Microwave browning vessel

Abstract

Browning vessels to be used with microwave ovens which comprise electroconductive films demonstrating improved detergent durability are described. The durable films provide stable resistance heating characteristics over a longer period, extending the useful lifetime of the browning vessel.

Description

BACKGROUND OF THE INVENTION

Microwave cooking provides a rapid and efficient means of processing foodstuffs, but generally does not result in surface browning of the food. To provide the surface browning which is in many cases preferred, some form of direct surface heating, either by contact or irradiation is required.

Conventional means of providing browning with microwave energy typically comprise the use of a microwave-heatable apparatus in the microwave chamber which acts as a supplemental heating source to brown the food by irradiation or contact. Such an apparatus is rendered microwave-heatable through the incorporation therein of electroconductive members which are heated by internal currents generated by the microwave field.

A preferred form of microwave browning apparatus is a browning vessel such as a plate, platter, dish or pan composed of a glass, glass-ceramic or ceramic material to which has beeen applied an electroconductive tin oxide film. The film typically has an electrical resistance value in the range of 90-350 ohms per square which renders it efficiently heatable in a microwave field. Upon exposure to microwave energy, the film and subsequently the vessel are heated to a degree sufficient to brown the food by contact. U.S. Pat. No. 3,783,220 describes a microwave browning vessel of this type, consisting of a glass, ceramic or crystallized glass (glass-ceramic) vessel having on its exterior surface a thin electroconductive coating of tin oxide. Tin oxide films such as described in U.S. Pat. No. 3,564,706 to Mochel, consisting predominately of tin oxide but also containing about 0.001-13percent Sb.sub.2 O.sub.3 by weight, are also suitable for this application.

Microwave browning vessels of the kind described in the aforementioned patent have recently become available commercially and initially provide an acceptable solution to the problem of browning with microwave energy. However, sustained usage of commercially available browning vessels of this type has uncovered a service life problem, apparently associated with the stability of the electroconductive tin oxide film, wherein a premature deterioration in the resistance heating characteristics of the film is observed. This deterioration in heating characteristics means that longer and longer heating times and increased quantities of microwave energy are required to obtain the desired browning performance. In many cases, the erratic electrical behavior of degraded films ultimately causes vessel failures through localized melting or fracture. We have recently discoverd that this problem is largely attributable to poor detergent durability demonstrated by the electroconductive tin oxide film on commercially available ware. Hence, accelerated service testing of commercially available ware with strong detergents produces rapid deterioration of the tin oxide film and erratic electrical resistance characteristics. Examination of this service-tested ware reveals a tin oxide film characterized by high resistivity and film defects, including spalled areas and pinhole clusters which are likely sources of the electrical arcing which is responsible for catastrophic vessel failures. Similar defects are demonstrated by films on ware subjected to prolonged periods of actual use.

Our attempts to improve the detergent durability of conventional tin oxide films by modifying the composition of the films and/or the method by which the films are deposited on the glass-ceramic vessels have not been entirely successful. Tin oxide films are normally applied to ceramic substrates by a process comprising heating the substrates to an elevated temperature and spraying the heated substrates with a solution of a thermally decomposable tin compound. Upon contact with the heated substrate, the solvent is volatilized and the tin compound is decomposed to yield an integral tin oxide coating which is electrically conductive. Although variations in substrate temperature, spraying method, solvent composition, tin compound composition, and, within limits, film thickness, have been attempted, no modifications have been discovered which are entirely effective to improve the detergent durability of these films. Nor have any composition additives been found which act to improve the tin oxide film durability.

SUMMARY OF THE INVENTION

We have now discovered that the low detergent durability of tin oxide films employed as above described is not due to any inherent weakness in the films, but rather is somehow related to the problem of bonding the film to the substrate. Hence, we believe that the poor detergent durability of prior art tin oxide-coated glass-ceramic browning dishes is largely attributable to bonding defects between the film and the glass-ceramic substrate, with respect to which the chemical nature of the glass-ceramic supporting surface plays a major role. Based on this discovery, we have been able to produce glass-ceramic browning vessels such as plates, dishes, pans and the like having electrically conductive tin oxide films of greatly improved detergent durability by selecting glass-ceramic substrate materials having surfaces which meet specified minimum requirements of chemical stability. The practical effect of our durable film-substrate combination is a microwave browning vessel offering stable resistance-heating characteristics over a significantly prolonged period of use.

