REFERENCE TO RELATED APPLICATION
This application claims the benefit of priority of application serial No.
60/058,148, filed September 8, 1997.
FIELD OF THE INVENTION
This invention relates to non-stick coated article, with a decorative pattern
having a three dimensional effect. The instant invention also relates to producing
a decorative pattern in coated cookware while maintaining a smooth non-stick
surface which allows for easy release of food particles.
BACKGROUND OF THE INVENTION
It has long been desirable to produce coated cookware which has
decorative appeal and maintains good release properties. One attempt to produce
patterned cookware which exhibits an illusion of optical depth is described in GB
1,131,038 (Tefal). The specification discloses a process for producing a pattern of
flaked magnetic particles in a polytetrafluorethylene (PTFE) matrix as a coating
on a substrate. The process is carried out by mixing the flakes with an aqueous
dispersion of PTFE and coating the dispersion onto the substrate. After the
coating step, a magnet is placed on the underside of the substrate (base), and the
magnetic field from the magnet causes the flakes to be attracted toward the
magnet. As shown in Figure 3 of the '038 patent, this movement includes the
vertical and near vertical orientation of the flakes within the coating thickness and
the flakes are entirely contained within the coating, which means that their largest
dimension is smaller than the thickness of the coating. This requires either thick
coatings or very small flakes (small largest dimension). The problem with small
flakes, however, is that they tend not to form a distinguishable pattern in the
coating. Consequently, thick PTFE coatings are necessary to produce a visible
pattern. Even then, the vertical orientation of the flakes by the magnetic lines of
force inevitably causes flakes near the top surface of the coating to protrude from
the surface, causing roughness of the baked coating, which is undesirable for a
release coating. The '038 patent also discloses that the base has cavities in it, i.e.,
it has a rough surface, which enables the flakes to be immobilized during the
baking of the coating. Among the problems with the magnetic patterning of the
release coating by the process of the '038 patent is the need for an excessively
thick PTFE coating, which nevertheless fails to completely contain all of the
flakes within its thickness and the need for a roughened substrate for adhering the
coating to the substrate and immobilizing the flakes during sintering.
Another problem with the pattern formed by the process of the '038 patent
is that the pattern is "fuzzy", i.e., lacks clarity. When the coated substrate is
placed directly on the magnet of Figure 1 of the '038 patent, the annular pole
piece of the magnetic is reproduced in the coating as a toroid ring, deviating from
the shape of the circular ring of the pole piece serving as the pattern. When a
shaped plate is laid across the top of the magnet, the resultant imprint of the
shaped plate is especially fuzzy where the magnetic force is directed through the
bulk area of the shaped plate as shown in Figure 2 of the '038 patent. The "fuzzy"
image is a manifestation of the of the '038 patent method producing unwanted
field lines (magnetic background effects); such method also produces a rough
decorative surface. If a stronger magnet is used in the method of the '038 patent,
to try to eliminate the fuzziness of the image, i.e. sharpen the image, another
unwanted background effect occurs, namely reproduction of the shape of the
magnet in the pattern in the coating.
In addition to design, cookware often includes liquid level markings on the
inside sidewalls of pots and pans or the like. Traditionally, such markings have
been achieved by embossing the metal base prior to overcoating with nonstick
finish. However, the depressions protrusions formed by embossing can interfere
with the release properties of the surface, causing a buildup of food deposits and
becoming a source of corrosion.
SUMMARY OF THE INVENTION
The present invention in its various embodiments solves the problem of
excessive coating thickness while still being able to produce smooth release
coatings containing magnetically induced flake patterns within the coating,
enables smooth substrates to be used and provides patterns of improved clarity,
e.g. line patterns, including novel patterns forming liquid level indicators. In one
embodiment, the present invention provides a substrate having a baked release
coating thereon which comprises fluoropolymer and magnetizable flakes, a
portion of said flakes being oriented in the plane of the substrate and another
portion of said flakes having been localized magnetically reoriented from the
plane of the substrate, the portion of said flakes which are magnetically reoriented
having a different appearance in reflected light than the portion of said flakes
oriented in the plane of the substrate, whereby the portion of said flakes which
have been magnetically reoriented forms a pattern in said coating, said flakes
having their longest dimension being greater than the thickness of said coating.
In the application of the coating composition in liquid form to the
substrate, the flakes orient themselves generally parallel to the plane of the surface
of the substrate, and the localized magnetic reorientation of the flakes causes the
flakes to tilt (reorient) from the original planar orientation. This tilt will vary from
perpendicular to the original planar orientation, i.e. perpendicular to the surface of
the substrate being coated, to less than perpendicular to the original plane. The
planar oriented flakes reflect incident light back to the viewer, while the reoriented
flakes do not. Thus, where the magnetic reorientation of the flakes is present in
the coating, this gives the appearance of a pattern in the coating. It is important
that the flakes be able to reflect light back to the viewer, and this is the reason why
large flakes (long dimension greater than the coating thickness) are used. Small
flakes are insufficiently reflective to give a distinct difference in appearance
between the area of reoriented flakes and planar disposed flakes, or in other words
to give a distinct pattern in the coating.
Because of the long dimension of the flakes being greater than the release
coating thickness, the reoriented flakes may protrude from the surface of the
coating, while the flakes which lie in the plane of the coating, i.e., not tilted, will
generally not protrude from the surface of the release coating. Even though some
of the reoriented flakes protrude from the surface of the release coating, the
protruded portions of such flakes are coated with the composition of the release
coating to form "mounds" of release coating encasing the protruding portions of
the flakes. The profile of these mounds, tapering into the flat surface of the
coating, enable the coating (after baking) to serve as a release coating. By running
one's finger over the surface of the baked coating, one can feel that the overall the
surface of the patterned release coating is smooth, and that the area of the pattern
that appears dark to reflected light, is slightly less smooth than the area that
reflects light, but nevertheless serves as a release coating, e.g., releasing food
cooked thereon.
In one embodiment of the present invention, the pattern is decorative.
When the substrate is cookware or bakeware, the pattern can be present on the
cooking (baking) surface and give the appearance of being three dimensional even
though the release coating on the substrate is smooth. In a preferred embodiment
the release coating is smooth, the smooth surface characterized by a surface
roughness of less than 1.5 micrometers. In another embodiment, the pattern is in
the form of liquid level indicia in the sidewall of the release-coated vessel. This
sidewall marking information is provided by the magnetic reorientation of the
flakes without any embossing of the cookware or bakeware sidewall and with the
coating containing the magnetically reoriented flakes being sufficiently smooth
surfaced to still serve as a release coating.
In another embodiment, the substrate surface is smooth and the coating is
adhered to the substrate through a primer layer on the substrate. In a preferred
embodiment, the substrate smoothness is characterized by an average surface
roughness of less than 1.5 micrometers. In another preferred embodiment, the
coating containing the flakes is in two parts. a midcoat layer and a topcoat layer.
