Note: Descriptions are shown in the official language in which they were submitted.
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Mo 3400-CIP
MD-90-11-IC-CIP
ENAMEL COMPOSITIONS
CROSS-REFERENCE TO RELATED AppLIcA-rIoN
This application is a Continuation-in-Part of U.S.
Application Serial Number 07/490,215, filed March 8, 1990.
Field of the Invention
The present invention relates to enamel compositions which
are suitable for the production of opaque cover coats and which
do not contain fluorine and have a high boron content.
SUMMARY OF THEI NVENTION
o Fluorine-free frit compositions having a high content of
boron oxide and a relatively low amount of titanium oxide are
provided. A small amount of cobalt oxide is included in a
preferred embodiment. These compositions are suitable for the
preparation of enamel cover coats and are useful in both the
dry and wet methods of application of frits. In particular,
the frits ~re suitable for use in the electrostatic method of
frit application on steel substrates. The sompositions of the
invention are particularly suitable in "two coat-one fire"
applications. The enamel produced from the frit of the
invent;on is characterized in its virtual freedom from pitting,
relatively high acid resistance and exceptional color
stability.
BACKGROUND OF THE INVENTION
Fluorine has long been recognized and used as a fluxing
component in the manufacture of ena~el frits. In addition,
traditional raw materials for the preparation of frits
generally contain fluorine as a flux. Alkali and alkaline
; earth metal cations are routinely introduced as fluorides.
Recently, however, it has been recognized that fluorine
: 30 emissions have an adverse effect on the ecology and regulations
relative to its disposal have been promulgated and enforced.
~; Development of fluorine-free compositions have therefore been
an objective of enamel and of frit producers and the present
invention represents an advance in this technology.
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Enamel compositions containing titanium oxide and boron
are known. For instance, in U.S. Patent 4,469,798 there is
disclosed an enamel frit which contains 15 to 20% B203 and 10
to 17% TiO2. While the composition contains no lead and is
5 thus described as ;nvolv;ng no ecolog;cal problems, it is noted
to require 2 to 10% of fluorine. Fluorine is said to function
in agitating the frit and in keeping its acidity and is -
regarded there as an essential component of the composition. ~-
U.S. Patent 3,216,847 discloses a frit composition having
o high contents of boron oxide and titanium oxide. The
disclosure is silent in regard to the presence of fluorine and
no cobalt oxide is noted as a necessary component.
U.S. Patent 3,017,279 also discloses a frit composition
having high contents of boron oxide and titanium oxide. While
the reference discloses that fluorine is an optional component,
all of the examples are directed to the preparation of
fluor~ne-containing frits. Therefore, the examples of this
reference are consistent with the previous statement that
fluorine-free frits are not readily available.
U.S. Patent 4,361,654 is directed to the preparation of
fluorine-free frits which are suitable as ground coats for
sheet iron. This reference does not suggest the use of the
fluorine-free frits for the production of cover coats and the
examples demonstrate that the fluorine-free frits would not be
suitable for this purpose. Even though a broad range of
titaniu~ oxide is set forth, the highest amount used in any of
the examples is 4.1X by weight. At this low amount the
titanium oxide remains dissolved in the frit. Only when the
amount increases to about 8 to 10% does the titanium oxide
begin to crystallize out in the resulting frit such that the
frit starts to become opaque. As this amount increases to in
excess of 15%, the frit becomes completely opaque and attains
the bright white or almond appearance which is necessary for a
cover coat. The presence of 4.1X by weight or less in the
examples of U.S. Patent 4,361,654 is consistent ~ith the use of
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the fluorine-free frits as ground coats as opposed to cover
coats.
It is an objective of the present invention to provide a
fluorine free frit which ;s suitable for the preparation of an
.s op~que, white- or almond-colored, acid resistant enamel cover
coat.
It is a further objective of the invention to provide an
ecologically compatible frit composition having a desirable set
of properties.
DETAILED DESCRIPTION OF THE INVENTION
The frit of the present invention is characterized in that
(i) it flows to form a smooth and uniform coating at a
sufficiently low temperature, (ii) the coefficient of thermal
expansion of the resulting enamel is sufficiently low so as to
be compatible with most ferrous substrates, ~iii) the resulting
enamel is acid resistant, and (iv) upon the firing process, it
partially devitrifies and becomes opaque.
In terms of 1ts content, expressed as weight percent on -
the basis of its oxides, the frit of the present invention
contains
Oxide Wt %
B203 17 - 22
. SiO2 40 - 45
TiO2 15 - 18
K20 3 - 9
Na20 5 - 10
Al 23 1 - 4 ~ .
: Li20 0.25 - 1.0
In a preferred embodiment, the frit also contains
CoO 0.02 - 0.1
optionally, the composition may further contain, the amounts
noted as percent by weight,
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MgO O - 1.0
ZnO O - 1.0 -
P205 o - 3.0
Nb205 0 - 0.1
NiO - 0-5
Cr203 0 - 0.1 ; .
