US 3859493 A
A method of and means for controlled heating with microwave energy is disclosed for materials exhibiting a rapid exponential rise in dielectric loss characteristic above a certain critical temperature. Such materials are referred to as having a "runaway" characteristic, when exposed to microwave energy. A base material, such as nylon, is coated with a liquid having a high dielectric loss and lower boiling point than that of the underlying material. The lossy liquid initially rapidly absorbs the microwave energy and heats the underlying material by conduction. After the liquid is vaporized, the base material can be heated further or the process may be stopped before complete varporization.
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Description (OCR text may contain errors)
Peterson MICROWAVE HEATING TEMPERATURE CONTROL Robert A. Peterson, Canton, Mass.
Raytheon Company, Lexington, Mass.
Filed: Nov. 21, 1973 Appl. No.: 418,151
 References Cited UNITED STATES PATENTS 3,643,054 2/1972 Foster 219/1055 3,702,785 11/1972 Knechtges et al ll7/l38.8 M X Primary Examiner-Bruce A. Reynolds Attorney, Agent, or Firm-Harold A. Murphy; Joseph D. Pannone; Edgar O. Rost  ABSTRACT A method of and means for controlled heating with microwave energy is disclosed for materials exhibiting a rapid exponential rise in dielectric loss characteristic above a certain critical temperature. Such materials are referred to as having a runaway characteristic, when exposed to microwave energy. A base material, such as nylon, is coated with a liquid having a high dielectric loss and lower boiling point than that of the underlying material. The lossy liquid initially rapidly absorbs the microwave energy and heats the underlying material by conduction. After the liquid is vaporized, the base material can be heated further or the process may be stopped before complete varporizatron.
4 Claims, 1 Drawing Figure HIGH VOLTAGE SUPPLY MICROWAVE HEATING TEMPERATURE CONTROL BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates to controlled processing materials having a substantially exponential rate of rise of dielectric loss characteristics relative to increases in temperature above a critical temperature utilizing microwave energy.
2. Description of the Prior Art The processing of poor thermally conductive materials such as paper, wood, leather, foodstuffs, as well as various plastic materials, has advanced in recent years with the use of energy sources operating in the microwave electromagnetic spectrum, such as the magnetron. The assigned frequencies of operation are, typically, 915 :13 MHz or 2450 i 50 MHz. The term microwave is defined as electromagnetic energy radiation having wavelengths in the order of 1 meter to 1 millimeter and frequencies in excess of 300 MHz.
The use of microwaves provides for dielectric heating by the very high frequency oscillating electric and magnetic fields to create heat by molecular friction. The degree to which such, energy is absorbed is dependent on the dielectric constant and loss characteristics of the material being processed. Any substance which readily absorbs microwave energy is termed lossy. Certain materials could not be processed, heretofore, with microwave energy due to the fact that a rapid change in the rate of heating takes place beyond a specific temperature with the rate of rise being substantially exponential. As a result the heating process is difficult to control and the product being treated may be damaged by melting. Nylon is exemplary of the socalled runaway materials which exhibit drastic temperature changes when exposed to heating with microwave energy. Higher temperatures cause the dielectric loss factor to increase rapidly and the microwave energy is absorbed at a faster rate than desired. It is, therefore, extremely difficult to process certain poor thermally conductive materials with microwave energy where the materials have this runaway characteristic. Ideally, material having high dielectric constant values readily admit microwave energy during heating but the material should also display low dielectric losses so that a controlled heating with the microwave energy will ensue.
