JP2016520948A - Low crystallinity susceptor film - Google Patents

Abstract

The microwave energy interactive structure includes a polymer film having a crystallinity (derived from density measurements) of about 1% to about 50% before being exposed to microwave energy, and a microwave on the polymer film. And a layer of energy interactive material. The layer of microwave energy interactive material is effective to convert at least a portion of the incident microwave energy into thermal energy.

Description

CROSS REFERENCE TO RELATED APPLICATIONS This application is filed in US provisional application 61/208379 filed February 23, 2009, US provisional application 61/273090 filed July 30, 2009, and 2009. July 2010, a continuation-in-part of US patent application Ser. No. 12 / 709,628, filed Feb. 22, 2010, claiming the benefit of US provisional application No. 61 / 369,925, filed Aug. 26 This is a continuation-in-part of US patent application Ser. No. 12 / 84,159 filed on the 29th. Each of the above applications is incorporated herein by reference in its entirety.

  It is known to use susceptors to improve browning and / or crispness of adjacent foods in microwave heated packages. In one embodiment, the susceptor absorbs at least a portion of incident microwave energy and converts it into thermal energy (ie, heat) due to resistive losses in the layer of microwave energy interactive material. A thin layer of wave energy interactive material. The remainder of the microwave energy is reflected or transmitted by the susceptor. A typical susceptor generally includes aluminum having a thickness of less than about 500 angstroms, such as about 60 to about 100 angstroms, and has an optical thickness of about 0.15 to about 0.35, such as about 0.17 to about 0.28. Has a density.

  As schematically shown in FIG. 1, a layer of microwave energy interactive material (ie, susceptor) 102 is typically supported on a polymer film 104 and defines a susceptor film 106. In most conventional susceptor films, the polymer film comprises biaxially oriented and heat treated polyethylene terephthalate, although other films are suitable. The susceptor film typically provides dimensional stability to the susceptor film and uses an adhesive or otherwise to protect the metal layer from damage, such as a support layer 108, such as paper or Bonded (eg, laminated) to the paperboard. The resulting structure 110 is referred to as a “susceptor structure”.

  The first commercialized microwave susceptor film, and subsequently introduced and used as a commercial susceptor packaging, is a highly oriented, highly crystallized, biaxially made from polyethylene terephthalate polymer, ie PET. Relied on the use of oriented and heat-set films (PET means such a biaxially oriented film unless otherwise specified herein). Typically, such films are highly oriented, that is, the draw ratio during the orientation process is about 3.5: 1 to about 4: 1 in the machine direction (MD) and in the cross direction (CD). About 3.5: 1 to about 4: 1. Biaxially oriented PET films made from this polymer can have some or all of a good combination of transparency, gloss, smoothness, water vapor and oxygen barrier properties, good mechanical strength and moderate thermal dimensional stability. It is universally used for a wide range of packaging and non-wrapping applications where combinations are useful. Commercially available films used in standard susceptor structures typically include films that fit this general description, and their properties have generally been optimized for high volume use other than use in microwave susceptors.

  This standard configuration includes several of these films, and thus may limit the heating performance of microwave susceptor packaging, packaging parts or composite susceptors and electromagnetic field modification or shielding packaging or parts. There are important elements. Those skilled in the art of producing microwave susceptor packages have a broad understanding of the phenomenon of self-limiting heating, the visual evidence of which is commonly referred to as crazing. During the heating induced in the susceptor layer itself due to the interaction of the susceptor material with either or both of the electrical and / or magnetic components of electromagnetic microwave energy, the temperature of the susceptor substrate film increases. Without wishing to be bound by theory, for the purposes of this discussion, an example where evaporated metal vapor deposits on a biaxially oriented film (vacuum metallization) and interacts primarily with the electrical component of microwave energy. When used, the residual shrinkage force in the susceptor substrate film allows the adhesive / support substrate ability to hold the susceptor substrate film in its original desired dimensional configuration (especially in a plane parallel to the width and length of the film). Above this, cracks appear in the susceptor substrate film, causing discontinuities in the microwave interactive susceptor material, which is thought to inhibit current flow in the metal layer. As crazing progresses, cracks intersect each other, and a network of intersecting lines divides the plane of the susceptor and gradually reduces the conductive islands. As a result, the total reflectance of the susceptor decreases, the total transmittance of the susceptor increases, and the amount of energy converted to sensible heat by the susceptor decreases.

  If this self-limiting action occurs too early (ie too early in the heating cycle), the susceptor is unable to generate the amount of heat necessary for a particular food heating application. In some instances, in contrast, runaway (ie, uncontrollable) heating of the susceptor causes excessive charring or scorching of nearby food and / or any support structure or substrate, such as paper or paperboard This self-limiting action may be advantageous where possible. Thus, for each application, the need for sufficient heating must be balanced with the requirement to prevent unwanted overheating. Unfortunately, in conventional highly oriented PET susceptors, the temperature at which crazing occurs can only be slightly controlled by changing the thickness of the metal layer, the type and amount of adhesive, and the uniformity of adhesive application, for example. .

  Although delaying the occurrence of crazing in susceptor structures has been a subject of significant effort, little or no meaningful improvement has been obtained with conventional thinking. However, in the present invention, by making significant and previously unforeseen changes to the base polymer materials used and / or the method of processing them into films, the occurrence of crazing can be increased or delayed and controlled. It is shown that there is a high possibility of microwave heating.

  In order to understand the uniqueness of the solutions employed in this disclosure, it is useful to first review previous attempts to create improved heated susceptor films through improvements to the methods of manufacturing these films. The discussion of film characterization also reveals a new understanding that supports the uniqueness of aspects of the present invention.

  Dimension at high temperature, because biaxially oriented hardware used to make standard PET susceptor substrate films allows highly oriented films to be conditioned or heat-set at controlled temperatures by heat It was the goal of researchers to increase the heat setting temperature and dwell time to obtain films with improved stability. For example, commonly assigned US Pat. Nos. 4,851,632, 4,993,526, 5,003,142, and 5,177,332 and US Patent Application Publication No. 2007/0084860 Al disclose the use of “thermally stabilized PET” films. In US Pat. No. 5,177,632, “the shrinkage when heated to 150 ° C. for 30 minutes is less than about 2%, preferably the shrinkage when heated so is about 1.5% or 1%. Less than, most preferably PET treated to about 0.6% or less. According to US Pat. No. 5,177,132, “thermally stabilized PET is made from conventional grade PET film by a stabilization process that includes a series of heat treatment and relaxation steps and is known to those skilled in the art. Thermal stability of PET. The process is described in more detail in Bulletin E-50542, “Thermal Stabilization of Mylar®” of EI Du Pont de Nemours and Company ”. This bulletin is not publicly available for review, but in the context of its description, particularly in US Pat. No. 5,177,332, the thermal stabilization process involves the production of “standard PET film for food packaging”. It is clear that an auxiliary heat treatment is included in addition to the heat treatment normally provided, and in all cases PET or PET film refers to a biaxially oriented film.

  Other researchers have attempted to use higher melting point (ie, melting temperature) polymers to achieve higher heating performance (eg, biaxial orientation that initiates melting in the range of about 260 ° C to 300 ° C). See US Pat. No. 5,571,627, which teaches the use of susceptor-based films, or US Pat. No. 5,126,519, which teaches the use of films made from PCTA copolyesters having melting points above 500 ° C.). Certain copolyesters are also disclosed, such as polyethylene naphthalate and polycyclohexylene-dimethylene terephthalate (PCDMT), which have inherently higher melting points (see, for example, US Pat. Nos. 5,527,413 and 5,571,627). Despite the claim that the heating performance has been improved, susceptors using these films are not believed to have proved suitable for commercial use. Difficulties encountered include excessive additional costs, manufacturing problems, and uncontrollable heating, resulting in unacceptable scorching or burning of food, packaging and / or packaging parts. . For example, US Pat. No. 5,527,413 discloses that PCDMT can become too hot, thus burning or carbonizing the paper in the susceptor structure or burning food that comes into contact with the susceptor. Therefore, it appears that using a high melting point copolyester has not been proven to be sufficient to create a commercially useful susceptor film.

  Thus, a higher heat flux and / or higher temperature can be controllably achieved than conventional susceptor structures, thereby browning the food better and / or improving the crispness without the risk of excessive carbonization There is a need for a susceptor structure that can be made.