The precise nature of the substrate instability giving rise to poor detergent durability in the subsequently applied tin oxide film is not completely understood. It appears certain that low detergent durability on the part of the glass-ceramic substrate is not a principal cause. Detergent attack is generally attributed to the alkalinity of detergent solutions, yet durable tin oxide films may be produced on glass-ceramic substrate materials demonstrating poor resistance to alkaline attack as well as on substrates having good alkali durability.

Our present belief is that the surface alkali ion mobility of the glass-ceramic substrate is an important factor limiting the durability of tin oxide films conventionally applied thereto. Hence, we have found that glass-ceramic substrate surfaces exhibiting low alkali ion mobility, as evidenced by a low degree of surface leachability in acidic media, provide supporting bases capable of providing a highly durable tin oxide film. On the other hand, glass-ceramic surfaces exhibiting high degree of alkali ion mobility provide bases which appear to reduce the detergent durability of supported tin oxide films to unacceptably low levels. It has been suggested that surfaces of high alkali ion mobility inhibit tin oxide bonding or lead to greater "pinholing" in the resultant film; however, the exact mode of film failure under alkaline conditions has not been determined.

The selection of glass-ceramic substrate materials suitable for incorporation into the browning vessels of the present invention is based on a determination of the acid leachability of the surfaces of candidate glass-ceramic materials, which is thought to be a function of the mobility of the alkali ions therein. Glass-ceramic materials exhibiting a weight loss due to leaching in a standard acid solution at a standard temperature over a fixed time interval of less than a specified weight per square centimeter of surface area have been found to be excellent materials for the production of improved tin oxide-coated microwave browning vessels according to the invention. Glass-ceramic materials exhibiting substantially greater weight losses due to acid leaching under equivalent conditions, on the other hand, produce browning vessels of poorer detergent durability and reduced service life.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A simple test for determining the suitability of glass-ceramic materials to be incorporated into browning vessels according to the present invention comprises exposing sample pieces of candidate materials to an aqueous five weight percent solution of hydrochloric acid (5 grams HCl per 100 grams HCl solution) at 95.degree.C. for 24 hours. The surface areas of the sample pieces are determined prior to acid treatment and the samples are weighed before and after treatment so that the weight loss per unit surface area of each sample due to acid leaching may be calculated. The resulting value, expressed as a weight loss in milligrams per square centimeter of glass-ceramic surface area exposed to the leaching solution, provides a useful measure of the acid leachability of a material and thus its suitability for use as a tin-oxide coated microwave browning vessel. We have found that glass-ceramic materials exhibiting a weight loss due to acid leaching of less than about 0.2 milligrams of material per square centimeter of leached surface area upon exposure to an aqueous 5 weight percent solution of hydrochloric acid at 95.degree.C. for 24 hours provide useful supports for tin oxide coatings and are desirable constituents of microwave browning vessels according to the invention. In contrast, glass-ceramic materials exhibiting significantly greater losses under equivalent conditions, e.g., 0.28-0.30 milligrams or more per square centimeter of leached surface area, are found to provide supported tin oxide surface films of markedly inferior detergent durability.

The testing procedure above described is important to the invention because it provides a screening method which is dependent only on the surface characteristics of the glass-ceramic material being tested. Unfortunately, these surface characteristics may not be readily predicted in advance merely on the basis of the composition of a candidate glass-ceramic or the identity of its principal crystalline phase, since other factors such as nucleation mode and heat treatment affect the extent of crystallization, the composition and configuration of residual glassy phases, and other variables which ultimately control the acid leachability of the glass-ceramic surface. For example, lithium alumino-silicate glass-ceramics comprising beta-spodumene and and beta-eucryptite solid solution crystal phases are widely used to fabricate cooking vessels because they are significantly stronger than glass and provide greatly superior thermal shock resistance; however, whereas some lithium aluminosilicate glass-ceramic materials demonstrate very low acid leachability so as to provide excellent components of browning vessels according to the invention, other such materials exhibit extensive acid leachability and provide browning vessels of poor detergent durability and abbreviated service life. Thus a screening method which provides direct information about the surface characteristics of the material is the best way of selecting glass-ceramic substrate materials useful for the purposes of our invention.

The wide variation in acid leachability demonstrated by some commercially available glass-ceramic materials is demonstrated by the data set forth in the Table below. The data shown was generated by preparing small samples of glass-ceramic materials, determining the weight and surface area of each sample, totally immersing each sample in an aqueous 5 weight percent solution of hydrochloric acid at 95.degree.C. for a 24 hour period, removing each sample from solution, rinsing in distilled water, drying at 140.degree.C. for 1/2 hour, and finally re-weighing each sample to determine the weight loss per unit surface area incurred during leaching.

