Shielding and field modification – thick metal films

10

Shielding and field modification ± thick metal films

T H B O H R E R , Pac Advantage Consulting, LLC, USA

Abstract: Thick metal elements produce shielding and field modification when incorporated into microwave packages. Designs with materials that maintain conductivity and reflectivity at oven field intensities are used to create even heating, controlled differential heating and enhanced browning and crisping, offering food manufacturers the ability to create conventional oven food quality food in microwave cooking times. This chapter reviews the history behind the use of thick metal films, describes the package functions and designs possible from the use of thick metal elements and provides examples of the benefits resulting from their use. Key words: thick metal, shielding, field modification, foil, even heating, differential heating, browning, crisping, chemical etching, conductive, reflective, antenna, transmission.

10.1

Introduction

This chapter discusses the use of thick metal films in active microwave packaging. Section 10.2 frames the rationale for the use of thick metal films by highlighting the cooking problems that have and continue to motivate researchers to use these materials, and summarizing the evolution of the specific technologies evaluated and commercialized. The theory behind the use of thick metal films in microwave cooking is discussed in Section 10.3, which introduces and details the functionalities available from the use of these materials and why they can improve the quality and convenience of cooked foods in meaningful ways. The understanding of the basic principles involved leads to a discussion of desired design options for the different effects possible. Examples of packaging structures employing these designs are presented. The effective use of thick metal in microwave packaging requires the ability to accurately and economically pattern this material. Section 10.4 outlines approaches that have been proposed and/or introduced in the marketplace. Chemical etching, the primary approach utilized commercially, is given more detailed treatment. Section 10.5 focuses on attempts to use antenna structures to collect and transmit or transfer microwave power within a food/package system. Different approaches that have been proposed are reviewed.

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Commercial examples demonstrating the significant benefits of the use of thick metal films are highlighted in Section 10.6, which also describes the range of configurations possible using today's technologies. Finally, Section 10.7 wraps up the key points of the chapter, providing comments on the future use of thick metal films in microwave packaging.

10.2

History

10.2.1 Objectives of use As microwave food heating emerged as a viable food service and domestic alternative to conventional methods of cooking or heating foods (such as hot air ovens and frying utensils), an early objective was to find ways to use stamped foil trays, which were commonly used for retail frozen foods and for reheating foods in food service applications. Durable, heat resistant and relatively easy to form into complex shapes, including multiple compartments, these trays are manufactured from thick metal films. Unfortunately, the metal in these packages acts as a complete shield to incident microwave energy, and heating in traditional metal containers made with continuous thick conductive sheets is plagued by cold food near the walls, with particular problems at the container bottom, where energy can only reach the food after penetrating the food above it. This phenomenon is even worse in bottom corner areas and greater container depth exacerbates the effect; these difficulties have severely limited the use of foil trays or other metal containers as primary microwave compatible containers. This chapter will briefly mention approaches designed to overcome this limitation, but will focus on the use of patterns of thick metal films in conjunction with materials possessing meaningful levels of microwave transparency and/or absorption. The primary objectives for using patterns of thick metal films in packaging structures are to overcome two fundamental limitations of microwaving food items in largely transparent containers ± the tendency for many items to have overcooked edges and undercooked centers, and the difficulty of coping with the different component heating rates encountered when heating multi-component meals or foods. Overcoming the large temperature variability present when heating in conventional, passive microwave packages provides more benefits than convenience and consumer satisfaction; eliminating cold spots helps ensure that minimum temperatures for food safety can be met in a wide range of ovens and cooking conditions.

10.2.2 Introduction to shielding and field modification Thick, conductive metal films behave quite differently from the thin, resistive metal susceptor films described in Chapter 9 and form the basis for shielding and

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field modification packages and package components. These materials also have a role to play when difficult to brown and crisp products are encountered. Shielding and field modification approaches use similar principles; strictly speaking, a shielding component or indeed any object that interacts with the electromagnetic energy generated by the oven magnetron acts to modify the microwave field. The two approaches are treated somewhat separately here due to the different degrees of complexity associated with the design and production of effective packaging structures exploiting their effects. For this discussion, shields are considered to be structures which exclude by reflection all or a portion of the microwave energy that would otherwise impinge on a specific part of a food item, thus concentrating the available energy on those parts of the food that are not shielded (or shielded as much). Shields are those structures or packages that create macro or gross impacts on the energy distribution in the oven, and thus in the food. Field modification packages go a step further by affecting in a controlled and predictable fashion either local energy distribution (by intensifying, moderating, leveling or even accentuating small-scale energy distributions), or by transferring energy from the point of incidence to another point where this energy can be more advantageously used. Designers generally create shields by utilizing large contiguous areas or large-scale patterns of thick metal; they create field modification structures by employing more complex, smaller-scale patterns of thick metal to achieve heating objectives. When thick metal is used in conjunction with the resistive susceptors of Chapter 9, hybrid structures can be created that can achieve even more sophisticated effects, combining enhanced browning and crisping with controlled heating.

10.2.3 Early thick metal approaches Previous chapters describe the parameters that define heating rates of food items and components exposed to microwave energy and illustrate the significant differences that exist among the foods and ingredients of interest to food designers. This section uses selected US patents to provide a partial overview of early attempts to incorporate thick metal films to overcome heating challenges. Early users of food service microwave ovens quickly learned that reheating a multi-component food or meal in a microwave oven was an exercise in suboptimization and sought ways to overcome this challenge. In an early example of the search to apply conductive metal shielding, a patent issued in 1952 disclosed a means of heating a food package in a microwave oven and controlling the heating effect by an electrically conductive shield positioned in proximity to a portion of the food package and acting to prevent access of the high frequency waves to portions of the package and to cause the high frequency waves to reach and heat some portions of the package to a greater degree than other portions thereof.1

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10.1 Shielding appliance for cooking microwave sundae (from US Patent 2,600,566).