  In related US patent application Ser. No. 12 / 709,628 (U.S. Patent Application No. 2010 / 0213191A1), an unoriented film, such as a generally or essentially amorphous polyethylene terephthalate (APET) film (eg, having a crystallinity of about 1 to It was found that about 25%) can be used in susceptor films that have better heating characteristics than conventional biaxially oriented PET film susceptors. However, it has also been observed that the APET film does not have sufficient strength for processing. Therefore, the present inventors have proposed various means for increasing the strength of the APET film. Nevertheless, there is a need for alternative susceptor base films and susceptor structures made therefrom.

U.S. Pat. No. 4,851,632 U.S. Pat. No. 4,993,526 U.S. Pat. No. 5,031,142 US Pat. No. 5,177,332 US Patent Application Publication No. 2007 / 0084860A1 US Pat. No. 5,177,632 US Pat. No. 5,177,132 US Pat. No. 5,571,627 US Pat. No. 5,126,519 US Pat. No. 5,527,413 US Patent Application No. 12/709628 (US Patent Application No. 2010 / 0213191A1) U.S. Pat. No. 4,943,456 U.S. Pat. No. 5,0028,226 US Pat. No. 5,118,747 US Pat. No. 5,410,135 U.S. Pat. No. 4,283,427 US Patent Application Publication No. 2010 / 0213192A1 US Patent Application No. 13/804673 US Pat. No. 6,204,492 US Pat. No. 6,433,322 US Pat. No. 6,552,315 US Pat. No. 6,677,563 U.S. Pat. No. 7,019,271 US Pat. No. 7,351,942 US Patent Application Publication No. 2008 / 0078759A1 US Pat. No. 5,421,187 US Pat. No. 5,530,231 US Patent Application Publication No. 2008 / 0035634A1 International Publication No. WO2007 / 127371

Bulletin E-50542, “Thermal Stabilization of Mylar (registered trademark)” I. Du Pont de Nemours and Company J. et al. Brandrup, E .; H. Immergut, E .; A. Edited by Grulk, Polymer Handbook, 4th edition, John Wiley & Sons (1999), p. V / 113 Polymer Handbook, J. et al. Brandrup, E .; H. Immergut, E .; A. Grulk, 4th edition, John Wiley & Sons, Inc. 1999, ISBN 0-471-16628-6. "Polymer Chemistry, An Introduction 3rd Edition", Malcol Stevens, 1999, Oxford University Press, page 344.

  The present disclosure is generally directed to polymer films (or films) for use as a base film or substrate in a susceptor film, methods of making such polymer films, and susceptor films including polymer films. The susceptor film is bonded to the support layer to form a susceptor structure. The susceptor film and / or susceptor structure can be used to form a number of microwave energy interactive structures, microwave heated packages, or other microwave energy interactive structures.

  The susceptor structure may generally have a browning reaction rate that exceeds the browning reaction rate of a susceptor structure made from a conventional biaxially oriented PET film. Accordingly, the susceptor structure of the present invention can provide a significant improvement in browning and / or enhancing the crispyness of food heated with the susceptor structure.

The polymer film may be non-oriented or oriented to various degrees. If the film is oriented, the orientation and heat setting conditions can be customized to achieve the desired level of crystallinity, residual orientation, and thus the desired heating performance for the particular application of the susceptor film. it can. The film of less, that the refractive index of less than about 1.64 _(n z), the birefringence of less than about 0.15 _(n z _-n x), and less than about 50% crystallinity (all herein The crystallinity value is characterized by having one or more of (derived from density measurements as described below). However, in some cases, other crystallinity, refractive index, birefringence, and respective ranges are preferred. By way of comparison, commercially available biaxially oriented homopolymer films typically used as susceptors have a refractive index (n _z ) of about 1.6447 to about 1.6639, and birefringence of about 0.1500 to about 0.1700. It may have _(n z -n _x).

  Polymer films having relatively low crystallinity, orientation, and / or residual shrinkage can be used for susceptor films and susceptor structures and can achieve greater heat flux and / or higher temperatures than conventional susceptor structures. Was found. Furthermore, the inventors have recognized that there is a relationship between the various base materials, process conditions, and crystallinity of the resulting film. As a result, the design flexibility of the base film and the resulting susceptor structure has been greatly increased.

  In some exemplary embodiments, the polymer film may include amorphous polyethylene terephthalate (APET), amorphous nylon, various copolyesters, or any combination thereof. As will be appreciated by those skilled in the art of polymer film manufacture, when the term amorphous is used to describe one or more of these films, this is largely due to the amorphous portion of the form. A film made in such a manner, and this type of film will typically have a crystallinity of from about 1% to about 25%. These are films that are made by rapid cooling from the melt and thus prevent the appreciable degree of crystallization from occurring. The copolyester may generally have a melting temperature that is comparable to or lower than the melting temperature of standard PET polymers, such as from about 250 ° C. to less than 260 ° C. However, many other polymers are also conceivable.

  If desired, one or more additives (ie, polymers) may be incorporated into the polymer film to improve the strength and / or processability of the polymer film. In addition, or as an alternative, the strength and / or processability of the polymer film may be improved by using a multilayer polymer film, in which case one or more of such layers are desirable for the polymer film. Gives a degree of robustness. Thus, multilayer films are characterized by increased tear strength, improved toughness, and improved dimensional resistance so that the film can be processed (eg, metallized, chemically etched, laminated, and / or printed) by a fast conversion operation, and various Susceptor structure and / or packaging material.

  If desired, additional functional characteristics can be imparted to the multilayer film by selecting a polymer having the desired properties. For example, a multilayer film can have barrier properties that can make the polymer film suitable for many applications, such as refrigerated microwave-processable food packaging that requires extended shelf life.

  Other features, aspects, and embodiments of the invention will be apparent from the description below.

1 is a schematic cross-sectional view of an exemplary microwave energy interaction structure. FIG. FIG. 6 is a diagram representing refractive index (n _z ) and temperature rise (ΔT, ° C.) for various exemplary susceptor base films and susceptor structures. FIG. 5 represents the relative browning kinetics and pizza browning pixel counts for various exemplary susceptor structures. FIG. 6 is a diagram representing refractive index (n _z ) and temperature rise (ΔT, ° C.) for various exemplary susceptor base films and susceptor structures. FIG. 5 represents the relative browning kinetics and pizza browning pixel counts for various exemplary susceptor structures. It is a figure showing the temperature response of the dynamic dimension about susceptor base film 6-2. It is a figure showing the temperature response of the dynamic dimension about susceptor base film 1-6. It is a figure showing the temperature response of the dynamic dimension about susceptor base film 7-1. It is a figure showing the temperature response of the dynamic dimension about susceptor base film 6-9. It is a figure showing the temperature response of the dynamic dimension about susceptor base film 6-11. FIG. 9 is a diagram representing a dynamic dimensional temperature response for a susceptor base film 7-8.

  The present disclosure is generally directed to polymer films (or films) for use as base films or substrates in susceptor films, methods of making such polymer films, and susceptor films and structures comprising polymer films. .

  In one embodiment, the base film can generally have a crystallinity of greater than 0% to about 50%, such as from about 1% to about 50%, before the film is exposed to microwave energy. In some embodiments, the degree of crystallinity is about 1% to about 49%, about 1% to about 48%, about 1% to about 47%, about 1% before the film is exposed to microwave energy. % To about 46%, about 1% to about 45%, about 1% to about 44%, about 1% to about 43%, about 1% to about 42%, about 1% to about 41%, about 1% to About 40%, about 1% to about 39%, about 1% to about 38%, about 1% to about 37%, about 1% to about 36%, about 1% to about 35%, about 1% to about 34 %, About 1% to about 33%, about 1% to about 32%, about 1% to about 31%, about 1% to about 30%, about 1% to about 29%, about 1% to about 28%, About 1% to about 27%, about 1% to about 26%, about 1% to about 25%, about 1% to about 24%, about 1% to about 23%, about 1% to about 22%, about 1 % To about 21%, about 1% to about 20%, about 1% to about 19%, about 1% to about 18%, about 1 About 17%, about 1% to about 16%, about 1% to about 15%, about 1% to about 14%, about 1% to about 13%, about 1% to about 12%, about 1% to about 11%, about 1% to about 10%, about 1% to about 9%, about 1% to about 8%, about 1% to about 7%, about 1% to about 6%, about 1% to about 5% About 1% to about 4%, about 1% to about 3%, about 1% to about 2%. Other crystallinity ranges within and / or overlapping with the above ranges are also contemplated, such as about 4% to about 35%, about 10% to about 40%, about 37% to about 50%, and the like.

