the cold section is where the temperature of the glass
falls below a threshold temperature; c. pumping a gas mixture
comprising less than 3% hydrogen based on volume into at least the
lower plenum of the hot section.
17. A method according to claim 16 wherein the pumping comprises
pumping a gas mixture comprising less than 1% hydrogen based on
volume into at least the lower plenum of the hot section.
18. A method according to claim 16 wherein the glass composition
comprises: from 65 to 75 weight percent SiO.sub.2; from 10 to 20
weight percent Na.sub.2O; from 5 to 15 weight percent CaO; from 0
to 5 weight percent MgO; from 0 to 5 weight percent
Al.sub.2O.sub.3; from 0 to 5 weight percent K2O; and from 0 to 2
weight percent Fe2O.sub.3, with weight percents being based on the
total weight of the glass composition.
19. A method according to claim 16 wherein the melting occurs in an
oxy-fuel furnace.
Description
RELATED APPLICATION
[0001] This application claims the benefits of U.S. Provisional
Application Serial No. 60/414,516 filed Sep. 27, 2002, which
application is herein incorporated by reference in its
entirety.
FIELD OF THE INVENTION
[0002] The present invention relates to a float glass chamber used
to produce flat glass by the float glass process, and more
specifically float glass chambers that can be used to yield glass
having reduced defect density.
BACKGROUND
[0003] The float glass process is well known for making sheets of
glass. In a typical float glass process, batch materials are heated
to form molten glass. The molten glass is then poured onto a bath
of molten tin. The molten glass is drawn along the bath of molten
tin and simultaneously cooled and attenuated to form a
dimensionally stable continuous sheet of glass, typically referred
to as a glass ribbon. The sheet is then removed from the bath for
further processing.
[0004] Two types of furnaces are used in the float glass
process--an air-fuel furnace and an oxy-fuel furnace. In an
air-fuel furnace, fuel is mixed with warm air and combusted to
provide heat to melt the glass batch materials.
[0005] In an oxy-fuel furnace, oxygen, not air, supports
combustion. As a result, an oxy-fuel furnace provides a much more
efficient melt than an air-fuel furnace because energy is no longer
being wasted heated up nitrogen in the air and oxy-fuel flames have
a higher flame temperature which radiates more efficiently. The
increased melting efficiency allows more tonnage to be processed
through an oxy-fuel furnace than through a similarly sized,
air-fuel furnace.
[0006] Both air-fuel and oxy-fuel furnaces have water in their
atmospheres. The head space (the area of the furnace above the
molten glass) in an oxy-fuel furnace has a higher concentration of
water than in an air-fuel furnace because the oxy-fuel atmosphere
lacks the nitrogen provided in an air-fired furnace that dilutes
the total water formed by combustion. Stoichiometrically, the water
typically constitutes about 66% by volume of the head space in an
oxy-fuel furnace versus 18% in an air-fired furnace. Since the
amount of water in the glass melt is proportional to the square
root of the concentration of water in the head space, glass melted
in an oxy-fuel furnace has a 1.7 to 2 times higher water
concentration than glass melted in a conventional air-fuel furnace.
Typically, glass melted in an oxy-fuel furnace contains more than
0.045 weight percent water based on the total weight of the
composition.
[0007] At the stage of the float glass process where molten glass
is poured onto molten tin, the molten tin temperature in the float
bath ranges from 1800.degree. F. to 1900.degree. F. (981.degree. C.
to 1037.degree. C.). At 1800.degree. F., at the glass-tin
interface, water that diffuses out of the molten glass dissociates
into hydrogen and oxygen. Because hydrogen isn't very soluble in
tin at 1800.degree. F., much of the hydrogen does not dissolve in
the tin but remains in the atmosphere of the bath. Some of the
hydrogen from the disassociation of water gets trapped at the
interface between the molten glass and tin and ultimately impinges
on the bottom surface of the glass ribbon and form defects along
the ribbon surface typically referred to as open bottom bubbles.
