Note: Descriptions are shown in the official language in which they were submitted.
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~^- ' 10 Back~round of the Invention
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.~ ! The present invention concerns a kitchen utensil that
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' has a vitreous cooking surface, and that is rendered resist-
ant to food sticking by application of a highly adherent,
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_ selected metal oxide coating. The coating is useful on both
~.~ cooking and baking ware, and facilitates the removal of food
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after cooking, especially in subsequent cleaning of the
cooking utensil.
Heretofore, the property of non-sticking has been
imparted to cooking utensils by coating the inside surface
of the utensils with an organic resin, especially a resin
containing fluorine, such as polytetrafluoroethylene, or a
silicone resin. However, these organic resin coated utensils
- tend to lack resistance to scratching and to overheating.
;i ~ Thus, in order to avoid damage of the coating by scratching,
..I,~ff* ~.
it is frequently recommended that metal implements, such as
spoons and knives, should not be used in contact with the
coating. Likewise, it is necessary to avoid overheating so
that the organic resin is not decomposed, since such decom-
position will not only destroy the characteristic non ~ cking
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- character of the coating, but also may release toxic pro-
_ duct.
It is then the object of the present invention to pro-
vide a kitchen utensil with an improved non-sticking coating
which is not subject to decomposition on overheating, which
is not subject to mechanical damage from metal implements,
~ and which otherwise avoids or minimizes the problems encoun-
`~';' tered with prior non-sticking coatings.
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~ State of the Art
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"~ 10 United States Patent No. 2,464,242, granted February
r 22, 1949 to P. S. Webb et al., discloses and claims a cook-
, ing implement having a silicone coated surface to facilitate
cleaning. United States Patent No. 3,393,086, granted July
16, 1968 to J. H. Keating, describes prior practice of
applying polytetrafluoroethylene to the surfaces of cooking
;^ and baking utensils to prevent food sticking. This patent
al`so points out the problem of poor adhesion of the coating,
and proposes a solution wherein a heavy metal frit is fired
on a rough aluminum surface to provide a base for the resin
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coating. I
~- Superficial oxide coatings applied to the external
surfaces of bottles and jars in order to décrease the fric-
tion between such glass articles and to improve resistance
~h~ to surface scratching are already known. For example,
United States Patent No. 2,831,780, granted April 22, 1958
to A. J. Deyrup, discloses that the scratch resistance and
; strength of glass articles are improved by pyrolyzing an
alkyl titanate, zirconate, or aluminate, on a hot glass
surface to form a corresponding metal oxide coating thereon.
However, it is not disclosed that such metal oxide coatings
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have the necessary mechanical and chemical durability required in cooking
utensils, nor is it disclosed that such coatings impart any resistance to
the adherence of food in the course of cooking or subsequent standing in
contact with food.
Statement of Invention
The present invention provides a cooking utensil comprising a
vitreous cooking surface containing silica in the compoisition thereof and
having covalently chemically bonded thereto a highly adherent, mechanically
and chemically inert, coating of a metal oxide that is resistant to food
sticking, the metal oxide being selected from the group consisting of
alumina, zirconia, titania, and mixtures thereof.
The invention further contemplates a method of rendering the ~-
vitreous cooking surface of a cooking utensil resistant to food sticking,
I said vitreous cooking surface containing silica in the composition thereof,
which comprises applying a metal oxide coating selected from the group
consisting of alumina, zirconia, titania and mixtures thereof to the
vitreous surface and then thermally treating the metal oxide coated
vitreous surface at a temperature of at least 475C., but below the
temperature at which the utensil deforms, to covalently chemically bond the
metal oxide of the coating to the silica of the vitreous surface.
General ~escription
The cooking utensil of the present invention may be of any
;~ construction that embodies an internal vitreous surface, that is, a vitreous
cooking surface on the utensil. Thus, it may be a molded utensil of any
~i conventional glass or glass-ceramic material, such utensil being molded
from glass and either used as such or thermally converted to a glass-ceramic
u~ensil. Alternatively, the utensil may be glazed ceramic ware or enameled
metal ware. The single requisite of the utensil, as such, is a vitreous
cooking surface.