  The inventors have determined that low crystallinity films made by various methods can be superior base films compared to their high crystallinity counterparts. Specifically, low crystallinity and low residual orientation levels result in a susceptor film and / or susceptor structure, even when the base film has a high absolute orientation level similar to conventional base films. It was found that it generally corresponds to the high heating capacity in This is significantly different from the conventionally used susceptor film having high orientation and high crystallinity. Several attempts have been made to understand the self-limiting action of susceptors, but the crystallinity and dynamic dimensional temperature response characteristics of oriented films used in microwave susceptor films and the resulting susceptor performance. The relationship is generally not considered or recognized by others.

  As noted above, the crystallinity values disclosed herein are derived from density measurements unless otherwise noted. As will be appreciated by those skilled in the art, there are various ways to determine the percent crystallinity of a polymer film, and the same sample tested in different ways will show different crystallinity results. Some methods (eg, density or differential scanning calorimetry (DSC)) are considered indirect methods because they use physical constants in calculations to obtain crystallinity values, while others (eg, X-ray diffraction or A similar method) is considered a direct method because it does not rely on such constants. Thus, each test method provides a suitable means to compare relative values measured by the same method, but it is not easy to compare absolute values of percent crystallinity between different test methods. .

  Specifically, the percent value of crystallinity presented in this specification was calculated using equation (1).

ここで、ρ_ｓは試料の実測密度、ρ_ａは結晶化度０％の材料の密度、ρ_ｃは結晶化度１００％の材料の密度である。

Here, ρ _s is the measured density of the sample, ρ _a is the density of the material with _a crystallinity of 0%, and ρ _c is the density of the material with a crystallinity of 100%.

As ρ _a and ρ _c , 1.333 and 1.455 g / cm ³ were selected, respectively (J. Brandrup, E. H. Immergut, EA Grulke, edited by Polymer Handbook, 4th edition, John Wiley & Sons (1999). ), P.V / 113) to obtain equation (1.1).

  In another aspect, it has also been found that in many cases the refractive index and / or birefringence of the polymer film performs better than the criteria described in the prior art. The refractive index of a material is the ratio of the speed of light in vacuum to the speed of light in the material. By polarizing light in a particular direction of the material, the refractive index in that direction can be measured. For amorphous materials without molecular alignment, a single value can be used to optically define the material. For materials capable of molecular orientation, such as semi-crystalline polymers, the absolute value of the refractive index increases with increasing crystallinity and orientation, and different values of refractive index measured in different directions may result in structures such as films. Can be used to characterize the anisotropy of The difference in refractive index in two directions of the polymer film is defined as birefringence between these two directions and can be used to understand the orientation difference between these directions. Since birefringence is defined as the difference in the value of the refractive index between the two directions, birefringence can be a positive or negative number depending on the particular sample morphology and the direction chosen for the refractive index. It is widely accepted that the large absolute value of birefringence is related to the large difference in orientation in these two directions.

Some polymer films useful for forming susceptor structures according to the present disclosure are less than about 1.64, such as about 1.57 to about 1.62 refractive index ( _nz ), where _nz is the film The refractive index in the machine direction). In each of the various independent embodiments, the polymer film is less than about 1.63, less than about 1.62, less than about 1.61, less than about 1.60, less than about 1.59, or less than about 1.58. It may have a refractive index (n _z ). Additionally or alternatively, the polymer film generally less than about 0.15 birefringence (n z _-n x) _(where n _x of (negative including birefringence values) means the refractive index in the thickness of the film ). In each of the various independent examples, birefringence _(n z -n _x) is less than about 0.14, less than about 0.13, less than about 0.12, less than about 0.11, less than about 0.10, Less than about 0.090, less than about 0.080, less than about 0.070, less than about 0.060, less than about 0.050, less than about 0.040, less than about 0.030, less than about 0.020, about 0 Less than about .015, less than about 0.010, or less than about 0.0050. However, other suitable refractive indices, birefringence values, and ranges thereof are also contemplated.

  As discussed above, prior art films have typically been characterized based on their static shrink properties (eg, “1% shrinkage at 150 ° C. for 30 minutes”). However, the inventors have determined that this conventional performance definition leads to inadequate and erroneous conclusions and uses this definition as a primary criterion for material and process selection to identify susceptor substrate films. Manufacturing may have limited the performance of microwave susceptor substrate films, susceptor components, susceptor packaging and susceptor packaging / electromagnetic field modification or the development of shielding packaging materials and components. confirmed.

  In order to realistically characterize the microwave heating action of a susceptor structure, the inventors have developed a dynamic temperature that better represents the actual conditions exposed during microwave heating using the susceptor structure. It has been found that the dimensional response of the susceptor substrate film to exposure to This understanding has been found to be particularly useful for designing superior susceptor substrate films and useful microwave heating structures for food applications or other industrial applications.

  This can be very important for the heating of food. In these susceptor packaging materials, browning and crispy changes in microwave-heated foods are highly desirable quality parameters for consumers, natural and appealing surface discoloration and the development of aroma and flavor This is because it often relies on the Maillard browning reaction to achieve the above, and it often relies on changes in the moisture content of the surface to achieve a crisp feel and mouthfeel. The progress of the Maillard reaction is also associated with a lower water activity in the food than is normally present at the start of cooking. The kinetics of the Maillard reaction begin to reach a useful reaction rate to achieve browning in the time frame of interest for susceptor packaging at temperatures above 155 ° C, and also because of the desired short cooking time in the microwave oven Raising the temperature is very beneficial for both aspects of the Maillard reaction, decreasing water activity and increasing reaction rate, since it increases as a function of power with increasing temperature.

  In particular, many of the films described as superior in the prior art (eg, consistent with the definition of “thermally stabilized PET” in US Pat. No. 5,177,132), as discussed further in connection with the examples. Shows a slight improvement, if any, over the standard susceptor structure using standard biaxially oriented PET film and the performance of the susceptor structure made from the base film of the present disclosure. Is over. For example, a film that exhibits shrinkage on the order of 1% at 150 ° C. for 30 minutes yields a susceptor film that exhibits typical thermal limiting and crazing behavior, which is overcome by the film of the present invention. We measured the dynamic dimensional temperature response of an example of a susceptor structure made from “thermally stabilized PET” that claims to provide improved susceptor performance, which is useful in microwave heating conditions. It shows that it is inferior in stability to the susceptor structure of the present disclosure. Thus, a typical method for characterizing heating capacity by prolonged exposure to temperatures that are at most the lower limit of the temperature range subject to microwave susceptor heating is that the actual event of interest, i.e. the part of the susceptor packaging material, is 150 Advance understanding of the actual heating performance achieved in microwave food heating in a microwave oven, a dynamic process often desired to reach temperatures significantly above typical static test conditions of ℃ Contradict that.

  In view of the above findings, the inventors have determined that a number of materials and processes can be used to form a susceptor base film having properties that provide excellent heating performance under microwave heating conditions. For example, a susceptor structure formed from a base film generally has a browning reaction rate that exceeds the browning reaction rate of a susceptor structure made from a conventional biaxially oriented PET film; Significant differences can be observed in and / or improved crispy.

  In one embodiment, the susceptor film can include a minimally oriented film. A minimally oriented film may be non-oriented (ie non-oriented) or slightly oriented (ie greater than 0% and up to about 20% oriented as discussed below). An unoriented polymer film is a film that has not been stretched in either or both the machine direction (MD) and the transverse direction (CD) at temperatures below the melting point of the polymer. Non-oriented polymer films can be quenched, resulting in a low crystallinity of, for example, less than about 25%. This can generally be attributed to the residual melt orientation associated with drawing the melt down to the desired final film thickness. In contrast, highly oriented films of the kind used in conventional susceptor films and structures have a high level of orientation and / or crystallinity due to strain and a high level of residual shrinkage.

