Note: Descriptions are shown in the official language in which they were submitted.
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NON-STICK COATING FOR ALUMINUM COOKING VESSELS
BACKGROUND OF THE INVENTION
Field of the invention
The present invention concerns a non-stick coating
- 5 for aluminum cooking vessels such as saucepans and frying
pans.
Description of the prior art
Most non-stick coatings are based on
polytetrafluorethylene (PTFE).
To make such coatings adhere to the surface of the
aluminum the latter is usually etched with acid to create
microcavities encouraging the a&esion of the PTFE
coating or a surface roughness of approximately
12 microns is produced by mechanical treatment to enable
the adhesion of a primer layer.
A coating adhering to an aluminum substrate that
has not received any chemical or mechanical treatment to
roughen its surface is known from PCT application
WO 91/02773.
The coating described in this PCT application
comprises an undercoat layer, a primer layer and a top
finish layer.
The undercoat layer is obtained from an aqueous
phase containing a surfactant, silica, a polyamide-imide
resin (PAI), polyphenylene sulfide or polyether sulfone.
The primer layer is obtained from an aqueous
dispersion of polytetrafluorethylene (PTFE) to which are
added polyamide-imide resin (PAI) and tetrafluorethylene
and poly (propyl vinyl ether) (PFA) copolymer.
The top finish layer is obtained from an aqueous
dispersion of PTFE.
The above layers are all applied to the substrate
by spraying in the aqueous phase.
After drying, these layers are baked in an oven to
sinter the coating.
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The undercoat layer has the drawback of requiring
transformation of the organic phase of the PAI resin into
the aqueous phase and the presence of surfactants.
The aim of the present invention is to create a
non-stick coating that shows excellent adhesion to
aluminum substrates whether or not they have been
mechanically or chemically treated to roughen their
surface.
SUMMARY OF THE INVENTION
Accordingly, the invention concerns a non-stick
coating for aluminum cooking vessels comprising an
undercoat layer applied directly to the aluminum, a
primer layer applied to said undercoat layer and a top
finish layer applied to said primer layer, said primer
layer being obtained from an aqueous dispersion of
polytetrafluorethylene to which are added polyamide-imide
resin and tetrafluorethylene and poly (propyl vinyl
ether) copolymer, said top finish layer being obtained
from an aqueous dispersion of PTFE and said undercoat
layer being obtained from a polyamide-imide resin in
solution in an organic solvent.
Tests have shown, surprisingly, that an undercoat
layer of this kind containing an organic solvent adheres
to the substrate just as well as the prior art aqueous
phase undercoat layer.
This undercoat layer does not require any complex
transformation from an organic phase to the aqueous phase
in the presence of surfactants and amides.
The undercoat layer may be obtained from a mixture
of polyamine-imide resin and polyphenylene sulfide.
The organic solvent used in the undercoat layer is
preferably selected from the following compounds:
N-methylpyrrolidone, dimethylformamide or an NMP/xylene
or DMF/xylene mixture.
The undercoat layer may contain inert charges such
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as mineral pigments, metal flakes, alumina, quartz.
The undercoat layer of the coating of the invention
shows excellent adhesion to aluminum even if the surface
of the latter has not been subjected to any form of
chemical or physical treatment other than degreasing.
In one version of the invention the primer layer
and the top finish layer are applied by the silkscreening
or roller coating process.
The method of applying a non-stick coating by the
silkscreening process and the attendant advantages are
described in French patent 2 689 037 in the name of
SEB S.A.
In this case, the primer layer and the top finish
layer also contain a thickening agent such as an acrylic
resin.
There follows one example of the composition (in
parts by weight) of the undercoat layer of the coating of
the invention.
. dry PAI (polyamide-imide) resin 30
. N-methylpyrrolidone 230
. xylene 0 - 10
. pigment (mineral or carbon black) 0 - 25
. aluminum flakes 0 - 40
. PPS (polyphenylene sulfide) 0 - 80
The aluminum flakes encourage adhesion of the
primer layer to the above undercoat layer. Their
presence at the surface of the undercoat layer confers on
the latter a roughness equivalent to that of mechanical
treatment and provides an armature for the final coating
in the manner of a mechanical treatment.
To this end, their dimensions must not exceed
25 microns.
The above undercoat layer may be sprayed onto an
aluminum disc that has merely been degreased or applied
by the silkscreening process or any other conventional
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means.
The undercoat layer is then dried to eliminate the
organic solvent.
The primer layer and the top finish layer are then
applied in succession by the silkscreening process as
described in French patent 2 689 037 or any other
conventional means.
The primer layer is made up of an aqueous
dispersion of PTFE to which has been added a PAI resin, a
PFA dispersion and, in the case of application by the
silkscreening or roller coating process, an acrylic
resln.
The top finish layer applied after the primer layer
has dried is made up of an aqueous dispersion of PTFE,
mica flakes, a pigment such as carbon black and, in the
case of application by the silkscreening or roller
coating process, a thickening agent such as an acrylic
resln .
After drying the top layer, the three layers are
baked simultaneously in an oven for 1' - 10' at
approximately 400C - 420C.
Tests show that the coating obtained in this way
adhere perfectly to the support.
When an aluminum disc is coated with these three
layers by the silkscreening or roller coating process,
for example, it can be formed after sintering into a
cooking vessel such as a saucepan, frying pan, cake tin,
etc without degrading the adhesion of the coating to the
support.
