MXPA00007093A - Microwave food scorch shielding - Google Patents

Abstract

A microwave container (10) which morphs from a relatively microwave transparent condition to a relatively microwave blocking condition in response to microwave irradiation. The container wall section has a plurality of discrete, unconnected microwave reflective material elements (20) initially permitting the transmission of microwave energy into the container (10) and either a microwave absorptive material (22) or a thermally responsive material active to coalesce the microwave reflective material elements (20) into a connected array or pattern to block the transmission of microwave energy from entering the container (10) after absorbing a predetermined amount of microwave energy.

Description

PROTECTIVE COVER AGAINST BURNS FOR MICROWAVE FOOD BACKGROUND OF THE INVENTION The invention relates to the field of packaging materials for food products, specifically to the field of packaging of food products for microwave irradiation. In the past, such packaging contained the food products and could include a susceptor to concentrate the thermal energy to heat or cook the food contained in the package. Said packaging typically did not protect food products from overheating or overcooking, and in certain embodiments, they did not protect them to reduce or eliminate the concentration caused by the susceptor or in the folds of said package. A typical example is microwave popcorn, which is conventionally made in a paper bag that carries a susceptor. Once the popcorn burst, it has been discovered that it burns easily by continuous exposure to microwave irradiation. The prior art has therefore not solved said continuous exposure of food products to microwave irradiation that is too long. In US-A 4,228,334, a slot in an opaque layer for microwave energy can be reduced by removing a bridge or slot member in a position that partially covers the slot due to shrinkage of a strap when heated up the default temperature. In EP-A 0001311 a thermoplastic film which shrinks with heat in a microwave reflecting material having apertures is laminated, the apertures are reduced in size due to shrinkage of the film to partially reduce the amount of microwave energy passing to the slot and introduce the food contained in the package. The present invention overcomes said shortcomings of the prior art by providing an apparatus that is substantially transparent initially for microwave irradiation (allowing heating and firing of normal microwaves). Upon reaching a predetermined temperature, the structure of the present invention transforms or changes its own shape, to a structure of protection against microwaves, avoiding additional heating and cooking (or burning) of the food products.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a perspective view of a microwave popcorn bag useful in the practice of the present invention. Figure 2 is a detailed plan view of a structure useful in the practice of the present invention before being irradiated by microwave energy. Figure 3 is a detailed plan view of the structure of Figure 2 after undergoing a transition in response to irradiation by microwave energy. Figure 4 is a side sectional view of a portion of the bag of Figure 1 showing the structure of Figure 2, taken along lines 4-4 in Figures 1 and 2. Figure 5 is a side section view similar to that of figure 4, except that it shows the structure of figure 3. figure 6 is a view composed of several modalities useful in the practice of the present invention in simplified schematic form before and after irradiation of microwave. 7 is a perspective view of a paper layer having conductive material printed thereon, similar to FIGS. 2 and 4. FIG. 8 is an alternative embodiment to that shown in FIG. 7, with coating material in FIG. powder that replaces the printed conductive material. Figure 9 is an alternative embodiment in addition to that shown in Figures 7 and 8 with particles of conductive material suspended in an insulating solvent. Figure 10 is a composite view of a weld point embodiment of the present invention showing the views in side and top section of a microcircuit before and after microwave irradiation. Figure 11 is a simplified side view illustrating the dispersion of particles. Figure 12 is a simplified perspective view illustrating the union of the particles. Figure 13 is a top plan view of the effect of dispersion and bonding of the particles. Fig. 14 is a simplified side view of a composite powder coating showing a composite material made of metal and flow before and after microwave irradiation. Figure 15 is a perspective view of the embodiment of Figure 9 before and after microwave irradiation. Figure 16 is a perspective view of the embodiment of Figure 9 illustrating certain aspects of the present invention.