The approach employed a sturdy metal cup (item 6 in Fig. 10.1) to partially enclose a paperboard container (item 1) holding ice cream (item 3) topped with cake (item 4) and syrup (item 5); the cup shielded the ice cream from microwave energy, keeping it frozen, while allowing the syrup to heat, creating a frozen ice cream sundae topped with hot syrup. Another inventor described similar features in a patent issued in 1955,2 and a series of patents describing patterns of apertures and spatial deployment of conductive metal films issued over the next few decades, although few of these approaches ever saw commercial light beyond food service or in the form of permanent appliances. (A partial listing of early patents is included in an earlier publication.3) Inventors created shielding devices in the form of rigid apertured metal sleeves designed to slide over plated meals prior to microwave heating.4,5 While durable and reusable, these sleeves were bulky, expensive and inflexible in use. Each specific pattern of apertures only worked well with specific combinations and placements of foods, limiting food types, portion sizes and arrangements. The significant thickness and durable construction of these permanent devices made them largely free of problems with arcing, so long as they were kept away

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from the microwave oven walls and not placed in close proximity to other conductive packaging elements. Others recognized the potential advantage of using lightweight packages with foil elements and were granted a patent describing a food package made of dielectric material and partially overlaid with a single, continuous conductive sheet arranged to expose multiple food articles to different amounts of microwave radiation.6 In Fig. 10.2, area 40 is the foil sheet that is positioned on the paperboard blank, item 14. This patent acknowledged that package arcing and charring could result from certain conductive film configurations and described avoiding sharp points through the use of smooth, rounded corners. Fabrication, however, was simply described in the specification as laminating the two materials together, with no mention of specific means to prepare and adhere the conductive material to avoid defects that could cause arcing or charring. Developing safe, predictable and economical means to incorporate thick metal films became the primary impediment to the commercialization of disposable microwave packages incorporating shielding and field modification elements. Scientists at Raytheon, the company which introduced the first RadarangeÕ microwave ovens for commercial and home use, experimented with fringing heating (concept described below) to intensify heating using metal elements.7,8 Decades later, others would attempt to harness the fringing phenomenon in disposable packages.

10.2 Continuous foil sheet attached to folding carton blank for partial shielding (from US Patent 3,865,301).

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It was also known that microwave absorbing materials could be mounted on thick metal films and function effectively if of proper thickness.9 Absorbers exploiting both the magnetic and electrical components of the microwave field could be made this way and were proposed as packaging elements.10 Later, manufacturers of pressed aluminum containers would use this phenomenon to find ways to increase the acceptance of these containers for microwave use.11 None of these approaches would see meaningful use in disposable packages. While the primary focus of shielding technology using thick metal films was on rigid packages, significant effort was also expended on flexible packaging structures. A series of patents issued starting in 1979 to The Procter & Gamble Company described `microwave energy moderating bags'.12±16 Perforated foil sheets adhered between thermoplastic films provided partial shielding for bag contents, with meat roasts often the stated target food. These patents described both static and dynamic shielding capabilities; shrinkable films reduced foil aperture sizes as the films shrank due to exposure to elevated temperatures, providing greater shielding than pre-shrunk structures as cooking progressed. More recently, flexible packages have been proposed to protect popped kernels of popcorn from further heating.17,18 These designs attempted to overcome the tendency of popped kernels of microwave popcorn to continue to be exposed to significant direct microwave heating, which further dries and toughens the kernels, potentially leading to scorching and, in extreme cases, burning of the product. The first approaches use an expandable apertured shield to limit not only the amount of energy that penetrates the shield, but through proper aperture sizing, the depth to which a portion of the energy can propagate. (The penetration depth limiting phenomenon is described below as evanescent propagation.) This design permits continued power delivery to the susceptor for further popping while protecting popped kernels from excessive exposure to microwave energy and can be incorporated into bag structures. The second approach uses an expandable storage chamber made from a continuous shield material; popped kernels are displaced into the shield by the force of popping and are held there protected from further microwave energy exposure. The commercialization rate for patented structures is, in general, not high, and the packaging structures described in this section as well as the many other patents covering thick metal structures are no exception. Some were not commercialized due to functional failure, material availability or cost, safety or the lack of cost-effective converting equipment to produce reasonable packages. In some instances, the packaging technology described was well ahead of the requisite complementary food technology. These patents and the work they represent are significant manifestations of the intense desire on the part of package producers and users to manage electromagnetic radiation exposure in the microwave oven. A continuing evolution of understanding and innovation led to commercially viable packages that played an important role in changing the way consumers prepared popular food items.

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10.2.4 Commercializing thick metal packages The Pillsbury Company introduced the first widely distributed disposable microwave shielding component as part of the package for a 10 cm square frozen microwave pizza commercialized for convenience stores about 1980. Foil laminated paperboard folded and glued into an inverted tray served as the lid for what also was the first commercial metalized film susceptor package. Apertures in the lid vented moisture and permitted controlled ingress of microwave energy to cook the pizza toppings while diverting the majority of the energy to the susceptor pad under the pizza crust. Patents describing a multi-component package (using a different susceptor technology) and specifics of the shielding lid issued to Pillsbury and American Can, respectively.19,20 The lid and a simple tray for containing food are illustrated in Fig. 10.3. This shielding lid was produced from an overall lamination of aluminum foil to paperboard by die cutting circular apertures in the top panel and rounded corners on the periphery of the blank, and required well-adhered glue tabs to create a low impedance electrical connection between the foil on the glue tabs and the foil of the sidewalls of the lid which the tabs overlay. This low impedance connection minimized the possibility of creating a sufficiently high electric potential between two foil layers that could lead to arcing. The patent specifically taught the risk of arcing from glue tabs that became detached and also described shielding lids using glued web corners instead of glued tabs.

