MICROWAVE APPLICATOR EQUIPMENT FOR RAPID UNIFORM HEATING OF RECEPTIVE POLYMER SYSTEMS

BACKGROUND OF DISCLOSURE

Field of the Disclosure

[0001] Embodiments disclosed herein relate generally to microwave emitters and the use of microwave energy to selectively heat thermoplastic polymer systems. The polymer systems may either be inherently responsive to microwave energy or modified by incorporating appropriate microwave responsive additives in the polymer or as components on the backbone of the polymer.

Background

[0002] Thermoplastic polymer pellets typically must be melted, re-shaped and cooled in a primary conversion process, such as extrusion or injection molding, in order to make parts of commercial value. In some instances, a secondary fabrication process, such as thermoforming, which involves further heating, reshaping, and cooling is required to achieve parts of commercial value. In both primary and secondary processes, heat energy is applied to the thermoplastic and is subsequently removed after reshaping has occurred.

[0003] Conventional heating mechanisms for thermoplastic polymer systems in many instances rely on contact or radiant heat sources. Radiant energy, commonly referred to as infrared, has a wavelength in the range of 1 to 10 microns and will penetrate absorbing materials to a depth of approximately 1 to 2 microns before half of the available energy has been dissipated as heat. The process of heat transfer continues through a process of conduction (in the case of a solid material) or a combination of conduction, convection and mechanical mixing in the case of a molten material. Contact heating similarly relies on conduction (or a combination of conduction, convection, and mixing) from the hot contact surface to heat the "bulk" of the material.

10004] The rate of heat transfer (RHT) associated with a conductive heat transfer process can generally be described by the relationship: RHT = f(A, Ct, Delta T), where A is the area available for heat transfer, Ct is the thermal diffusivity of the material, and Delta T is the available temperature driving force, which will decrease with time as the temperature of the material being heated increases. The thermal diffusivity, Ct, of unmodified thermoplastics is inherently low, thereby impeding the heat transfer in a conventional radiant or contact heating system. Furthermore, the heat conduction process may result in an undesirable temperature gradient with the surface of the part being heated (such as a sheet material) being substantially hotter than the center of the part being heated, and being highly dependent on the thickness distribution of the part being heated.

[00051 By ^wa Y °f contrast, microwaves have a wavelength of approximately 12.2 cm, large in comparison to the wavelength of infrared. Microwaves can penetrate to a much greater depth, typically several centimeters, into absorbing materials, as compared to infrared or radiant energy, before the available energy is dissipated as heat,. In microwave- absorbing materials, the microwave energy is used to heat the material "volumetrically" as a consequence of the penetration of the microwaves through the material. However, if a material is not a good microwave absorber, it is essentially "transparent" to microwave energy.

[0006] Some potential problems associated with microwave heating include uneven heating and thermal runaway. Uneven heating, often due to the uneven distribution of microwave energy through the part, may be overcome to a certain extent, such as in a conventional domestic microwave oven, by utilizing a rotating platform to support the item being heated. Thermal runaway may be attributed to the combination of uneven heating outlined above and the changing dielectric loss factor as a function of temperature.

[0007] Microwave energy has been used, for example, to dry planar structures such as wet fabrics. Water is microwave sensitive and will evaporate if exposed to sufficient microwave energy for a sufficient period of time. However, the fabrics are typically transparent to microwaves, thereby resulting in the microwaves focusing on the water, which is essentially the only microwave-sensitive component in the material. Microwave energy may also be used to heat other materials, such as in the following references.

[0008] U.S. Patent No. 5,519,196 discloses a polymer coating containing iron oxide, calcium carbonate, water, aluminum silicate, ethylene glycol, and mineral spirits, which is used as the inner layer in a food container. The coating layer can be heated by microwave energy, thereby causing the food in the container to brown or sear.

[0009] U.S. Patent No. 5,070,223 discloses microwave sensitive materials and their use as a heat reservoir in toys. The microwave sensitive materials disclosed included ferrite and ferrite alloys, carbon, polyesters, aluminum, and metal salts. U.S. Patent No. 5,338,611 discloses a strip of polymer containing carbon black used to bond thermoplastic substrates.

[0010] WO 2004048463A1 discloses polymeric compositions which can be rapidly heated under the influence of electromagnetic radiation, and related applications and processing methods.

[0011] A key limitation to the use of microwaves for heating polymeric materials is the low microwave receptivity of many useful polymers. The low microwave receptivity of the polymers thus requires either high powers or long irradiation times for heating such polymeric systems. In polymers designed specifically for microwave absorption, there is often a trade-off between their microwave properties and mechanical or thermal properties, i.e., the mechanical and thermal properties are often less than desirable.

[0012] Another key limitation to the use of microwaves for heating polymeric materials is the limited availability of microwave heating devices suitable for or capable of effectively processing and heating polymeric materials on a continuous or semi-continuous basis. This is especially true where the materials to be processed are large in size.

[0013] U.S. Patent Application Publication No. 20030183972 discloses a method and apparatus for molding balloon catheters employing microwave energy. Microwave energy generated by a gyrotron is directed toward the mould, to heat the polymeric material without heating the mould. The balloon can be further heated by additional microwave energy. Also disclosed is a polymer extrusion apparatus utilizing microwave energy for heating polymer feedstock material within the extruder tip and die prior to product formation.

[0014] WO2004/009646 discloses the use of microwave energy to aid in altering the shape and in post-production processing of fiber-reinforced composites. A silane based sizing on the fibers is thermally degraded in the pre-heating die leaving carbon deposits on the fiber. The fibers are then pultruded and coated in extruded thermoplastic. The carbon deposits then allow the use of microwave energy in the post-production processing of the article, e.g. heating for physical deformation and welding.

[0015] U.S. Patent No. 3,843,861 discloses an apparatus for the microwave heating and vulcanization of rubber or synthetic material. U.S. Patent No. 6,211,503 discloses a device and method of heating components made of microwave absorbing plastic. The device uses a microwave generator, antenna, and a tube-like device to process the material. The tube-like device into which the microwaves are injected shields the outside world from microwaves and is designed with an inside diameter smaller than half the wavelength (about 12 cm for microwaves), to form a very strong, single mode field within the cavity. This device may allow roughly homogeneous heating of parts, but only for very small parts (< 6 cm in size).

[0016] U.S. Patent No. 7,034,266 discloses a tunable microwave apparatus for use in the manufacture of disposable absorbent articles. The microwave activates a binder fiber material to operatively provide a plurality of interconnections between absorbent fibers and binder fibers. The microwave apparatus may be used for the microwave heating of a continuous web of interconnected materials or a series of individual absorbent bodies connected by a web of tissue, non- woven, or other carrier material.

[0017] Accordingly, there exists a need for microwave heating apparatuses

(equipment), and processes using the same, for the rapid, volumetric heating of polymeric materials using microwave energy. Additionally, there exists a need for materials, equipment, and processes that have the ability to heat or melt only a portion of a polymeric material, sufficient to render the bulk material capable of flow, facilitating the shaping or further processing of the polymer.