DETAILED DESCRIPTION OF THE INVENTION With reference to the figures, and more particularly to Figure 1, a food package compatible with microwaves in the form of a bag of popcorn can be observed and which is useful in the practice of the present invention. The bag 10 is preferably a layered construction, having an inner layer 12, an outer layer 14 and a central layer 16. The inner and outer layers 12, 14 are preferably each formed of transparent microwave material such as paper or plastic. The central layer 16 is an interrupted pattern or dispersion of the microwave reflection material, such as metal. Said pattern or arrangement can be observed in plan view in Figure 2, and in more detail in the side sectional view in Figure 4. In addition to (and separated from) the structure for the present invention, the bag or package 10 it may have a conventional susceptor 18 attached thereto. It should be understood that the structure of the central layer 16 can be used in addition to a central layer while another remains within the spirit and scope of the present invention; for example, the pattern of microwave reflection material described with respect to the core layer 16 may be placed "off center" in a laminated construction, or may be used as an outer layer, if desired. As shown in Figures 2 and 4, in this embodiment the interrupted pattern of the central layer 16 is preferably formed of separate metallic elements 20, 22. The elements 20 can be printed conductive material such as a plurality of separate metal segments. , which can be formed as hyphens. The elements 22 are similarly separate conductive segments, which can be formed as separate points but which do not contain dashes 20. It should be understood that the scripts are preferably of a material that is not affected by microwave irradiation, nor by the temperatures reached in the practice of the present invention, although the points 22 are designed to be affected by said microwave irradiation, or more particularly, by the thermal effects of said irradiation of the food products or packaging (or both). The present invention provides a structure that is transparent for microwave irradiation during an initial exposure period and then becomes reflective for microwave energy after the predetermined exposure, to protect the contents of the bag or package against burns or overheating during the continuous application of microwave energy. In the embodiment shown in Figures 1-5, the points 22 will be merged in the application of the predetermined microwave exposure by raising the temperature to a predetermined melting point, in which occurrence the elements 22 will come into contact with the elements 20, forming an uninterrupted pattern to provide microwave protection later. Figures 3 and 5 show the post-irradiation pattern (protection). In practice, once the temperature of the core layer 16 exceeds a predetermined value, the points 22 will undergo a phase change and will be electrically cut to the adjacent elements 20, resulting in an uninterrupted pattern 26, as shown in the figures 3 and 5. As will be evident with respect to other modalities, the pattern may be regular, or irregular or random, with the condition that it will initially allow the passage of microwave energy (preferably without substantial impediment), and with the condition besides that in its final state of protection, it is substantially impermeable (preferably of reflection) with respect to the microwave irradiation of impact. When the central layer becomes reflexive,? = D2 / h? «L (1) with the equivalent condition: sh» 3x1020O "1 (2) where? Is a microwave interaction parameter, d is the depth of penetration of the electromagnetic field in the central