1. Technical Field
This invention relates to microwave technology and, more specifically, to microwave susceptors, i.e. materials capable of generating thermal energy from microwave energy. The invention focuses upon a group of refractory solid materials, porous and relatively microwave transparent per se, that become microwave responsive by a simple process which deposits finely subdivided microwave responsive substances on at least one accessible surface. These microwave susceptible elements are uniquely suited for the storage of microwave generated heat and its delivery to load objects.
2. Description of the Prior Art
It is well known among practitioners of microwave cooking that, speed of preparation notwithstanding, microwave ovens produce results which are quite different from those obtained in conventional ovens. Microwaves heat food essentially throughout by acting upon microwave susceptible components such as water, salts, sugars and the like. Food components which are less microwave interactive do not absorb microwave energy as readily, but heat up by their close proximity to and admixture with receptive components in a constant process of thermal equilibration. In contrast, conventional ovens heat foods by conduction, radiation and convection from the outside in. This method of heating produces surface effects such as browning and crisping which are often desirable but not attainable in an all-microwave oven.
A few microwave ovens now offer added radiant or convective heat in attempts to simulate conventional ovens. Manufacturers of microwave cookware are also trying to address this need by specialized cooking utensils, described as browning grills or skillet browners, which feature microwave susceptible surfaces made of ferritic materials, magnetite and the like. A few noteworthy inventions of this type, some more recent, include a cooking container described in U.S. Pat. No. 4,751,358 and two microwave heating utensils disclosed in U.S. Pat. Nos. 4,800,247 and 5,057,659. Devices in this category are sturdy and reusable. They employ susceptor materials, particulates or matrices, which are permanently bound to or incorporated into thermally conductive substrates, to form integral structures.
Similar considerations have been given to the development of cook-in packaging for foods. In this case, microwave susceptors disposed on the packaging substrate provide directed heat to promote crispness. These are exemplified by a variety of cripsing boards which carry microwave susceptors based on vacuum metallized or metal-sputtered coatings on a polyester film which is laminated to the packaging material. A more recent example of this type, disclosed in U.S. Pat. No. 5,126,519, utilizes a film substrate with a melting point above 500.degree. F. Microwave interactive packaging in this category are commonly used for crisping such foods as french fries, fish sticks, pizza and the like, clearly one-use applications.
Another group of microwave susceptor packaging materials is claimed to be less expensive, yet perform as well as the metallized boards. They are based on particulate susceptor components which are fixed into position with polymeric binders and permanently bound to the packaging substrate. A few notable examples of this type are disclosed in U.S. Pat. Nos. 4,917,748, 4,959,516, 5,021,293 and 5,132,144. Most of them employ finely subdivided solid susceptors in liquid media and methods akin to printing for disposing the susceptors into position. That is followed by similar overcoating steps with heat curable protective substances, which make the finished structure suitable for direct contact with foods.
Presently available microwave susceptors and devices which carry them have much in common. They are designed for relatively fast and intense delivery of heat, in order to produce special effects such as browning and crisping of foods. They are intended primarily, if not exclusively, for use inside the microwave oven. They employ a variety of microwave responsive substances, ranging from metallized to particulate components, frequently more than one. Susceptor coatings employ binders to achieve integrity. They are permanently bonded to their substrates and usually covered by protective layers against abrasion and direct contact with foods. Most susceptor coatings are made by intricate multi-step methods of fabrication and complex processes. Many such coatings include extra components, for special effects, such as flame retarders, heat atenuators, masking agents or visual modifiers.
Where differences do exist, they relate to the specific types of application. Packaging-related susceptors are obviously made to be disposable. Their substrates are poor thermal conductors and rightfully so. Hence, the load object, food, is located on the susceptor side of the substrate. Most of the substrates have limited but sufficient heat stability, for their intended performance, and no heat storage capacity to speak of. By comparison, cooking devices augmented with susceptors are, obviously, permanent and reusable. Their substrates, be they metals or ceramics are good thermal conductors. Hence, their susceptor may be located on sides opposite to and away from the load object. They may also be imbedded in vitreous ceramic structures. Cooking devices employ substrates which, by necessity, must be temperature stable. However, they are neither intended or able to store substantial amounts of heat, given the weight and specific heat of the materials used. Even when they reach extremely high temperatures, they tend to give up their heat quickly by virture of their heat thermal conductivity.
It is clear from the foregoing discussion that microwave susceptors and devices of the prior art which carry them lack certain attributes, among them:
1. Ability to store heat which can extend beyond microwaving. PA0 2. Ability to deliver moderate heat over extended periods. PA0 3. Simple and inexpensive susceptor components and substrates. PA0 4. Simple methods of fabrication. PA0 5. Susceptor components which are safely away from abrasion or contact with load objects, without binders, overcoats and the like. PA0 6. Variety in size, shape and functionality. PA0 1. Unrestricted temperature stability. PA0 2. Ability to store heat. PA0 3. Ability to deliver moderate heat for extended periods. PA0 4. Simple and inexpensive susceptor components and substrates. PA0 5. Simple methods of fabrication. PA0 6. Loosely but practicably surface-bound susceptor components. PA0 7. Safe usage without danger of susceptor component abrasion. PA0 8. No direct contact by susceptor components with heating load objects. PA0 9. Versatile functionality. PA0 10. Dual ovenability.
Moving in the direction of stored heat and prolonged delivery of such heat, it is noteworthy that many solid materials are naturally microwave responsive. Certain items of pottery and ceramics, all of mineral origin, are known to be microwave interactive by virtue of their chemical and ionic structure. Examples of such materials are Corning's Visions glass, presently in commercial use, and a glass-ceramic containing nepheline which is no longer in use. The latter was actually considered microwave unsafe. Plates made of that material, known as Pyroceram, reached extremely high temperatures in the microwave. Many shattered in explosive force, and their production was discontinued. The use of all-susceptor solids for storage of heat and its sustained delivery is clearly negated by their properties of low specific heat and relatively good thermal conductivity. The first necessitates the use of substantial mass for sufficient heat storage capacity. The second makes high temperature extremes, at outer surfaces, virtually inevitable.
One way the buildup and delivery of heat may be moderated is by making the solid susceptors porous. As the density of the material decreases, it tends to give up stored heat more slowly, by virtue of diminishing thermal conductivity. As the volume of the expanding mass increases, it also presents a larger target for the microwave energy. That may limit the penetration of microwaves into the solid. The solid would thus tend to heat up unevenly, more likely from the outside in. Moreover, its thermal storage capacity would not be fully utilized, even when its surface temperature becomes extremely high. It would clearly be advantageous, therefore, to have the microwave susceptor disposed on the surface of the solid in the first place, rather than throughout the solid. That would greatly diminish the cost of materials and fabrication. It would also make it possible to use, as substrate, any number of pre-existing solid objects which are made to be porous anyway. Such solids, inexpensive and readily available, could in fact be relatively microwave transparent per se.
Accordingly, the objects of this invention are to propose materials and methods for making solid susceptor elements with performance characteristics which include:
The feature of dual ovenability is of particular importance in a changing marketplace. Since many all-microwave devices have fallen short of expectations, manufacturers are now inclined to offer dual ovenability to consumers who are not ready to give up their conventional ovens.