Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Including a second component containing structurally defined...
Reexamination Certificate
1998-09-01
2001-06-19
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Including a second component containing structurally defined...
C428S332000, C428S421000, C428S422000, C428S461000, C220S573200
Reexamination Certificate
active
06248435
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to release coatings to form a non-stick finish on an article and to compositions therefor.
BACKGROUND OF THE INVENTION
Non-stick finishes (coatings) for such articles as cookware are well known. The anomaly in this art is that the coating must release the food cooked on the coating, but must not release from the substrate, i.e., the metal, glass, or 10 ceramic material of construction of the cookware. This paradox has been addressed by (a) treating the substrate surface to promote adhesion, e.g. by grit blasting, and/or (b) multilayer coating processes for the substrate, wherein the primer layer has both a thermally stable non-fluorinated polymer for bonding to the substrate and a fluoropolymer for bonding to an overlying fluoropolymer release layer. Nevertheless, the interface between the substrate and the overall release coating is still a major region of failure of the coating, caused by the repeated heating and cooling of the cookware in use. The heating and cooling subjects the interface to the stress of differential expansions between the substrate and the polymer-containing release coating, which can eventually cause the coating to separate from the substrate, giving the coating on the cookware and bakeware a blistered appearance and exposing the coating to easy puncturing and loss of release property. The repetitive exposure of the coating to heating during cooking also tends to cause degradation of the primer layer at the substrate interface, which leads to blistering of the overall coating or reduction in the bond between the release coating and the substrate such that the release coating can be scraped off.
SUMMARY OF THE INVENTION
The present invention overcomes this problem by providing a release coating on cookware which enables the cookware to be used at lower cooking temperatures to achieve the same cooking result.
The present invention solves this problem by providing a substrate having a baked fluoropolymer release coating thereon, said coating containing magnetizable flakes oriented primarily in the thickness direction of said coating so as to be free of pattern. The thickness direction orientation of the flakes improves the thermal conductivity of the release coating. When heat is applied to the underside of the substrate, as is the case when the substrate is cookware, the temperature at the cooking surface of the release coating, which is the cooking temperature, will be higher as compared to the cooking temperature from a release coating which does not contain the thickness-direction oriented flakes. Thus according to the present invention, the heat applied to the underside of the substrate, such as by the cooktop burner, can be reduced, which in turn reduces the temperature at the interface between any primer used as part of the release coating to adhere it to the substrate. A reduction of only 20° F. (11° C.) at the substrate/primer interface can reduce the tendency of the primer to degrade by at least one-half. The present invention can provide better improvement than that. With reference to the cooking temperature (release coating surface), when the release coating without the thickness-oriented magnetizable flakes is heated to 400° F. (204° C.) through the coating substrate, the release coating used in the present invention provides a cooking temperature of at least 20° F. (11° C.) higher, preferably at least 30° F. (16.7° C.) higher, and more preferably at least 40° F. (22.2° C.) higher. These temperature differentials are based on the average cooking temperatures. The improved heat transfer of the release coating manifested by these higher cooking temperatures enables the applied heat to the underside of the substrate to be reduce accordingly to achieve the 400° F. cooking temperature, and the substrate/primer interface temperature will also be reduced accordingly.
Since the substrate will normally be in the form of cookware which has a flat bottom and sidewall extending upwardly from the flat bottom and since release coatings are normally applied to both the bottom and sides of the interior surface of the cookware, the magnetizable flakes will be present in the coating applied to both the bottom and sidewall. Heat is applied to the underside of the bottom of the cookware, i.e., the flat substrate portion thereof, to accomplish the cooking. Thus, it is only necessary for the magnetizable flakes in the coating on the flat bottom (substrate) to have the thickness direction orientation for conducting the applied heat to the cooking surface of the release coating. The importance of the absence of pattern will be explained below.
The thickness direction orientation of the magnetizable flakes in the release coating is obtained by the process of applying the release coating composition in liquid form to the substrate and while the resultant coating is still wet, subjecting the coating to magnetic force which orients the magnetizable flakes primarily in the thickness direction of the coating, followed by baking the coating to form the release coating and fix the flakes in their thickness-direction orientation. This thickness-direction orientation acts to improve the heat transfer between the substrate and the cooking surface of the release coating. Preferably the flakes include flakes having a longest dimension which is greater than the thickness of the layer of the release coating which contains the flakes, or the entire release coating if only one layer is used to form the coating. Such flakes extend through most or the entire thickness of the layer to provide optimum heat transfer effect.
Since improved heat transfer is desired over the entire flat surface of the cookware, because it is common to expose such cookware to heating over the entire underside of the cookware, the thickness direction orientation of the magnetizable flakes extends over the entire flat surface, i.e., in the portion of the coating that is on the flat surface. If magnetic force were not applied to the wet coating composition, the flakes would tend to orient in the plane of the coating, i.e., essentially parallel to the flat substrate surface, which would not provide efficient heat transfer through the thickness of the coating. The same would be true if the magnetic force were applied to only selected regions of the coating overlying the flat substrate, in which case the magnetizable flakes would form a pattern which would be visible in light reflected from the surface of the release coating. The parallel flakes (not reoriented by the magnetic force) would reflect the incident light while the coating-thickness oriented (reoriented) flakes would not reflect the incident light. The reflected light would give the coating a light-colored appearance, while the coating region where light would not be reflected would have dark appearance, the combination of light and dark regions in the coating providing a pattern to the viewer. The presence of a pattern is avoided in the present invention by having the flakes primarily thickness direction oriented, whereby, the entire interior bottom of the cookware (the release coating thereon) will have a dark appearance, i.e., no pattern.
A preferred composition of the present invention comprises 75 to 98 wt % fluoropolymer, 1 to 15 wt % magnetizable flakes, and 0.1 to 15 wt % of compatibilizing polymer, to total 100 wt % based on the combined weight of these three ingredients. The presence of the relatively small amount of compatibilizing polymer enables the flakes to have a long dimension which exceeds the thickness of the layer formed from the composition to be covered by fluoropolymer, even where tips of the flakes extend from the exposed surface of the layer. Such tips will thus be embedded in minute “mounds” of fluoropolymer from the layer composition, which thereby enable the layer to serve as a release coating. The compatibilizing polymer attracts the fluoropolymer to the flakes, so that when they are magnetically oriented in the thickness direction of the coating, the fluoropolymer is carried wit
E. I. Du Pont de Nemours and Company
Nakarani D. S.
Paulraj Christopher
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