metal layer 26, h is the thickness of the metallic central layer 26,? is the wavelength of the electromagnetic energy field and s the conductivity of the metal core layer 26. In order to confirm that the pre-irradiation dimensions of the core layer 16 do not result in microwave monitoring, b »4p? ha / c (3) where b is the space between the adjacent metallic elements 20, 22,? is the radial frequency of the microwave field, h is the thickness and a is the amplitude of the microwave elements 20, 22, and c is the light speed (3x1010cm / s). It has been found that yes b »1μm, the central layer (in its initial state) will not provide any substantial microwave monitoring at 2450 MHz. It should also be understood that the length of each of the elements 20, 22 is much less than a quarter of the wavelength of the microwave frequency of interest. Here, with the microwave frequency at 2450 MHz, the wavelength is 12.25 cm. The coefficients of reflection and absorption (the ratios, respectively, of the reflected and absorbed energy for the incident energy) of an arrangement of metallic particles of radius R deposited each on a plane surface with density n (per unit area) are : (where K = 0.026 for R «d, and K = 0.002 for R» d), and aabs = (nR23d) / 2? for R »d (5a) aabs = [(nR23d) / 2?] (2pRd /? 2 for R« d (5b) For R = 0.1 mm, d = 0.01 mm, and nR2 = 0.01, ref ~ 1014 and aa s ~ 10"4. (It should be understood that the symbol ~ as used herein refers to" in the order of "or" in the scale of. ") In addition, a sheet made of said particles to have a thickness h = nR3 will have: aref = { 1 -? / p for h «d, 1-d / 4p? for h» d.}. (6) aref = {? / p for h «d, d / 4p? For h »d.}. (7) If arßf is set to" 0.999999 and aabs is set to "0.00001 (the conditions of a relatively good reflector and bad susceptor) it is found that the restriction on the particle radius it's from R >1 microns (It should be understood that the symbol "as used herein refers to" around "). To avoid arching of the interparticles, it is assumed that the particles are ellipsoidal, characterized by a long dimension a, and a short dimension (transverse) b. the linear dimension of the space between the adjacent particles is d. The field between isolated and closely adjacent conductive ellipsoids is: E == E0 (a / b) 2 (1 + b / d) (8) and when it is observed that the dielectric resistance for several materials is approximately Eds = 107 to 108 V / m, and the resistance of the electric field in the Conventional microwave ovens is of the order of E0 = 1 KV / m, the conditions of non-arcing is: max. { (a / b), (a / o) < (Eás / E0 = 100 (9) In order to have metallic particles that follow the packing temperature, it has been found that it is desirable to make the particle radius R much smaller than 1 mm to avoid any significant time delay due to the thermal mass and the consequent thermal tia of the particle with respect to the overall packing temperature Obviously, in certain circumstances it may be desirable to delay the transition to the protection state, and in such instances, the particle size may be increased to provide such delay With reference now to Figure 6, it is contemplated that within the scope of the present invention there is a structure that modifies or changes its shape from a transparent phase of microwaves (dielectric) to a phase of microwave (protection) reflection, Illustrated by the method of isolated segments of connection to undergo the change as shown from form 16 to form 26, or reach the result of the desired protection by the discrete melting particles 30 to achieve a connected pattern 32, or to precipitate the conductive particles from an isolated suspended state 34 to a conduction, precipitated state 36.

