Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
107S'~70
SPECIFICATI ON
This invention is in the field of applying porcelain
enamel coatings and, more specifically, is directed to a method
of pre-treating a ferrous surface for the reception of a porcelain
enamel coating, the pre-treating being done by means of a soft
glass composition preferably in fritted form which reacts with
the underlying ferrous surface to provide a means for bonding the
enamel coating to the underlying surface.
Ever since sheet steel has been provided with
porcelain enamel coatings, it has been considered necessary to
subject the steel to a varied series of plating and preparation
steps in order to improve the adhesion and the appearance of the
porcelain enamel. The only exception to this rule has been in
connection with very thin hollow ware of a very inexpensive nature
where surface preparation was kept at a minimum. In any
quality work, however, it was always considered necessary to
prepare the surface by chemical and mechanical procedures before
the porcelain enamel frit was applied. A typical series of
treatment steps for a steel sheet involved first repeatedly dipping
the sheet in an alkaline cleaner at temperatures ranging from
about 140 to 212F (60 to 100C). This was followed by a warm
rinse with water at about 120 to 140F (49 to 60C). Following
the warm rinse, there was usually a cold rinse with water at
room temperature. Then came a pickling step with which the
washed material was treated with a solution of 5 to 10~, sulfuric
acid at temperatures ranging from about 150 to 160"F (66 to 71CC).
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Following the pickling step, the material was again rinsed with
water contazning some small amount of sulfuric acid~at room
temperature. Following the cold rinsing step, a flash nickel
deposition took place using temperatures of 130 to 180F (60 to
82C) to deposit about 0.02 to 0.6 grams of nic~el per square
foot. Fol~owing the nickel deposition, the steel was again cold
rinsed with water and a small amount of sulfuric acid to prevent
- formation of ferric iron. Finally, the surface was neutralized
with agents such as sodium carbonate and borax at temperatures
of about 120 to 130F (49 to 54C). ~he equipment required
for this extensive heat treatment necessitated substantial capital
investment and the time and labor involved provided a substantial
portion of the cost of the enameled product.
There have been a few disclosures in the prior art
15 which sought to avoid the extensive and time-consuming pickling
and nickel coating steps in enameling steel but to our knowledge
they had little or no success commercially. One such disclosure
appears in the expired Zimmerman U.S. Patent No. 2,828,218
which described a frit which was applied as a ground coat to a
20 metallic surface at an application weight of 1/8 to 3/8 ounces
per square foot dry weight. The frit composition included
materials such as flint, feldspar, dehydrated borax, soda ash,
sodium nitrate, fluorospar, calcspar and red iron oxide. A
conventional porcelain enamel cover coat was applied over the
25 fired ground coat.
1075'~70
An improvement to this type of process is found in
U.S. Patent No. 2,786,782 issued to Zimmerman et al. In this
patent, Zimmerman et al. suggested adding black iron oxide
(magnetic) and zinc oxide to frit compositions in order to improve
5 the adherence. This material was applied in the conventional
manner with mill additions as a ground coat, and after firing
of the ground coat, a conventional porcelain enamel frit was
applied and fired. Typically, the ground coat of this patent had
from 5 to 13% magnetite, 2 to 7% zinc oxide, 35 to g2% silica,
20 to 25% boric oxide, 11 to 22% of one or more alkali metal
oxides, 4 to 6% calcium oxide and 3 to 5% alumina.
A third reference which-refers to the possibility of
eliminating surface preparation such as etching or metal plating
is U.S. Patent No. 2,864,721 issued to King et al. This
disclosure deals with a ground coat produced by milling a slip
airectly from raw batch enameling materials, without fritting,
and applying the slip to the metallic article at a dry weight of
8 to 17 grams per square foot. The preferred ground coat
included from 1 to 15% magnetite, 0 to 9% zinc oxide, 30 to 45~70
silica, 17 to 28% boric oxide, 11 to 22% of an alkali metal oxide,
3 to 8~ calcium oxide, and 0 to 7% alumina.