10.3 Apertured foil laminated shielding lid and microwave transparent food support tray (from US Patent 4,345,133).

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Overall foil laminations patterned with post die cutting, while effective for relatively simple applications like this first use (single, small and uniform product), fail to provide sufficient versatility for larger, multi-component products and more complex heating requirements. The foil needs to be combined with other continuous layers or structures resistant to food penetration or leakage if the thick metal functionality is to be incorporated into barrier packaging components such as trays or lids. Other means to selectively incorporate patterns of thick metal films with intricate shapes were needed to satisfy the requirements of food developers. Alcan patented a series of structures incorporating large-scale patterns of thick metal films primarily aimed at field modification through the creation of higher mode heating configurations in a container.21±26 The fundamental energy mode in the body of a substance to be heated is characterized by a concentration of energy at the periphery of the substance, or the container if the substance fills the container. Higher heating modes increase the number and reduce the scale of the standing waves that develop. A producer of aluminum foil and pressed foil containers, one of Alcan's primary objectives was overcoming the very poor temperature profiles that develop in foods microwave heated in these trays. Much of Alcan's technology was based on the concept of conductive elements placed above the food creating a multiplicity of smaller standing waves beneath them, leading to smaller dimensions for heating variations. Smaller variations should result in more uniform temperatures at the end of the cook cycle, and should equilibrate more quickly during post-heating rest or wait time than larger-scale variations resulting from unmodified microwave energy distributions. While improved cook performance was achieved using these elements, no simple, cost-effective and robust methods were developed to place the required multiple small aluminum patches into either rigid or flexible lid structures. Adhesive-backed patches are susceptible to mechanical damage and variability in placement; adhesive failures, particularly at patch edges could lead to arcing. This technology was widely evaluated by food companies, but little commercial activity resulted. Chemical etching of largely opaque evaporated aluminum coatings on film was introduced to create patterns that served as decorative windows in bags and pouches. Sodium hydroxide readily reacts with and dissolves aluminum, offering the potential to create patterns in sheets of the metal. Previously available etching technology was largely confined to batch processing, which was unsuitable in both capacity and cost for the large areas required for high-volume food packages. Beckett Packaging developed a high-productivity roll-fed process to etch patterns in evaporated aluminum coatings, which served as the launching point for extending etching to thick metal films; this important process will be described in more detail later in this chapter.27 Concurrent with the development of this process came package designs capable of providing functional, reasonable cost shielding and field modification

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effects in paperboard based structures.28±34 Introductions came first in Europe, for high-value frozen foods enabled by this technology, but introductions were limited by sufficient converting capability to translate the complex foil patterns now available into commercial packages. The etching hardware and technology were acquired by Fort James Corporation, later part of Graphic Packaging International. Its further research, investment and access to downstream converting capability have resulted in significant commercialization activity for food products in Europe and North America, with growing activity on other continents. This remains the primary thick metal patterning capability for microwave packages today, and introductions continue for new products.

10.3

Physics and design principles

10.3.1 How thick is a thick metal film? For the purposes of microwave package design, metal films become `thick' when they move from the realm of having meaningful transmittance or absorbance of 2450 MHz microwave energy to being effectively 100% reflective. This reflectance corresponds to the metal films being essentially completely conductive and not generating bulk-resistive losses as they carry the electrical component of the microwave field in the oven. Complete conductance or reflectance at the expected field strength exposure is the property that matters, and care must be taken when measuring the performance of potential structures to understand reflective performance at real world oven field strengths. Most often, the reflectance, absorbance, transmittance (RAT) properties of microwave packaging materials are non-destructively characterized in low-power microwave network analyzers. While useful for characterizing initial properties of materials, the low electric currents induced in such units fail to identify potential electric breakdown of materials at typical oven cooking conditions. Practically, what this means for metal films is that to be considered `thick', they should be at least one skin depth thick, as discussed below, preferably at least two skin depths thick. For this discussion, skin depth is defined as the thickness of the layer at the surface of an electrically conductive material in which a large portion of microwave-induced electric currents are contained and carried. The skin depth can be calculated as follows: s 2 10:1 s ˆ !a where s ˆ skin depth,  ˆ conductivity, ! ˆ angular frequency, and a ˆ absolute permeability. Inserting the properties of pure aluminum at 2450 MHz yields a skin depth of 1.7 m for the material of most commercial interest and practicality for

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microwave packaging. The skin depth equation is developed for radiation incident from one side and as microwave field exposure may occur from both sides of the metal layer, thicknesses below two skin depths, or roughly 3.4 m for aluminum, may experience stress amplification in the bulk and risk breakdown at power levels normally experienced in microwave oven heating. Standard 7.24 m thick packaging grade aluminum foil exceeds two skin depths in thickness, is largely free of structural defects and easily handled on common packaging converting equipment. Properly designed microwave packaging elements from this material provide stable performance at required oven power levels. High optical density evaporated aluminum layers have been proposed for use as shielding materials due to their foil-like appearance and temptingly high reflectance characteristics as measured on low-power network analyzers. To date, however, these materials fail to perform as effective shields at commonly encountered electric field strengths in typical domestic or food service microwave ovens. Previously published data showed a sharp drop-off in reflectance for 3.5 optical density metalized films laminated to paperboard at incident power levels between 100 and 200 W in a VR430 waveguide.35 For this waveguide, these power levels represent electric field strengths between 5600 and 7900 V/m. To put this in perspective, electric field strengths of 9400 V/m and higher are measured in typical domestic microwave ovens, placing the reflectance breakdown power level squarely in the range of expected field strength exposure.36 Owing to the low loss factors of frozen foods, interactive microwave packaging components are subjected to almost all of the electric field intensity immediately after power is applied in the heating cycle. It is at this earliest portion of the heating cycle that the reflectance properties of high optical density evaporated films are lost, rendering them ineffective as shielding devices and incapable of carrying sufficient current to act much different from partially degraded standard thin film susceptor materials. By comparison, in these same tests standard thickness foil showed no deterioration at the full power capability of the test equipment (2300 W, which corresponds to field strength of 26 700 V/m). It should be noted that the reason this test data indicated less than 100% reflectance for foil is that a 3.175 mm gap was left around the edge of the foil or metalized film sample so perimeter currents characteristic of microwave oven operation would be imposed on the material, which factor into the total electric stress experienced by fully or partially conductive elements. Despite the lack of demonstrated success for the high optical density evaporated films as conductive and reflective elements at real world oven conditions, researchers continue to explore means to employ them. Lower initial cost and relative ease of patterning these thinner films compared with 7.24 m foil are powerful incentives for the development of creative solutions to their