The open-bottom bubbles can be described as voids in glass that
generally have an inverted-U shape cross-section. The presence of
open bottom bubbles increases the overall defect density of the
glass.
[0008] Customers set requirements for the defect density of glass
for certain applications. The standards are very difficult to meet
with conventional float glass processes due to the presence of open
bottom bubbles.
[0009] The present invention provides a novel apparatus and method
that yields float glass having a lower total defect density as a
result of reduced open bottom bubble defects.
SUMMARY OF THE INVENTION
[0010] In one embodiment, the present invention is float glass
chamber comprising:
[0011] a hot section having an atmosphere in at least the lower
plenum comprises less than 3 percent hydrogen based on volume;
and
[0012] a cold section, wherein the boundary line between the hot
section and the cold section is where the temperature of the glass
falls below a threshold temperature.
[0013] In another embodiment, the present invention is method for
making float glass with reduced defect density comprising:
[0014] a. melting a glass composition to form a glass melt; and
[0015] b. pouring the glass melt in a float chamber having a hot
section and an cold section, the boundary line between the hot
section and the cold section is where the temperature of the glass
falls below a threshold temperature,
[0016] wherein the hot section has an atmosphere in at least the
lower plenum comprises less than 3 percent hydrogen based on
volume
DRAWINGS
[0017] FIG. 1. is a sectional view of a float chamber according to
the present invention, with portions removed for clarity.
DESCRIPTION OF THE INVENTION
[0018] As used herein, spatial or directional terms, such as
"left", "right", "inner", "outer", "above", "below", "top",
"bottom", and the like, relate to the invention as it is shown in
the drawing figures. However, it is to be understood that the
invention may assume various alternative orientations and,
accordingly, such terms are not to be considered as limiting.
[0019] Unless otherwise indicated, all numbers expressing
dimensions, physical characteristics, quantities of ingredients,
reaction conditions and so forth, used in the specification and
claims are to be understood as being modified in all instances by
the term "about". Accordingly, unless indicated to the contrary,
the numerical values set forth in the following specification and
claims can vary depending upon the desired properties sought to be
obtained by the present invention. At the very least, and not as an
attempt to limit the application of the doctrine of equivalents to
the scope of the claims, each numerical parameter should at least
be construed in light of the number of reported significant digits
and by applying ordinary rounding techniques. Moreover, all ranges
disclosed herein are to be understood to encompass any and all
subranges subsumed therein. For example, a stated range of "1 to
10" should be considered to include any and all subranges between
(and inclusive of) the minimum value of 1 and the maximum value of
10; that is, all subranges beginning with a minimum value of 1 or
more and ending with a maximum value of 10 or less, e.g., 5.5 to 10
or 3.2 to 7.8.
[0020] Conventional float glass processes are typically carried out
using a float chamber as shown in FIG. 1. Non-limiting examples of
float glass processes are disclosed in U.S. Pat. No. 3,083,551,
U.S. Pat. No. 3,961,930, and U.S. Pat. No. 4,091,156, which are all
hereby incorporated by reference.
[0021] In a conventional float glass process, a glass batch
composition is heated to a molten state and poured into the float
chamber. Typically, the float chamber has a refractory roof 3 that
divides the chamber into an upper plenum 1 and a lower plenum 2.
The lower plenum contains the glass 4 and the tin 5. The upper
plenum contains all of the overhead electrical heating elements to
provide controlled heating of the liquid metal float bath and the
formed glass ribbon. A controlled atmosphere is maintained in the
chamber via gas inlets 6 and gas outlet(s) 7.
[0022] The novel float glass chamber of the present invention
comprises at least two sections--a hot section and a cold section.
The boundary line between the hot section and the cold section is
where the temperature of the glass falls below a predetermined
temperature, hereinafter referred to as the "threshold
temperature," required for glass in the hot section. In a
non-limiting embodiment of the present invention, there is no
physical barrier between the hot section and the cold section.