In recent times, the technique of applying a ground
coat and a cover coat of porcelain enamel by means of electrosta-
tic deposition has become more popular. Particular emphasis
has been placed upon providing a two coat-one fire system for
steels such as cold rolled steel. Examples of such current
107S'~70
practicc will be found in publications sucll as "Rationalization in the
Inamelling Industry With Elcctrostatic Dry Procedures", a paper given at the
International Congress in Vitreous Enamelling, in October, 1975; an article in
"Iron Age" for December 15, 1975 entitled "Enamelers Place Hopes in Dry Frit
Spraying", pages 49 to 52; and the article entitled "Powder: A Shot in the
Arm for Porcelain Enameling" appearing in Industrial Finishing, January, 1976
pages 29 to 31. In each of these articles, however, the two coat-one fire
system was applied to regularly pickled workpieces.
The present invention provides an improved method for applying high
quality porcelain enamel to a workpiece, particularly to workpieces composed
of steels such as cold rolled steel which have heretofore been enameled only
with great difficulty.
In a first aspect this invention provides a method of applying a
porcelain enamel coating to a ferrous workpiece which comprises cleaning said
workpiece to leave a clean, unpickled, nickel-free surface, electrostatically
applying a hydrophilic soft fritted glass free of iron oxide, antimony oxide,
and molybdenum oxide to said clean surface, said coating having the capability
of reacting with the underlying workpiece surface at a firing temperature,
said coating having a melted composition as follows:
SiO2 16-45% by weight
2 1~7-12~1%
Na2O or K2O
or both 10-25% " "
B2O3 10-26% " "
CaO or BaO
or both 2-20% " "
said composition also including a bond promoting oxide which is CuO, NiO or
both, electrostatically applying a dry porcelain enamel frit to the thus-
treated workpiece and firing the thus-coated workpiece at a temperature
sufficient to cause reaction of the soft fritted glass with the underlying
workpiece surfaceJ followed by fusi.on of said porcelain enamel frit into an
ena~el coating.
'`~
10'75~70
In a second aspect this invention provides a fritted reactive glass
composition suitable for electrostatic application to a ferrous surface prior
to the application of a porcelain enamel cover coat thereover, said reactive
glass composition having a matrix of the following melted composition:
SiO2 16-45% by weight
2 1.7-12.1% " "
Na2O or K2O
or both 10-25% " "
2 3 10-26% " "
CaO, or BaO
or both 2-20% " "
said composition also including a bond promoting oxide which is CuO or NiO or
both.
The process of the present invention makes it unnecessary to
employ either a pickling step or a flash nickel coating or surface roughening
in preparing the metal surface for enameling. Instead, a reactive coat is
first deposited electrostatically on the surface of the workpiece, the
reactive coat being classified as a "soft" glass meaning that it is high
fluid, i.e., has a relatively low viscosity at temperatures below the firing
temperature for the cover coat. Typically, the soft glass is used in
accordance with the present invention at low viscosities at temperatures as
low as 1,200F (648C) or lower. This fluidity at relatively low
temperatures, coupled with the
- 4a -
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chemical nature of the glass permits the reactive coat to actually
etch and react with the underlying ferrous surface before the
overlying porcelain enamel cover coat melts.
The reactive- coat is applied to a thickness of
about 0.5 to 2 mils (12 to 51 microns). The preferred coating
application for the reactive coat is in the range from 1 to 1.5
mils (25 to 38 microns). To aid in the electrostatic deposition
of the reactive coat, the coating particles are preferably
encapsulated to provide an electrical resistivity in the range from
1012 to 1016 ohm centimeters.
The application of the reactive coat is followed by
the application of a dry porcelain enamel cover coat, also by the
electrostatic deposition process. Preferably, this cover coat
also is encapsulated to provide an electrical resistivity for the
particles in the aforementioned range. Then, the thus-coated
workpiece is fired at a temperature sufficient to cause fusion of
the porcelain enamel frit in the cover coat, and reaction of the
soft glass or active coating with the underlying workpiece surface.
Typical firing temperatures range from about 1300 to 1600~F
(704 to 871C) and preferably from 1400 to 1500~ (760 to 816C).
ON THE DRAWINGS
.
Other objects, features and advantages of the
invention will be readily apparent from the following description
of a preferred embodiment thereof, taken in conjunction with the
accompanying drawing, although variations and modifications may
1~7 5~70
be effected without departing from the spirit and scope of the
novel concepts of the disclosure, and in which:
The single figure of the drawings illustrates
schematically one continuous coating apparatus which may be
S employed to practice the method of the present invention.