In each of the various embodiments, the minimally oriented film of the present disclosure is characterized by having one or more of the following.
Less than about 1.59, such as from about 1.57 to about 1.58, or from about 1.5727 to about 1.58, such as about 1.575 (eg, Polymer Handbook, J. Brandrup, E. H. Immergut, E .; A. Grulke, 4th edition, John Wiley & Sons, Inc., 1999, ISBN 0-471-16628-6), or a refractive index of about 1.5723 to about 1.5727, for example about 1.5725 (n _z ),
Less than about 0.005, less than about 0.004, less than about 0.0035, or less than about 0.0028, for example from about 0.0012～ about 0.0022, for example from about 0.0016 birefringence _(n z -n _x ),
Greater than 0%, such as at least greater than about 1% and less than about 25%, less than about 22%, less than about 20%, less than about 18%, less than about 15%, less than about 12%, less than about 10%, about Less than 7%, less than about 5% crystallinity. In one particular embodiment, the film may have a crystallinity of about 4%. However, other crystallinity, refractive index, birefringence, and respective ranges may be preferred.

  Any suitable polymer can be used to form the susceptor base film or substrate. In one embodiment, the film is amorphous PET (APET), such as Pure-Stat Technologies, Inc. (APIST film commercially available from Lewiston, Maine) may be included. However, other suitable APET films and / or other polymer films are also used.

  The present inventors have found that a susceptor film having a low level of orientation tends to resist crazing more greatly than a conventional highly oriented, high crystallinity biaxially oriented PET film. While not wishing to be bound by theory, it is believed that the large residual contraction force has a prominent role in the generation and propagation of crazing of the susceptor structure. It is believed that minimally oriented films tend to resist crazing more than highly oriented polymer films because low oriented films exhibit much lower heat-induced dimensional shrinkage than highly oriented films. Thus, minimally oriented films, such as APET, that have inherently low or very low shrinkage force or no shrinkage force, such as APET, are conventional highly oriented films that have inherently high shrinkage force, such as high orientation. There is a tendency to resist crazing more than PET. This is believed to be clearly different from the conventional way of designing susceptor films taught in the art.

  Alternatively or additionally, and not wishing to be bound by theory, the crystallinity of the minimally oriented polymer film increases during the heating cycle, thereby increasing the heat resistance of the polymer, Therefore, it is conceivable to increase the thermal stability. As a result, the stability of the susceptor film will increase during the heating cycle.

  In another embodiment, the susceptor film is a moderately oriented polymer film, i.e., at least one dimension of the film is 20% to about 200%, such as about 20% to about 150%, such as about 30% to about 70%, For example, it may include a polymer film that has been subjected to an orientation process that increases by about 50%. This is about 1.2: 1 to about 3: 1, such as about 1.2: 1 to about 2.5: 1, such as about 1.3: 1 to about 1.7: 1, such as about 1.5: 1 corresponds to each draw ratio (where the draw ratio is equal to the draw percentage divided by 100 plus 1). Such an alignment process is useful when the unoriented film does not have sufficient strength for a particular application.

In each of the various embodiments, the moderately oriented film of the present disclosure is characterized by having one or more of the following.
Less than about 1.62, less than about 1.61, less than about 1.60, or less than about 1.59, such as about 1.57 to about 1.59, such as about 1.5733 to about 1.5848, such as about 1 .5791 refractive index (n _z ),
Birefringence (n _z − of less than about 0.05, less than about 0.035, less than about 0.01, or less than about 0.0024, such as about −0.013 to about 0.0024, such as about −0.0029. n _x ), and greater than 0%, such as at least about 1% and less than about 50%, less than about 48%, less than about 45%, less than about 42%, less than about 40%, less than about 38%, about 37% Less than about 36%, less than about 35%, less than about 34%, less than about 30%, less than about 25%, less than about 20%, less than about 17%, less than about 15%, less than about 12%, about 10% Less than, less than about 7%, or less than about 5% crystallinity. In one particular embodiment, the crystallinity of the film may be about 4%. However, other crystallinity, refractive index, birefringence, and respective ranges may be preferred.

  The orientation may be biaxial or bi-directional (ie both machine direction (MD) and cross machine direction (CD)), or uniaxial or unidirectional (ie either MD or CD). Surprisingly, the inventors have determined that under certain process conditions, the film can be given strength without bringing the level of crystallinity close to that observed in conventional biaxially oriented PET. . Specifically, conditions can be selected to minimize crystallization due to strain and crystallization due to heat typically associated with normal orientation processes. Thus, the orientation process conditions can be customized to provide the desired level of crystallinity and thus the desired heating performance for a particular application of the susceptor film.

  As will be appreciated by those familiar with the orientation process, stretching all around Tg (about 80 ° C. for polyethylene terephthalate), if all else is equal, will give more force to obtain a given draw ratio. And requires greater stress on the polymer. This is a typical path sought to develop high crystallinity, high residual orientation and high mechanical strength properties for a standard highly biaxially oriented PET polymer film. The resulting film often has a crystallinity greater than 50%. However, the inventors have found that it may be useful to leave the conventional practice of pursuing high levels of crystallinity and residual orientation. Instead, the films of the present disclosure are generally stretched at temperatures much higher than Tg to minimize crystallization due to strain. As an example, in a laboratory test, a film stretched at a temperature of at least about 105 ° C. and annealed at a temperature of at least about 170 ° C. exhibits significant strength without sacrificing the heating performance of the unoriented film. Achieved an improvement. This is discussed further below in connection with the examples. These temperatures are exemplary only and were obtained with laboratory scale equipment (see the Examples), and such temperatures are for commercial scale equipment for a given polymer system. It is noted that it may or may not be applicable.

  In another aspect, we have developed a susceptor film that can provide relatively low crystallinity even at high levels of orientation if the polymer is correctly selected, and thus has a higher heating capacity than conventional films. It has also been found that it can be manufactured. By way of example, it has been found that susceptor films with excellent heating properties can be made from copolyesters having the same or lower melting point as standard PET homopolymers. While not wishing to be bound by theory, consideration of the steric hindrance of the subject copolyesters in this disclosure slows the onset of crystallinity and typically even highly oriented films made from these materials. It is believed that the absolute crystallinity level commonly achieved with PET homopolymers cannot be reached. Even though these films approach the maximum orientation crystallinity possible during processing, the copolymers cannot reach the level of absolute crystallinity possible with homopolymers. Thus, films made from the types of copolyesters disclosed herein are biaxially oriented standard PET, whether unoriented, slightly oriented, moderately oriented, or highly oriented. It has refractive index and birefringence values lower than those typical for films and heat stabilized standard PET films.

More particularly, in each of the various embodiments, the highly oriented film of the present disclosure is characterized by having one or more of the following:
Less than about 1.64, for example about 1.56～ about 1.63, for example about 1.58～ about 1.61, for example about 1.5769～ about 1.6124, for example from about 1.5920 refractive index of _{(n z} ),
Less than about 0.15, less than about 0.14, less than about 0.125, or less than about 0.11, such as from about 0.0030 to about 0.10, such as from about 0.0029 to about 0.1022, such as about 0 birefringence of .0437 _(n z _-n x), and greater than 0%, such as at least about 1%, and less than about 50%, less than about 48%, less than about 45%, less than about 40%, less than about 38% Less than about 37%, less than about 36%, less than about 35%, less than about 33%, less than about 30%, less than about 28%, less than about 26%, less than about 25%, less than about 22%, less than about 20% A crystallinity of less than about 17%, less than about 15%, less than about 12%, or less than about 10%. In one particular embodiment, the crystallinity may be about 7%. However, other crystallinity, refractive index, birefringence, and respective ranges may be preferred. These values reflect avoidance of a significant amount of crystallinity induced in the film by strain or by heat during the orientation process. Accordingly, highly oriented susceptor films and structures comprising a base film comprising a copolyester have superior heating characteristics comparable to the non-oriented, slightly oriented and / or moderately oriented base films described above. It was shown to have.

  These results are unexpected and in sharp contrast to the prior art where copolyesters with higher melting points than homopolymer PET were used in an attempt to create greater temperature resistance. There has also been an attempt to create a susceptor base film having a high heating capability by heat-fixing a biaxially oriented film at a high temperature for a long time. The results of these attempts varied. The inventors have shown a slight increase in resistance to crazing with a slight increase in the temperature at which the release of the significant shrinkage forces present in highly oriented homopolymer PET films begins, with reduced performance and limited heating. It was found that the heat generation ability was brought about. However, we have a thin, high mechanical strength, even in highly oriented films that have better heating performance than conventionally used susceptor films by minimizing crystallinity and residual orientation. We found that we can create a film.