The problem of food adherence to dishes in general, and cooking
dishes in particular, is well recognized. As pointed
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out earlier, a great deal of effort has been expended in the
development of organic resin ~oatings to provide a surface
f- ~ resistant to such food sticking or adherence. In at least
some instances, these coatings have been eminently successful
with respect to the prevention of food sticking, until they
were mechanically damaged by metal cutlery or by other
mechanical scraping or impact. In spite of intensive study,
the problems of mechanical damage and lack of adherence have
persisted with the known coatings. In contrast, the coating
of the present invention is a highly adherent, mechanically
and chemically inert coating that avoids the problems inherent
" in prior organic type coatings.
.j The present coating is a very thin oxide coating which
may consist of alumina, zirconia, or titania, either singly
or in mixture, as desired. These coatings are mechanically
resistant to scraping or scratching with metal cutlery or
other implements, and are chemically inert with respect to
such materials as food acids and detergents. Further, they
can be rendered highly adherent to vitreous surfaces. The
_ 20 coating must of course be a continuous coating, that is must
not leave any open areas or discontinuities on the coated
surface. Otherwise, thickness is not particularly important,
except as greater thicknesses become economically wasteful
of material, or may tend to spall or crack in case of excess-
: ~ 3 ively thick coating
The oxide coating may be applied by any conventional
coating practice. Thus, the selected oxide may be vaporized,
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for example by electron bombardment, and the vapors con- i
densed on the vitreous surface~ The technique described in
earlier mentioned United States Patent No~ 2,831,780, that
~~~-- is, pyrolysis of an organo-metallic compound, is also a
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very useful technique. Cathodic sputtering is another
frequently used method of oxide application. Other thermal
7''t decomposition and vapor deposition techniques are known and
~~ may be used when particular conditions warrant.
Thus, the formation of an oxide coating on a surface is
a well known technique, and the present invention is not
based on either this principle, as such, or on a specific
method of applying an oxide. As mentioned earlier, oxide
coatings have been applied to the external surfaces of
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~s3 lo bottles and jars as a lubricant to minimize sticking of such
glass surfaces to one another, and consequent scratching and
.z~ damage thereto. Such external coatings on containers were
not exposed to food sticking as occurs in cooking utensils.
Neither are they exposed to the repeated usage that is
-customary with cooking utensils. Efforts to substitute such
coating for organic resin coatings on cooking surfaces have
heretofore proved unsatisfactory.
The excellent adherence obtained between the oxide film
and the vitreous surface, in accordance with the present
invention, is attributed to a superior bond between the
oxide constituents thereof. In our preferred practice, we
thermally treat an oxide coated vitreous surface after
deposition of the oxide coating.1 Such thermal treatment may -
be at a temperature above 450C. but necessarily below the
deformation temperature of the ware being treated. It is
our belief that a covalent chemical bonding is established
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between the metal oxide of the coating and the silica of the
vitreous network of the cooking utensil surface.
The kitchen utensils conforming to the present inven-
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~' ~ 30 tion can be characterized by the following test method,
~ which demonstrates the non-adherence of food and the fact
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that this property is conserved in spite of subsequent
chemical and mechanical treatment.
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Two cooking vessels, one treated according to the pre-
~` ~ sent invention and one untreated, both containing a piece of
meat, are placed side by side in a domestic oven the meat
being cooked in the normal manner. When the meat is cooked
_ the two dishes are equally blackened, on almost all their
interior surface, either by spattering onto the sides or by
-r- ~ burnt juices on the base.
o After taking out the meat and quickly emptying out the
remaining contents, the two vessels are placed side by side
in an automatic dish washing machine where they are sub-
~r-r~ mitted to a normal washing cycle. After removal, the treated
vessel is perfectly clean whereas untreated vessel is still
black on almost all its surface and must be scoured.
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' . i J The treated vessel is now submitted to a test of 4
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~ ~ ~ hours duration in a boiling 0.1% solution of a commercial
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detergent (trademark Mach I) recommended or use in industrial
dishwashing machines. This test simulates about 2000 actual
machine cycles and is generally equivalent to approximately
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10 years service for a cooking vessel.