The flakes are in the midcoat layer and the topcoat can either insure that no flakes
protrude from the surface of the overall coating or can smooth out the mounds
which encase flakes protruding from the midcoat layer, depending on the
thickness of the topcoat. The thickness of the midcoat layer and preferably the
combined thickness of the midcoat and topcoat layers is less than the length of the
long dimension flakes, in which case while smoothing out the surface of the
midcoat, the topcoat will telegraph the tops of the underlying mound through the
flat surface of the topcoat. This smoothing out provided by the topcoat further
improves the release character of the release coating. If a roughened substrate is
used, which does not require a primer layer, the midcoat described above will be
the bottom layer or undercoat layer.
The coated substrate of the present invention is preferably made by a
process wherein with the application of an aqueous dispersion comprising
fluoropolymer and the magnetizable flakes to the substrate, the resultant liquid
coating is subjected to localized magnetic force to produce the pattern of
reoriented flakes desired. Preferably the aqueous dispersion is applied
simultaneously to the substrate with the application of the magnetic force.
Another departure from the process of British patent 1,131,038 is how the
magnetic force is applied to the flakes, namely from a diffuse magnetic field
rather than directly from the magnet itself. The magnet which is the source of the
magnetic force is spaced from the substrate being coated. The magnetic force is
communicated across the space between the magnet and the flakes in the coating
from a diffuse magnetic field intervening between the magnet and the coating
through a die of magnetizable material positioned between the diffuse magnetic
field and the coating on the substrate. The diffuse magnetic field isolates the
coating from direct exposure to the magnetic field of the magnet, eliminating
unwanted background effects from the pattern, thereby improving pattern clarity.
The magnetizable die has reduced " background effects" on the pattern, i.e.,
greater clarity, than when the coating is subject to direct exposure of the magnetic
field of the magnet. By background effects is meant that the magnetic force
operates on flakes lying outside the edges of the desired pattern causing such
background flakes to move out of planar configuration. These background effects
cause unwanted fuzziness or increased darkness of the pattern edges. Another
unwanted background effect is reproduction of the shape of the magnet in the
pattern formed in the coating. Thus, in accordance with the present invention, the
shape of the pattern can both be sharp and be independent of the shape of the
magnet and the pattern can be in the form of lines rather than thick imprints of the
source of the magnetic force as in the '038 patent. The magnetizable material can
be considered the die for the pattern.
In one embodiment, the die is of sheet metal construction. e.g., forming an
annulus, with the "knife" edge of the sheet metal shape (looking like a "cookie
cutter") serving as the die. In another embodiment, the die is one or more pins.
The edge of the sheet metal die forms a line pattern in the coating corresponding
to the shape of the edge (s) of the die. Depending on the spacing of the pins from
one another, the ends of the pins form a pattern of disconnected non-reflective or
connected non-reflective (lines) regions . Such a configuration of pins is
particularly useful for patterning sidewalls which have curved surfaces such as
with liquid level markings. In still another embodiment the die can be a plate
having a configured edge and/or cut-outs. Instead of the plate being positioned
"on-edge" to form the pattern in the coating, a lateral face of the plate is aligned
with the bottom of the substrate to be coated, whereby the pattern present in the
plate being subjected to the diffuse magnetic field is reproduced in the coating.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 shows in schematic side elevation an equipment arrangement for
forming a magnetically induced pattern in a fluoropolymer release coating on one
embodiment of substrate.
Figure 2 is a perspective view of the magnetizable die used to form the
pattern in Figure 1.
Figure 3 shows a plan view of the substrate (frying pan) of Figure 1 with
the magnetically induced pattern visible in the release coating on the substrate.
Figure 4 shows in side elevation and enlarged cross-section the
magnetically reoriented magnetizable flakes deflecting incident light on the
release coating to produce the pattern shown in Figure 3.
Figure 5 shows in side elevation and enlarged cross-section a preferred
embodiment of the release coating of the present invention.
Figure 6 shows in perspective another embodiment of magnetizable die
useful in the present invention
Figure 7 shows in plan view of the substrate the magnetically induced
pattern in the release coating obtainable from the die of Figure 6.
Figure 8 shows in plan view another embodiment of magnetizable die for
forming a magnetically induced pattern in the form of a liquid level marking in a
release coating in accordance with the present invention.
Figure 9 shows in schematic side elevation one use of the die of Figure 8
for forming the liquid level marking in the release coating on the sidewall of the
frying pan.
Figure 10 shows in schematic side elevation an equipment arrangement
using a configured plate aligned with the underside of a substrate (frying pan) to
form a magnetically induced pattern in a fluoropolymer release coating.
Figure 11 shows a plan view of the plate used in the equipment
arrangement of Figure 10.
Figure 12 shows a plan view of the substrate of Figure 10 with the
magnetically induced pattern visible in the release coating on the release coating.
DETAILED DESCRIPTION
Reference will now be made in detail to the present invention as illustrated
in the accompanying drawings.
In Figure 1 is shown the substrate to be coated and magnetically patterned
in accordance with the present invention, the substrate being in the form of a
frying pan 2 of non-magnetizable material such as aluminum, copper, stainless
steel, glass or ceramic. The frying pan 2 is shown to have a handle 4. A liquid
dispersion of a mixture of fluoropolymer resin and magnetizable flakes is applied
as a spray 6 onto the interior surface of the frying pan 2 to form a release coating 8
thereon as best shown in Figure 4. The flakes 10 in the sprayed composition tend
to orient themselves generally parallel to the surface of the substrate as shown in
Figure 4, except in the region of magnetic force applied by magnetic die 12, which
causes the flakes 10' in such region to reorient out of the plane of the substrate,
i.e., such flakes form an angle with the plane of the substrate, whereby incident
light on the release coating either is reflected at an angle away from the
perpendicular path of the incident light as shown in Figure 4 or is not reflected at
all when the reoriented flakes are parallel to the incident light. The flakes 10'
which are tilted to the perpendicular or near perpendicular protrude from the
surface of layer 8, but the protruding portions of the flakes are encased in release
composition of which layer 8 is composed to formed small mounds 11 of release
coating protruding from the otherwise flat surface of the coating 8. Where the
flakes 10 are parallel to the surface of the substrate, the incident light is reflected
directly back to the viewer. The difference in reflection of the incident light gives
the release coating a magnetically induced pattern in the shape of the
magnetizable die.
The magnetic force is applied to form the pattern as further shown in
Figure 1. The magnetizable die 12 is made of sheet metal, e.g., 0.1 mm to 4 mm
thick, and is in the form of a morningstar pattern as best shown in Figure 2. The
sheet metal forming the die 12 is at an angle with respect to the plane of the
underside of the fry pan 2. so that the upper edge and not the face (side) of the
sheet metal forms the pattern of localized magnetic force in the coating 8. The
upper edge of the sheet metal can be as thin as a knife edge as well as thicker, e.g.,
up to the 4 mm thickness mentioned above. The die 12 in essence looks like a
cookie cutter, with its size depending on the size of the pattern to be formed in the
release coating. In order to stabilize the sheet metal walls forming the die, the
interior space 14 of the die can be filled in by nonmagnetizable solid material such
as wood (not shown).