  Any suitable copolyester can be used. The copolyester may generally have a melting point of less than about 260 ° C, such as from about 200 ° C to about 260 ° C, such as from about 220 ° C to about 260 ° C. In some examples, the copolyester may have a melting point of less than about 250 ° C, such as from about 200 ° C to about 250 ° C, such as from about 220 ° C to about 250 ° C. The melting point may be similar to or lower than the melting point of standard PET polymer. Standard PET polymers have peak crystal melting points in the range of 250 ° C. to 260 ° C. as measured by second DSC heating, but some literature reports that the melting point of PET homopolymers is as high as 265 ° C. ("Polymer Chemistry, An Introduction 3rd Edition", Malcol Stevens, 1999, Oxford University Press, page 344).

  Copolyesters can also generally withstand the very high level (> 50%) crystallinity progression typically associated with standard biaxially oriented PET or heat stabilized PET films. In some examples, the base film has a high biaxial orientation (similar to the stretch ratio common to biaxially stretched standard or heat stabilized PET polymer films) despite being subjected to a high biaxial orientation. It can have a crystallinity of less than%. It is expected that similarly low or even lower crystallinity values will be observed for unoriented, slightly oriented or moderately oriented films made from copolyesters.

  One specific example of a copolymer suitable for use in a susceptor base film is SKYPET-BR (SK Chemicals, Seoul, Korea). This is a broad classification of materials represented by polymers of CAS index number 25640-14-6, CA index name 1,4-benzenedicarboxylic acid dimethyl ester, 1,4-cyclohexanedimethanol and 1,2-ethanediol. Is included. SKYPET-BR has a melting point of 236 ± 2 ° C. which is much lower than the typical melting point of standard PET polymer. An alternative polymer with similar properties is Eastman PET 9921 manufactured by Eastman Chemical Company, Kingsport, TN. These materials are part of a wide variety of 1,4-cyclohexanedimethanol (commonly abbreviated as CHDM) based copolyester materials, and poly (ethylene terephthalate) is modified by 1,4-cyclohexanedimethanol. Alternatively, it can be formed by modification of poly (1,4-cyclohexylenedimethylene terephthalate) with ethylene glycol or isophthalic acid.

  Other possible suitable materials are prepared by condensation of dimethyl terephthalate or terephthalic acid and ethylene glycol with one or more of dimethyl isophthalate, isophthalic acid, and diethylene glycol (diethylene glycol-isophthalate). Modified polyethylene terephthalate copolyester. The resulting polymers vary with respect to the level of modification or copolymerization, resulting in a variety of properties that can be used in modifying the performance of substrate susceptor films made from these polymers. Can do.

  While several examples are provided herein, it is recognized that numerous other possibilities and combinations thereof are possible, including copolyesters developed after the date of this disclosure.

  In yet another aspect, the inventors have found that a high orientation temperature (all the same as well) can result in a high heating capability. Without wishing to be bound by theory, it is believed that this is a result of the high mobility of the polymer chains at high temperatures and thereby making the chains slippery during orientation. This can act to reduce the stress required to achieve a given degree of stretching and to reduce crystallization due to strain that minimization may be advantageous. There are two rotamers in polyester, Gauche and Trans. Gauche isomers are generally characterized by being in a relaxed state, while trans isomers are in an extended high energy state. Notably, the Gauche isomer is found only in the amorphous domain or region of the structure, while the trans may be present in both the crystalline and amorphous regions. Only trans is present in the crystalline region.

  During strain crystallization, it is understood that some of the Gauche isomer content is converted to trans in the amorphous region, increasing the overall crystallinity as well as the residual orientation. In some process situations, such as heat setting without inducing further crystallization, it may be possible to reverse some of the conversion and consequently reduce the crystallinity in a possible advantageous way. While not wishing to be bound by theory, it is believed that minimizing (or somewhat reversing) Gauche-trans conversion contributes to the performance of the disclosed susceptor films and structures, Although the content of Gauche and trans in the amorphous and crystalline domains has not been directly measured, the choice of materials and processes disclosed herein can lead to standard heat-stabilized films that are representative of past practice. It can be inferred that the overall crystallinity level of the resulting susceptor substrate film can be minimized compared to a characteristic level of crystallinity typically exceeding 50%. The high Gauche isomer content in the films of the present disclosure is also consistent with the low refractive index and birefringence values reported herein.

  In yet another aspect, as discussed in connection with the examples, depending on the film, annealing at higher temperatures may provide additional heating capability beyond that achieved by the homopolymer film. It is advantageous.

It will be appreciated that the above-described copolyesters can also be used to produce unoriented films, slightly oriented films, and moderately oriented films. In either case, and for highly oriented films, the copolyester is either in a film of uniform structure made only from copolymer or in coextrusion combining separate layers of copolymer and homopolymer. Can be used. Blends of copolymer and homopolymer polyesters can also be used advantageously compared to 100% homopolymer structures. The incorporation of the copolyester results in a film having properties superior to a base film that includes 100% homopolymer as the susceptor base film, even in the presence of the homopolymer. When using a copolyester in a moderately oriented film, the level of crystallinity, either alone or in combination with one or more other polymers, compared to a susceptor base film containing only PET homopolymer It is noted that larger orientations can be used without exceeding 50%. In each of the various examples, the film comprising the copolyester and PET homopolymer is characterized by having one or more of the following:
A refractive index (n _z ) of less than about 1.64, such as about 1.60 to about 1.63, or about 1.5975 to about 1.6280, such as about 1.6123,
Less than about 0.15, less than about 0.14, less than about 0.125, or less than about 0.11, such as about 0.065 to about 0.14, such as about 0.0654 to about 0.1355, such as about 0 birefringence of .1031 _(n z _-n x), and greater than 0%, for example greater than at least about 1%, and less than about 50%, less than about 48%, less than about 45%, less than about 42%, about 40 %, Less than about 38%, less than about 37%, less than about 36%, less than about 35%, less than about 34%, less than about 30%, less than about 25%, less than about 20%, less than about 17%, about 15% %, Less than about 12%, or less than about 10% crystallinity. However, other crystallinity, refractive index, birefringence, and respective ranges may be preferred. It is contemplated that other polymers may be used alone or in any combination to form various susceptor base films having the properties described herein in connection with the base film of the present invention.

  For any of the susceptor base films described herein or contemplated herein, the crystallization kinetics of the polymer film can be manipulated to achieve the desired level of crystallinity at various points in the heating cycle. Time, temperature, and use of nucleating agents are variables that can be adjusted as needed to obtain the desired susceptor film performance. In addition, because different food products require different heating cycles for optimal preparation, additional degrees of freedom associated with controlling the initial level of crystallinity and further increase in crystallinity during heating. Dynamics are expected to allow for greater customization capabilities, which further enhances the usefulness and uniqueness of the susceptor films described herein.

  In some cases, the susceptor film may be intended to be used more than once. In that case, the degree of crystallinity of the polymer film will be high for the second use and any subsequent use.

  The polymer film can be formed by any suitable method. In one example, the polymer film substrate may be a water cooled film, cast film, or any other type of polymer film formed using a quench process. However, many other processes and systems are used. It will be appreciated that if such a film does not undergo a conventional post-extrusion orientation process, in some cases the film may be difficult to handle and / or convert to a susceptor structure. Thus, to improve the processability of the film, it is conceivable to subject it to a minimal orientation process in which the film is oriented (ie, stretched) slightly (eg up to 20%, eg about 5% to 20%). Since such orientation is relatively small compared to a standard highly oriented film stretched about 350-450% in each direction, such a slightly oriented film is substantially non-oriented herein. Is considered.

  If desired, the crystallinity of the minimally oriented film can be increased under control by heat treatment or conditioning after extrusion. As noted above, additives that improve crystallization kinetics can also be used.

  Additionally or alternatively, an additive can be incorporated into the film to improve its properties to facilitate processing or provide a more robust microwave heating performance. As an example, a reinforcing additive (eg, a polymer) can be used to make a slightly more fragile thin cast APET film stronger. Examples of suitable additives include ethylene methyl acrylate copolymers, ethylene-octene copolymers, or any other suitable polymer or material that improves the strength and / or processability of the polymer film. Other additives that provide various functions or advantages are also used. Any of such additives in any suitable amount, for example, up to about 15% by weight of the polymer film, up to about 10% by weight of the polymer film, up to about 5% by weight of the polymer film, or any other suitable Can be added in an amount. In other examples, the additive is in an amount of about 1% to about 10%, about 2% to about 8%, or about 3% to about 5% of the weight of the polymer film, or any suitable amount or amount. It can be used in the range.