SUMMARY OF DISCLOSURE

[0018] In one aspect, embodiments disclosed herein relate to an apparatus for heating a thermoplastic material, wherein the thermoplastic material includes a microwave-sensitive polymeric region, the apparatus comprising: two or more microwave heating apparatuses; wherein each microwave heating apparatus includes: a microwave generator; a circulator operatively disposed between the microwave generator and a horn; a waveguide and an iris plate disposed between the circulator and the horn; and a resonant cavity; wherein the waveguide transmits microwave energy from the microwave generator through the iris plate to the horn; wherein the horn disperses the microwave energy to generate a uniform energy density spread in the resonant cavity; wherein the circulator impedes energy from passing from at least one of the iris plate, the horn, and the resonant cavity to the microwave generator; wherein the resonant cavity includes at least one inlet and one outlet for passing the microwave sensitive polymeric material through the resonant cavity; wherein the microwave heating apparatuses are coupled; and wherein the inlets and outlets of the multiple emitters form a single passageway for passing the thermoplastic material through the resonant cavities; a microwave choke proximate the passageway inlet; and a microwave choke proximate the passageway outlet.

[0019] In another aspect, embodiments relate to a method for processing a thermoplastic material having a microwave-sensitive polymeric region. The method may include exposing the microwave-sensitive polymeric region to microwaves in a microwave heating apparatus, and processing the thermoplastic material, wherein the exposing causes an increase in the temperature of the polymeric region.

[0020] Other aspects and advantages of embodiments disclosed herein will be apparent from the following description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS [0021] FIG. 1 illustrates an embodiment of a microwave heating device containing multiple microwave circuits. [0022] FIG. 2 illustrates a microwave heating apparatus useful in embodiments disclosed herein.

[0023] FIG. 3 illustrates another microwave heating apparatus useful in embodiments disclosed herein. [0024] FIG. 4 illustrates a microwave heating device useful in embodiments disclosed herein. [0025] FIG. 5 illustrates one embodiment of a multilayered sheet incorporating a microwave sensitive layer. [0026] FIG. 6 presents Izod impact data for PP and ABS with and without microwave sensitive additives. [0027] FIG. 6A presents Falling Dart data for coextruded PP with and without microwave sensitive additives. [0028] FIG. 7 presents the time-temperature response for the microwave heating of a polypropylene sheet useful in embodiments disclosed herein. [0029] FIG. 8 presents the measured heating rate as a function of microwave power for several polymers containing Zeolite A, a microwave receptive additive. [0030] FIG. 9 presents the temperature profile measured for a sample of sheet being heated in a microwave heating apparatus, illustrating the uniform heating that can be achieved with selective microwave heating. [0031] FIG. 10 presents a temperature profile measured for an A/B/A sheet sandwich sample being heated in a microwave heating apparatus, illustrating the uniform heating that can be achieved with selective microwave heating. [0032] FIG. 11 presents the heating characteristics measured for two PP samples of a three layered sheet comprising a core layer of microwave sensitive material. [0033] FIG. 12 presents a temperature snapshot of the heating profile for one of the two PP samples in FIG. 11, where the snapshot is taken two minutes into the heating process. [0034] FIG. 13 presents a temperature snapshot of a heating profile for a PP sample heated using a microwave heating / thermoforming apparatus similar to that illustrated in FIG. 4. [0035] FIGS. 14-19 present microwave heating results for various samples processed dynamically, where a microwave-sensitive polymeric sheet is passed through a microwave cavity at a fixed speed, similar to that illustrated in FIG. 4.

DETAILED DESCRIPTION

[0036] In one aspect, embodiments disclosed herein relate to a microwave heating apparatus for heating polymers, hi another aspect, embodiments disclosed herein relate to a microwave heating apparatus having multiple microwave emitters, useful in processing large polymeric structures. The polymers may incorporate microwave-receptive components, either on the backbone of the polymer or as polymeric or non-polymeric additives in the polymer, which may allow the polymer to be heated rapidly and controllably through the application of microwave energy. In other aspects, embodiments relate to methods for processing polymers incorporating microwave receptive components in a microwave heating apparatus.

[0037] Compared to alternative methods of heating, such as radiant, convective, or contact heating, the use of microwave energy may result in very rapid, volumetric heating. The use of microwave energy may overcome two fundamental limitations of the conventional heating systems: the dependence on the thermal conductivity of the polymer to transport heat energy form the surface of the part; and the maximum allowable temperature of the polymer surface which in turn determines the maximum available temperature driving force.

[0038] A polymer may inherently be receptive to microwaves based upon its chemical composition. Alternatively, a microwave sensitive polymer composition may be formed by combining a microwave receptive additive with a base polymer which is non-receptive to microwaves. Suitable base polymers, microwave receptive polymers, and microwave receptive additives useful in embodiments disclosed herein are described below. The resulting microwave receptive or microwave sensitive polymers may be heated using microwave energy, in lieu of or in combination with radiant, convective, or contact heating. The heated polymer may then be mixed, transferred, shaped, stamped, injected, formed, molded, extruded or otherwise further processed, such as in a primary conversion process or a secondary fabrication process to form useful articles.

[0039] Embodiments disclosed herein relate to the efficient conversion of thermoplastic materials using electromagnetic energy, by selectively heating a portion of the volume of the thermoplastic material, that portion being sufficient to render the material processable in a subsequent forming technique. As used herein, processable

means the provision of sufficient melt-state or softening of at least a portion of the thermoplastic in order for the bulk plastic to be mixed, transferred, shaped, stamped, injected, extruded, etc., to form a product. The heating of the thermoplastic substrate maybe achieved by the exposure of the thermoplastic to electromagnetic energy, such as microwaves, which have the ability to penetrate through the entire volume of the substrate and to be preferentially absorbed in microwave sensitive regions.

[0040] By applying microwave radiation, heat may be generated locally at a predetermined region of the volume, bulk, or part of the polymer specimen. Thus, the amount of energy applied may be carefully controlled and concentrated, as other regions may consist of non-absorbing materials which are transparent to the radiation used. For example, untreated polypropylene and polyethylene are transparent to microwave radiation. By focusing on materials that are receptive to microwaves, the energy used may be reduced, the cycle times shortened and the mechanical and other properties of the final material may be adapted and optimized for various requirements and applications.

[0041] Sites within the microwave sensitive material may be either favorable or non-favorable for absorption of the electromagnetic energy. Sites that are favorably absorptive will readily and rapidly heat under the influence of electromagnetic energy. In other words, only a specified portion of the volume of the substrate will be strongly affected by the electromagnetic energy, relative to other regions of the material.

[0042] In this manner, the electromagnetic energy interacts with only certain regions of the substrate, which will increase in temperature when electromagnetic energy is present. The heating of neighboring regions within the bulk material will subsequently occur due to thermal conduction and other such mechanisms. As the bulk material is heated volumetrically, the material may be converted into a processable state more rapidly as compared to conventional heating techniques. Moreover, because that material may contain less heat energy than would normally be present had the entire bulk material been heated via surface conduction (infrared heating), there may be considerable savings in energy. For example, infrared heating results in significant energy losses to the atmosphere, and requires that the surface temperature of the part is significantly higher than the desired bulk temperature in order to effect an acceptable rate of heat transfer from the part surface to the part core and raise the core temperature to that required for processing. In contrast, microwave

selective heating, which causes the temperature of the microwave sensitive polymer to heat rapidly and volumetrically to processing temperature, may result in a significantly lower polymer surface temperature, especially in such cases that comprise microwave transparent surface layers. Microwave heating may also have less tendency for energy to be lost from the system, transferring energy primarily to where it is needed, i.e. the microwave sensitive polymer. Microwave heating may also result in considerable savings in cycle time for a conversion process. The heating time may be reduced, not only because the microwave heating mechanism occurs rapidly throughout the bulk (in contrast to thermal conduction), but the total energy content of the part is less. The cooling cycle may also be reduced as the unheated regions of material effectively act as heat sinks to draw heat out of the neighboring heated regions, significantly enhancing the overall cooling rate of the bulk material.