The magnetizable die is not the source of the magnetic force. Instead, the
source of the magnetic force is magnet 16 which can be a permanent magnet or as
shown in Figure 1 can be an electromagnet having a central pole 18 surrounded by
electrical coil 20 and in turn by an annular pole 21. The magnet 16 generates the
magnetic force necessary for the invention. The magnet 16 is spaced from the
frying pan 2, and the magnetic force from the magnet is communicated to the
release coating through the die 12. The spacing of the magnet from the underside
of the substrate can be great enough that the coating on the substrate is not directly
exposed to the magnetic force of the magnet or the magnetic force of the magnet
16 is diffused into a magnetizable metal plate 22 interposed between the magnet
16 and die 12. In either case, the die communicates the magnetic force from a
diffuse magnetic field rather than the coating 8 being exposed directly to the
magnetic field of the magnet. This enables the magnetically induced pattern in the
release coating to be precisely controlled by the configuration of the magnetizable
die 12, wherein the pattern closely conforms to the shape of the die facing the
underside of the substrate. The momingstar pattern 24 as a hollow line pattern in
the release coating resulting from the use of die 12 is shown in the base of the
frying pan 2 in Figure 3. This pattern is visible to the naked eye by virtue of light
being reflected from the surface of the release coating, i.e. from the surface inside
and outside the pattern.
Application of the magnetic force to the flakes in the release coating
through the magnetizable die such as die 12 is effective to localize the
reorientation effect on the flakes in the coating composition to provide the faithful
reproduction of the die. The flakes are assumed to be reoriented, because in the
absence of magnetic force, the flakes will be oriented substantially in the plane of
the coating, so as to be light reflective. The magnetic force is not so strong that
the die itself creates unwanted background fuzziness in the pattern, but is strong
enough to produce the pattern in the coating. The diffuser plate 22 also enables
the magnet to be any size, i.e. independent of the size of the pattern to be
magnetically induced in the release coating, except that the area of the face of the
magnet should be smaller, and totally contained within, the area of the diffuser
plate, so that lines of force of the magnet cannot pass directly to the substrate
being coated. Thus, one size magnet can be used to create a wide variety of
pattern sizes and shapes, depending on the magnetizable die used.
A key to producing cookware which is both decorative and still retains its
release properties is proper modulation of the magnetic force applied to the release
coating by the die. Such modulation can be achieved by the height of the
magnetic die and/or by use of the diffuser plate and can be facilitated by including
additional spatial gaps of non-magnetizable material as needed to produce the
pattern effects desired. Such a gap can be achieved by using nonmagnetizable
spacing sheets (not shown) between the diffuser plate and the die or the magnetic
die can be spaced from the underside of the frying pan instead of being in contact
therewith as shown in Figure 1. Another spatial gap can be achieved by the
thickness of the cookware substrate thereby instituting a gap between the tips of
the magnetizable die and the magnetizable flakes in the release coating. Any gap
in addition to the thickness of the substrate (uncoated frying pan), spacing of the
die from the substrate and/or the diffuser plate is selected to eliminate
background effects of the magnetic field of the magnet, while allowing the
magnetic force to penetrate the gap and via the magnetic die, to act on the release
coating.
In the case of point and edge effects, field strength has been determined to
drop by a factor of 1/d7 where d is the distance of the spatial gap between the tips
of the magnetizable die and the magnetizable flakes. So even a small spatial gap
will greatly affect the magnetic strength By reducing the strength of the magnetic
field and eliminating or decreasing certain lines of force, magnetic background
effects are reduced. This results in a smooth decorative surface on the substrate.
While the magnetizable flakes still in the liquid state of the coating are
mobile, it has been found that clarity of the pattern is improved when the coating
is exposed to the magnetic force from the magnetizable die simultaneously with
the step of applying the liquid coating composition to the substrate. To facilitate
these steps being carried out simultaneously, the magnetic die is preferably
positioned on the underside side of the substrate to be coated with the release
coating instead of on the coating side thereof.
The resultant liquid coating, containing the magnetically-induced pattern,
is then dried and baked to sinter or otherwise fuse the fluoropolymer to form the
release coating, by heating the coating typically to temperatures of 350°C to
420°C, depending on the fluoropolymer resin used. The flakes in the release
coating should be made of material that while magnetizable, are unaffected by
such heating. Examples of material from which the flakes can be made include
such metals as iron and nickel and alloys containing these metals, with stainless
steel being the preferred material. For simplicity, the fluoropolymer resin/flake
coating is referred to as a release coating both before and after the baking step.
when in,fact the baking step is necessary before the release (non-stick)
characteristic is realized.
The baking stabilizes (affixes) the magnetically induced pattern of
reoriented flakes within the release coating on the substrate. The substrate can be
roughened such as by grit blasting or chemical etching to create cavities to which
the release coating can anchor. Preferably, however, the substrate as shown for the
frying pan 2 surface in Figure 4 is smooth. Even when smooth, the magnetically
induced pattern of reoriented flakes obtained in accordance with the present
invention remains in place during the baking process, whereupon the pattern
becomes permanent within the coating. In accordance with the preference for a
smooth surfaced substrate, the release coating is preferably adhered to the
substrate via an intervening primer layer 30 such as shown in Figure 5. In another
preferred form of the present invention, the release layer or coating is in two parts
(layers), the layer 8 which contains the flakes 10, and a topcoat 32 which is free of
such flakes. The layer 8 is thereby present as a midcoat. The topcoat 32 contains
minute mounds 33 extending from its surface, telegraphing the presence of the
mounds 11 from layer 8, but smoothing them out. The presence of the topcoat 32
thus provides a smoother exposed surface for the release coating, and if thick
enough can mask the mounds 11 in the underlying layer altogether. The topcoat
adds to the aesthetics of the decorative surface by improving the gloss.
Figure 6 shows another embodiment of magnetizable die 40 comprising a
wooden plate 42 having holes drilled therein to accommodate magnetizable metal
pins 44 which are preferably tightly engaged in their respective holes. This die
can be used in place of die 12, with the bottom ends of the pins in contact with
the diffuser plate 22 and the top ends in contact with (or adjacent to) the underside
of the frying pan 38 which is similar to frying pan 2. Each pin, being at an angle
to the plane of the underside of the frying pan 38, communicates the magnetic
force from the diffuse magnetic field of the plate 22 to the coating to form a
pattern visible in reflected light as a plurality of dark points (dots) 45 within the a
light-appearing coating, with the diameter of the dots in the pattern being slightly
larger than the diameter of the rods pins as shown in Figure 7. The pattern
(placement and frequency) of pins can be varied as desired and can be combined
with an annular pattern such as that momingstar pattern shown in Figure 3. The
dots formed within the coating can have the optical appearance of depressions
lending an impression of optical depth and therefore thickness to the cookware
article, while yet retaining a smooth, nonstick surface. For convenience, the
structure forming the magnetic die, e.g. the sheet metal forming the die in Figure 2
or the pins 44, will be positioned perpendicular, i.e. the die itself can be
considered as being perpendicular, to this plane of the underside of the substrate
bearing the liquid coating composition.