An alternative detergent test is to place the vessel in
a boiling solution of 1% trisodium phosphate Ma3PO4 for 30
minutes. According to the German standard DIN 51 035, this
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test is equivalent to 5000 machine cycles.
After one of these detergent tests, the vessel con-
forming to the invention has not changed in appearance. In
order to verify that there has been no effect on the property
of non-adhesion the vessel is resubmitted to the cooking and
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washing test. The result obtained is identical to that
~ obtained previously.
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The following are some non-limi~ing examples of pro-
cedures used to produce non-adherent cooking utensils con-
forming to the invention:
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Example 1
A PYREX~ brand cooking vessel molded from a borosili-
' cate glass, previously degreased, is placed in an enclosure
~ ~ under vacuum. Titanium monoxide, TiO, is evaporated by
'-r~ ~~ means of electronic bombardment and deposited onto the glass
- ~ forming a thin film, pale brown in color. ~he vessel is
then submitted ~o a thermal treatment of l hour at 500C.
";,~ The titanium monoxide coating, TiO, is converted to a higher
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oxide TinO2n_l or TiO2 and the film becomes transparent,
possibly slightly colored because of the phenomenon of light
interference.
Example 2
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A TAMARA~ brand cooking vessel molded from a borosili-
cate opal glass is heated at 500C. for 10 minutes. A
_ solution of titanium isopropylate in isopropyl alcohol is
then sprayed onto the hot vessel with the aid of a spray gun
as is described in U.S. Patent No. 2,831,780 or Dutch Patent
64 04159 published on October 19, 1964. The film of titanium
oxide thus obtained is not sufficiently adherent to the
` ~ substrate. Adequate adhesion of the film is obtained by a
;,`t~ t~ subsequent heat treatment of 1 hour at 600C. This treatment
. can be replaced by heating directly in a flame.
The oxide film finally obtained is transparent and
possibly irridescent as that of Example 1.
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108t~S63
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Example 3
A steel frying-pan with a vitreous coating and an -
enamelled cast iron frying-pan are coated with a film of
titanium monoxide, TiO, following the method described in
Example 1. They are then heat-treated for 1 hour at 475C.
Example 4
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~ ; A PYREX~ brand cooking vessel molded from a borosili-
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"~ cate glass is heated to 400C. and then exposed for 30
.,S seconds to the vapor of aluminum isopropylate as described
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` 10 in U.S. Patent No. 2,831,780 previousIy cited. The film
$ thus obtained is not durable.
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The vessel then undergoes a subsequent heat-treatment
of 1 hour at 650C.
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Example 5
J` In vacuum equipment furnished ~ith apparatus for cathodic
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sputtering, a glass cooking vessel is placed on the anode
and alumina at the cathode.
Under the effect of the high tension existing between
the two electrodes, the alumina volatilizes into a plasma
which is attracted to the anode and thus onto the glass
vessel.
With a film thickness of about 0.1 micron and in any
case more than 0.05 micron the vessel possesses the required
qualities of non-adherence and durability.
Example 6
A PYREX~ brand vessel molded from a borosilicate glass
is placed in a vacuum chamber, under very low pressure.
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Under the effect of electronic bombardment, alumina is
evaporated onto the vessel to form a thin film. The film
thus obtained is not durable. Adequate durability is obtained
by a subsequent heat treatment, in the circumstances by
direct flame treatment.
Example 7
A solution of 1% by weight of hydrated zirconyl chloride,
--i''' ZrOC128H20, is prepared. This solution is sprayed onto a
d'~ PYREX~ brand cooking vessel previously heated to 400C. The s
q
vessel then undergoes a thermal treatment of 1 hour at
;;i~,.~J 600C. or at 700C. One obtains a thin transparent uncolored
film.
~ Example 8
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' r~ . A glass cooking vessel is placed in a vacuum chamber
under very low pressure. Under the effect of electronic
bombardment a mixture of titanium monoxide and alumina,
essentially equal in proportion, is evaporated onto the
_ vessel.
The film thus obtained is only durable after a subse- I
quent thermal treatment, in the circumstances a treatment of
1 hour at 550C.
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