MnO2 ~ 1
Fe203 0 - 1 . O
- The frit is fluorine-free and has a relatively high -.
lo content of boron oxide and a relatively low content of titanium
oxide. A small amount of cobalt oxide is i~cluded in a -
preferred embodiment. In addition, the frit is characterized
in the absence of barium oxide therefrom. :
While the frit of the invention is suitable for preparing
enamel cover coats by using either a dry or a wet process, it -
has been surprisingly found that the composition of the
invent~on is particularly suitable for use in the dry method,
preferably the electrostatic meth~d, for applying frits. The
resulting enamel cover coat was surprisingly and unexpectedly
20. found to be uniform, color stable and resistant to chemical
attack by acids. This is particularly surprising in view of
the recognition in the industry that the dry method, and more
particularly the electrostatic method, is generally less ~`
forgiving, i.e., has a narrower margin for error, than other ;
25 methods of frit application.
The electrostatic method for applying frits to a metallic
substrate is well known in the art and described in U.S. Patent
; 3,928,668, the disclosure of which is incorporated herein by ~-
reference. In accordance with this method, a frit, optionally
30~ having first been coated with a suitable agent to impart a high
` resistivity, is electrostatically deposited by spraying it
uniformly over a substrate and fired to form an enamel. It has
long been recognized that the method, although offering several
processing advantages and resulting in an excellent enamel
s coating, is less~forgiving than the known wet method of
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application. This sensitivity to small variations in the
process may lead to poor quality in the resulting enamel. It
was therefore surprising and unexpected to find that the
composition of the present invention is very suitable for
application by electrostatic deposition and that process
variations do not result in enamels of poor quality.
In accordance with the present invention the frit may be
directly applied to a metal substrate or it may be applied to a
ground coat. In a preferred embodiment the frit is applied to
the ground coat before the ground coat is fired and then both
coats are fired. This method is referred to as "two coat, one
fire" process. Also, in accordance with the present invention
it is not necessary to blend the frit with other additives
prior to applying it to a substrate using the wet or dry
processes.
A high boron content in a frit composition is generally
associated with the poor acid resistance of the resulting
enamels. Accordingly, the good acid resistance of the enamels
of the present invention is also surprising and unexpected.
Specifically, while compositions containing about 20 percent
boron oxide are rated, at best, B in accordance with the PEI -
Porcelain Enamel Institute - acid resistance test, the
composition of the present invention, although containing a
high level boron, is consistently rated at least A on the same
scale.
In accsrdance with the method of application of the
composition of the present invention, the frit is applied on
the substrate at a rate of approximately 30 grams per square
foot. This rate of application results upon firing in an
enamel having a thickness of about 5 mils.
PROCEDURE OUTLINE
The following is a description of the procedure which was
used for preparing a composition within the scope of the
invention. The raw materials were weighed (the raw material
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formula is set forth in the table below), dry blended and
charged into a glass melting furnace.
Material Weight
Silicon Dioxide 1480.0
Sodium Borate (Na2B4O7) 623.0
Zinc Oxide 12.5
Potass;um Nitrate 135.0
Boric Acid 17.9
Lithium Carbonate 41.6
Titanium Dioxide 611.0
Potassium Carbonate 219.0 .
Aluminum Oxide 113.0
Sodium Tripolyphosphate 38.6
Magnesium Oxide 17.0
Sodium Borate Pentahydrate (Na2B4O7.5H2O) 674.0
Cobalt Hydrate/Silicon Dioxide 18.6
Compositions within the scope of the invention were melted
in a laboratory scale gas-air fired rotary smelter for about
12-18 minutes and at a temperature of 2000 to 2500F. The
resulting molten glass was poured into water to produce the
frit. Additionally, frit compositions were melted in
production scale continuous gas-oxygen fired smelters at a
temperature of about 2200-2400F. The production smelted
material was made into frit form by passing it through
water-cooled rollers. ;
The fritted material was comminuted using conventional
means. Typically, 80-85 percent by weight of the thus
comminuted frit has an average particle size of less than 45
micrometers (325 mesh).
3Q The thus produced frit was used to provide an enamel cover
coat to steel samples ùsing conventional wet and dry
application techniques.
Steel coupons (20 gauge extra low carbon cleaned steel)
were used as the coated substrate. Frit in accordance with the
invention was applied and the sample was fired at a temperature
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of 1350 to 1600-F. Holding times rang;ng from 3 to 10 minutes
were used to simulate commercial firing conditions. Also, a
high humidity condition was induced to simulate a worst case
condition in terms of the resulting surface appearance.
Standard evaluation tests were performed on the fired
samples, including the Porcelain Enamel Institute (PEI3 acid
resistance test, color stability under varying firing
conditions and the loaded beam stress test to determine the
compressive stress present in the fired coating. In all cases,
the surface quality of the enamels prepared from the frit of
the invention exceeded that of currently available commercial
products.
Modifications and variations to the above described
invention are possible and might occur to those skilled in the
art in view of the present disclosure. It is however to be
understood that such changes are within the scope of the
invention as defined by the following clai~s. -
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