Figure 8 shows in enlarged plan view another embodiment of a
magnetizable die 46 based on pins 48. In this embodiment, the pins are of smaller
diameter, e.g. 1 mm in diameter as compared to 3 mm in diameter for the pins 44
of Figure 6. The pins 48 are spaced closely together, e.g. pin heads are in close
proximity or touching contact with each other but can be held in place the same
way, namely by a wooden plate or foam block, 50, having holes which tightly
accommodate the pins 48. As shown in Figure 8, the pins 48 form information
instead of decoration, namely to show a liquid level and label of "1 CUP" for the
liquid level. This die can be used to apply this information to the sidewall of the
frying pan 38, or other release coated vessel, such as shown in Fig 9, wherein the
die is shown positioning its pins against the sidewall of the frying pan and against
diffuser plate 52, beneath which is the magnet 54 which is the source of the
magnet force reaching the flakes in the coating composition. The close spacing of
the pins 48 creates a pattern of continuous lines in the coating, providing volume
information appearing on the frying pan without any indentation being present in
the substrate forming the frying pan or without any change in smoothness of the
release coating which contains this liquid level indicia. In this embodiment, the
pins 48 can be made in different lengths to account for the curvature of the
sidewall of the frying pan. This embodiment of die can also be made of sheet
metal formed in the pattern of information desired and held in place by a wooden
base or foam block. The use of pins, however, as in Figures 8 and 9 facilitates the
forming of a wide variety of patterns of indicia, such as additional liquid level
markings, including letter description thereof, e.g. oz. or ml. The pins used as the
magnetic die in the present invention can have any diameter desired depending on
the pattern desired, but typically, they will have a diameter of 0.5 mm to 5mm.
Figures 10-12 show a different embodiment, wherein the
magnetically indiced pattern in the release coating is formed using a configured
plate. the face of which is oriented in the same direction as the bottom of the
substrate to be coated. In Figure 10, the configured plate 60 of magnetizable
material is positioned in contact with the bottom surface of frying pan 62 which is
similar to frying pan 2. Instead of diffuser plate 22 used in Figure 1, a diffuser
block 64 of magnetizable material is used, and a magnet 66 is positioned beneath
block 64. The height of block 64 is such that for the strength of the magnet 66
used, sufficient magnetic force reaches the magnetizable flakes in the release
coating (while still flowable) to cause the flakes to orient away from the plane of
the substrate so as to reproduce the pattern of plate 60. While Figure 10 shows the
under-surface of the frying pan, the plate 60, block 64, and magnet 66 all being in
sequential contact with one another. an air gap or non-magnetizable spacer can be
introduced between any on the elements forming this equipment arrangement, so
as to modulate the magnetic force emanating from the magnet. Such modulation
can be used for example if it is desired for space reasons to use a diffuser plate
like that of Figure 1 instead of block 64. The area of the face of magnet 66 is
smaller than the bottom area of the diffuser block 64, and the magnet is
positioned within the bottom area of the diffuser block, so that all of the magnetic
force reaching the plate 60 does so by passage through the block 64. Figure 11
shows the configuration of the edge of plate 60, consisting of a solid center region
68 having tapering arms 70 radially extending therefrom. Preferably the diffuser
block which is in this embodiment an upstanding cylinder, because the plate is
derived from a circular plate, has an outer diameter which is about the same as the
diameter of the region constituting the solid center 68 of the plate 62. The pattern
72 of configured plate 60 is reproduced magnetically in the release coating on the
cooking surface of frying pan 62 as shown in Figure 12 as a dark region
corresponding to the pattern of plate 60 surrounded by a light region, with the
dark region appearing to be recessed below the light region, giving the cooking
surface of the frying pan a three dimensional appearance. Other configurations
which depart from a circular pattern from which the plate 60 is derived can be
used
Heat resistant materials especially useful in forming the primer layer and
the release coating in clude fluoropolymer resin components. Such resin contains
at least 35 wt% fluorine. One particularly useful fluoropolymer is
polytetrafluoroethylene (PTFE) which provides the highest heat stability among
the fluoropolymers. Optionally, the PTFE contains a small amount of comonomer
modifier which improves film-forming capability during baking, such as
perfluoroolefin, notably hexafluoropropylene (HFP) or perfluoro(alkyl vinyl)
ether (PAVE), notably wherein the alkyl group contains 1-5 carbon atoms, with
perfluoro(ethyl or propyl vinyl ether) (PEVE and PPVE, respectively) being
preferred. The amount of modifier may be insufficient to confer melt-fabricability
to the PTFE, generally no more than about 0.5 mole%. The PTFE,
can have a single melt viscosity, usually about 1 x 109 Pa.s, but, if desired, a
mixture comprising PTFE's having different melt viscosities can be used to form
the fluoropolymer component.
In one aspect of this invention, the fluoropolymer component, is melt
fabricable fluoropolymer, either blended with the PTFE, or in place thereof.
Examples of such melt- fabricable fluoropolymers include tetrafluoroethylene
(TFE) copolymers with one or more of the comonomers as described above for
the modified PTFE but having sufficient comonomer content to reduce the
melting point significantly below that of PTFE. Commonly available melt-fabricable
TFE copolymers include FEP (TFE/HFP copolymer) and PFA
(TFE/PAVE copolymer), notably TFE/PPVE copolymer. The molecular weight
of the melt-fabricable tetrafluoroethylene copolymers is sufficient to be film-forming
and be able to sustain a molded shape so as to have integrity in the primer
application. Typically, the melt viscosity of FEP and PFA will be at least about I
x 102 Pa.s and may range to about 10 - 400 x 103 Pa.s as determined at 372°C
according to ASTM D-1238.
The fluoropolymer component is generally commercially available as a
dispersion of the polymer in water, which is the preferred form of the composition
for this invention for ease of application and environmental acceptability. By
"dispersion" it is meant that the fluoropolymer particles are stably dispersed in an
aqueous medium, so that settling of the particles does not occur within the time
when the dispersion will be used. The stability of the dispersion can be achieved
as the result of the relatively small size of the fluoropolymer particles, typically on
the order of 0.2 micrometers, and the use of one or more surfactants in the
aqueous dispersion. Such dispersions can be obtained directly by the process
known as dispersion polymerization, optionally followed by concentration and/or
further addition of surfactant. Examples of suitable surfactants include at least
one of octylphenoxytriethoxyethanol, triethanolamine oleate, among others.