[0043] The microwave sensitive polymer may be used during the primary conversion or secondary fabrication processes. For example, in some embodiments, the microwave sensitive polymer may be used during the fabrication of polymeric articles including films, foams, profiles, compounded pellets, fibers, woven and non- woven fabrics, molded parts, composites, laminates, or other articles made from one or more polymeric materials. In other embodiments, the microwave sensitive polymer may be used in conversion processes such as sheet extrusion, co- extrusion, foam extrusion, injection molding, foam molding, blow molding, injection stretch blow molding, and thermo forming, among others.

[0044] Microwave Emitter

[0045] An industrial microwave oven typically includes three main components: an oven cavity where objects can be bombarded with microwaves, a magnetron which produces the microwaves, and a wave guide which transfers microwaves to the oven cavity. A continuous microwave oven typically includes a vestibule which may act to trap all non-absorbed microwave energy so that radiation is prevented from escaping into the surroundings.

[0046] Current microwave heating equipment is limited with respect to the size of materials that may be processed. For example, as the size of a waveguide used to direct microwave energy is increased, the uniformity of the energy density spread exiting the waveguide may decrease. In order to increase the size or aspect ratio of materials that can be heated and to reduce the occurrence of uneven heating and the

consequent tendency for thermal runaways of conventional microwave ovens, as discussed above, a microwave apparatus has been designed which incorporates multiple resonant cavities.

[0047] The use of multiple resonant cavities may provide a uniform energy density and high field strength, resulting in rapid, uniform heating of a microwave- sensitive material. Multiple resonant cavities may be preferred where the material to be heated is larger than could be effectively heated using a single emitter, such as for a polymeric sheet having substantial width.

[0048] One possible configuration for a microwave heating apparatus having multiple resonant cavities is a linear array of heating units. Microwave energy may be provided to the heating units using one or more microwave generators, for example, where one microwave generator provides microwave energy that is directed into two or more resonant cavities. In some embodiments, a single microwave generator may provide microwave energy to a single resonant cavity. In this case, each unit may sit next to the adjacent unit in a line.

[0049] In the linear array, the heating units may be closely coupled to one another mechanically, such that the inlet and outlet feed slots of the multiple cavities collectively form a single passageway, capable of handling a material greater in size than any single emitter could handle individually. The separation between the adjacent resonant cavities may be relatively small and designed such that a relatively uniform temperature rise may be achieved across the entire material during heating. The individual heating units may heat adjacent regions of the material passing through the multiple cavities. For example, individual units may establish a uniform high intensity microwave field across their respective portions of a polymeric sheet and - rapidly heat the sheet as it moves through the array to the temperature required to process (e.g., shape or form) the material as desired, reducing the overall cycle time of the heating step prior to the processing step.

[0050] Another possible configuration for a microwave heating apparatus having multiple resonant cavities is a staggered array of heating units. For example, the heating units may be staggered such that the outer bounds of the energy density spread in each heating unit are adjacent or overlapping. In a linear array, portions of the material may not be exposed to microwave energy due to space required for the

component parts of individual heating units. By using a staggered array, it may be possible to expose the entire width of the material being heated to microwave energy.

[0051] Referring now to FIG. 1, one configuration for a multiple resonant cavity array is illustrated. The heating apparatus 40 may include one or more microwave circuits 41, including at least one microwave generator and other equipment (described below) to control or direct the microwave energy to the multiple resonant cavities 43. The microwave energy may then impact a microwave-sensitive or microwave receptive material 46, such as a microwave-sensitive polymeric sheet, in the resonant cavities 43. Microwave chokes 47 may be used to minimize the leakage of microwave energy from the array.

[0052] As illustrated, heating apparatus 40 contains a bank of 12 microwave circuits 41 (2 rows of 6), which may operate in conjunction to uniformly heat sheet 46. As opposed to the linear arrangement described above, the use of two rows of emitters may allow complete coverage of the heating areas, if desired. In this manner, any separation between heating cavities, such as due to the material thickness 45x of horns 45, for example, may be minimized or eliminated, resulting in a uniform temperature rise across the material being heated.

[0053] Regarding the microwave circuits and other equipment that may be used to control or direct microwave energy to the multiple resonant cavities 43, any equipment that may be used for processing microwave energy may be used. For example, section 42 may include equipment to direct and control energy from a microwave generator to resonant cavities 43, including tuning devices and other circuitry to minimize feedback of reflected energy to the microwave generator; and a waveguide 44 may direct microwave energy through horn 45, which may provide a uniform microwave energy density spread to resonant cavities 43. Other equipment that may be used includes: horns, waveguides, microwave antennae, circulators, isolators, duplexers, phase shifters, twin stub tuners, four stub tuners, EH tuners, network analyzers, e-field probes, infrared pyrometers, variable power sources, and other equipment known to those skilled in the art.

[0054] Feed slot 49, as mentioned above, may be a single passageway (the array having, overall, one inlet and one outlet) for the material to be heated to be passed through the resonant cavities. Microwave chokes 47 may be configured to minimize microwave leakage from the multiple resonant cavities through the inlets and outlets.

Additionally, feed slots 49 may be adjustable to accommodate various sizes (thickness and/or width) of sheet passing through the inlet and outlet, and may also be adjustable in relation to cavities 43 such that the sheet may pass through a maxima in e-fϊeld in the resonant cavities 43.

[0055] Movable pistons 48 may be vertically adjusted to change the effective length of the resonant cavity. Movable pistons 48 may, for example, effectively adjust the length from the iris plates to the bottom of the resonant cavity, allowing for tuning of the resonant cavity in relation to the microwave frequency, allowing a standing wave to develop. A control system may control the multiple cavity array as a single unit. In this manner, the individual pistons may be uniformly adjusted to tune the resonant cavities and to adjust the microwave chokes based on the thickness of the thermoplastic material passing through the feed slots. Due to minor variations in the operating parameters for each microwave emitter - resonant cavity combination, the ability to fine tune each microwave circuit may be preferred.

[0056] Although Figure 1 illustrates an embodiment having 12 microwave circuits, other embodiments may contain one or more emitters to heat sheet specimens. For example, the number of microwave circuits may be based upon the size of the microwave generator(s), the size of the material being processed, and the heating rate desired, among other variables.

[0057] Referring now to FIG. 2, a simplified circuit diagram for a microwave heating apparatus 1 that may be used in accordance with embodiments of the present disclosure is illustrated. A circuit for microwave heating apparatus 1 may include a microwave generator 2, such as a klystron, a magnetron, a gyrotron, or a traveling wave tube. Energy produced in the microwave generator 2 may be passed through a circulator 3 and through a waveguide 4 with matching iris plates 4a. The energy then passes through a horn 5, where the microwave energy may be spread to uniformly enter the resonant cavity 6. The horn 5 may also prevent excitation of orthogonal modes within the microwave energy being transmitted. Cavity 6 may include inlets and outlets I/O to enable passage of a material to be heated through the cavity 6.