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power handling shortcomings. There is no present commercially demonstrated solution to this shortcoming; therefore, the remainder of this chapter focuses on the use and preparation of metal films greater than two skin depths in thickness.

10.3.2 The roles of thick metal films in microwave packaging Thick metal films provide valuable options to the package designer, expanding available package functions beyond those achievable in passive, transparent containers or in packages solely incorporating energy absorbing susceptor elements. Modest shielding and field modification occur when using susceptors, but rarely to the extent necessary to make significant energy distribution changes. Susceptor placement is also often not consistent with the needed placement of shielding or field modification elements. The early microwave pizza example demonstrates this dilemma well; the susceptor was placed under the bottom of the crust to assist browning and crisping, while the pizza topping required shielding located above the pizza to prevent overcooking. Reflect, conduct, redirect, redistribute and reradiate are a few of the terms that can be used to describe the effects thick metal film elements have on microwave energy in microwave packages incorporating them. These effects lead to three main results in microwave packages: · even heating; · controlled differential heating; · browning and crisping. While the first two effects can most often be achieved solely with thick metal elements, browning and crisping effects are typically achieved in conjunction with susceptor elements. Even heating Large-scale thick metal patterns exclude incident energy from adjacent sections of a food product; proper placement of the metal reduces the heating rate of those food sections absolutely compared with an unmodified package and also relatively compared with non-shielded areas. Where the unmodified heating of the food product or products is known to result in undesirable temperature distributions, designs can be created that generate proper distributions through the redistribution of energy to overcome common tendencies for edges (especially corners) to be overcooked and centers (especially the bottom center of the food mass) to be undercooked. The simplest shielding approaches provide shielding on sidewalls of trays while leaving the majority of the tray floor transparent. An example of this approach is shown in Fig. 10.4, where the rim, sidewalls and a small portion of the edges of the floor of the tray (areas numbered 3, 4 and 5) have a layer of foil,

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10.4 Tray with foil shielding on sidewalls, rim and extending partially into base (from US Patent 4,351,997).

while the majority of the tray floor (area numbered 2) does not have this metal layer and is completely transparent.37 A tray with this type of thick metal pattern would be best used for a product that is relatively uniform in thickness and whose heating characteristics in the length and width dimensions vary little, and may also incorporate a shielding lid to force more energy through the microwave transparent bottom of the tray. Food weights above roughly 350 g exceed the ability of this relatively simple design to provide even heating.38 Smaller-scale metal patterns aim to modify the microwave energy pattern more locally, and are also aimed at creating even heating of relatively homogeneous foods. The arrays of foil patches applied to the lids of containers mentioned earlier were designed to create higher order modes of heating than would occur with purely passive packaging. The theory behind these approaches is to create smaller heating `cells' in the food, yielding smaller temperature differences over smaller dimensions and resulting in more uniform final temperature profiles. Figure 10.5 illustrates an array of such patches (squares numbered 606 in a regular array adhered to a rigid lid on a tray) designed to create higher order heating modes and greater temperature uniformity.39 More complex patterns can be used in conjunction with large-scale shielding to enhance the transfer of energy to the most difficult place to reach in a food item, the center of the bottom. In addition to providing more uniform heating, such systems can make the heating results more tolerant of variations in oven performance, product or total load in the oven (i.e. cooking more than one item, including cooking several different items). Resonant loops can be used to increase energy absorption as well as conduct energy across the floor of a tray, biasing the distribution of energy to the portion of the food needing the largest heating boost. Figure 10.6 shows an example of an oval tray combining sidewall

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10.5 Top view of field effect foil patch array on tray lid (from US Patent 4,831,224).