[0023] In one non-limiting embodiment of the invention, the
threshold temperature is 2100.degree. F. In another non-limiting
embodiment of the invention, the threshold temperature is
1800.degree. F. In another non-limiting embodiment of the
invention, the threshold temperature is 1600.degree. F. The lower
the threshold temperature for the hot section, the larger the hot
section and the smaller the cold section and visa versa.
[0024] In a non-limiting embodiment of the present invention, the
hot section of the chamber is approximately 90 to 100 feet from the
point where the molten glass is poured onto the tin. The cold
section of the chamber is the next approximately 70 to 140 feet of
chamber behind the hot section, depending on the size of the
bath.
[0025] In a non-limiting embodiment of the present invention,
numerous gas inlets and outlets are present in the upper plenum and
lower plenum of the float chamber. Various gaseous mixtures can be
pumped into the chamber through the gas inlets or out of the
chamber through the gas outlets to control the atmosphere within
the chamber.
[0026] In a non-limiting embodiment of the invention, the gas
inlets to at least the lower plenum over the hot section of the
chamber deliver in a gas comprising less than 1 weight percent
hydrogen based on volume. The remainder of the gas can be an inert
gas, such as but not limited to nitrogen. Under normal operating
conditions, in one non-limiting embodiment of the present
invention, the atmosphere of the lower plenum over the hot section
of the chamber can comprise 3 percent hydrogen based on volume. In
another non-limiting embodiment of the present invention, the
atmosphere of the lower plenum over the hot section of the chamber
can comprise 1 percent hydrogen based on volume.
[0027] Various mixtures of hydrogen and nitrogen or argon or
ammonia in placed of mixed gases can be pumped into the atmosphere
of at least the lower plenum over the cold section of the chamber.
In a non-limiting embodiment of the invention, the gaseous mixture
can comprise up to 10 percent of the hydrogen based on volume. The
rest of the gas can be nitrogen.
[0028] The gas outlets in the float chamber can be used to remove
gas from the chamber. In one non-limiting embodiment of the
invention, up to 40 volume percent based on volume of the total
flow of the gas pumped into the chamber as discussed above can be
removed from the hot section. In this embodiment, it may be
necessary to adjust the level of nitrogen in the atmosphere to
prevent hydrogen from flowing upstream into the hot section of the
chamber.
[0029] By reducing the hydrogen in the hot section of the float
chamber, the present invention reduces the level of saturation of
molten tin, specifically with respect to hydrogen, at the hot
section of the float chamber. The molten tin is able to dissolve
more hydrogen from the disassociation of water so open-bottom
bubble defects in the glass are reduced.
[0030] The present invention also encompasses a method for
producing glass. According to the present invention, glass can be
produced via the following steps: adding glass batch materials to a
furnace; melting the batch materials; pouring molten glass from the
furnace into the float chamber; and removing the float glass from
the float chamber.
[0031] The first step of the present invention comprises adding
glass batch materials to a furnace. The furnace can be an air-fuel
furnace or an oxy-fuel furnace. The glass batch materials can be of
any conventional type including, but not limited to, conventional
soda-lime-silica glass batch materials. A conventional glass
composition can be characterized as follows:
[0032] from 65 to 75 weight percent SiO.sub.2;
[0033] from 10 to 20 weight percent Na.sub.2O;
[0034] from 5 to 15 weight percent CaO;
[0035] from 0 to 5 weight percent MgO;
[0036] from 0 to 5 weight percent Al.sub.2O.sub.3;
[0037] from 0 to 5 weight percent K.sub.2O; and
[0038] from 0 to 2 weight percent Fe.sub.2O.sub.3.
[0039] All values are in weight percent based on the total weight
of the glass composition.
[0040] The second step of the present invention comprises melting
the batch materials in the furnace. The melting processes can be
accomplished using techniques that are well known in the art. For
example, in an oxy-fuel furnace, the batch materials can be melted
by supplying oxygen and fuel to melt the batch materials.
