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Description  |
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BACKGROUND OF THE INVENTION
Cookware is often coated with nonstick coatings. For high temperature
top-of-the-range applications, fluorocarbon coatings are preferred.
However, such coatings require relatively high baking temperatures with
consequent high energy consumption, and need to utilize materials for the
cookware substrate which will not warp or give off gases at the baking
temperatures used.
Cookware intended for use in an oven, known as bakeware, need not be
designed to withstand as high temperatures as top-of-the-range cookware.
This permits more use of silicone or polysiloxane coatings. Such coatings,
in comparison with fluorocarbon coatings, are often less durable and less
adherent to the substrate.
Multi-layer coatings have been used for cookware coatings, especially for
fluorocarbon coatings. There remains a need for bakeware coatings with
superior durability, adherence and aesthetics, along with low-cost and
low-energy requirements for applying the coatings.
SUMMARY OF THE INVENTION
The invention provides an article bearing a multi-layered coating
comprising:
a primer coat comprising an epoxide-nitrogen-formaldehyde resin and
optionally aluminum flake,
an intermediate coat comprising a polymethylphenylsiloxane resin derived
from a cross-linkable precondensate having a silicon-bonded hydroxyl
content of at least about 3%, preferably about 5%, based on the weight of
the precondensate, an epoxy resin, talc, and aluminum flake, and
a topcoat comprising a polymethylphenylsiloxane resin derived from a
cross-linkable precondensate having a silicon-bonded hydroxyl content of
about 2-4%, based on the weight of the precondensate, and aluminum flake.
Preferably, the epoxide-formaldehyde-nitrogen resin of the primer is the
reaction product of an epoxy resin and a urea-formaldehyde resin wherein
the epoxy resin is a condensation product of epichlorohydrin and bisphenol
"A" having an epoxy equivalent weight of 1700-2500 and the urea
formaldehyde resin has a combined molar ratio of urea:formaldehyde:butanol
of about 1:2:1. (Percentages and proportions herein are by weight except
when indicated otherwise.)
DETAILED DESCRIPTION OF THE INVENTION
The epoxide-formaldehyde-nitrogen resin primer of the invention provides
good bonding to substrates of steel, tinplated steel, stainless steel and
aluminum.
The reactive polysiloxane-epoxy intermediate coat containing aluminum flake
provides desirable aesthetics and a good base for the topcoat which uses a
less reactive but more elastic polysiloxane that would provide a
less-durable coating without the intermediate coat. The talc can act as an
extender but, combined with the aluminum flake, it also aids in
reinforcing the intermediate coat. The appearance of coatings with coarse
aluminum flake is less dulled by the talc than would be that of coatings
with fine aluminum flake.
The elasticity of the topcoat makes it more resilient and less subject to
deterioration of appearance as a result of use and abuse during cooking.
The aluminum flake aids in distributing heat through relatively thick
coatings and aids in bonding the intermediate and topcoat together. Flakes
of metal other than or in addition to aluminum, such as stainless steel or
nickel, may also be useful. In addition, titania-coated mica pigment can
give desirable property and appearance effects, such as "Afflair"
pigments, products of E. M. Laboratories, Inc., of Elmsford, New York,
generally described in U.S. Pat. Nos. 3,087,828 and 3,087,829 - Linton
(1963).
Preferred thicknesses are 5-8 microns (.mu.m) for the primer, 15-20 .mu.m
for the intermediate coat, and 10-12 .mu.m for the topcoat, for a total
thickness of 30-40 .mu.m.
In the epoxide-nitrogen-formaldehyde resin of the primer, it is desirable
to use as the epoxide a condensation product of epichlorohydrin and
bisphenol "A" such as Shell Oil Company's "Epon 1007" which has an epoxy
equivalent weight of 1800-2400. Epoxy equivalent weight of 1700-2500 is
desirable for use with the invention. Instead of urea-formaldehyde resin,
one can use other nitrogen-formaldehyde resins such as
melamine-formaldehyde or benzoguanamine-formaldehyde resins.
A suitable epoxide-nitrogen-formaldehyde resin is described in U.S.
application Ser. No. 871,697, filed Jan. 23, 1978--Wald now abandoned.
A variety of epoxy resins can be used in the primer as well as in the
intermediate coat.
The reactivity, hardness and durability of polysiloxane resins are
determined by several factors. The silicon-bonded hydroxyl content of the
precondensate is one important factor. A higher hydroxyl content causes
more cross-linking and greater hardness in the resulting resin. Suitable
polysiloxanes are described in U.S. application Ser. No. 829,824, filed
Sept. 1, 1977--Vassiliou.
Preferably, the polysiloxane resins used herein are derived from a
resin-forming precondensate having one or more of the units
##STR1##
wherein X is a functional group (preferably a hydroxyl group) which allows
cross-linking at its site, and a, b, c, d and e are of a magnitude and in
proportions which provide the resin with a degree of substitution,
measured as the ratio of phenyl plus methyl groups to silicon atoms, of at
least about 1.0, and a ratio of phenyl groups to silicon atoms of at least
about 0.3.
Precondensates having a random arrangement of the repeating units in any
order are, of course, included.