The release coating, which in one embodiment may be a midcoat and a
topcoat, used in this invention is generally derived from a dispersion of one or
more fluoropolymers to which has optionally been added a dispersion of an
acrylic polymer. Suitable midcoat and topcoat are described by U.S. Patent
Nos. 4,180,609 (Vassiliou); 4,118,537 (Vary & Vassiliou); 4,123,401
(Berghmans & Vary); 4,351,882 (Concannon) hereby incorporated by reference.
The composition forming the midcoat and topcoat used in the present
invention can contain in addition to the fluoropolymer component, a dispersion of
a polymer of monoethylenically unsaturated monomers, such as the acrylic
polymer dispersions described in U.S. Patent Nos. 4,123,401 (Berghmans and
Vary) and 4,118,537 (Vary and Vasilliou); hereby incorporated by reference. The
coating composition typically shows improved coalescence on curing if a polymer
of monoethylenically unsaturated monomers have been added to the
fluoropolymer component. The polymer of monoethylenically unsaturated
monomers can be any suitable polymer or copolymer (in the sense of being
composed of two or more types of monomers) of ethylenically unsaturated
monomers which depolymerize. and whose depolymerization products vaporize,
in the temperature range of about 150°C below the fusion temperature of the
fluoropolymer used to about the fluoropolymer's decomposition temperature and
thus vaporizes during the baking step. It may be desirable that the polymer of
monoethylenically unsaturated monomers be in solution in a solvent compatible
with the rest of the system or be present as a stable dispersion of small particles.
For desired results, the average particle size is generally below 1 micrometer.
Illustrative of acrylic polymers which can be used as an additive are
polymers of one or more monoethylenically unsaturated monomers which also
contain one or more monoethylenically unsaturated acid units. Representative of
the monomers are alkyl acrylates and methacrylates having 1-8 carbon atoms in
the alkyl group, styrene, alpha-methyl styrene and vinyl toluene. Representative
of the acid units are acrylic acid, methacrylic acid, fumaric acid, itaconic acid and
maleic acid (or anhydride). Mixtures of these polymers can also be used. The
acid units of these polymers can optionally be esterified with glycidal esters of
4-14 carbon atoms. Such a polymer is ordinarily present at a concentration of
about 2-300% by weight of the fluoropolymer, and preferably about 5-20%. The
preferred polymer additive is an acrylic latex of a methylmethacrylate/ethylacrylate/methacrylic
acid 39/57/4 terpolymer.
The release coat, in particular the midcoat used in the present invention,
contains an effective amount of light reflecting magnetizable flakes to produce a
pattern in the coating upon localized reorientation of the flakes. The release
coating generally contains from 2 - 6 wt. % of magnetizable flakes, based on the
dry weight of the coating composition. Some of these flakes may have a longest
dimension which is less than the thickness of the coating, e.g., less than 50 wt. %
of the flakes, but this condition may exist because of the flake size distribution in
the flakes that are commercially available. The "short" flakes make little
contribution to the visibility of the pattern. Particularly useful are 316L stainless
steel flakes having an average longest dimension of from 20 to 60 micrometers,
and normally, the flakes will be a mixture of sizes in which a substantial
proportion, preferably at least 40 wt%, has a longest dimension of at least
44 micrometers.
The compositions forming the primer, intermediate and top coatings used
in the present invention often contain one or more pigments, normally in a mill
base medium that is either soluble in or miscible with the water of the
fluoropolymer aqueous dispersion. However, judicious care is needed in selecting
the pigment and quantities of pigment for use in the midcoat and topcoat used in
this invention in order not to mask the pattern created by magnetic induction. The
pigment mill base is normally produced by milling (grinding) pigment in its liquid
medium, which deagglomerates the pigment and produces dispersion uniformity.
The preferred medium is water which contains an amount of a surfactant sufficient
for the mill base to become an aqueous dispersion of the pigment by the milling
process. Pigments for use in cookware applications have limitations imposed on
their use by the U.S. Food and Drug Administration (FDA) because of food
contact. Pigments to be used in this invention must be heat stable and nontoxic.
Suitable pigments include at least one member from the group of carbon black,
titanium dioxide, iron oxide, and zeolites such as ultramarine blue, cobalt blue,
among others.
The compositions forming the topcoat when used in this invention often
contain mica particles, and mica particles coated with pigment. Such particles
impart scratch resistance to the articles on which they are coated. These particles
have an average longest dimension of about 10 to 200 micrometers, preferably
15-50 micrometers , with no more than 50% of the particles of flake having
longest dimensions of more than about 500 micrometers. For use in this
invention, mica particles coated with pigment having a longest dimension of 1-15
micrometers are preferred. Small particle size mica flakes, whether present in the
coating which contains the flakes and/or in the topcoat when used, allow the
magnetically induced pattern to be seen without scattering light or showing
metallic luster, yet provide reinforcement for the topcoat. The mica particles
coated with pigment preferred for this invention are those described in U. S.
Patent Nos. 3,087,827 (Klenke and Stratton); 3,087,828 (Linton); and 3,087,829
(Linton); hereby incorporated by reference. The micas described in these patents
are coated with oxides or hydrous oxides of titanium, zirconium, aluminum, zinc,
antimony, tin, iron, copper, nickel, cobalt, chromium, or vanadium. Titanium
dioxide coated mica is preferred because of its availability. Mixtures of coated
micas can also be used. The mica or coated mica is ordinarily present in the
topcoat at a concentration of about 0.2-20% by dry weight of the composition.
The primer coating when used in this invention is generally derived from
an aqueous dispersion of at least one fluoropolymer and a water soluble or water
dispersible film-forming binder material. A suitable primer is described by the
,US Patent Nos. 4,087,394 (Concannon); 5,240,775 (Tannenbaum) and 5,562,991
(Tannenbaum); hereby incorporated by reference.
The film-forming binder component that can be used in forming the primer
coating is composed of polymer which is thermally stable. This component is
well known in primer applications for non-stick finishes. for adhering the
fluoropolymer-containing primer layer to substrates and for film-forming within
and as part of the primer layer. The binder is generally non-fluorine containing
and yet adheres to the fluoropolymer. Preferred binders are those that are soluble
or solubilized in water or a mixture of water and organic solvent for the binder.
which solvent is miscible with water. This solubility aids in the blending of the
binder with the fluorocarbon component in the aqueous dispersion form. An
example of the binder component is polyamic acid salt which converts to
polayamideimide upon baking of the composition to form the primer layer. This
binder is preferred because in the fully imidized form obtained by baking the
polyamic acid salt, this binder has a continuous service temperature in excess of
about 250°C. The polyamic acid salt is generally available as polyamic acid
having an inherent viscosity of at about 0.1 as measured as a 0.5 wt % solution in
N,N-dimethylacetamide at about 30°C. It is dissolved in a coalescing agent, such
as N-methylpyrrolidone, and a viscosity-reducing agent, such as furfuryl alcohol
and reacted with tertiary amine, preferably triethylamine, to form the salt, which is
soluble in water, as described in greater detail in U.S. Patent Nos. 4,014,834
(Concannon) and 4,087,394 (Concannon); the disclosure of both is hereby
incorporated by reference. The resultant reaction medium containing the
polyamic acid salt can then be blended with the fluoropolymer aqueous
dispersion, and because the coalescing agent and viscosity-reducing agent are
miscible in water, the blending produces a substantially uniform coating
composition. The blending can be achieved by simple mixing of the liquids
together without using excess agitation so as to avoid coagulation of the
fluoropolymer aqueous dispersion. The proportion of fluoropolymer and binder in
compositions of the present invention can be in the weight ratios of about 0.5 to
2.5:1. The weight ratios of fluoropolymer to binder disclosed herein are based on
the dry weight of these components in the primer layer, which in essence is the
same as the relative weight in the primer layer after baking the composition after
application as a coating to a substrate. When the composition of the invention is
in the preferred aqueous form, these components will constitute about 5 to
50 wt. % of the total dispersion.