Various embodiments of the central layer 16 can be seen in Figures 7, 8 and 9. In Figure 7, a printed microcircuit 38 having reactive non-microwave particles 40 and solder points 42 is secured to a paper substrate or layer 44. In Figure 8, the conductive particles 46 (made, for example, of metal) are applied to a substrate 44 by powder coating. In Figure 9, the metal or other conductive particles 46 are kept in suspension by an isolation solvent 48, such as a resin or volatile material capable of being removed by heat. It should be understood that, as shown, the particles in Figures 8 and 9 are considerably enlarged from the scale of the particles 40 in Figure 7. Referring now to Figure 10, a non-wettable microcircuit embodiment can be observed 38 In this figure, side section views 50, 52 are taken along l BB and DD, respectively, and upper section views 54, 56 are taken along l AA and CC, respectively. It should be understood that views 50 and 54 are before microwave irradiation, and views 52b and 56 are as the microcircuit appears after microwave radiation. Said embodiment utilizes a "lobed" weld shape 58 located between a protective layer 60 (such as plastic) and a substrate 63 (such as paper). The microcircuit elements 64 are separated from the welding element 58 before irradiation, as can be seen in views 50 and 54. At that time, the elements 64 and 58 do not significantly impede the microwaves from penetrating the composite package made of a layer. protective 60, microcircuit elements 58 and 64, and substrate 62. As the mode shown in views 50 and 54 are heated, the weld will change shape to that shown in views 52 and 56 to affectively form a protective microcircuit of microwave due to the "relaxation" of the welding element to the shape 66. The characteristic reforming time is determ by the viscous flow in response to the surface tension once the solder material is liquefied. The time of reformation, tr can be estimated as: t r =? R2 /? H (10) when? is the viscosity, and? the surface tension. (It should be understood that the symbol = as used herein refers to "approximately equal to" with, for example, a scale factor omitted). For R = 0.1 cm and h = 0.01 cm, tr can be as short as a second. Care must be taken to avoid perforation or penetration of the protective layer and paper substrate due to the tendency of welding to assume a spherical shape. Assuming the contact angle f is the small estimate (typical for non-wettable surfaces) p = (4? Cosf) / h (11) gives p = O4 at 105 dynes / cm2 which is considerably less than a typical ultimate paper strength of around 1010 dynes / cm2. In the microcircuit mode, it should be understood that the fusion of the welding spots 42 must occur before the food has the opportunity to burn. In addition, even the non-wettable metallic elements 40 can be used with dots or other formed shapes of welding, such as those illustrated in Figure 10. With regard to the use of the powder coating to form the interchangeable microwave protective layer, they should consider the procedures for the dispersion and binding of powdered particles. With reference to Figure 11, the dispersion of the particles is illustrated graphically with a single particle of an initial radius 68 Ro and a dispersion length 70 R, where the dispersion time, t ^ can be estimated by: t = (vR / ??) (R / R0) 3 = (10"3 ~ 10_5) (R / R0) 3 sec (12) where ?? is the energy of humidification (of the same order of magnitude as the energy of surface) The bonding time, tc, can be estimated similarly as: tc =? R2 / h? = (10-3 ~ 10-5) (R / h) sec (13) where R is the initial radius 72 and H is the thickness 74. In this way it can be seen that the dispersion time and the bonding time can be considerably less than one second.A macroscopic upper plan view of the scattering and joining phenomena is shown in Figure 13, in where a paper layer 76 is initially coated with discrete metal particles 78 using a conventional powder coating process.The dispersion of the particles 78 is illustrated with the number 80, with eventual bonding in a relatively continuous metal sheet 82 (the which may have some remaining openings 84.) As is well known, the openings will not adversely affect the protection, with the condition e that the dimensions of each opening are much smaller at a wavelength of the applied microwave field. In addition to the powder coating all metal particles are used, it should be understood that it should be within the scope of the present invention to utilize a composite powder coating technology such as that illustrated in Figure 14, with metal particles. 86 embedded in an organic flow 88 (such as epoxy resin) to form composite particles 89 having a desired melting temperature to achieve a protective structure 90 formed of contact metal particles in the substrate 92. In this embodiment, the particles of metal 86 may remain intact or, alternatively, fused to form a relatively continuous sheet 82 such as that shown in Figure 13. In the powder coating practice the layer serving as a microwave protection, tin-based powders they can be used with a particle radius of around 10 mm and with a melting temperature in the range of 40 to 316 ° C. Alternatively, a clumping of metal powder can be used to form a (protective) conductive layer. Referring now to Figures 15 and 16, yet another approach is to use metal particles 94 dispersed and suspended in a solvent-containing coating 96. It should be understood that the coating 96 should be physically stable in conventional storage and ambient temperatures and that it is capable of volatilization at a desired predetermined elevated temperature. The initial volume fraction of the metal particles of the total volume is preferably less than about 10%. As the solvent evaporates, the volume fraction of the metal particles rises, and a microwave protection structure 98 is formed on the substrate 100 as the metal particles 94 come into contact with each other. The characteristic solvent evaporation time, t «, depends on the parameters of the solvent material and the porosity of the paper: t < lo / [na3 (1 + 1pna2)] (14) where n is the saturated vapor concentration, v is the molecular velocity, a is the molecular radius.a is the porosity of the paper, l0 is the thickness of the solvent layer 102 , and lp is the thickness of the covering paper (protective layer) 104. The invention should not be considered as limited to all the details thereof since modifications and variations thereof can be made without departing from the spirit and scope of the invention. the invention.