[0058] For a given length of cavity 6 and frequency of microwave radiation emitted from microwave generator 2, a standing wave may be established within the cavity 6 between a bottom of the cavity 6 and the iris plates 4a. This standing wave may enable very high electric field strengths to be established within the cavity 6.

Additionally, the shape and size of the iris may be optimized to enable matching of the system frequency to the frequency range of the materials to be heated using microwave heating apparatus 1.

[0059] During operation of the microwave heating apparatus 1 , energy may be reflected from the cavity 6 through the iris plates 4a. Energy may also be back- reflected from the iris plates 4 toward microwave generator 2. This reflected energy may interact negatively with the microwave generator 2. The circulator 3 may impede the reflected energy from interacting with the microwave generator 2. Circulators are described, for example, in U.S. Patent Nos. 4,771,252 and 5,384,556, which are hereby incorporated by reference. In some embodiments, isolators, duplexers, and the like may be used in lieu of or in combination with circulator 3. For example, where circulator 3 is connected to only two components, as shown, circulator 3 may function as an isolator.

[0060] Referring now to FIG. 3, a simplified circuit diagram for a microwave heating apparatus 1 including optional components that may be used in various embodiments disclosed herein is illustrated. As described above with respect to FIG.

2, energy produced in the microwave generator 2 may be passed through a circulator

3, a waveguide 4, and a horn 5, entering resonant cavity 6.

[0061] Microwave heating apparatus 1 may include a lower movable piston 7 to adjust a length of resonant cavity 6. As described above, for a given cavity length and frequency of microwave radiation, a standing wave may be established within resonant cavity 6. This standing wave may enable very high electric field strengths to be established within resonant cavity 6. The variable length of resonant cavity 6 (in the direction of the standing wave) afforded by the lower movable piston 7 may enable the fine tuning of the resonant cavity 6. The ability to fine tune resonant cavity 6 may allow the microwave heating of materials having varied sizes and dielectric properties. Moreover, the position of movable piston 7 may be used to reduce or minimize the amount of leakage of microwave energy from resonant cavity 6 through the cavity inlets and outlets I/O. Microwave chokes (not shown) may also be used to prevent leakage of microwave energy through the cavity inlets and outlets I/O.

[0062] Microwave heating apparatus 1 may include a phase shifter 8. Phase shifter 8 may be used to modulate a frequency of the energy emitted by microwave generator 2. Phase shifter 8 may also provide for minor adjustments of the electrical

length between iris plates 4a and microwave generator 2. These minor adjustments may be used to prevent any back reflected energy from the iris plates and the cavity from causing adverse effects on the microwave generator.

[0063] Phase shifter 8 and circulator 3 may also be used in a tuning circuit. For example, U.S. Patent No. 4,162,459, which is hereby incorporated by reference, describes a tuning circuit including a circulator and a phase shifter. Phase shifter 8 may include electrically (e.g., diodes, non-linear dielectrics, and ferro-electric materials), magnetically (e.g., ferritic compounds), and mechanically (e.g., a trombone line) controlled phase shifters.

[0064] Microwave heating apparatus 1 may also include a tuning device 9 disposed between the microwave generator 2 and the iris plates 4a. Tuning device 9 may include EH tuners, twin stub tuners, four stub tuners, and the like. Tuning devices 9 may be used in lieu of or in combination with the phase shifter 8.

[0065] Although illustrated in Figures 2 and 3 with the microwave energy being directed downward into resonant cavity 6, the microwave energy may be directed in any direction (upward, downward, side-to-side, etc.). In some embodiments, referring to Figure 1 , the microwave energy may be directed at an angle α relative to a plane P connecting the inlet and outlet passageways, where the angle α may range from 30 to 90 degrees; angle α may range from 60 to 90 degrees in other embodiments; from 75 to 90 degrees in other embodiments; and from 85 to 90 degrees in yet other embodiments. The microwave energy is preferentially directed substantially perpendicular to a plane P connecting the inlet and outlet passageways.

[0066] In some embodiments, a second resonant circuit may be created between the iris plate and the microwave generator. A tuning device may be used in the waveguide section above the iris plate in order to fine tune this second resonant circuit, if desired.

[0067] As described above, the microwave heating apparatus may be tuned to generate a standing wave, to reduce leakage, to minimize adverse effects of reflected energy, and to match the resonant frequencies of materials to be heated with the microwave heating apparatus. Tuning may include phase shifters, tuning devices, varying the position of the iris plate relative to the microwave generator, varying the length of the resonant cavity, and varying the position of a material to be heated within the cavity.

[0068] In some embodiments, a network analyzer or an e-field probe may also be used to tune the microwave heating apparatus. A network analyzer, typically used when the microwave generator is not operational, may inject a small amount of microwave energy into the system and analyze back reflection. The back reflection may be minimized by altering the position of the movable piston, or by altering the settings on the phase shifter or tuning devices that may be used. An e-field probe may measure the electric field within the resonant cavity. The system may be tuned by altering the settings of the phase shifter, tuning devices, or movable piston to maximize the electric field within the resonant cavity.

[0069] In other embodiments, microwave apparatus disclosed herein may include other components typically used in a microwave system and known to those skilled in the art. For example, the microwave systems disclosed herein may include directional couplers, amplifiers, attenuators, transformers, transmission lines, antennas, connectors, couplers, splitters, oscillators, and microwave impedance tuners, among others.

[0070] Referring now to FIG. 4, a portion of a microwave heating apparatus 10 that may be used in accordance with embodiments of the present disclosure is illustrated. Microwave heating apparatus 10 may include a microwave generator (not shown), such as a klystron, a magnetron, a gyrotron, or a traveling wave tube, a circulator (not shown), and an upper waveguide section (not shown) supplying microwave energy to the apparatus 10. Additional components of microwave heating apparatus 10 may include tuning device 12 (including tuning pistons 11), waveguide 14 and matching iris plates 13, horn 15, resonant cavity 16, and microwave choke 17. A lower moveable piston 18 may be used to adjust a length of the resonant cavity 16. Feed slots 19 may provide an inlet and outlet to facilitate the processing of polymer sheets through the microwave heating apparatus 10.

[0071] Resonant cavity 16 may provide a uniform field pattern in a direction transverse to the direction the material to be heated is passed through resonant cavity 16. Microwave energy may be fed into the waveguide 14 from the microwave generator (not shown). The microwave energy may then pass through horn 15, where the microwave energy may be spread to uniformly enter the resonant cavity 16. At the bottom of resonant cavity 16, the energy reflects off the top face of movable piston 18. For a given cavity length and frequency of microwave radiation, a standing

wave may be established within resonant cavity 16. This standing wave may enable very high electric field strengths to be established within resonant cavity 16.

[0072] In some embodiments, the length of resonant cavity 16 may be fixed, as by not including a moveable piston 18. As illustrated in FIG. 4, the variable length of resonant cavity 16 (in the direction of the standing wave) afforded by the lower moving piston 18 may enable the fine tuning of the resonant cavity 16. The ability to fine tune resonant cavity 16 may allow the microwave heating of materials having varied sizes and dielectric properties. Moreover, the position of moveable piston 18 may affect the amount of leakage of microwave energy from resonant cavity 16, sideways from the feed slots 19. Microwave chokes 17 may prevent leakage of microwaves through feed inlets and outlets, feed slots 19, and moveable piston 18 may allow this leakage to be further minimized. By adjusting the moveable piston 19 such that the wall current at the boundary of feed slot 19 point is zero, the side leakage may be minimized. The system may then be fine tuned to the magnetron frequency via adjustment of the tuning device 12 and tuning pistons 11. By- adjusting the moveable piston 18, tuning device 12, and tuning pistons 11, the electrical length may remain equal and resonance may be maintained.