In the Figure reference numeral l0 has been applied
generally to a powdered frit source which contains the fritted
reactive coating particlesO A vacuum pump 11 propels the
particulate material into a fluidizing chamber 12 wherein the
10 particles are suspended in a stream of air~ The fluidized
particle stream is then passed by means of a conduit 13 to a
mani~old 14 disposed in a treating chamber generally indicated
at reference numeral 15. The chamber 15 is provided with a
pair of opposed slots 16 and 17 through which an overhead
15 conveyor 18 extends. The conveyor 18 is arranged to deliver
individual plates 19 to the treating vessel 15, the depths of the
slots 16 and 17 being sufficiently long to accomplish this purpose.
Within the treating cham~er 15, a plate 19 is
subjected to spraying by means of a plurality of electrostatic
20 spray guns 20 and 21 which are fed with a fluidized suspension
of particles from the manifold 14 through feed lines 22 and 23
The guns 20 and 21 are also provided with compressed air
through suitable inlet lines (not shown). Electrostatic spray
guns 20 and 21 are commercially availa~le articles~availa~le,
25 for example, from the DeVilbiss Company of Toledo, Ohio.
107S'~'70
Generally speal~irlg, such guns include a charged electrode past
which the fluidized powder particles issue and become charged.
An opposite charge is provided on the plate 19, typically by
having the conveyor 18 at ground potential. The electrostatic
5 spray guns 20 and 21 can be continuously operated or intermittent-
ly operated as desired. To prevent powder spray from leaving
the treating chamber 15, there can be provided a pair of air
seals 24 and 25 which provide curtains of air across the slots
16 and 17, the curtains terminating in collectors 26 and 27,
10 respectively. Any powder trapped in the curl:ains is recirculated
by means of conduits 28 and 29 to the fluidizing chamber 12 or
elsewhere.
Excess powder which appears in the chamber 15
settles by gravity onto an inclined bottom 30 of the treating
15 chamber 15 and is recycled to the fluidizing chamber 12 by
means of a conduit 31.
The figure in the drawing illustrates a continuous
electrostatic application system for applying the reactive coat to
a plate 19. Such a plate need only be cleaned by conventional
20 alkaline cleaners or the like,rinsed and dried. No pickling or
flash nickel coating is required. It will be understood, of
course, that a similar system can be used for applying the
cover coat of porcelain enamel frit electrostatically.
The only pretreatment required for the ferrous
2S workpiece in accordance with the present invention is a simple
1~'7~'~70
cleaning, typically with an alkaline-type cleaner, followed by
rinsin~ Llnd drying. Neither acid pickling nor nickel flash
coating is required to secure adequate adhesion of the porcelain
enamel layer onto the workpiece.
The nature of the reactive coating has a great
deal to do with the improved results obtained according to the
present invention. The reactive coating is characterized as a very
soft glass, i.eD, one that has a low viscosity at relatively low
temperatures of 1200F or less. The chemical nature of the
soft glass also, we believe, renders it very reactive toward the
~errous surface so that it actually chews or attacks the metal,
creating a strong anchoring bond to which the fused porcelain
enamel cover coat can readily adhere.,
The reactive coating must melt and flow out before
the porcelain enamel cover coat melts or else specks of the coat-
ing will be visible in the cover coat. To determine the- proper
relationship between the viscosities of the molten reactive coating
and the molten cover coat, we used the standard fusion flow
test. The test details are given in ASTM Test Method C374-70
"Fusion Flow of Porcelain Enamel Frits (Flow Button Methods)"O
We used 3.0 gram frit samples and made the tests at 827C
(1520F). We found that the flow button formed from the reactive
coating should be at least twice as long as that for the cover coat,
and can be as long as 20 times. Preferably, the flow from the
1075Z70
reactive coating button should be from 3 to 10 times the flow
measured for the cover coat button.
The chemistry of the reactive coating can vary
widely as long as it has the low viscosity and chemical nature
S required to react with the underlying metal sur~ace.