Claims (22)

NOVELTY OF THE INVENTION CLAIMS

1. - An apparatus for the protection of food products against burns in a microwave field comprising: a) a generally transparent microwave base material that forms a generally closed container for food products; b) a reactive energy material located in the base material having: i) an initial configuration that allows the transmission of microwave energy in the container to heat food products located within the container; and ii) a final configuration that at least partially blocks the transmission of microwave energy in the container to prevent the burning of the food products, characterized in that in the initial configuration a plurality of discrete, separate elements (20, 22) of material of Microwave reflection are individually sized and sufficient separation is provided to allow such transmission of microwave energy, and in a final configuration achieved after a predetermined exposure to microwave energy, most discrete separate elements (20, 22) it is in contact with each other to form a continuously extending arrangement (26) that substantially blocks the transmission of microwave energy in the container.

2. - The apparatus according to claim 1, further characterized in that the energy reactive material undergoes a phase change between the initial configuration and the final configuration.

3. The apparatus according to claim 1 or 2, further characterized in that the reactive energy material is a carrier with a microwave reflection material initially dispersed therein and the transition acts on the carrier to cause the material of Microwave reflection generally forms a microwave reflection layer.

4. The apparatus according to one of claims 1 to 3, further characterized by an interrupted pattern of microwave reflection elements (20, 22) having grooves therebetween; wherein the reactive energy material comprises elements (22) located in the slits between the elements (20) of the interrupted pattern of microwave reflection material and initially separated therefrom and further where the transition to the final configuration occurs in the reactive energy elements (22) in the slits connecting the interruptions in the pattern of the microwave reflection material so that the pattern becomes substantially uninterrupted and where the pattern is sized in the final configuration (26) substantially blocking the passage of the microwave energy through it to prevent the burn later.

5. The apparatus according to one of claims 1 to 4, further characterized in that the predetermined exposure to the microwave energy corresponds to a predetermined temperature.

6. The apparatus according to one of claims 1 to 5, further characterized in that the reactive energy material reacts to the microwave energy directly.

7. The apparatus according to one of claims 1 to 6, further characterized in that the reactive energy material reacts at a predetermined elevated temperature resulting from the predetermined exposure to microwave energy.

8. The apparatus according to one of claims 1 to 7, further characterized in that the energy reactive material is a reactive material thermally located in the base material and suffers a transition from the initial configuration to the final configuration upon reaching a default temperature.

9. The apparatus according to claim 8, further characterized in that the thermally reactive material is metal.

10. The apparatus according to claim 9, further characterized in that the metal is at least partially fused to form the final configuration.

11. The apparatus according to claim 8, further characterized in that the thermally reactive material is a carrier containing metal particles.

12. The apparatus according to claim 11, further characterized in that the thermally reactive material is a solvent.

13. The apparatus according to claim 12, further characterized in that the metal particles are precipitated to form the final configuration.

14. The apparatus according to claim 8, further characterized in that the thermal reactive material further comprises metal particles that touch at least one another to form the final configuration.

15. The apparatus according to claim 8, further characterized in that the thermally reactive material is powder coated in the base material.

16. A method of protecting food products against burns in a microwave field comprising the steps of: a) forming a generally closed container (10) of a generally transparent microwave material to contain food products; b) forming a layer of reactive energy material in the base material in an initial configuration allowing transmission of microwave energy in the container to heat food products located within the container; and c) applying microwave energy to the container so that the reactive energy material is modified to a final configuration to at least partially block the transmission of microwave energy in the container to prevent burns of the food products inside the container, characterized because the layer of reactive energy material is formed by a plurality of discrete, separate elements (20, 22) of microwave reflection material individually sized and separated sufficiently to allow the transmission of microwave energy in the container; and in the reactive energy layer having received a predetermined exposure to microwave energy, a substantial majority of discrete, separated elements having contacted each other and having formed a continuously extending arrangement (26) that substantially blocks the transmission of microwave energy in the container.

17. The method according to claim 16, further characterized in that the reactive energy layer is applied to the base layer by printing a microcircuit thereon.

18. The method according to claim 16 or 17, further characterized in that the microcircuit contains reactive elements at an elevated temperature to complete the microcircuit and form a protective microwave layer.

19. The method according to one of claims 16 to 18, further characterized in that the reactive energy layer is applied to the base layer by powder coating.

20. The method according to claim 19, further characterized in that the powder coating includes microwave reflection particles dispersed and generally unconnected in the initial configuration and where in addition the microwave reflection particles are joined in the final configuration to form a microwave protective layer.

21. The method according to claim 16, further characterized in that the reactive energy layer is a solvent containing scattered and generally unconnected microwave reflection particles.

22. The method according to claim 21, further characterized in that the solvent evaporates in step c), causing the microwave reflection particles to precipitate and form a protective microwave layer.