It is thought that the invention can use a broad variety of polysiloxane
resins. Such resins are provided in coating compositions in the form of
resin-forming precondensates of varying molecular weight containing
functionality, generally silanol functionality, which causes cross-linking
upon curing, such as by heat or catalysis, to form a rigid
three-dimensional macropolymer matrix.
The precondensate will generally be of relatively low molecular weight,
such as 1000 or 1100, and enough of the precondensate molecules will have
three or more functional groups to cause the formation of a resin with a
rigid, cross-linked, three-dimensional matrix upon condensation or curing.
Preferably, the matrix of a polysiloxane resin will be hard and have a low
degree of elongation, such as 2-4%, in contrast with elastomers which are
soft and have a high degree of elongation, such as 30% or more. Typically,
polysiloxanes to be used in the intermediate coat will have a pencil
hardness of F at 25.degree. C. and B at 260.degree. C., while the softer
polysiloxanes to be used in the topcoat will also have a pencil hardness
of F at 25.degree. C. but of 5B at 260.degree. C.
Particularly useful in the topcoat of the present invention is
Rhone-Poulenc's Rhodorsil resin 2104 which has a silcon-bonded hydroxyl
content of about 2-4%, a ratio of phenyl groups to silicon of 0.4 and a
ratio of methyl groups to silicon of 0.9.
Polysiloxane resins and resin-forming precondensates of the invention can
be prepared by techniques known in the art. Generally, desired proportions
of dimethyldichlorosilane, methylphenyldichlorosilane, and sometimes
diphenyldichlorosilane are hydrolyzed to form cyclic structures, and then
the cyclic structures are polymerized with acid or base to form the
polysiloxane resin-forming precondensate. It is evident that appropriate
proportions of the units selected for the structural formulas must be
provided in order to achieve the ratios of the preferred compositions and
in order to obtain the desired resin-forming precondensates. For instance,
those skilled in the art would know that if a, b, c and e are each zero
and the structure is made entirely of
##STR2##
the resulting composition would be hydrated silicon dioxide which is not a
resin-forming precondensate. Also if b, c and d were all zero and the
resin were made entirely of
##STR3##
the result would be an elastomer rather than a resin.
The three coatings can also contain such conventional additives as flow
control agents, surfactants, plasticizers, antioxidants, oxidation
catalysts, colorants, etc., as are necessary or seem desirable. Such
additives are used in nonstick coatings in accordance with customary
practices by those skilled in the art.
The invention is illustrated by the following example.
EXAMPLE
The primer, intermediate and topcoat coating compositions are formulated by
mixing the ingredients described below. Grinding is used to make
dispersions used in the formulations. Colorants can be added to the
intermediate coat for desired effects. The primer is applied by any
suitable technique, such as spraying or brushing, and allowed to dry in
air at ambient temperature (about 25.degree. C.) for 5-7 minutes until it
is tack-free, or it can be force-dried at 60.degree. -80.degree. C. for
1-2 minutes. Then the intermediate and topcoat compositions are similarly
applied. After all three coatings are in place, the coated article is
heated at low temperatures to cure the multi-layer coating. A suitable
curing schedule is 10 minutes at 220.degree. C., or 5 minutes at
250.degree. C., or 20 minutes at 200.degree. C., or 3-4 minutes at
275.degree. C.
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Parts by
Weight
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PRIMER
Epon 1007 - solution in xylene
33.91
and methyl isobutyl ketone
(epoxy equivalent weight 1800-2400)
Diacetone alcohol 20.59
Urea-formaldehyde resin 4.58
Carbon black - dispersion in melamine
6.15
formaldehyde and butyl carbitol
Hydrocarbon solvent - BP 150.degree.-190.degree. C.
12.93
Xylene 2.02
Butyl acetate 17.60
Fine aluminum flake - particle size -
1.32
median 7 .mu.m - Alcoa 1593 paste
INTERMEDIATE COAT
Reactive polymethylphenylsiloxane
35.13
resin, 5% OH - Dow Corning 6-2230
or Rhone-Poulenc Rhodorsil 6352E
Xylene 5.58
Butyl acetate 9.05
Cellosolve acetate 5.14
Methyl isobutyl ketone 2.24
Talc - millbase containing 34.5%
27.47
laminar IT Extra Micro Talc -
Norwegian Talc Company - particle size
median 2-4 .mu.m dispersed in above
siloxane and solvents
Mica flake coated with titania
0.85
Afflair.RTM. pigment - E. M. Laboratories,
Inc.
Epon 828 (epoxy equivalent weight 180)
4.50
Zinc octoate - solution in mineral
0.51
spirits containing 12% Zn.sup.+2
Coarse bright aluminum flake - particle
4.58
size - median 40-44 .mu.m Silberline
SS-3166-ER
TOPCOAT
Polymethylphenylsiloxane resin
99.75
35.5% solution in xylene, glycol and
n-butanol, 2% OH containing iron
octoate catalyst - Dow Corning
DCR-9503
Coarse bright aluminum flake
0.25
Silberline SS-3166-ER
or -
Polymethylphenylsiloxane resin
99.50
60% solution in xylene, glycol and
n-butanol, 2-4% OH - Rhone-Poulenc
Rhodorsil 2104 with added iron octoate,
cerium octoate or zirconium octoate
catalyst
Coarse bright aluminum flake
Silberline SS-3166-ER
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