An inorganic filler film hardener component may be present in the primer
composition. The film hardener is one or more filler type materials which are
inert with respect to the other components of the composition and thermally stable
at baking temperatures which fuse the fluoropolymer and binder. Preferably the
film hardener is water insoluble so that it is uniformly dispersible but not
dissolved in an aqueous dispersion. By filler-type material is meant that the filler
is finely divided, generally having a particle size of about 1 to 200 micrometers.
usually 2 to 20 micrometers, which is usually obtained by the film hardener
component and which imparts durability to the primer layer by resisting
penetration of sharp objects that may penetrate the fluoropolymer overcoat.
Examples of the film hardener include one or more metal silicate
compounds such as aluminum silicate and metal oxides, such as, titanium dioxide
and aluminum oxide. Examples of such film hardeners are described in U.S.
Patent 5,562,991 (Tannenbaum) and U.S. Patent No. 5,250,356 (Batzar); the
disclosure of which is hereby incorporated by reference.
The primer composition used in the present invention in aqueous
dispersion form may also contain such other additives as adhesion promoters,
such as colloidal silica or a phosphate compound, such as a metal phosphate, e.g.,
Zn, Mn, or Fe phosphate.
The coatings used in the present invention, whether single coating
containing the magnetizable flakes, or multiple coatings, such as primer, midcoat
(containing the flakes) and topcoat, can be applied to substrates by a variety of
techniques and to a variety of substrates. Roller, dip, and spray coating can be
utilized. It is only necessary that the coating composition which contains the
magnetizable flakes be applied as a liquid composition so that the flakes can be
localized magnetically reoriented to form the pattern. The layer containing the
magnetizable flakes will be thinner than the longest dimension of the flakes and
will generally be 5-40 micrometers thick, preferably 5-30 micrometers thick, more
preferably 5-25 micrometers thick (0.2-1 mil). When the release coating is a
combination of midcoat (containing the flakes) or undercoat and topcoat, the
combined thickness will generally be 5-50 micrometers thick, preferably 5-40
micrometers thick. Preferably, the flake-containing layer will be the thicker layer,
constituting 60 to 90% of the total thickness of the to layers, and more preferably
70 to 85%. The magnetizable flakes are chosen to have a longest dimension which
is greater than the thickness of the flake-containing layer, and more often, thicker
than the total thickness of the flake-containing layer and the topcoat, if present.
The primer layer, if used will generally have a thickness of 0.5 to 10 micrometers,
more often 5 to 10 micrometers (0.2-0.4 mils). The layer thicknesses disclosed
herein refer to the dry film thickness (DFT).
The substrates can be any non-magnetizable material which can withstand
the relatively high bake temperatures used to fuse the coatings. Such substrate
materials include metals and ceramics, such as aluminum, anodized aluminum,
stainless steel, enamel, glass, pyroceram, among others. The substrate can be
gritblasted (roughened) or smooth, and cleaned prior to coating. For pyroceram
and some glass, improved results are obtained by activation of the substrate
surface such as by slight, chemical etch, which is not visible to the naked eye.
The substrate can also be chemically treated with an adhesion agent such as the
mist coat of polyamic acid salt disclosed in U.S. Patent No. 5,079,073
(Tannenbaum); hereby incorporated by reference.
The compositions described above are particularly used to provide an
article of cookware, having a cooking surface which comprises a multi-layer, non-stick
coating on a substrate which coating minimizes sticking by food residues
and which is heat resisting by being stable above about 300°C. The present
invention provides for a coated substrate having a magnetically induced image
pattern and preferably having an average surface roughness,(abbreviated Ra), less
than 1.5 micrometers. as determined using a Hommel Profilometer, model T-500.
Typically, the surface roughness will be at least 0.5 micrometers. The substrate
itself preferably has the same smoothness, preferably less than 1.5 micrometers
and more preferably less than 1.25 micrometers.The coated substrate of the
present invention may be in the form of numerous articles of decorative cookware
such as frypans, pots, bakeware, casseroles and the like. Although items of
cookware are herein illustrated, numerous other household or industrial
applications of this technology are contemplated. By example, the sole plate of an
iron may be provided with a magnetically induced pattern. Processing tanks or
vats having a release finish may benefit from liquid level marking or the like.
Further, industrial coaters may choose to apply identification markings or a logo
to release coated surfaces by the disclosed magnetic inducing techniques.
EXAMPLE 1
A pattern is magnetically induced in a release coating on an aluminum
substrate which has the form of a frying pan. The setup for applying the coating is
similar to that illustrated in Figure 1
Aluminum frying pan 2 has a diameter of 25.4 cm and is typically 1.5
-3.2 mm thick. The frying pan is positioned over a magnetizable die 12 which is
akin to a mold or "cookie cutter" being formed from magnetizable sheet metal
into a morning star pattern as shown in Figure 2. The die is formed from 1010
steel alloy sheet of 1.6 mm thickness. The die has a pattern of an 8 pointed star
having an apparent diameter of 22.9 cm inches with edges that are 10 cm high.
The magnetizable die 12 is positioned over a diffuser plate 22 which rests
on a platform 9 (not shown). The plate is a carbon steel plate having the
dimensions of 30.5 x 30.5 x 0.65 cm. Positioned between the diffuser plate 22
and the magnetizable die 22 are two nonmagnetizable spacer sheets (not shown)
of aluminum having the dimensions 30.5 x 30.5 x 1.3 cm. The platform is
positioned over magnet 16 and provides a shield between diffuser plate 22 and
magnet 16 and prevents plate 22 from adhering to the magnet. Magnet 16 is a
permanent magnet of Neodimium-Iron- Boron Alloy of 10 cm diameter with a
capability of generating 2 tesla (20,000 gauss) manufactured by Dexter Magnetics
of Sunnyvale, CA 94086. Diffuser plate 22 absorbs upwardly emanating
magnetic fields and drives the fields horizontally creating a larger workable
magnetic area equal to the breadth of the diffuser plate, but of weakened magnetic
force.