  Any of the susceptor base films may comprise a multilayer film comprising at least two separate layers, each of which may comprise one or more polymers and optionally one or more additives. The layers may be coextruded or formed separately and bonded together using adhesives, tie layers, thermal bonding, or any other suitable technique. Other suitable techniques include extrusion coating and coextrusion coating.

  If desired, each layer of the multilayer film may be a quenched film, i.e. a film formed under conditions where the polymer melt is frozen very rapidly after exiting the extrusion die opening. By rapidly freezing and further cooling the solidified polymer film in this way, the progress of the micro or macro structure of the crystal can be minimized. As noted above, when using a low crystallinity film to form a susceptor film, the susceptor film is more susceptible to microwave heating during microwave heating compared to conventional susceptors made from biaxially oriented polyethylene terephthalate. High temperatures and large heat fluxes can be achieved.

  If desired, additional functional properties can be imparted to the multilayer film by selecting a polymer having the desired properties. For example, ethylene vinyl alcohol (EVOH) can be used to provide oxygen barrier properties. Polypropylene (PP) can be used to provide water vapor barrier properties. Such properties make the film useful for controlled or improved atmosphere packaging, especially for chilled or cold-stored foods that typically require higher oxygen and water vapor barrier properties than frozen foods. Become. Many other possibilities are possible.

  Many multilayer films are contemplated by the present disclosure. For purposes of illustration and not limitation, some exemplary structures include (a) APET / olefin, (b) APET / tie layer / olefin, (c) APET / tie layer / olefin. / Tie layer / APET, (d) APET / tie layer / PP / tie layer / APET, (e) APET / tie layer / PP / tie layer / amorphous nylon 6 or nylon 6,6, (f) APET / Tie layer / APET, (g) APET / tie layer / EVOH / tie layer / APET, (h) APET / tie layer, (i) APET / tie layer / all layers of reclaimed material / tie layer / EVOH / tie Layer / APET, (j) APET / tie layer / EVOH / tie layer / amorphous nylon 6 or nylon 6,6, (k) APET / tie layer / olefin / tie layer / EVOH / tie layer / APET, ( ) APET / tie layer / olefin / tie layer / EVOH / tie layer / nylon 6,6, (m) PET homopolymer / copolyester / PET homopolymer, and (n) copolyester / PET homopolymer / copolyester. It is done.

  In Examples ac and kl, and any other multilayer film contemplated in this disclosure, the olefin layer may be any suitable polyolefin, such as low density polyethylene (LDPE), linear low density polyethylene (LLDPE), Medium density polyethylene (MDPE), high density polyethylene (HDPE), polypropylene (PP), copolymers of any of these polymers, and / or metallocene catalyzed of these polymers or copolymers.

  In Example i and any other multilayer film contemplated in this disclosure, the reclaimed material layer may include film edge scrap and any other recyclable material according to conventional practice. Any of the various other examples (eg ah or jl), or other films contemplated in this disclosure, may include such a regrind layer. In some cases, the pulverized recycled material layer may require a tie layer to better adhere them to adjacent film layers.

  In examples bj and any other multilayer film contemplated in this disclosure, the tie layer may comprise any material suitable for providing the desired level of adhesion between adjacent layers. In some exemplary embodiments, the tie layer may include DuPont's Bynel®, Equistar's Plexar®, Basell's Lyondell, or Exxon's Exxlor ™. The exact choice of the tie layer depends on the rheological properties that ensure a uniform layer distribution in the adjacent polymer and coextrusion process in which it is to be joined. For example, DuPont's Bynel 21E781 is part of the Bynel 2100 series of anhydride-modified ethylene acrylate resins most often used to adhere to PET, nylon, EVOH, polyethylene (PE), PP, and ethylene copolymers. Plexar® PX1007 is one type of ethylene vinyl acetate copolymer that can be used to adhere a similar range of materials to the above-mentioned Bynel resin. The Exxlor® grade can be used to improve the impact resistance performance of various nylon polymers. In addition, tie layers and other resins can be selected for conventionally approved use in high temperature films for applications such as retort pouches that must minimize resin extract in foods.

  Amorphous nylon 6 or nylon 6,6 can replace APET in any of the multilayer film structures described above, or in any other structure within the scope of this disclosure. Many other structures are also possible.

  Many techniques are used to form a multilayer film. Film casting is a widely used rapid film manufacturing technique, but adaptation of the air-cooled blown film process can also create a cooling rate suitable for making the multilayer film of the present disclosure. By applying cooling air outside the “bubbles” of the blown film, the cooling rate can be increased compared to the use of room temperature air directed only at the outer surface of the bubble. Furthermore, the production rate can be increased by replacing the cooling air for cooling the internal bubbles. Although the cooling rate can be increased by using a water-cooled mandrel in contact with the inside of the bubble, this process is relatively less flexible with respect to the width of the film that can be produced. This is because increasing the cooling rate can only be achieved from intimate contact between the polymer bubble and the mandrel, so that different mandrels are required to produce films of different widths.

  Another way for the tubular film blowing process is the tubular water cooling process (TWQ). TWQ involves direct contact between the cooling water and the outside of the polymer bubbles, which results in a very high heat transfer rate and very rapid cooling of the extruded polymer film. Some TWQ processes combine direct contact between the outside of the bubble and water and an internal mandrel for support and further cooling. Another TWQ process uses only direct contact with water on the outer surface of the bubble, but sometimes it is aided by the exchange of cooling air inside the bubble. In some situations, the latter TWQ process may be used more advantageously. This is because a device without an internal mandrel has low manufacturing and operating costs and a large degree of freedom in changing the film width. Such a TWQ extrusion line is available, for example, as an AquaFrost® system from Brampton Engineering, Canada. However, many other processes and systems are also used.

  The basis weight and / or thickness of the polymer film, whether single layer or multilayer, may vary depending on the respective application. In some embodiments, the thickness of the film can be about 12 to about 50 microns, such as about 12 to about 35 microns, such as about 12 to 20 microns. However, other thicknesses are possible.

  To form a susceptor film, a layer of microwave energy interactive material (ie, a susceptor or microwave sensitive coating) is disposed on one or both sides of the polymer film. The microwave energy interactive material may be a conductive or semiconductive material, such as a vacuum deposited metal or metal alloy, or a metal ink, organic ink, inorganic ink, metal paste, organic paste, inorganic paste, or any combination thereof. May include. Examples of metals and metal alloys that may be suitable include, but are not limited to, aluminum, chromium, copper, inconel alloys (nickel-chromium-molybdenum alloys with niobium), iron, magnesium, nickel, stainless steel, tin, Titanium, tungsten, and any combination or alloy thereof.

  Alternatively, the microwave energy interactive material may include metal oxides, such as aluminum, iron and tin oxides, optionally in combination with a conductive material. Another metal oxide that may be suitable is indium tin oxide (ITO). ITO has a more uniform crystal structure and is therefore transparent at most coating thicknesses.

  Alternatively, the microwave energy interactive material may comprise a suitable conductive, semiconductive, or nonconductive artificial dielectric or ferroelectric. The artificial dielectric includes a conductive subdivided material in a polymer or other suitable matrix or binder and may include conductive metal, such as aluminum flakes.

  In other embodiments, the microwave energy interactive material may be carbon-based, as disclosed, for example, in US Pat. Nos. 4,943,456, 5,002826, 5,118,747, and 5,410,135.

  In yet other embodiments, the microwave energy interactive material may interact with the magnetic portion of the electromagnetic energy in the microwave oven. A correctly selected material of this kind can self-limit based on the loss of interaction when the material reaches the Curie temperature. An example of such an interactive coating is described in US Pat. No. 4,283,427.

  Other microwave energy interactive materials that can be combined with the film of the present invention to create a microwave susceptor structure represent another embodiment of the present invention.

  The susceptor film is then laminated or otherwise bonded with another material to produce a susceptor structure or packaging material. In one example, the susceptor film is laminated or otherwise bonded to paper or paperboard to create a susceptor structure having a greater heat flux output than conventional paper or paperboard based susceptor structures. The paper may have a basis weight of from about 15 to about 60 pounds / ream (pounds / 3000 square feet), such as from about 20 to about 40 pounds / ream, such as about 25 pounds / ream. The paperboard may have a basis weight of about 60 to about 330 pounds / ream, such as about 80 to about 140 pounds / ream. The paperboard generally can have a thickness of about 6 to about 30 mils, such as about 12 to about 28 mils. In one particular example, the paperboard has a thickness of about 14 mils (0.014 inch). Any suitable paperboard, such as a solid bleached sulfate board, such as a Fortless® board available from International Paper Company, Memphis, TN or a SUS® board available from Graphic Packaging International. Bleached sulfate board can be used.