[0073] The resulting electric (electromagnetic) field within resonant cavity 16 may result in a uniform band of heating across the material being heated. By moving the material (such as a sheet) through slots 19, the material may pass through the resonant cavity 16 and heats upon exposure to the field. The rate of heating of the material moving through resonant cavity 16 may be varied, such as by varying the speed of passage of the material through resonant cavity 16 or by varying the electric field strength within resonant cavity 16.

[0074] Microwave heating apparatus 10 may heat a thermoplastic material having a microwave-sensitive region by passing the material through the resonant cavity 16. The microwave energy channeled from the microwave generator to the resonant cavity 16 may heat the microwave sensitive polymeric region, allowing the thermoplastic material to be processed, as will be described below.

[0075] Microwave heating apparatus 10 may be capable of rapid and uniform heating of polymers, and may adapt to the nature of the microwave sensitive polymer (receptor type, receptor concentration, matrix type, etc.) and the form of the material being processed (thickness, shape, etc.). For example, apparatus 10 may include a

variable power source (not shown); horn 15 may provide a uniform energy density spread; and tuning device 12 may allow for fine tuning of the wavelength emitted. In this manner, the microwave emitter may be tailored to efficiently heat a particular polymer.

[0076] Analytical measurement devices (not shown) may also be provided to monitor or enhance the performance of the microwave heating apparatus. A thermal imaging device, such as an infra-red pyrometer, temperature sensors, thermocouples, and the like installed within horn 15, outside of cavity 16, or any other suitable location, may monitor the temperature of the material being processed, and may provide a real-time temperature reading of the material. These thermal imaging devices may be used to monitor temperature evolution during the process, usually prior to forming of the heated material. For example, an infra-red pyrometer may be placed within the horn, looking down onto the material being heated within the cavity. The infra-red pyrometer may monitor the real-time sample surface temperature. Data from the infra-red pyrometer may be fed to a controller which in turn may alter the speed of transit of the material being heated, microwave power input, and other process variables to attain the desired degree of heating. Control of heating in this manner may enable a final uniform temperature distribution across the material being heated, both axially and perpendicular to the axis.

[0077] A network analyzer may be used to tune resonant cavity 16. A network analyzer may inject a small amount of microwave energy into the system and observe the back reflection. Minimizing the back reflection by altering the settings on tuning device 12 and moveable piston 18 tunes the resonant cavity 16. An E-field probe may be located within resonant cavity 16 such that the tuning device 12 and moveable piston 18 may be adjusted to maximize the electric field within resonant cavity 16.

[0078] Tuning of the microwave heating apparatus may also be affected by the length of the upper waveguide section (not shown) between the tuning device 12 and the circulator and/or microwave generator (magnetron). Magnetrons working at a set temperature and power emit EM radiation at a constant frequency. However, as the magnetron power is increased, and the anode current increases, the magnetrons frequency may change by values of up to 2 MHz per Amp. If an incorrect length for the waveguide section between the tuning device and the magnetron is used, then the resonant frequency of the horn 15 will move in an opposite direction to the change in

frequency of the magnetron. To enable easier tuning of the microwave heater, the length of waveguide section between the tuning device 12 and the magnetron may be set to a predetermined value based upon the power setting, such that the resonant frequency of the applicator will move in the same direction as the change in frequency of the magnetron when the power of the magnetron is changed.

[0079] In certain embodiments, one or more emitters may be used to heat a microwave sensitive polymer. For example, the efficiency of the horn 15 in providing the energy density spread may become decreased as the overall size of the microwave heating apparatus is increased. Multiple emitters may thus allow processing of a specimen having a greater size (width) than could be effectively processed by an individual emitter. This will be discussed in greater detail below.

[0080] The selected power rating for the microwave emitter used may depend on the size or thickness of the polymer specimen being heated. The power rating may also be selected based on variables such as the cycle time for operations occurring upstream or downstream from the heating stage. In certain embodiments, a variable power source may be employed, providing process flexibility, such as the ability to vary a part size or composition (amount or type of microwave receptive additive).

[0081] In some embodiments, the microwave emitter may have a constant or variable power rating in the range from 100 W to 1,000 kW. In other embodiments, the power rating may range from 500 W to 500 kW; from 1 kW to 100 kW in other embodiments; from 5 kW to 75 kW in other embodiments; and from 10 kW to 50 kW in yet other embodiments. In certain embodiments, the power rating may range from 15 kW to 40 kW; and from 20 kW to 30 kW in yet other embodiments. In other embodiments, the power rating may range from a lower limit of 10, 20, 50, 100, 500, 1000,-or 5000 W to an upper limit of 5, 10, 15, 20, 25, or 30 MW.

[0082] Other embodiments may contain one or more emitters to heat sheet specimens, where the number of emitters employed may be based upon the emitter size, sheet size, heating rate desired, and other variables. In some embodiments, sheet thicknesses may range from 0.01 mm to 10 cm; from 0.1 mm to 7.5 cm in other embodiments; and from 0.25 cm to 5 cm in yet other embodiments. In other embodiments, multiple emitter arrays disclosed herein may be used for thick sheet applications, where the sheets may have a thickness up to 15 cm; up to 10 cm in other

embodiments; up to 5 cm in other embodiments; and up to 2.5 cm in yet other embodiments.

[0083] Multiple emitters arrays described herein may also allow for the processing of sheets having substantial widths. For example, embodiments disclosed herein may process sheets having a width of 10 feet or more; 8 feet or more in other embodiments; 6 feet or more in other embodiments; 4 feet or more in other embodiments; and 2 feet or more in yet other embodiments.

[0084] The aspect ratio of sheet that may be processed in a multiple emitter array may range from 1 to 5000 in some embodiments, where the aspect ratio is defined as average width divided by average thickness. In other embodiments, the aspect ratio may range from 10 to 2500; from 50 to 1000 in other embodiments; and from 100 to 500 in yet other embodiments.

[0085] For the above described sheet thicknesses, widths, and aspect ratios, the sheet length may be any desired length. Sheet length may depend on whether the downstream processes are configured to process a continuous sheet, such as from a roll, for example, or configured to process a sheet of finite length. Accordingly, sheet length may vary from a few centimeters to an infinite length.

[0086] Regardless of sheet width, length, or thickness, multiple emitter arrays disclosed herein may provide for selective heating of selected sheet regions in some embodiments, and may provide for rapid, uniform heating of the sheet in other embodiments. As used herein, rapid heating may refer to the heating of at least a portion of the sheet at a rate of at least 5°C per second in some embodiments; at least 10 ⁰ C per second in other embodiments; at least 20 ⁰ C per second in other embodiments; at least 30 ⁰ C in other embodiments; and at least 50 ⁰ C in yet other embodiments. As used herein, uniform heating may refer to the heating of a sheet, or at least a selected portion of a sheet, wherein the heated portion has a maximum temperature variance of 10 ⁰ C or less in some embodiments; 7.5°C or less in other embodiments; 5°C or less in other embodiments; 4°C or less in other embodiments; and 3°C or less in yet other embodiments. By comparison to conventional infrared heating, the heating rates and temperature variances afforded by various embodiments of the microwave heating apparatuses disclosed herein may provide an advantage in cycle times, reduce the deleterious effects on the polymer due to excess heat exposure, as well as provide for improved processing.