SUMMARY OF THE INVENTION In accordance with the present invention an additive is proposed for use in microwave heating of materials having asubstantially exponentially rising dielectric loss characteristic to produce runaway" heating. An illustrative embodiment of the invention includes the application of a lossy liquid having a high energy absorbing characteristic, as well as a boiling point below that of the material being treated. An exemplary material is triethylene glycol. A material, such as nylon, is immersed in the liquid and is then exposed to the microwave energy. Initially, the base material becomes heated faster because of the high dielectric loss of the coated liquid which absorbs the energy and heats the underlying nylon material. The liquid boiling point is below that of the base material and, therefore, once this temperature is achieved the coated liquid rapidly vaporizes. The underlying material can then be heated directly with microwaves or the operation can be terminated before complete vaporization. The heating process can, therefore, be maintained under careful control with the utilization of lossy coatings which permit the rapid transfer of absorbed energy to the underlying material by conduction up to a critical temperature. After processing any lossy coating chemical residue is readily removed by rinsing. Numerous poor thermally conductive materials may now be processed with microwave energy with the control of the dielectric loss characteristic during the heating with the microwave energy with the described lossy coatings of the invention. The combination of the different materials will enlarge the scope of microwave processing.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the single FIGURE shown, the invention will now be described in detail. A base material 10 of the type having a high temperature dielectric loss characteristic is coated with a lossy liquid selected from the group having high dielectric loss characteristics and boiling point lower than the base material. The coated material 10 to be treated is disposed within a microwave oven apparatus 12 comprising an enclosure 14 defined by conductive walls 16. An access opening 18 is provided in one of the walls and is closed by a door member 20 having a handle 22 of the drop-down type supported by hinge 24. The enclosure 14 is radiated by microwave energy from a source 26, such as a magnetron, by means of a probe 28 extending within waveguide transmission means 30 having an open end 32. A high voltage supply 34 provides for the control and energizing of the magnetron source. The energy is distributed by such well-known means as a mode stirrer 36 having a plurality of paddle members 38 driven by a motor 40. The coated material 10 to be treated is supported by a dielectric plate member 42 which is in turn supported on a pedestal 44 resting on bottom wall 16 to provide for heating of the material uniformly on all sides. The absorption of the microwave energy by the lossy coating rapidly heats this material and the heat is transferred by conduction to the underlying material. As the temperature is increased over time the liquid reaches its boiling point and vaporizes to partially or fully expose the underlying material to the microwave energy.
The heating may be continued or terminatedas desired. The lossy liquid coating having a lower boiling point is vaporized after the necessary temperature is reached and the underlying material will have been shielded from being overheated during the initial processing. Afterwards the material may be safely exposed to further microwave heating to complete the process without possible damage such as melting caused by the high temperature runaway characteristics leading to uncontrolled heating. After a predetermined temperature is achieved, the underlying material may be united with other components, formed or drawn into desired shapes, such as filaments or wires from rods. In the practice of the invention, therefore, previous problems with melting due to the high temperature and dielectric loss heating factors can now be adequately controlled to permit the processing of further materials and products having inherently poor thermally conductive characteristics.
While the invention has been described with relation to the material nylon, it is evident that there are numerous other materials having runaway characteristics which can enjoy microwave heating advantages through the utilization of lossy coating additives having a lower boiling point and high dielectric loss for controlled heating to reduce prior art adverse effects, such as melting. Numerous variations, modifications or alterations will suggest themselves to those skilled in the art and, therefore, it is intended that the foregoing description of the invention be considered in its broadest aspects and not in a limiting sense.
1. A method of controlling microwave heating of material having a substantially exponential rate of rise in dielectric loss characteristic above a critical temperature comprising the steps of;
coating said material with a lossy high energy absorbing liquid having a melting point lower than that of the underlying material; and
heating said coated material with microwave energy for a period of time sufficient to vaporize said coated liquid and heat said underlying material to a temperature below said critical temperature.
2. The method according to claim 1 wherein said material comprises nylon and said lossy liquid comprises triethylene glycol.
3. In combination;
a source of microwave energy;
a source ofa material having a substantially exponential rate of rise in dielectric loss characteristic above a critical temperature coated with a lossy high energy absorbing liquid and having a lower melting point than the underlying material; and
means for radiating said member with said energy.
4. The combination according to claim 3 wherein said member comprises a nylon material and said liquid coating comprises triethylene glycol.