  Alternatively, the susceptor film may be laminated or otherwise joined with another polymer film. The polymer film is considered to shrink little or not at all, as does the corresponding base film, so that the performance attributes of the susceptor film are not adversely affected. It is contemplated that such polymer films may be transparent, translucent, or opaque as required for a particular application. It is further contemplated that the laminated (or otherwise joined) structure can be thermoformed. The shallow drawn shape is expected to maintain the functionality of the susceptor in all ranges except the maximum stretch range during thermoforming, and to customize the degree of susceptor functionality, The design can be advantageously used to adjust the local draw ratio. High crystallinity materials are not easily molded, especially in in-line bag-fill-fill-seal packaging machines, whereas the film crystallinity of the present disclosure is inherently low, which makes film formability advantageous. . The shaped structure may later be crystallized by methods common to those skilled in the art.

  If desired, the susceptor base film may be subjected to one or more treatments to modify the surface prior to placing the microwave energy interactive material on the polymer film. By way of example and not limitation, the polymer film may be subjected to a plasma treatment to modify the surface roughness of the polymer film. While not wishing to be bound by theory, such a surface treatment provides a more uniform surface for receiving microwave energy interactive materials, thereby increasing the heat flux and maximum of the resulting susceptor structure. The temperature is thought to rise. Such processing is discussed in US Patent Application Publication No. 2010 / 0213192A1 and US Patent Application No. 13/804673 filed March 14, 2013, both of which are hereby incorporated by reference in their entirety. Incorporated into.

  If desired, the susceptor film may also be used in combination with other microwave energy interactive elements and / or structures. A structure including a multilayer susceptor is also conceivable. It will be appreciated that the use of the susceptor film and / or structure of the present invention with such elements and / or structures provides improved results compared to conventional susceptors.

  As an example, the susceptor film may be used in combination with a foil having a sufficient thickness to reflect most of the incident microwave energy or a high optical density deposition material. Such elements are typically conductive and reflective metals or metal alloys, such as generally having a thickness of about 0.000285 inches to about 0.005 inches, such as about 0.0003 inches to about 0.003 inches. Formed from aluminum, copper, or stainless steel in the form of a solid patch having. Such other elements may have a thickness of about 0.00035 inches to about 0.002 inches, such as 0.0016 inches.

  In some cases, a microwave energy reflective (or reflective) element may be used as a shielding element when the food tends to burn or dry during heating. In other cases, smaller microwave energy reflecting elements may be used to diffuse or weaken the intensity of the microwave energy. One example of a material that uses such a microwave energy reflecting element is Graphic Packaging International, Inc. (Marietta, GA) can be purchased under the trade name MicroRite® packaging material. In other examples, a plurality of microwave energy reflecting elements may be arranged to form a microwave energy distribution element to direct microwave energy to a specific area of the food product. If desired, the loop may be made long enough to cause microwave energy resonance, thereby improving the distribution effect. Examples of microwave energy distribution elements are described in US Pat. Nos. 6,204,492, 6,433,322, 6,552,315, and 6,677,563, each of which is incorporated herein by reference in its entirety.

  In yet another example, the susceptor film and / or structure may be used with or to form a microwave energy interactive insulating material. Examples of such materials are provided in US Pat. Nos. 7,019,271, 7,351,942, and US Patent Application Publication No. 2008 / 0078759A1, published April 3, 2008, each of which is incorporated by reference in its entirety. As incorporated herein.

  If desired, any of the many microwave energy interactive elements described herein or contemplated herein may be substantially continuous, i.e., without substantial breaks or interruptions, or For example, it may be discontinuous by including one or more cuts or openings through which microwaves pass. The cut or opening may extend through the entire structure or may only pass through one or more layers. The number, shape, size, and location of such cuts or openings is dependent on the type of structure being formed, the food being heated in or on it, the desired degree of heat, browning, and / or crispy Whether or not direct exposure to microwave energy is necessary or desirable to uniformly heat the food, the need to adjust the temperature change of the food due to direct heating, and the need for ventilation Depending on the situation, it can vary depending on the particular application.

  By way of example, the microwave energy interactive element may include one or more transparent regions to increase the efficiency of dielectric heating of the food product. However, if the microwave energy interaction element includes a susceptor, such openings reduce the overall microwave energy interaction area and thus improve food heating, browning, and / or surface crispness. Reduce the amount of microwave energy interactive material available. Thus, in order to obtain the desired overall heating characteristics for a particular food product, the relative amounts of the microwave energy interaction region and the microwave energy transparent region must be balanced.

  As another example, heating, browning, and / or crispiness are improved rather than being lost to heating food or portions of food that are not intended to browning and / or crispiness In order to ensure that microwave energy is efficiently concentrated in the power region, one or more portions of the susceptor may be designed to be microwave energy inactive. Additionally or alternatively, it may be advantageous to provide one or more discontinuities or inert areas to prevent overheating or charring of structures containing food and / or susceptors.

  As yet another example, one or more “fuses” that limit the propagation of cracks in the susceptor and thereby control overheating in areas of the susceptor where heat transfer to food is low and the susceptor tends to be too hot. Element may be incorporated into the susceptor. The size and shape of the fuse can vary as needed. Examples of susceptors including such fuses include, for example, U.S. Pat. Nos. 5,421,187, 5,530,231, U.S. Patent Application Publication No. 2008 / 0035634A1, published Feb. 14, 2008, and Nov. 8, 2007. Published International Publication No. WO 2007/127371, each of which is incorporated herein by reference in its entirety.

  Any such discontinuities or openings in the susceptor may include physical openings or voids in one or more layers or materials used to form the structure or structure, or It is noted that non-physical “openings” may be used. A non-physical aperture is a region that is transparent to microwave energy that allows microwave energy to pass through the structure without actual voids or holes cut through the structure. Such regions can be simply deactivated by removing microwave energy interactive material from a specific region or by mechanically deactivating a specific region by not applying the microwave energy interactive material to a specific region. (The region is electrically discontinuous). Alternatively, the microwave energy interactive material is chemically deactivated in a specific region, thereby converting the microwave energy interactive material in that region into a material transparent to microwave energy (ie, microwave energy). A transparent or inactive region containing the microwave energy interactive material in a deactivated condition). Both physical and non-physical apertures allow food to be directly heated by microwave energy, but physical apertures carry away water vapor or other vapors or liquids released from food Also provides ventilation performance.

  The invention will be further understood by the following examples which are not intended to be limiting in any manner. All information provided represents approximate values unless specified otherwise.

  Calorimetry was performed to determine the heat flux produced by the various susceptor structures and the maximum temperature reached. As shown in Table 1, various polymer films were used to form susceptor structures. The polymer films are DuPont Mylar® 800C biaxially oriented PET (DuPont Teijin Films ™, Hopewell, Va.), Pure-Stat APET (Pure-Stat Technologies, Inc., Lewiston H Oriented PET (DuPont Teijin Films ™, Hopewell, VA) and Toray Lumirror® F65 biaxially oriented PET (Toray Films USA, Kingstown, RJ). The physical properties of the untreated film (some of which were obtained from the manufacturer's data sheet) are shown in Table 1.

  The control films (Samples 1-1, 1-6, and 1-7) were measured to have a crystallinity of 52-55%, as shown in Table 2, to form a susceptor film This is consistent with the information provided by the manufacturer of the BOPET film that is generally used.

  About 1.5 to about 2.0 pounds / ream of the following adhesives: Royal 20469 (Royal Adhesives & Searants, South Bend, IN), Royal 20123 (Royal Adhesives & Sealants, South Bend, IN), South Bend, IN Each susceptor structure was made by bonding the susceptor film to the paperboard support layer using one of 5380M5 (Henkel Adhesives, Elgin, IL). However, other suitable adhesives may be used.

Refractive index measurements were made at 633 nm using a Metricon 2010 Prism Coupler (Metricon Corporation, Pennington, NJ). The results for the film were summarized as (n _z ) (machine direction, MD), (n _y ) (lateral or transverse direction, CD), and (n _x ) (thickness) directions. Birefringence (n z _{-n x)} is calculated from the refractive index represents the refractive index difference in MD and the thickness of the film throughout the specification. All of these films were transparent without the addition of colorants or pigments.