10.6 Tray with shielded sidewall and containing resonant loop in base (from US Patent 5,593,610.

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shielding extending just into the bottom wall (areas 32 and 34) with an annular ring (area 36) in the base of the container; the tray was designed for use in conjunction with a lid incorporating cooperating conductive elements to provide a robust heating packaging system.40 De-tuning unloaded resonant elements To reiterate a previous important point, the larger the total food load, the more likely thick metal elements are required to achieve even heating results consistent with minimum consumer quality, cook time and food safety requirements. As the food load increases, however, so does the likelihood that consumers may eat only a portion of the food at the first sitting and place the remaining food in the refrigerator in the original package for later reheating. This practice presents a significant safety challenge when highly interactive resonant loops or other field modification features have been incorporated into a package. These features must interact strongly to function effectively to modify the field or transmit energy when adjacent to a meaningful food load, but in the absence of such a load into which energy can be constructively dissipated, have the potential to cause excessive local energy concentration and heating of the package and any remaining food residue during a subsequent microwave heating cycle. Package damage and food residue scorching or even burning could result where food has been completely or partially removed. In the highly unlikely instance that a consumer would cut through the protective film layer and damage the thick metal element, it could lead to arcing in an unloaded condition. A solution has been created for this problem that uses the presence or absence of food adjacent to the loops to selectively tune or de-tune their functionality.41 By appropriately segmenting loops or other conductive metal elements, DC electrical discontinuity is created between the segments, rendering them largely inactive in the absence of an adjacent load. When food is adjacent to the loops, it dielectrically loads the gaps between the segments, raising the capacitance and creating a continuity effect or coupling between the segments. This capacitive coupling allows the segments to function as effective resonant loops, power transmission lines or reflective areas, albeit at lower efficiency than unsegmented features of similar overall dimensions. In the absence of an adjacent food load, the pattern is detuned from a resonant state and becomes effectively non-interactive with incident microwave energy, preventing arcing, overheating, scorching or burning. Figure 10.7 shows an example of segmented patterns which provide shielding that automatically tunes and de-tunes depending on whether a food load is present or absent.42 Further work has resulted in a growing number of abuse-tolerant structures incorporating patterns of thick metal films.43±46

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10.7 Pattern of repeated segmented metallic interactive elements (from US Patent 6,204,492).

Controlled differential heating Highly heterogeneous or multi-component food products represent a more complex heating challenge; different food items react very differently to microwave energy and can heat at significantly different rates. Differences in shapes and sizes of different components further complicate finding an optimal heating protocol, with the unfortunate outcome of sub-optimizing the heating and resultant quality of all components. Different target temperatures (such as desired in the earlier ice cream sundae example) are likely for the different food products as well. Creating differential heating microwave packages starts with creating an in-depth understanding of the cooking dynamics of each component and the optimal end state at the time of serving. Food density and moisture content, microwave absorption characteristics, thermal conduction, tendency to crisp, brown, toughen, etc., all of which change as the food heats, affect the final results. Cooking individual components separately begins the process of understanding the nuances of how each component behaves in a microwave oven, and also begins to quantify the nature and complexity of the challenge being faced. Once the heating performance parameters for each component are understood, the next step is to determine how the components will be arrayed in the final package. Multi-compartment trays or separate containers for each component

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may be used, or the components may just be arranged in a predictable (sometimes not so predictable!) way in a single-compartment tray. Interactions between the individual items must be accounted for and first approximation packaging elements are then designed and tested. Sound experimental procedures that include monitoring oven power input, cook time, resultant food temperature profiles and food product characteristics accelerate the iterative process of tuning conductive elements to optimize performance. Once performance is satisfactory in the single oven that should be used for first evaluations, it is prudent to test performance in a selection of relatively popular ovens known to represent a wide variety of cook behavior. Conductive element design changes to accommodate the expected range of consumer oven variability can improve predictability of heating results, making consumer heating instructions simpler and more reliable, and increasing the likelihood of satisfied customers. The photo in Fig. 10.8 of a simple two-compartment paperboard tray with full bottom and sidewall shielding in the smaller compartment shows the low end of the range of pattern complexity that can be envisioned. The more complex patterns presented in other figures and references represent the wide range of design possibilities available for the package designer. Different patterns can be incorporated into different compartments or sections of the package to overcome heating shortcomings that might otherwise have to be dealt with through the inclusion of additional package components or more complex handling requirements on the part of the consumer. Since simple package designs that minimize the number of discrete package components and manipulation by the consumer before, during or after heating

10.8 Two-compartment paperboard tray with full shielding in smaller compartment (courtesy Graphic Packaging Int'l, Inc.).

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offer the best chance for commercial success, building functionality in through complex thick metal patterns is a powerful route to packages that robustly satisfy cooking and safety requirements. An additional differential heating (and browning and crisping) concept to be considered is that of evanescent propagation heating.47 Properly sized apertures in reflective material create limited propagation on the side of the reflector opposite the incident microwave radiation. The energy that can be transferred decays exponentially with distance and can be used to increase the ratio of surface to bulk heating. The previously cited popcorn package used this mechanism to limit the exposure of popped kernels to energy while maintaining energy transfer to the microwave susceptor to complete the popping cycle.48 Browning and crisping The susceptors of Chapter 9 cannot provide browning and crisping for all food items. As product mass, dimensions and moisture content increase, standard susceptors are less able to raise the temperature of all the surfaces to be browned or crisped. Once again, bottom centers represent the greatest geometry challenges in heating, and creating satisfactory texture and color development completely across crusts for large pies or pizzas has proven especially difficult. High moisture content breaded or enrobed dough products require a delicate balance between creating appropriate color and texture on the surface while avoiding drying out or toughening the center of the product. Thick metal films offer the ability to enhance the heating of standard susceptors by helping to ensure that all portions of a susceptor are exposed to the energy required to raise their temperature to desired levels. Figure 10.9 shows a blank for a rectangular pressed paperboard tray that incorporates complex combinations of conductive element patterns that can be used alone or in hybrid structures when positioned to overlay a metalized film susceptor.49 Designs of this type can operate by exploiting multiple functionalities, combining shielding and field modification with fringe heating to enhance susceptor performance and energy transfer (see Section 10.5) to activate susceptor areas that otherwise would experience very little incident microwave energy and would have little ability to convert sufficient energy to achieve browning and crisping temperatures in the food. Fringe heating with thick metal films can occur when elongated apertures or spaced transmission lines are created in reflective material.50 Strong energy dissipation occurs when a product is immediately adjacent to these apertures or lines and these elements have been explored in the search for a means to create grill marks on food placed adjacent to such elements. Unfortunately, the thermal mass of foods typically desired to exhibit such marks, such as meat portions, has exceeded the ability of these constructions to create this effect. They are,

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10.9 Blank for rectangular pressed tray with conductive element apertures, arrays and segmented transmission loops (from US Patent 6,677,563).

however, capable of increasing surface heating, and if used with susceptors can enhance susceptor performance.