The reactive coating may be based upon a
borosilicate glass matrix having the following composition in its
melted form:
Broad Preferred
In~redient~tan~e Range
SiO2 16-45 % by weight 19-39 % by weight
F2 1. 7-12. 1 " "3. 4-8. 6 " ~' "
Na~ or K2O 10-25 " " 17-23
or th
B2O3 10-26 " " 17-~6 " "
CaO or BaO 2-20 " " 11-17 " "
or both
The above oxides and other elements form the base
composition which gives a glassy frit to hold the bond promoter and
other oxides. The ~ond promoter oxides are cupric oxide (CuO) and
nickel oxide (NiO). The cupric oxide is added in amounts of 1 to
12~7o o~ the melted composition and preferably from 2 to 9% by
weight. The nickel oxide can be used in amounts of from 0.5 to
1.6~37o by weight of the melting composition, and preferably from
0.9 to 1.3~7o by weight.
Certain other oxides are desirably added but are
not essential in the sense that the bond promoting oxides can work
1075Z70
without the added oxides. These added oxides are materials
such as cobalt oxidc (COO) in amounts of f~om 0 to 1% and
preferably from 0.5 to 0.9~70 of the melted composition. Another
oxide which can be added ~s man~anese oxide (MnO) in amounts
5 of ~om 0 to 5% by weight of the melted composition, and
preferably from 0 tO l~o by weight.
Still other oxides can be added for various purposes,
including improvement of bond or adjustment of the flow rate.
These miscellaneous oxides are given in the following table:
Broad Preferred
Oxide Ran~e Range
~nO 0-5 % by weight 2-4 ~0 by weight
Al2O3 0-6 " " 0-2 " "
P2O5 0-5 " " 0-2 " "
TiO2 0-5 " " 0-1 " "
Li20 Or2 . 2 ~ O- l . 5
While many different oxides can be used in the
reactive coating, as noted above, there are several oxides which
should not be used because they tend to destroy the bond when
added to the reactive coating. These oxides are iron oxide,
antimony oxide and molybdenum oxide. The efîect of iron oxide
is just the contrary to its effect in the compositions described in
the Zimmerman U.S. Patent Nos. 2~786,782 and 2,828~218, and
Burnham et al. U.S. Patent No. 2,864~721 who teach the use of
iron oxide as an adherence metal oxide in conventional ground
-10-
1075Z70
coat frits. When used in the compositions of the present
invention, iron oxides give no bond and provide pits in the
coating.
The reactive coating can be smelted in the
5 conventional manner for the frits. Typically, we use an 1800F
(982~C) rotary smelter with a 30 minute residence time.
5pecific reactive coating compositions have been
made up from the following raw batch compositions:
1~7S'~70
~1~ u~, ~, o I ~ o~ ~D ~ ~ O O O r~ ~ ~ ~ ~1
a~ ~ ~ ~o a~ ~ ~ a~ ~ co r~ ~ o ~ ~Dlo
dP ~ ~ O ~ O ~ ~~ ~ I 1 ~1
O ~ ~ ~D ~r ~ ~ o o ~ ~ ~i ~ I ~1
o ~ ~ o~ o ~ u~lo
X~ ~ o ~D ~ ~ ~~ I_ ~ o o ~ ,i ~lo
o ~ In ~ o ~ u~ o In O ~ ~Dlo
~i 1` ~ ~i ~ O ~ O ~ ~ ~ O
o 1--~ oo o ~ o o~ oo ~o N r~ o
ml~P O ,_~ ~ O ~ ~ I~ u~ O ~D O O ~ ~ o