[0087] The above described microwave heating apparatuses may be used to heat various polymeric materials, including microwave receptive polymers and composites including polymeric materials and microwave receptive additives.

[0088] Microwave Receptive Additive

[0089] The microwave receptor, or the additive which may be blended with a base thermoplastic polymer to form a microwave sensitive polymer, may include conductive or magnetic materials such as metals, metal salts, metal oxides, zeolites, carbon, hydrated minerals, hydrated salts of metal compounds, polymeric receptive materials, clays, silicates, ceramics, sulfides, titanates, carbides, sulfur, inorganic solid acids or salts, polymer acids or salts, inorganic or polymeric ion exchangers, clays modified with microwave receptive compounds, inorganic or polymeric substances which contain a molecular or polymer microwave receptor, organic conductors, or other compounds that may be effective as microwave receptors that may impart receptivity and selective heating to a polymeric material.

[0090] Any of the above additives may be used separately or in combination to provide the desired effect of selective heating. In some embodiments, microwave receptive additives may exhibit a narrow band response to electromagnetic energy. In other embodiments, the microwave receptive additive may be heated by irradiation across a broad band of frequencies. In one embodiment, the additive may be regarded as having a receptive nature over a frequency range froml MHz to 300 GHz or above. In other embodiments, the additive may be heated in a frequency range from 0.1 to 30 GHz or above; from 400 MHz to 3 GHz in other embodiments; and from 1 MHz to 13 GHz or above in other embodiments. In yet other embodiments, the additive may be heated in a frequency range from 1 to 5 GHz.

[0091] Polymer

[0092] Polymers which may be combined with one or more microwave receptive additives to form a microwave sensitive polymer include resins selected from polyolefins, polyamides, polycarbonates, polyesters, polylactic acid and polylactide polymers, polysulfones, polylactones, polyacetals, acrylonitrile-butadiene-styrene resins (ABS), polyphenyleneoxide (PPO), polyphenylene sulfide (PPS), styrene- acrylonitrile resins (SAN), polyimides, styrene maleic anhydride (SMA), aromatic polyketones (PEEK, PEK, and PEKK), ethylene vinyl alcohol copolymer, and copolymers or mixtures thereof. In certain embodiments, polyolefins and other

polymers which may be combined with a microwave receptive additive include polyethylene, polypropylene, polystyrene, ethylene copolymers, propylene copolymers, styrene copolymers, and mixtures thereof. In other embodiments, polymers which may be combined with a microwave receptor include acrylonitrile- based polymers, hydroxyl group-containing polymers, acryl- or acrylate-based polymers, maleic anhydride-containing or maleic anhydride-modified polymers, acetate-based polymers, polyether-based polymers, polyketone-based polymers, polyamide-based polymers, and polyurethane-based polymers.

[0093] In some embodiments, the microwave sensitive polymer may be incorporated as a discrete layer (or several layers) in a multi-layered structure in such a way that the discrete layer (or layers) may be preferentially heated prior to subsequent fabrication. Heat energy may then be conducted from these layers to adjacent layers of polymer that may be essentially "transparent" to microwave energy, thereby allowing the total polymer structure to reach the required fabrication temperature more rapidly than with a conventional heating system.

[0094] In certain embodiments, the microwave sensitive polymer may be formed by combining from 0.1 to 200 parts by weight microwave receptive additive per hundred parts polymer. In other embodiments, the microwave sensitive polymer may be formed by combining from 1 to 100 parts by weight microwave receptive additive per hundred parts polymer; from 2 to 50 parts in yet other embodiments; and from 3 to 30 parts in yet other embodiments.

[0095] In certain embodiments, the content of the microwave receptive additive may comprise from 0.1 to 25 weight percent of the microwave sensitive polymer, In other embodiments, the content of the microwave receptive additive may comprise from 1 to 20 weight percent of the microwave sensitive polymer; and from 2 to 15 weight percent in yet other embodiments.

[0096] In some embodiments, the microwave sensitive polymer may be in the form of powder, granules, pellets, uneven chippings, liquid, sheets, or gel. The microwave sensitive polymer may be crystalline, semi-crystalline, or amorphous. In some embodiments, the microwave sensitive polymer may include colorants, reinforcing or extending fillers, and other functional additives such as flame retardants or nanocomposites.

[0097] Applications

[0098] As described above, the microwave heating apparatuses disclosed herein may be used to heat a polymer for subsequent processing, such as being mixed, transferred, shaped, stamped, injected, formed, molded, extruded, or otherwise further processed. In some embodiments, microwave heating apparatuses disclosed herein may be useful in thick sheet thermoforming processes, such as for forming refrigerator liners, for example. La other embodiments, microwave heating apparatuses disclosed herein may be useful for heating, binding, or processing air laid binder fibers, for example. In other embodiments, microwave heating apparatuses disclosed herein may be useful for blow molding processes, such as for the formation of blown bottles, for example.

[0099] In other embodiments, microwave heating apparatuses disclosed herein may be useful in applications where the polymer being processed is not completely molten. For example, microwave heating apparatuses may be used to selectively heat a select portion of the polymer passing through the apparatus, thereby concentrating the heat energy to only that portion being further processed, such as by a forming, molding, or stamping process. This may enhance the structural integrity of the material handled during processing, may reduce cycle times, and may reduce the energy required for processing the material into the desired shape.

[00100] In other embodiments, microwave heating apparatuses disclosed herein may be useful in processing embossed sheets, including embossed sheet thermoforming. In conventional infrared thermoforming, heat input must pass through the surface of the sheet, and often reduces the retention of the embossing structure or surface details. In addition to the reduced heating cycles, as described above, microwave heating apparatuses may allow for increased retention of embossing structures during processing due to the reduced energy footprint imparted to the sheet.

[00101] In other embodiments, selective heating may allow the use of microwave sensitive layers of polymer interspersed with non-sensitive layers. Layered polymers may provide for: optimum temperature profiling; the use of pulsed microwave energy during polymer processing; the selective placement of the microwave emitters providing for heating of specific regions of a part; and other manifestations which may provide for preferential or selective heating by virtue of the microwave sensitivity of one or more thermoplastic parts or layers.

[00102J As one example of sheet extrusion, a microwave sensitive layer may be incorporated into a multilayered sheet. For example, FIG. 5 illustrates one

embodiment of a multilayered sheet incorporating a microwave sensitive layer. The microwave sensitive layer B may form a sheet core, bounded by outer layers A not sensitive to microwave heating. Incorporation of a microwave sensitive core layer may facilitate subsequent processing of the sheet, such as during sheet thermoforming. In some embodiments, sheet thermoforming may be facilitated by use of a microwave selective polymer by enabling thick sheet thermoforming, selective drawability, and rapid, uniform heating of the sheet.