10.4

Patterning thick metal films

10.4.1 Patterning approaches explored Several patterning approaches were discussed in Section 10.2, and represent a subset of the approaches proposed by those who recognized the importance of patterning thick metal films for microwave packaging. A more complete listing with brief comments on each technique follows. · Die cutting rigid metal sheets to be formed into sleeves or boxes. These early, permanent appliances were durable, but costly to manufacture and limited in flexibility for different foods or arrangements. Since they were not suitable for disposable packaging, they principally found use in food service applications.

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· Post-trimming apertures in formed foil trays or containers. While effective in providing transmission openings on the bottom of foil containers, this approach is cumbersome since the resulting structure is not capable of holding food unless a leak-proof insert is sealed over the opening or the apertured tray is used to hold a separate, microwave transparent tray. Recently, a metal can with a microwave transparent bottom and selectively removable metallic lid has been proposed, which would essentially leave a continuous sidewall shield with microwave transparent ends.51 · Die cutting or punching apertures in self-supporting foil or foil/film laminates subsequently laminated to flexible films and/or rigid non-interactive sheets. Although mechanically removing segments from self-supporting foil or laminates with film is less capital intensive than chemical etching, cut quality is critical to avoid defects that can lead to arcing. There is also a limit to how large the apertures can become before web handling difficulties arise, and fine detailed patterns are difficult to achieve. · Die cutting foil (preferably mounted on carrier substrates) and spot adhering the resulting patch to a carton blank for later folding and setup. Suitable only for extremely simple, large-scale patterns, this simple process must be implemented carefully to avoid cut quality issues or edge damage to the foil during package fabrication and filling. · Laminating foil to paperboard or paper and die cutting the combined structure. The approach used for the first major commercial shielding introduction is relatively simple to implement, but patterning an overall foil lamination requires creating apertures in the structure. Unless these apertures are subsequently covered, other packaging components must provide barrier to contamination or permeation during storage. Strip laminating foil followed by die cutting can result in selective placement of foil on package blanks that can be subsequently folded to provide large-scale, simple patterns in trays.52 · Chemically etching patterns in foil mounted to film and post-laminating the structure to paper or paperboard. This versatile process has become the predominant choice for patterning thick metal and is discussed in more detail below. · Using pressure-sensitive label processes to create self-adhering patches of foil (generally laminated to a carrier substrate) that are attached to formed package components. With an understanding of the previously discussed limitations (potentially prone to damage, placement error and generally restricted to simple patterns), this approach may have merit for applications where the simplest shielding or field modification effects are desired. Application complexity grows when multiple patches are required on a single package, requiring either pre-patterning of one large pseudo-label or multiple application stations. A recent patent proposes applying a shield label to the cover or sidewall of an already filled food package.53 This patent anticipates possible damage to the foil and teaches foil thicknesses between 25 and

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500 m, 3 to 70 times the thickness of the foil used in commercial packages prepared by chemical etching. · Pattern adhering foil to paperboard, die cutting or laser cutting the foil component in register with the edge of the adhesive pattern and stripping the loose foil away to leave a solidly adhered pattern on the board, which can then be over-laminated with a protective film.54 This process simplifies foil handling compared with the pre-punching of foil described earlier, and is not as limited in terms of the amount of foil to be removed, but is still limited in the resolution of patterns that can be produced. For patterns of simple or moderate complexity, this process is attractive as it uses proven process elements, can conceptually be retrofitted into existing adhesive lamination equipment, and offers the opportunity to recycle unneeded foil. Mechanical die cutting is expected to result in a faster process, while laser cutting could offer the ability to create more complex patterns. · Using a laser to vaporize unwanted foil from a laminated structure to leave behind the desired foil pattern.55 Potentially only useful for patterns in which minimal foil is to be removed due to the high power requirements to remove large sections of metal. · Printing a shielding material in a pattern on a package member and protecting the shield from subsequent damage. While periodically claimed in patents,56 the paucity of specific formulation and performance data provided suggests that this elusive objective has yet to be achieved from a performance point of view, much less by using materials compatible with producing food packages at reasonable cost. The discussion in the previous chapter on printed susceptor structures described the difficulties in achieving practical formulations for these resistive structures; achieving highly conductive printable formulations capable of robustly carrying high current loads has yet to be demonstrated on a practical basis for packaging, although it will remain an objective for researchers.

10.4.2 Chemical etching Commercially, chemical etching is the method of choice for producing packaging structures incorporating thick metal films. The use of an aggressive chemical to selectively remove metals is well known in the art, and some of the difficulties associated with etching aluminum with hot aqueous alkali such as sodium hydroxide were being addressed in the 1950s.57 The growing availability of vacuum metalized films for packaging opened the door for high-speed, lowcost roll to roll selective etching of thick metal films to be developed. The basic process steps for selective patterning are: 1. 2.

mount the etchable metal film on an etchant resistant carrier web; print a chemical resist in a pattern corresponding to desired final metal pattern;

Shielding and field modification ± thick metal films 3. 4. 5.

257

expose the structure to an etchant capable of dissolving the metal to be removed; neutralize any etchant remaining after the web exits the etchant exposure area to prevent any further etching; and protect and prepare the etched pattern for subsequent converting operations.