Concurrently, alternate samples of the same glass-ceramic materials are provided with electrically conductive tin oxide films having thicknesses and film resistivities suitable for microwave heating, and these tin-oxide-coated materials are subjected to an accelerated detergent test simulating an extended period of actual use wherein they are immersed in an aqueous detergent solution containing 1200 cc of water, 45 grams of sodium hydroxide, and 45 grams of SUPER SOILAX detergent, a commercially available detergent, at 95.degree.C. for about 40-45 minutes. Following exposure to this solution the samples are removed and the quality of the tin oxide films is evaluated.

The Table sets forth a designation for each sample, the general type of glass-ceramic material comprising the sample, the acid leachability of each sample, expressed in milligrams of weight loss per square centimeter of sample surface area upon exposure to the 5percent by weight HCl solution at 95.degree.C. for 24 hours, and the effect of the accelerated detergent test on the electrically conductive tin oxide films with which the alternate samples were provided. Evaluation of the tin oxide films involves coating then with DY-CHEK dye penetrant, a commercially available dying composition, and then attempting removal of the dye by various means. The quality of the tin oxide films is then rated on a film quality scale according to the ease of removability of the dye as follows: AA--readily removable with a dry cloth; A--removable with a wet cloth; B--removable by scouring with abrasive detergent compositions; and C--not removable by conventional cleaning means (gross film defects). AA and A indicate excellent film quality and integrity. B indicates some opening of film structure and shorter effective life. C indicates potential vessel/coating failure conditions are present.

TABLE __________________________________________________________________________ Detergent Test Results Acid Leachability Glass-Ceramic 5% HCl-95.degree.C.-24 hrs. SnO.sub.2 Film Sample Designation Glass-Ceramic Material Type (mg/cm.sup.2) Quality Time in __________________________________________________________________________ Test Corning Code 9608 beta-spodumene/beta-eucryptite ss 0.12 A 40 minutes (TiO.sub.2 -nucleated) Corning Code 9617 beta-spodumene/beta-eucryptite ss 0.01 A 40 minutes (TiO.sub.2 -nucleated) Hercuvit 106 beta-spodumene/beta-eucryptite ss 0.01 A 45 minutes Glass-Ceramic (ZrO.sub.2 -TiO.sub.2 -nucleated) Corning Code 0336 beta-spodumene/beta-eucryptite ss 0.30 B 40 minutes (TiO.sub.2 -nucleated) Glass-Ceramic A.sup.1 beta-spodumene/beta-eucryptite ss 1.8 C 40 minutes (ZrO.sub.2 -TiO.sub.2 -nucleated) Glass-Ceramic B.sup.2 beta-spodumene/beta-eucryptite ss 24.0 C 45 minutes __________________________________________________________________________ .sup.1 Glass-Ceramic A is a material used in the fabrication of microwave browning vessels which are presently commercially available. .sup.2 Glass-Ceramic B is a material used in the fabrication of cooking vessels which are not presently commercially available.

From the above data, the correlation between the surface characteristics of glass-ceramic materials as reflected by their HCl acid leachability and the detergent durability of electrically-conductive tin oxide films deposited thereon is readily apparent.

Although the quantitative data in the Table has been set forth only with respect to beta-spodumene/beta-eucryptite solid solution glass-ceramics, the same correlation holds in other glass-ceramic systems, and even in glass systems as evidenced by the fact that certain glasses having low surface acid leachability provide chemically suitable supports for the production of highly durable tin oxide coatings.

The effects of low durability in the tin oxide film on the performance characteristics of glass-ceramic browning vessels and the advantages of the present invention as related to improved vessel performance are shown in more detail in the following examples.

EXAMPLE I (Prior Art)

A glass-ceramic microwave browning vessel of the commercially available type, consisting of a glass-ceramic dish having a base provided with an electrically conductive tin oxide film exhibiting resistance characteristics suitable for efficient heating by microwave energy, is selected for testing. The glass-ceramic dish is composed of Glass-Ceramic A of the above Table, a glass-ceramic material having a beta-spodumene/beta-eucryptite crystal phase and moderately high acid leachability, exhibiting a weight loss of about 1.8 milligrams per square centimeter of surface area upon exposure to a 5 weight percent hydrochloric acid solution at 95.degree.C. for 24 hours.

This microwave browning vessel is subjected to an accelerated detergent test comprising immersion in an aqueous detergent solution containing 0.30percent by weight of SUPER SOILAX detergent, a commercially-available detergent, at 95.degree.C. This test has been found to duplicate in a few days the effects on glass and glass-ceramic cooking articles of several years of use in actual dishwasher service. The microwave browning vessel is periodically removed from this detergent solution and the tin oxide film is examined for evidence of deterioration.