N 0 ~ 00 0 ~ O CO CO 1-- ~r ~ O O 1--lo
C~ o o ~r o ~ t~i ~` Ln o ~o o d' ~ ~i ~--i t~ lo
.~ r~ ~ o ~ o co ~ ~ ~ o o ~Dlo
~ ~ o D o ~ ~ o N O O ~ ~ ~ ~
æ, c~ ~ N ~ ~ er ~ N r~ lo
;~ ~1~ , u~o I ~ o o ~ I ~i II lo
~ I ~O o ~ ~ o ~o ~0 ~ ~ 00 ~r N N lo
al~ O ~ O O
O ~1 N C~ ~ ~ ~ ~ ~ t` r- Cl~ N O ~ a~ lo
N O ~ D ,i N 1,_~
P O ~1 ~ri O ~ i o o ~i ~i ~) lo
1-- ~ u~ CO ~ a~. Ll~ ~ ~ ~ O ~ ~ O ~ ~lo
~ o~o
2 ~ o o ~ N a~ ~> O O ~ --1 ~1 1 1 lo
~ ~ $ ~ $~
1075270
These raw batch compositions resulted in the
following me!ted composition:
-13-
~O~iZ70
:Z ~ ~ ~-- ~ ~ co ~ r~ o o _~ o ~ o a. ~ ~ o
N r-l i N I _ I r-l
oP 1~ 1~ ~ ~ CO ~D ~ r; O r; O r-i O ~ ~ ~ O
N r-i I N I r-i O
00 ~ ~In O t~D ~ ~ ~ r~ r~ ~ O
O r~ 0 N ~ r~D ~ r-l _I ~I r--I O
~; c~ ~ 00 ~ ~ ,; O O O ,; O ~ r~ ~ O
N _~ I N I ~_1 0~
co ~ ~ ~ ~n o 1~ r~ ~ o
O O r; ~~i ~ ~ o
N ~ ~ ~ 00 r` d' ~ r~l ~r ~ O
H d~ ~ ~ O ~ 00 N ~ O O ~ C~ ~ ~ i O
I ~1~1
~ ~ ~ r~ o r- oo r ~ u~ ~ O -l O
H 5~9 ~) ,; O O r; o '; N~ ~ ~ ¦o
H
cn o ~ ~ ~ Ln o t~9 In ~ ~ r~ ~ r~ ~ O
O ~ N r~l N I r~l r~l
~ O ~ D ~ ~ O 1~ 00 1~ ~ ~ r~ r~ ~ ~ O
~ ~ ~ r; OOr; O r;~ ~r I ~ ~
~ ~ ~O r~ ~ o
~ ~ D~ ~r ~ ~o o ~ ~ ~ ~
o 1~ r` ~D In ~ r~
O 0~i ~ r;L
oo,; o ~;
et~ ~ ~ ~ ~ o ~ I~ ~ In ~ ~ ~ 1~ ,~ lo
m d~ D ~ C~ ~D ~O O ,; ,; O ~r ~ ~ o
N ,~ I I NI ,~ ,~
o~ D ~ r; O O r; O r; O ~ ~ ~ O
N ~ ~I 0~ 0~
~¦ ~ ON ~ ~ ON ,~ ON
z m
--14--
107S~70
One of the features of the reactive coating of the
present invention which distinguishes it from the materials here-
tofore used in ground coats is the hydrophilic nature of the glass.
Indeed, the glass is sufficiently soluble in water so that it could
5 not be effectively used in conjunction with mill additions and the
like in forming an aqueous slip for application to a metal surface
as a conventional ground coat.
The reactive coating is preferably applied in fritted
form. Since the composition is highly susceptible to water, it is
10 recommended that the frit be produced by roll quenching instead of
by quenching under water.
The dry particles of frit making up the reactive
coating are applied by electrostatic deposition. This is a technique
which has received considerable attention in recent years. For
15 a background of electrostatic spraying techniques, reference is
invited to the article by M. L. Pouilly in the October, 1953 issue
of "Finish" magazine, pages 71 to 73. The Pouilly article refers
to the use of high voltage packs in electrostatic spraying for por-
celain enamel which are capable of producing 80,000 to 140,000
20 volts, and curren~3from l to 10 milliamperes.
It is also highly desirable that both the reactive
coating and the porcelain enamel should be finely divided for
deposition by electrostatic coatingO Generally, the frit in each
case is gr~und in a ball mill to a retention o~ 002 to 2~ on a 2Q0
~7S;270
mesh screcn ~3 to 12~70 on a 325 mesh screen).