[00103] Although illustrated in FIG. 5 as a three layered sheet, in other embodiments a microwave sensitive polymer may form a region or regions within a polymer structure. For example, the microwave sensitive polymer may form a discrete layer in a sheet having two or more layers. In other embodiments, the microwave sensitive polymer may form specific regions of a larger structure, allowing selective heating of those regions for further processing. In yet other embodiments, the microwave sensitive polymer may form one side of a side-by-side fiber structure. In yet other embodiments, the microwave sensitive polymer may form the core or the sheath of a core/sheath fiber structure. It should be understood that embodiments disclosed herein are applicable to materials having two or more layers.

[00104] In a foam extrusion process, for example, incorporation of a microwave sensitive layer may allow selective heating of the foam core and the solid, non- sensitive skin, enabling shorter heating cycles while preventing collapse of the foam structure. In other embodiments, incorporation of different concentrations of the microwave absorbing species in each of the layers may allow differential heating of each of the layers and hence optimization of any subsequent fabrication step, such as thermoforming. In other embodiments, incorporation of a microwave sensitive layer may allow selective foaming of a post-formed sheet.

[00105] In other embodiments such as injection molding or injection stretch blow molding, incorporation of a microwave sensitive layer may allow shorter cycles due to the internal cooling of the polymer, where the non-sensitive portions may act as heat sinks and therefore provide a reduced cooling time. Injection molding may also be facilitated by use of pulsed microwave energy, resulting in a mixture of molten and semi-molten material which can be injection molded, the semi-molten material acting as a heat sink during subsequent cooling of the part. Injection stretch blow molding

may also benefit from the optimized thermal gradient resulting from microwave selective heating, allowing for improved mechanical properties of the final product.

[00106] In some embodiments, a layered thermoplastic sheet, containing microwave sensitive and non-microwave sensitive layers, may be selectively heated prior to thermoforming. In other embodiments, layered or co-extruded pellets of thermoplastic materials may be selectively heated prior to subsequent processing in for example, an injection molding process. These may result in accelerated cooling due to the presence of "internal heat sinks" described above, and hence reduced cycle time, similar to the layered sheet case described above.

[00107] Li other embodiments, pulsed microwave energy may be used to create

"slices," or discrete regions, of molten polymer interspersed with layers of un-melted polymer prior to subsequent processing. This may also result in accelerated cooling and hence reduced cycle time, similar to the layered sheet case described above.

[00108] In other embodiments, selective placement of one or more microwave emitters may allow selective heating of specific areas of a sheet or other thermoplastic part prior to subsequent processing. This may be particularly useful in thermoforming processes where the sheet must be deep drawn in a particular area.

[00109] In other embodiments, a process may employ selective heating and consolidation of an absorbent core, such as that used in hygiene products which contain a bicomponent binder fiber containing a microwave sensitive component (in particular polypropylene fibers or fibers containing a microwave sensitive material such as a maleic-anhydride graft or other polar species) and cellulosic fibers. For example, in a fiber-forming process, the planar material may pass through a microwave heater with energy sufficient to partially melt the polymeric fibers and heat the cellulosic fibers, by virtue of their inherent moisture content. Subsequently the fibers may be consolidated into an absorbent core with in integrated network of polymeric fibers and cellulose. Alternatively, the construction may be a technical textile where the microwave sensitive fiber may be used to bind together the woven or non- woven structure as a covered yarn.

[00110] In other embodiments, processes may employ a blend of two polymers, one being receptive to microwave energy, the other being transparent, in such a way that the microwave receptive domains can be selectively heated. The relative proportion of each of the polymers, the phase morphology, the concentration of the

microwave sensitive component and the power applied may be used to control the rate of heating of the microwave sensitive phase and hence the rate of heating of the total composite.

[00111] In other embodiments, selective heating may allow the use of a microwave receptive reinforcing member within a transparent polymer matrix. The reinforcing member may take the form of a continuous mesh or net, a woven or non- woven fabric, continuous filaments or discontinuous, staple fibers. The reinforcing member may also be polymeric in nature or may comprise other non-polymeric, microwave-sensitive materials, such as carbon or metals.

[00112] In other embodiments, microwave receptive polymers may be used in the skin and/or core of a three (or more) layered foam structure (for example, a sheet), comprising solid skins and a foam core. The concentration of the microwave receptive components may be varied in each of the layers and the microwave power selected in order to achieve both rapid heating of each of the layers and the desired temperature distribution through the whole structure immediately prior to subsequent processing. This may eliminate the need for the very gradual heating required in infrared heating processes to achieve the desired thermoforming temperature profile without premature foam collapse.

[00113] In some embodiments, microwave receptive components in the form of zeolites, inorganic hydrates, or polymer hydrates in a thermoplastic polymer matrix (for example, a thermoplastic sheet) may be used. The zeolites may contain water within the zeolitic structure, may be heated using microwave energy, and the thermoplastic matrix subsequently re-shaped. For example, in the case of a sheet, the sheet may be formed into a container. The container may be further exposed to water to incorporate the latter into the pores of the zeolite within the formed container. The shaped container may subsequently be reheated, releasing the water from the hydrated additive as steam, which may act as a blowing agent causing the thermoplastic matrix to expand into foam.

[00114] In other embodiments, the use of microwave receptive materials on the skin layer of a packaging sheet used in the aseptic packaging process of food products to selectively heat the skin layer may eliminate the need for hydrogen peroxide or steam sterilization.

[00115] EXAMPLES

[00116] Example 1 : Heat Testing of Microwave Sensitive Polymers

[00117] Zeolite A (Aldrich, molecular sieves 4A, catalog no. 233668) and Fe ₃ O ₄

(Alfa Aesar, catalog no. 12374) are selected for evaluation in selective heating processes. Criteria used to select these two additives include effectiveness (response to microwaves), cost, and required loading of the additives, environmental, health and safety concerns. The expected impact that the additives may have on polymer properties (based upon particle size, morphology, and other properties) are also considered.

[00118] For example, FIG. 6 presents laboratory data illustrating the decrease in Izod

Impact that Zeolite A can have on PP and ABS. However, it has been found that impact values for coextruded sheet having exterior microwave transparent layers may not be significantly influenced by the microwave-receptive additive, as illustrated in FIG. 6A. FIG. 6A presents Falling Dart data for 20/60/20 coextruded polypropylene sheets having a 4 mm total thickness, where the interior layer contains zeolite at the indicated concentrations (control has zero zeolite). As indicated by the results, coextruded microwave-sensitive polymers may have similar physical properties, including impact and ductility.

[00119] The chosen additives are used at four loading levels (approximately 3, 6,

10, and 14 weight percent) in polymer sheets of varying thickness (3, 6, and 10 mm), where the polymers included ABS, HIPS, PP, and Conductive TPO. The microwave heating of the polymer sheets is then tested in an apparatus similar to that illustrated in FIG. 4, described above (the microwave heating apparatus may include components such as tuning pistons, an EH tuner, matching iris plates, waveguide, horn, microwave chokes, a lower moveable piston, and a sample feed slot). Sheets are processed through the microwave heating apparatus by feeding the samples through the sample feed slot. The test apparatus is capable of rapid and uniform heating of polymers, and could adapt to the material nature and form (receptor type, receptor concentration, matrix type, and sample thickness and shape). The test apparatus included a 2.54 GHz variable power source, and a WG9A waveguide connection into the horn, providing a uniform energy density spread. The iris plates and EH tuner allow for fine tuning of the wavelength emitted. Analytical measurement devices (not shown) are also provided to monitor the temperature of the polymer sheet being processed, among other variables.