Steps 1 and 2 are easily accomplished on separate lamination and printing hardware, but from a practical and cost-effectiveness point of view, steps 2 through 5 are best combined into a single continuous process which minimizes extra handling between key steps in the process. The continuous process avoids contamination or damage to surfaces that could take place during winding of rolls prior to or after etching and prior to protection of the etched pattern. Figure 10.10 shows a schematic of the etching process. The brevity of this description belies the complexity associated with consistently and economically operating this process. Aqueous sodium hydroxide, often referred to as caustic soda, or simply caustic, is the etchant of choice for etching aluminum foil laminated to oriented PET film. This strong base aggressively attacks aluminum-generating reaction products of aluminum hydroxide, aluminum oxide and hydrogen gas; the reaction is exothermic. Coatings resistant to caustic are readily printable using conventional application methods. Achieving high-speed etching of 7.24 m foil requires, in addition to very good web handling and tension control (especially in the caustic exposure area): means to contain the concentrated (~50%) caustic at elevated temperatures to increase reaction kinetics; sufficient exposure time to the caustic to completely remove the aluminum foil in the areas unprotected by the pattern resist coat; make-up caustic addition to replenish the strength of the etchant working on the exposed foil; means to extract the heat generated by the reaction; the ability to capture and neutralize any caustic mist and to react evolved hydrogen gas to prevent explosions; and filtration means to remove precipitated reaction products or other solid contaminants that would affect etching quality. Ultimately,

10.10 Schematic of chemical etching process.

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10.11 Packages containing thick metal film elements (courtesy Graphic Packaging Int'l, Inc.).

dissolved contaminants reduce etching efficiency and quality, requiring complete replacement of the etching bath; fortunately the neutralized spent etchant has value as a raw material for other industrial processes, eliminating potential disposal problems for this complex solution. With proper process control and selection of raw materials, precise replication of high-resolution patterns can be consistently reproduced and the resulting lamination to rigid substrates can be converted into a variety of packaging forms, including cartons, sleeves, cards, inserts, pressed and folded trays. Figure 10.11 illustrates examples of thick metal containing packages, including packages that combine thick metal elements with susceptors.58 While paper and paperboard have provided the structural rigidity for commercial packaging articles to date, combinations of thick metal elements with polymeric packaging components have been considered in the past and continue to be pursued.59,60

10.5

Antennas

It is not surprising that package designers have thought in terms of antennas as they attempted to incorporate thick metal films into packages to capture, transmit and reradiate energy; the basis for microwave cooking came out of work done during World War II to optimize performance of radar defense systems, systems which ultimately were a key element in the Allies' crucial victory in the Battle of Britain.61 By definition, an antenna is simply an arrangement of conductors that can either generate an electromagnetic field in

Shielding and field modification ± thick metal films

259

response to alternating voltage applied to the conductors or, when placed in such an electromagnetic field, have induced in the antenna an alternating current. This reminds us that the magnetron of a microwave oven is a field-generating antenna. The circuits discussed above created from thick metal films have primarily acted as receiving antennas, activated by incident electromagnetic radiation and dissipating energy into nearby food, or at most, food located a short distance away. Typically, in the package structures illustrated and referred to above, the receiving points are located immediately adjacent to the food load. Partly inspired by early and periodic examples of receiving antennas which were intended to protrude directly from food items,62±64 or which were incorporated into permanent browning or cooking dishes,65 package designs have been proposed which incorporate antennas with receiving or collection points separated from the food and generating or dissipating points adjacent to the food.66 Figure 10.12 is illustrative of this effort, with loop 10 representing the collection portion and lines 20 and 22 representing dissipation portions underlying a susceptor disk 40.67 The concept is to capture energy at a point substantially removed from the food product, where the strong receiving characteristics of the antenna can capture significant energy and transmit the power in current form down the linear tracks where it can interact with the food, a susceptor or other structures as desired. In this particular execution, the impedance of the dipole antenna is matched to the impedance of the destination transmission portion to minimize reflection and re-radiation by the antenna.

10.12 Conductive metal antenna with dipole collecting loop and linear transmission/dissipation lines (from US Patent 5,322,984).

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Packaging and products for use in microwave ovens

Since the antenna loops are intended to be strongly interactive as receivers of microwave energy, one of the difficulties in applying them practically to packages has been to find ways to limit the interactions when food loads are reduced, as has been done in segmented active thick metal elements as described above. The development of segmentation technology has been important in permitting the use of this special subset of antennas in packages.

10.6

Application examples

While packages incorporating thin film susceptors of the type described in Chapter 9 represent by far the largest commercial volume of interactive microwave packages, growing application is being made of packages exploiting the performance advantages of thick metal films. The ability to create uniform temperature distributions in large, multi-serving sized products offers the opportunity to dramatically reduce cook time compared with conventional oven cooking while achieving similar or even improved cook performance. This discussion will highlight the dramatic results achievable for large products, but single serve items can also be improved by the use of these designs; examples of a single serve style tray are shown in Figs 10.11 and 10.13 and a planar card for an enrobed dough hand-held snack is shown in the lower right corner of Fig. 10.11. The poor cook performance of large products, such as a 1 kg family size lasagne, in microwave transparent trays (the overdone edges and top surface experience excessive moisture loss while centers are undercooked) largely prevented these products from being viable microwave products, and relegated consumers to greater than one hour conventional oven preparation times when oven preheat cooking time is included. Dramatic cook time reductions and better