The additional nonmagnetizable aluminum spacer sheets further dampen
the strength of the magnetic field acting on magnetizable flakes 10' in release
coating 8 as the coating is applied to frypan 2. The distance between magnet 16
and magnetizable die 12 as illustrated in Figure 1 may be adjusted to deliver the
magnetic force of desired strength through the edges of die 12. The magnetic
force as measured at the tip of the magnetic die in contact with the frypan is 128
gauss. It has been found that by reducing the strength of the magnetic field and
eliminating or decreasing certain lines of force, that magnetic background effects
are reduced. This results in a decorative surface on the substrate that is smooth.
A primer having the composition of Table 1 is sprayed on a clean, lightly
etched aluminum frying pan having a surface smoothness of 1.25 micrometers to
dry film thickness (DFT) of 15 micrometers. The primer was dried at 66°C for
5 minutes. A midcoat with magnetizable flakes having the composition of
Table 2 is sprayed onto the frying pan to a DFT (dry film thickness) of
13 micrometers as magnetic force was applied through the magnetizable die in
accordance with the present invention, causing a portion of the flakes to
magnetically reorient into the pattern of the edges of the die. A topcoat having the
composition of Table 3 is sprayed over the midcoat to a DFT of 13 micrometers
while the midcoat is still wet also in the presence of magnetic force. The entire
system is baked at 427°C to 435°C for 5 minutes. The frying pan has a decorative
surface with a magnetically induced pattern and an average surface roughness,
(Ra) less than 1.5 micrometers , as determined using a Hommel Profilometer,
model T-500.
In all of the following Tables: "solvent-surfactant blend" corresponded to
approximately 19.5% butyl carbitol, 23.9% mixed aromatic hydrocarbons, 4.7%
cerium octoate, 37% triethanolamine, 8% lauryl sulfate, and the balance was
water; and "acrylic dispersion" corresponded to approximately 39/57/4 methyl
methacrylate/ethyl acrylate/methacrylic acid. The polymer comprised about 40%
of the dispersion, 9% triethanolamine, 8% sodium lauryl sulfate, and the balance
was water.
Primer | Coating Composition (Wt. %) | Solids Content in Finished Article (Wt. %) |
Furfuryl Alcohol | 1.85 | - |
Polyamic acid salt in N-Methyl Pyrrolidone | 18.3 | 30.39 |
Deionized Water | 48.8 | - |
Mica | 0.050 | 0.03 |
PTFE Dispersion | 8.04 | 27.38 |
FEP Dispersion | 5.95 | 18.10 |
Colloidal Silica Dispersion | 3.64 | 6.01 |
Carbon black dispersion | 8.09 | 13.43 |
Aluminum silicate dispersion | 5.25 | 4.64 |
Intermediate | Coating Composition (Wt. %) | Solids Content in Finished Article (Wt. %) |
PTFE Dispersion | 58.5 | 81.0 |
PFA Dispersion | 10.6 | 14.7 |
Deionized Water | 3.2 | - |
316L SS Flake | 1.9 | 4.3 |
Solvent-Surfactant blend | 13.1 | - |
Acrylic polymer dispersion | 12.7 | - |
Topcoat | Coating Composition (Wt. %) | Solids Content in Finished Article (Wt. %) |
PTFE Dispersion | 66.95 | 94.55 |
PFA Dispersion | 3.51 | 4.96 |
Deionized Water | 3.77 | - |
Mica (1-15 microns) | 0.21 | 0.49 |
Solvent-Surfactant Blend | 12.51 | - |
Acrylic polymer dispersion | 13.04 | - |
EXAMPLE 2
A pattern is magnetically induced in a release coating on an aluminum
substrate which has the form of the sidewall of a frying pan. The setup for
applying the coating is similar to that illustrated in Figure 9.
Aluminum fry pan 38 has a diameter of 25.4 cm and is typically 1.5
-3.2 mm thick. The fry pan is positioned over a magnetizable die 46 based on
pins 48 wherein the die is positioned against the sidewall of the frypan and against
diffuser plate 52 beneath which is placed magnet 54. as shown in Figure 8. The
die is formed from a plurality of straight pins of steel alloy having a 1 mm
diameter head and a length of 3 cm. The pins are spaced closely together, e.g. pin
heads are in touching contact with each other and are held in place by a foam
block 50 of polystyrene of 1.95 cm thickness which tightly accommodates the
pins . The pin heads are positioned flush to one surface of the foam block and in
contact with the frypan. The pin ends protrude through the opposite surface of the
foam block and are in contact with the diffuser plate. The die is a pattern of
liquid level marking "1 CUP".
The platform, diffuser plate and magnet are the same as those specified in
Example 1. No spacer plates are present. Preparation of the frying pan,
compositions of primer, midcoat, and topcoat. and method of application are the
same as those specified for Example 1.
The close spacing of the pins 48 creates a pattern of continuous lines in the
coating, providing liquid level markings appearing on the frying pan without any
indentation being present in the substrate forming the frying pan or without any
change in smoothness of the release coating which contains this liquid level
indicia.
EXAMPLE 3
Similar to example 1, two aluminum frying pans, but of differing
thicknesses, are coated with a magnetically induced pattern. One frying pan is
8 gauge, e.g., 3.2 mm, the other pan is 6 gauge, e.g., 4.1 mm. Using fry pans of
different thicknesses illustrates the differences of varying the spatial gap between
the tip of die and the magnetizable flake in the release coating. The die for this
Example 3 is formed by positioning sheets from 1010 steel alloy of 1.6 mm
thickness x 10 cm x 6.9 cm in alternating arrangement with sheets of
1.6 mm x 10cm x 5.7 cm inches in tightly fitting slots of a foam block to form
12 radiating edges that form a pattern of radiating lines (similar to the line
representation of a sun) with an apparent diameter of 17.8 cm . The edges of one
side of the die are positioned against the frying pan bottom with opposite edges of
the die positioned against the diffuser plate. The spatial gap between the tips of
the die and the magnetizable flakes differ by the thickness of the two frying pans.
The platform, diffuser plate and magnet are the same as those specified in
Example 1. No aluminum spacer plates are present. Preparation of the frying
pan, compositions of primer, midcoat, and topcoat, and method of application are
the same as those specified for Example 1. The magnetic force as measured at the
tip of the magnetic die in contact with the frying pan is 300 gauss.
Radiating line patterns are visible in both frying pans. However, the
pattern as determined by visual inspection, in the thicker (6 gauge) pan is
somewhat weak, yet has lines of greater clarity (less fuzzy) due to the increased
spatial gap. The pattern created in the thinner (8 gauge) pan is strong but the lines
are fuzzy. To correct the pattern in the thicker pan, a larger (stronger) magnet
which can produce a stronger magnetic force communicated to the coating by the
magnetic die is used. To correct the pattern in the thinner pan, spacer plates are
used to modulate the magnetic force delivered to the die.