At the end of 36 hours of exposure to the detergent solution, examination of the selected browning vessel reveals localized spalling of the tin oxide coating from the glass-ceramic vessel, particularly at locations where the coating is thin. Testing of the coating by dye application and removal indicates the additional presence of pinhole defects clustered at numerous locations throughout the coated area. The dye is not removable from many of these coating defects, indicating a rating of C on the aforementioned film quality scale.

This browning vessel with its deteriorated tin oxide coating is placed in a microwave oven to determine the effects of coating degradation on heating performance. As the oven is activated, some electrical arcing is observed in the tin oxide coating. After a short heating interval, breakage of the glass-ceramic vessel occurs. This breakage is attributed to large thermal gradients induced in the glass-ceramic vessel because of the extreme non-uniformity of heating exhibited by the degraded tin oxide film.

This example illustrates only one failure mode which has been observed in glass-ceramic browning vessels having tin oxide films of poor electrical uniformity; breakage of the vessel on cooling after use has also been observed in some cases. In addition, the electrical arcing observed in deteriorated tin oxide films is detrimental even when vessel failure does not result because it can cause damage to microwave oven components.

EXAMPLE II

A glass-ceramic cooking vessel composed of a glass-ceramic material having a weight loss on exposure to aqueous 5 weight percent HCl at 95.degree.C. for 24 hours of less than 0.2 milligrams per square centimeter is selected for treatment. This vessel is composed of Corning Code 9608 glass-ceramic material, a material described in the above Table as a beta-spodumene/beta-eucryptite glass-ceramic material exhibiting a weight loss on acid leaching of about 0.12 milligrams per square centimeter under the conditions described. An electrically conductive tin oxide film exhibiting resistance characteristics suitable for efficient heating by microwave energy is provided on the base portion of the vessel by conventional means. The microwave heating characteristics of this vessel are equivalent to the initial heating characteristics of browning vessels of the commercially available type described in Example I above.

The glass-ceramic browning vessel prepared as described is subjected to the accelerated detergent test described in Example I, wherein it is immersed in an aqueous detergent solution containing 0.3 percent by weight of SUPER SOILAX detergent at 95.degree.C. The vessel is periodically removed from the detergent solution, examined for film defects, and returned to solution.

After a total immersion time of 120 hours in the detergent solution, the vessel is removed and examined for tin oxide film defects by dye application and removal as herein above described. The film defects present are such that the dye is readily removed from the film by rubbing with a wet cloth indicating a rating of A using the aforementioned scale.

The browning vessel detergent-tested as above described is then placed in a microwave oven to evaluate the heating performance of the tin oxide film. Heating is rapid and uniform and no electrical arcing of any kind is observed. It is therefore concluded that the vessel would provide extended service in actual use under detergent dishwashing conditions.

The browning vessel of Example II comprises the best mode presently known for carrying out the present invention. Vessels composed of Corning Code 9608 glass-ceramic material are commercially available as Corning-ware skillets or pots and may be coated with electricallyconductive tin oxide films according to conventional methods to provide browning vessels offering improved tin oxide detergent durability and resistance heating characteristics substantially more stable than are provided by glass-ceramic browning vessels available in the prior art.

While the invention has been described with reference to the manufacture of microwave browning vessels having electroconductive tin oxide films of improved detergent durability, it is expected that the selection of glass-ceramic substrates as hereinabove set forth would permit the manufacture of improved microwave browning vessels wherein electroconductive films composed of other oxides such as indium oxide or other conductive materials such as carbon are alternatively employed.

Claims

We claim:

1. A microwave browning vessel demonstrating improved detergent durability and increased service life consisting of a glass-ceramic vessel having on at least a portion of the surface thereof an electrically conductive film consisting at least predominately of tin oxide, said glass-ceramic vessel being formed of a glass-ceramic material which exhibits a weight loss due to acid leaching of less than about 0.2 milligrams of material per square centimeter of leached surface area upon exposure of the leached surface area to an aqueous 5 weight percent solution of hydrochloric acid at 95.degree.C. for 24 hours.

2. A microwave browning vessel according to claim 1 wherein the glass-ceramic material is of lithium aluminosilicate composition and comprises a beta-spodumene/beta-eucryptite solid solution as the principal cyrstal phase.

3. A microwave browning vessel according to claim 1 wherein the electrically-conductive film consists essentially of tin oxide and antimony oxide (Sb.sub.2 O.sub.3), said antimony oxide being present in an amount of about 0.001-13 percent by weight.