[0041] The third step of the present invention involves pouring
molten glass from the furnace into the float chamber. As is well
known in the art, the molten glass flows onto the top of the molten
tin and moves along the top of the tin from the hot section of the
chamber to the cold section of the chamber. The temperature of the
glass in the hot section and the cold section of the chamber are as
discussed above. Also, the environments above the glass in the hot
section and the cold section of the chamber are as discussed
above.
[0042] The glass melt coming into the tin bath can contain water.
The glass melt can have a water content equal to or greater than
0.045 weight percent based on the total weight percent of the
composition.
[0043] The next step of the invention involves removing the float
glass from the bath as is well known in the art.
[0044] After the float glass is removed from the float chamber, the
glass is controllably cooled and cut into glass sheets. The sheet
can be further processed, e.g. cut to shape and heat processed, to
form a desired glass article.
[0045] The glass can also be coated. In a non-limiting embodiment
of the invention, the glass is coated. The coating can include one
or more coating layers and/or coating films. The coating can be of
any desired type. For example, but not to be considered as
limiting, the coating can be an electroconductive coating, a
heatable coating, an antenna coating, or a solar control coating,
such as a low emissivity coating. Non-limiting examples of solar
control and antenna coatings are disclosed in U.S. Pat. Nos.
4,898,789; 5,821,001; 4,716,086; 4,610,771; 4,902,580; 4,716,086;
4,806,220; 4,898,790; 4,834,857; 4,948,677; 5,059,295; and
5,028,579, which patents are herein incorporated by reference.
Non-limiting examples of electroconductive coatings are disclosed
in U.S. Pat. Nos. 5,653,903 and 5,028,759, which are herein
incorporated by reference.
[0046] Glass made by a float process typically ranges from a sheet
thickness of 2 millimeter to 20 millimeters. Glass having the
aforementioned thickness can be prepared on a conventional float
line having a line speed ranging from 100 to 800 inches per minute.
The required thickness of the glass is determined by the end use of
the glass.
[0047] The present invention provides glass having reduced defect
density; specifically open-bottom bubbles. Such defects in glass
can be measured using on-line and off-line methods. An Automatic
Inspection System manufactured by Inspection Technologies Inc. can
be used to measure defects on-line. Defects can be measured
off-line by visual inspection. The defect density of glass is
measured as number of defects per 100 square feet. The standards
for measuring defects in glass are well known in the art. For
example, defects can be measured in categories from <0.06" to
>0.25".
[0048] Glass produced according to the present invention can meet
the various commercial standards for defect density. For example,
car manufactures set standards for defect density for automotive
windshields. One automobile manufacture requires-automotive
windshield glass production to have less than 1 total defect per
100 square feet.
[0049] The glass produced according to the present invention can be
used as automotive transparencies, in colored glasses, laminated
products, etc. as is well known in the art. A laminated product can
comprise at least one piece of glass produced according to the
present invention. Such a laminated product can be a
windshield.
EXAMPLES
[0050] The invention is illustrated by the following non-limiting
examples. The following is an example of a control run where
hydrogen was in the lower plenum of the hot end and a run according
to the present invention.
1 Control Example of the Invention H.sub.2 in total chamber 1900
scfh 600 scfh H.sub.2 in at least lower 1300 scfh 0 scfh plenum of
hot end Open Bottom Bubble 1.36 per 100 sq. ft. 0.07 per 100 sq.
ft. defects Thickness of glass 3 mm 3 mm Tonnage 599 Tons per day
604 TPD Threshold Temperature 1769.degree. F. 1761.degree. F.
H.sub.2O in glass 0.049% 0.049%
CONCLUSION
[0051] The apparatus and method of the present invention allows
float glass to be produced which has substantially reduced
open-bottom bubble defects as compared to conventional float
glass.
[0052] The above examples are offered only to illustrate the
present invention. The scope of the present invention is defined by
the following claims.
* * * * *