10.13 Trays with sidewall shields and segmented resonant loops in base (courtesy Graphic Packaging Int'l, Inc.).

Shielding and field modification ± thick metal films

261

product quality are achieved when thick metal patterns are incorporated into trays for this product. Table 10.1 provides data for four commercial packages, comparing cook time in a conventional oven with cook time in packages incorporating aluminum foil elements, some also including susceptor film.68 In all cases, product weight loss during cooking was similar in the thick film containing microwave packages to conventional oven cooking. For the lasagne cooked in the passive microwave package, product weight loss was twice that in the pattern foil shielding package. Customers and consumers judged product quality of the food prepared in the thick metal microwave packages to be equal or superior to the products prepared in the conventional oven. The 1.1 kg lasagne was cooked in the tray on the left of Fig. 10.13, which incorporates partial sidewall shielding with segmented resonant loops on the package bottom wall.69 The design used for the rectangular meat pie with top and bottom raw dough crusts combines the unique challenge of balancing heating in a shape lacking radial symmetry with that of cooking, browning and developing texture in a frozen raw dough crust in contact with a moist filling; the complex structure is illustrated in the upper left corner of Fig. 10.11 and described in an issued patent.70 This product enablement offers the opportunity for food manufacturers to extend their range of convenient prepared food products when microwave cooking becomes a high-quality, time-saving way to heat products that previously required heating in a conventional oven. Reductions in preparation time of at least 70% compared with conventional ovens, and elimination of all too common requirements for constant consumer attention and package manipulation during cooking for passive packages creates a powerful tool for food manufacturers to convert long preparation time products into fast preparation foods competitive with the time and energy (and often higher cost) associated with stopping for take-out food. Instead, popular family meals and foods can be prepared quickly and simply from pre-purchased frozen foods. Growing sensitivity to energy use in the home also benefits from transitioning foods from conventional oven preparation to the microwave. Initial power consumption studies show reductions of at least 70% in the energy required to cook these products result from avoiding the conventional oven and using microwave cooking.71 This is consistent with power company published guidance to consumers suggesting microwave cooking uses 75% less energy than conventional ovens.72

10.7

Conclusions

From heavy, rigid metal sleeves with limited product adaptability, the use of thick metal films in microwave cooking to achieve shielding and field modification has evolved to a highly flexible system capable of incorporating a wide range of custom designs tailored to the specific needs of individual food

Table 10.1 Examples of benefits gained from thick metal containing microwave packages (data from Graphic Packaging International, Inc.) Product (all cooked from frozen)

Product weight

Thick metal film package

Conventional oven cook conditions

Passive package microwave cook conditions

Thick metal film package microwave cook conditions

Cook time savings, thick metal microwave vs. conventional oven (including preheat)

Lasagne

1100 g

Pattern foil pressed paperboard tray

10 min preheat + 60 min @ 190 ëC

24 min total 12 min on high, 12 min @ 50% power

15 min on high

78%

Round meat pie, raw dough bottom crust

625 g

Pattern foil and overall susceptor pressed paperboard tray

10 min preheat + 35±40 min @ 177 ëC

Not applicable, passive package unable to cook the crust properly

10 min on high

78±89%

Rectangular meat pie with raw dough top and bottom crusts

900 g

Pattern foil and overall susceptor pressed paperboard tray cooked in carton containing pattern foil and overall susceptor insert on inside cover

10 min preheat + 50±60 min @ 205 ëC

Not applicable, passive package unable to cook the crust properly

18 min on high

70±74%

Round fruit pie with raw dough top and bottom crusts

685 g

Pattern foil and overall susceptor pressed paperboard tray cooked in carton containing pattern foil and overall susceptor insert on inside cover

10 min preheat + 50±60 min @ 205 ëC

Not applicable, passive package unable to cook the crust properly

15 min on high

75±79%

Shielding and field modification ± thick metal films

263

products. Contrasted to mechanical fabrication of expensive implements unsuitable for consumer packages, chemical etching technology permits costeffective production and utilization of shielding and field modification packages for high-volume, fast-moving consumer goods. When this technology is combined with smart food formulation, more products can enter the realm where high quality and convenience are combined, increasing value for consumers. Shielding and field modification will continue to be valuable tools for microwave package designers and that interest will drive further evolution of structures and production approaches. Alternatives to foil will continue to be explored, with printable shielding materials or creation of shielding from thick evaporated layers requiring significant breakthrough technology before viable solutions are realized. Cross-fertilization of materials and fabrication methods from radio frequency identification (RFID) and other `smart' electronics technologies will require adding the ability to retain conductivity at much higher current densities than these emerging low-power technologies currently require. Creation of packages with more complex shapes than currently offered may attract interest and effort, and the extension of shielding and field modification technology using thick metal films into the use of microwave packages or components for health care or industrial processing applications could drive additional utilization and growth. In many respects, although this technology has been of interest since microwave cooking was discovered in the late 1940s, it has remained largely behind the scenes until more recently. A number of factors, including the increasing realization that the microwave oven is an energy-saving appliance compared with conventional hot air ovens, are favorable for increased utilization of this powerful technology.

10.8

Sources of further information and advice

This section provides several additional selected resources the reader may wish to examine to pursue specific interest and is followed in Section 10.9 with the references cited in the body of the chapter. Bouirden A, Ouacha A, Lefeuvre S and Keravec J (1989), `Microwave browning of foods', KEMA High Frequency/Microwave Processing Conference. Brody A L, Strupinsky E R, and Kline L R (2001), Active Packaging for Food Applications, Lancaster, PA, Technomic Publishing Company, Inc. Buffler C R (1993), Microwave Cooking and Processing, New York, Van Nostrand Reinhold. Collin R E (1998), Foundations of Microwave Engineering, New York, McGraw-Hill. Decareau R V (1992), Microwave Foods: New Product Development, Trumbull, CT, Food & Nutrition Press, Inc. Jackson J D (1992), Classical Electrodynamics, 3rd edn, New York, Academic Press. Maygar R J (2004), `A companion to Classical Electrodynamics 3rd edn by J.D. Jackson', Rutgers University, http://www.scribd.com/doc/SO9827/A-Companion-

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to-Classical-Electrodynamics-3rd-Edition-by-J-D-Jackson Osepchuk J M (1995), `Microwave technology', in Kirk-Othmer Encyclopedia of Chemical Technology, vol. 16, 4th edn, 672±700. von Hippel A R (1954), Dielectrics and Waves, New York, Wiley.

10.9

References

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