[00120] Referring now to FIG. 7, a polypropylene sheet, 6 mm thick, having 6 weight percent Zeolite A is heated using the microwave heating apparatus at a power setting of 1100 Watts, and the temperature of the sheet is measured as a function of time. The sheet increases from room temperature to approximately 155°C in about 17 seconds, indicating a rapid heating cycle.

[00121] Referring to FIG. 8, the measured heating rate as a function of microwave power for several samples containing Zeolite A is shown. Eight samples as described in Table 3 are prepared. The increase in temperature of the samples is measured as the samples are heated at a constant microwave power. At moderate to high power settings, Samples 1, 3, and 7 exhibit rapid heating rates (greater than about 7 ⁰ C per second); Samples 2, 6, and 8 moderate heating rates (2 to 6 ⁰ C per second); and Samples 4 and 5 slow heating rates (less than about 2°C per second).

Table 3.

[00122] Referring to FIG. 9, the temperature profile measured for a sample in motion being heated in a microwave heating apparatus is shown. The sample is a nylon strip, 6 mm thick, 85 mm in width, and 500 mm in length. The sample moves through the apparatus at a rate of 400 mm/min at a power setting of 500 W. FIG. 9 presents a snapshot in time of the stabilized temperature profile resulting from the heating, and illustrates the uniform heating that can be achieved with selective microwave heating.

[00123] Referring to FIG. 10, a temperature profile measured for an A/B/A sandwich sample being heated in a microwave heating apparatus is shown; the A layers are non-receptive to microwaves, the B layer is receptive. Experimental results confirm that the layered concept provides a uniform heating band, similar to that predicted by E-field modeling as described above.

[00124] Referring now to FIG. 11, the heating characteristics measured for two 3 mm PP samples of a three layered sheet comprising a core layer of microwave sensitive material bounded by outer layers of a polymer transparent to microwave energy are shown. For both samples, the core layer contained 10 weight percent Zeolite A. Sample 9 had a top layer (outer skin), whereas Sample 10 did not have a top layer. The samples are exposed to microwave energy at a power setting of 500 W. Sample 9, having a top layer, heats much slower than Sample 10, not having a top layer. FIG. 12 presents a time-temperature snapshot of the heating of Sample 9 two minutes from the start of the test, illustrating again how the outer layers lag behind as thermal conductivity transfers heat from the microwave sensitive layer to the outer layers. In this manner, the outer layers may act as a carrier for a low melt strength core.

[00125] Referring now to FIG. 13, a temperature snapshot of the heating of a polypropylene sample (4mm thick 20/60/20 co-extruded PP material with 14% zeolite A in the core material) using a microwave heating apparatus and thermoforming station as illustrated in FIG. 4 is illustrated. The polypropylene sheet contains 14 weight percent of a microwave receptive additive (Zeolite A) and is heated in a microwave heating apparatus having a power rating of 1.5 kilowatts as the polymer sheet is passed through the microwave heating apparatus at a rate of 3 mm/second. Microwave energy was directed only toward the central portion of the sheet, selectively heating that portion of the sheet to be molded. As can be seen in FIG. 13, the temperature profile across the heated section varied by only 4 degrees Celsius, illustrating sufficiently uniform heating across the sample width for forming the desired molded part.

[00126] Example 4

[00127] Referring now to FIGS. 14-19, results for the dynamic heating of microwave- sensitive polymeric samples in a microwave heating apparatus, similar to that illustrated in FIG. 1, are illustrated. Monolayer and co-extruded polypropylene sheets (20/60/20 co-extruded structure) polypropylene sheets (PP Dl 14) having a zeolite (Zeolite A) loading ranging from 3 to 14 weight percent were heated in a microwave heating apparatus, where the sheets were fed through the resonant cavity at a rate of 10 mm/second. Following the heating step, the polymer sheet was forwarded to a mold at the same transport rate as the feeding rate through the applicator. The mold

was positioned at a 300 mm distance from applicator. The power level of the microwave generator was varied, and the resulting temperatures of the polymer sheets were measured. Temperatures were measured 1) inside the resonant cavity, 2) as the polymer exits the cavity, and 3) at the location of the mold. Temperature versus power level results for the various samples are presented in FIG. 14 (coextruded PP samples) and FIGS 15-18. Additional monolayer samples were heated at a rate of 5 mm per second, results of which are presented in FIG. 19.

[00128] As another example, thermoforming a sheet into a refrigerator liner requires the polymer to have a good balance of stiffness and toughness, sufficiently high low-temperature impact properties, good ESCR, and good temperature resistance. Additionally, the polymer must have a deep draw processing window, having good melt strength and limited sag. The A/B/A layered polymer, having improved melt strength and decreased sag during thermoforming as a result of the reverse temperature profile may enable the thermoforming of TPO sheet for refrigerator liners and similar thick sheet applications.

[00129] Embodiments disclosed herein may provide for rapid, volumetric heating of a thermoplastic material. Embodiments may also provide for selective heating of discrete parts of a thermoplastic structure, such as individual layers in a laminated or co -extruded multilayer structure, for example. Other embodiments may provide for pulsed microwave energy resulting in regions of heated and unheated microwave receptive material. Some embodiments may provide for selective placement of the microwave emitters providing for heating of specific regions of a part. In other embodiments, selective microwave heating, having high penetration efficiency, may allow near simultaneous heating of the core layer and the skin layers, especially as compared to the slow conductive transfer of radiant heat from one or both outer layers through the polymer.

[00130] Embodiments disclosed herein may be used for the selective microwave heating of thermoplastic polymer materials. With regard to polymer processing, this technology may offer many advantages for designers and processors, including selective, rapid heating; reduced heating / cooling cycle times (high speed); high energy efficiency and other environmental benefits such as reduced emissions (as it is a dry and fumeless process) and increased recycling potential (through enabling the more widespread use of self-reinforced single material components); preservation of

properties in self-reinforced parts (reduces risk of reversion); increased productivity; improved part quality and strength; and minimization of thermal degradation due to reduced residence time in a thermal process, and therefore thermal stabilization additives can be reduced in polymer formulation.

[00131] Embodiments disclosed herein may provide a microwave heating unit providing uniform energy density and high field strength. The microwave heating apparatus may be capable of establishing very high electric fields to heat very weakly absorbing polymers rapidly and controllably through the application of microwave energy.

[00132] Advantageously, embodiments disclosed herein may provide reduced heating times, reducing overall fabrication cycle time and hence reduced piece part cost. Embodiments disclosed herein may also provide reduced cooling times as a result of the use of selective heating, introducing "heat sinks" within a material that is being processed. Additionally, volumetric heating eliminates the need for "surface" or "contact" heating and therefore eliminates the potentially deleterious effects of high polymer surface temperatures. Volumetric heating also eliminates the undesirable temperature gradient through the sheet thickness.

[00133] Embodiments disclosed herein may also advantageously provide improved productivity through reduced overall cycle times and reduced system energy requirements. Embodiments disclosed herein may also provide tailored thermal profiling providing optimum thermoforming conditions for all thermoplastic materials and, in particular, enabling the thermoforming of thick thermoplastic polyolefm sheet, which otherwise has an unacceptably narrow processing window.

[00134] While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. In particular, embodiments of the present invention may also use a single emitter rather than multiple emitters. Accordingly, the scope of the invention should be limited only by the attached claims.