EXAMPLE 4 (Comparative)
Similar to Example 1, an aluminum frying pan, is coated with a
magnetically induced pattern but instead of the set up as described in Fig 1 herein,
a pole piece in the form a of a shaped plate of magnetizable steel (8mm thick)
having the same morning star pattern is placed directly on (laid across) the
magnet. The shaped plate is in contact with the underside of the frying pan. The
pole piece is a flat plate with no hollow interior, and serves as a template akin to a
"dress pattern" used for sewing. The magnetic force is directed through the bulk
area of magnetic template acting on the magnetizable flakes of the release coating.
The magnetic force is sufficient to cause orientation of the flakes but not
excessive to obliterate the resultant pattern. Nevertheless, directing magnetic
force the bulk area produces unwanted field lines which result in a fuzzy outline to
the solid magnetic imprint and a roughened decorative surface on nonmagnetic
base 1. The roughened surface is unsuitable in that food particles tend to stick.
Further the surface is more susceptible to gouging because of flake has oriented
on an angle and is more likely to respond to be pulled from the coating.
The magnet used is 0.6 tesla (600 gauss), permanent magnet. No platform,
diffuser plate or spacer plate is present. Preparation of the frying pan,
compositions of primer, midcoat, and topcoat, method of application and
thickness of coatings are the same as those specified for Example 1. The
magnetic force of the die in contact with the frying pan measured as follows: at
the point of the star, 300 gauss; at the edge of the star, 180 gauss; at the interior of
the pattern. 120 gauss.
The frying pan has a decorative surface with a magnetically induced
pattern and an average surface roughness, (Ra), of between 1.5 - 3.0 micrometers.
EXAMPLE 5
In this Example, the equipment arrangement shown in Figs, 10-12 is used.
using a frying pan similar to that used in Example 1 having a smooth interior
surface. The magnetizable die is the configured plate of Figure 11 having a
diameter of 22.9 cm from tip to tip of the extending arms and 0.94 cm thick. The
diffuser block 64 is made of mild steel (alloy 1010) and is 6.35 cm in diameter
and 7.6 cm high. The magnet is a stacked pair of rare earth permanent magnets,
each being Neo-37® magnet obtained from Dexter Magnetics and providing a
magntic force of 3 tesla (30000 gauss). Each magnet It is 5.59 cm in diameter
and 0.78 cm thick, and the stack of the two magnets is about 1.5 cm thick.
Primer, midcoat and topcoat are applied to the cooking surface of the frying pan,
in a similar manner as disclosed in Example 1, except that the primer layer is
7.5 micrometers thick, the midcoat layer is 18 micrometers thick and the topcoat
is 5 micrometers thick, the thicknesses being controlled by the spray time used to
apply the coatings. As in Example 1, the midcoat, which contains the
magnetizable flakes is applied to the dry primer layer while being subjected to the
magnetic force using the equipment arrangement just described. The three-coat
system applied to the frying pan is baked as in Example 1 to obtain the pattern
shown in Figure 12 wherein the dark appearing pattern in the release coating is set
in a surrounding area of light-color, the dark-appearing pattern appearing to be
recessed below the plane of the light color area, to give the cooking surface of the
frying pan a three-dimensional appearance. The primer and topcoat compositions
are similar to the corresponding compositions used in Example 1, and the midcoat
composition was an aqueous dispersion having the following composition:
An mixture containing mixed aromatic hydrocarbons, cerium octoate,
triethanolamine, oleic acid, Triton® X-100 surfactant in proportions to provide
the composition in the following table is added to the blend of acrylic polymer
dispersion and fluoropolymer dispersion. The stainless steel flakes, Cab-O-Sil®
fumed silica, ethylene glycol, polyamic acid salt, sulfonate surfactant, Triton®
X-100 surfactant, and furfural alcohol in proportions to provide the composition
in the following table are milled together for addition to the blend of other
components. The acrylic polymer dispersion corresponds to approximately to
39/57/4 (wt. ratio) methyl methacrylate/ethyl acrylate/methacrylic acid. The
polymer comprises about 40% of the dispersion, 9% triethanolamine, 8% sodium
lauryl sulfate, and the balance to total 100 wt% is water.
Component | Wet Composition (Wt. %) | Solids Content Coating in Finished Article (Wt. %) |
PTFE Dispersion | 57.15 | 80.3 |
PFA Dispersion | 10.34 | 14.7 |
Deionized Water | 4.96 | - |
316L SS Flake | 1.8 | 4.3 |
Solvent-Surfactant blend | 10.67 | - |
Acrylic polymer dispersion | 12.7 | - |
Polyamic acid salt in N-methyl pyrrolidone | 0.20 | 0.5 |
Cab-O-Sil® fumed silica | 0.17 | 0.4 |
sulfonate surfactant | 0.04 | - |
Triton®X-100 surfactant | 0.68 | - |
ethylene glycol | 0.04 | - |
furfural alcohol | 0.02 | - |
cerium octoate | 0.60 | - |
diethyleneglycolmonobutylether | 2.51 | - |
triethanolamine | 4.75 | - |
1,2,4-trimethylebenzene | 1.01 | - |
cumene | 0.06 | - |
xylene | 0.06 | - |
aromatic hydrocarbon | 1.93 | - |
Notes: The polyamic acid salt converts to polyamideimide upon baking. The wet composition contains 36% by weight of water, based on the total wet composition, the water coming primarily from the aqueous dispersion form of the PTFE and PFA. The overall water content of the total composition is 36% primarily supplied by the aqueous media from the polymer aqueous dispersions. |
The polyamic acid salt in the composition provides the benefit of being
compatible with both the SS flakes and the fluoropolymer components in the
composition so that when the flakes reorient under the influence of magnetic
force, the portions of the flakes which protrude above the flat surface of the
midcoat will be enveloped by fluoropolymer, so that the reorientation does not
produce minute fissures (visible under 20X magnification) in the midcoat during
reorientation, i.e. tilting of the magnetically affected flakes from the horizontal
towards the perpendicular may leave empty space being in the midcoat. Athough
the midcoat is covered by a topcoat. minute fissues in the midcoat provide easy
pathways for moisture to permeate through all the layers to reach the substrate
(frying pan) and cause blistering of the coatings. Upon baking, the polyamic acid
salt coverts to polyamideimide and bonds the flakes to the fluoropolymer. The
midcoat obtained in this Example is free of minute fissures.
The surface of the baked coating on the frying pane is smooth to the touch,
having a smoothness of about 0.8 micrometers in the light-colored area and about
1.3 micrometers in the pattern (dark color) area.
The importance of having the block 64 present to diffuse the magnetic
force is indicated by reproducing this Example, but eliminating the block,
whereby the magnet 66 is positioned in direct contact with the underside of plate
60. The resultant image is less sharp, and the surface of the baked coating
(primer/midcoat/topcoat) is rougher, namely 1.75 to 2.5 micrometers in the pattern
area), which compromises the release property of the coating.