Since the reactive coat and the co~er coat are both
applied by electrostatic deposition, it is important that the
resistivity of the porcelain enamel powder be controlled for spray-
5 ing efficiency and also for adherence to the metal. Conse~uently,it is highly desirable to encapsulate the electrostatically sprayed
particles in a synthetic resin in order to increase the resistivity
of the particles to the range of 1012 to 1016 ohm centimeters. To
accomplish this, we preferably make use of the techniques des-
cribed and claimed in the Nedeljkovic U.S. Patent No. 3,930,062,
issued December 30, 1975 and assigned to the same assignee as
the present applicationO
Briefly stated, the Nedeljkovic patent describes a
method of pretreating borosilicate glass powders to reduce their
caking tendencies and to bring their resistivities up to where they
can be successfully sprayed by electrostatic deposition techniquesO
The anti-caking capabilities of the compositions are improved by
reacting the frit in particulate form with an alkoxysilane having the
formula:
Rn~i-(ocH3)4-n
where R is a methyl or phenyl group, or both, and n is 1 or 2,
These materials are combined with the frit in an
amount of about 002 to 008~ by weight of the mixture. Apparently
the alkoxysilane reacts with watex present to form a silanole
-16 -
1075270
which then presun~ably reacts with the hydroxyl groups in the
glass to improve the electrical resistivity.
The dry adherence of the frit to the substrate can be
improved by treating the frit with an adhesion promoter consisting
of silazane having the following formula:
R3 -Si-NH-Si- R3
where R is hydrogen, an alkyl radical, an aryl radical, or a
combination of alkyl and aryl radicals.
These materials are added in amounts of about 0. 05
to 0.5% by weight.
Additional benefits are derived in terms of improving
tfle deposition rate, when the mixture being sprayed also includes a
chlorosilane having the formula:
~n-Si-Cl4 n
where R is an alkyl or aryl radical, or both, and n is 1, 2 or 3.
The reactive coating is applied to the ferrous surface
to a thickness in the range of about 0. 5 to 2 mils (12 to 51 microns).
Preferably, the coating thickness is on the order of 1 to 1.5 mils
(25 to 38 microns).
After application of the reactive coating, a convention-
al porcelain enamel frit can be applied dry over the pre~coated
surface, also by means of electrostatic deposition. In order to
increase the electrical resistivity of the procelain enamel frit to
increase its adhesion tendencies, this ~rit is also treated by
encapsulation with a synthetic resin such as polyethylene, or with
_17-
1(~75Z7~
the silane compounds referred to in the aforementioned Nedeljkovic
patent. In any event, the electrical resistivity of the porcelain
enamel frit is modified until it is in the range of 1012 to 1016 ohm
centimeter .
S Following application of the cover coat, the coated
article is then fired in the normal way at temperatures ranging from
about 1300 to 1600F (704 to 871C) and preferably from 1400 to
1500F (760 to 816CC).
EXAMPLE
Pieces of cold rolled steel having thicknesses between
18 and 22 gauge ~0. 079 to 0.127 cm) were coated with a reactive
fritted glass composition having the following melted analysis:
SiO2 26. 8~o by weight
F2 7- 3%
Na2O 16. 6% " "
ZnO 3. 3% " "
CaO 8. 5% " "
BaO 6. 2% " "
K2O 3, 7%
Al2O3 1. 8% " "
CoO ~. 7% " "
MnO 0. 6% " "
~i2o 1 5% " "
P2O5 0. 6% " "
NiO 1. 3~0 " "
-18-
1C~7SZ70
~2C3 20.1% by weight
CuO 4. 19~, " "
103.1
Minus 0 for F2 3.1
100.0
The coating was applied to a thickness of about 1 mil
(25 microns) by means of a DeVilbiss electrostatic spray gun used
for the application of porcelain enamel coatings. The surface of the
cold rolled steel had merely been washed, rinsed and dried prior
to application of the coating. Next, a commercial cover coat
(Chi-Vit 14350) was milled to a fineness of 0.5 retention on a 200
mesh screen and it was then screened using a 60 mesh screen.
Approximately 0.5% of methyl trimethoxy silane and 0.2~ of
hexamethyl disilazane were added to the powder. This material
was electrostatically sprayed over the precoated part and then
fired at 14S0F (788~C) for 3-1/2 minutes. The resulting porcelain
enamel coating exhibited good adherence to the substrate and was
of good quality.
The advantages of the process of the present
invention are numerous. For one, it eliminates the pickling
ste~, the nickel plating tanks, associated rinse tanks and milling
equipment. Furthermore, it allows the use of non-remium steels
in one-fire enameling process. This, in turn, provides savings
in floor space, capital equipment ccst, energy, labor and materials,
as well as less water pollution. The quality of the finished part
is comparable to that obtained by current porcelaln enameling
- 25
processes"
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