Samples 1-1 and 1-6 made using commercially available standard heat-stabilized PET homopolymer films, respectively, in both MD and CD characteristic of prior art highly oriented, highly crystalline films A high refractive index (n _z and n _y, respectively) was exhibited. Samples 1-3 and 1-4 of the cast film system structure shown good performance, in practice, very close to the values reported for amorphous homopolymer PET polymer, much lower (n _z) and (n _y ) Indicates the value. Combining this data with more than 50% crystallinity for samples 1-1 and 1-6 and only about 5% crystallinity for samples 1-3 and 1-4, It can be confirmed that the film mainly represents an amorphous film having a slight residual orientation even if it exists. The difference in (n _z ) and (n _y ) values in Samples 1-1 and 1-6 represents a slight difference in MD and CD orientation or heat setting. However, these are within the expected range for films having a moderately balanced MD / CD orientation.

  Calorimetric data was collected using an 8-channel FISO MWS microwave workstation optical fiber temperature sensing device (FISO, Quebec, Canada) attached to a Panasonic 1300 watt consumer microwave oven model NN-S760WA. A sample about 5 inches in diameter was mounted between two circular Pyrex® plates each having a thickness of about 0.25 inches and a diameter of about 5 inches. The plate was loaded with about 250 g of water in a plastic bowl placed on a stretched polystyrene insulation sheet about 1 inch thick (so that the heat radiated from the water did not affect the plate). The bottom plate was raised approximately 1 inch above the glass turntable using three substantially triangular ceramic stands. In order to measure the surface temperature of the plate, a thermo-optic probe was attached to the upper surface of the upper plate. After heating the sample at full power for about 5 minutes in a microwave oven at about 1300 W, the average maximum temperature rise from the initial ambient temperature on the surface of the top plate was recorded in degrees Celsius. (The calorimetric finite element analysis model shows that the average maximum temperature rise is proportional to the heat flux produced by the susceptor structure.) Before performing calorimetry, the Delcom 717 conductivity meter (Delcom Instruments, Inc., Prescott, WI) were used to measure the conductivity σ (mmho / sq) of each sample, and five data points were collected and averaged. The results are shown in Table 3. Here, ΔT is the temperature rise of the sample, and ΔΔT is the difference between the temperature rise of the sample and the temperature rise of the control sample (structure 1-1, standard biaxial orientation, heat-fixed PET film). Visually evaluate and record the degree of crazing of the susceptor surface after exposure to microwave cooking. It is noted that the absolute temperature reached by each sample was about 22 ° C. higher than the reported ΔT since the initial temperature of all samples was about 22 ° C. (ambient temperature).

  In general, susceptor structures 1-3 and 1-4 provided maximum heating power and no visible crazing was evident. On the other hand, structure 1-1 showed lower heating power than structures 1-3 and 1-4, and showed the amount of crazing expected for a standard commercial susceptor. The structure 1-6 had slightly less crazing than the structure 1-1 and provided moderate heating power. Structures 1-7 were intermediate between 1-1 and 1-6 in both heating power and crazing.

  In particular, the structure 1-5 that had already been heated showed a larger output than the structure 1-1. Although no visible crazing was seen, the sample still showed some self-limiting action (proven by ΔTmax). Without wishing to be bound by theory, it is believed that this self-limiting action is at least partly the result of changes in the density of the polymer film during the microwave heating cycle. Specifically, it is known that the density of the polymer film decreases as the polymer film is heated. However, as the polymer film is heated, there is also an increase in crystallinity and a concomitant increase in density. It is believed that the magnitude of this density increase exceeds the magnitude of the initial density reduction, so that there is an overall density increase during the heating cycle. It is further believed that this increase in density causes the susceptor structure to collapse or microcrazing, which results in atomic-scale electrical discontinuities.

According to the calorimetric measurement described in Example 1, the microwave reflection, absorption, and transmission (RAT) characteristics of the conventional susceptor structure (structure 1-1) at various heating times are compared with the experimental susceptor structure ( Comparison with structure 1-3). Each sample evaluated for low power RAT was set on a HP8753A network analyzer. The output is used to calculate the reflection (R), absorption (A), and transmission (T) (collectively “RAT”) characteristics of the sample. The merit factor was also calculated at each heating time. Where merit factor = absorbance (A) / (1-reflection (R))
It is. The merit factor is a useful measure of the ability of the microwave susceptor structure to resist the progression of high permeability fractions that impair good browning performance. Maintaining a high merit factor during the cooking cycle is the ability to convert microwave energy into sensible heat to produce effective surface browning and energy to reflect excessive direct microwave heating inside the food It shows that the susceptor holds the ability to avoid

Furthermore, for several heating times of the structure 1-1, a new parameter (P / A, mm / mm ² ) obtained by dividing the peripheral length of the crazing by the area of the region was measured. Each sample after heating was examined by measuring the peripheral length of each crazing of each sample under magnification by image analysis. P / A was obtained by dividing the total length of the crazing by the area of the region. The results are shown in Table 4 and Table 5.

  In particular, when the heating time was increased, the structure 1-3 exhibited greater heating than the structure 1-1. A susceptor structure with a high merit factor limits the amount of direct microwave heating of the food while maximizing susceptor absorption, thus generally showing greater browning of the food surface and improved crispy. Thus, in practice, structures using low crystallinity polymer films advantageously provide high levels of surface browning and / or improved crispness while advantageously minimizing induction heating of food. it can.

  Image analysis was performed to measure the degree of browning of foods using various susceptor structures. In each example, a Stouffer microwaveable flatbread melt was heated on a susceptor structure in a 1000 W microwave oven for about 2.5 minutes. When the heating cycle was completed, the food was turned over and the side of the food heated near the susceptor was photographed. Images were evaluated using Adobe Photoshop. For the assessment of this browning level, the analytical procedure used included R = 79, G = 30, as best representative of the hue that this food developed when browned by the susceptor structure being tested. It was accompanied by selection of RGB hue set points of B = 13. However, a single lightness / darkness value of this hue cannot accurately characterize browning as assessed qualitatively by an experienced observer, and so three important lightness values Three brightness averages of 33, 82-85, and 109, corresponding to the types of, were selected and individual pixel counts were performed on each of these values. Here, a high average brightness value corresponds to a slow progress of the selected hue during cooking. Since three average brightness values were used, a fairly tight pixel selection tolerance of 20% was provided for each brightness value. The results are shown in Table 6.

  Both structures provided some browning and / or improved crispness, but the number of pixels for the lightness values corresponding to the dark areas was significantly higher, and structures 1-3 replaced the food or susceptor structure. It was clearly shown to provide the greatest degree of browning and crispness without burning. By repeating the test on the structure 1-3, the excellent browning performance of the structure was confirmed.

  Various films and susceptor structures were prepared for evaluation. Two film manufacturers for preparing APET films: SML Machinesellschaft mbH (Lenzing, Austria) (“SML”) (Sample 5-3) and Pure-Stat Technologies, Inc. (Lewiston, ME) ("Pure-Stat") (Samples 5-4 to 5-15) were used. Furthermore, Dartek (registered trademark) N201 nylon 6,6 (Liqui-box Canada, Whitby, Ontario, Canada) was evaluated (Sample 5-2). Mylar® 800 biaxially oriented PET (“BOPET”) (DuPont Teijan ™ Films, Hopewell, Va.) (Sample 5-1) was evaluated as a control material. To form the various susceptor structures, the film was then metallized with aluminum and (optionally) about 1 to about 2 pounds / ream of Royal Hydra Fast-en® 20123 adhesive (Royal Adhesives, South Bend). , IN) was bonded to 14 pt (0.014 inch thick) Fortless® paperboard (International Paper Company, Memphis, TN).

  Optema (TM) TC120 and Optema (TM) TC220 ExCo (ethylene methyl acrylate copolymer resin, ExxonMobil Chemical), Sukanim F535 (ethylene methyl acrylate copolymer resin, Sukan Polymers Corporation, Duncan, SC), 84 trademark, Eng. Various reinforcing additives such as octene copolymers, Dow Plastics), and Americem 60461-CD1 (composition unknown) (Americem Cuyahoga Falls, OH) were also evaluated.

  Pure-Stat Technologies, Inc. The process of forming the APET film used by was as follows. Traytuf® 9506 PET resin pellets (M & G Polymers USA, LLC, Houston, TX) were dried with a desiccant and transported to the hopper of a cast film line extruder. The additive pellets were weighed into the throat section of the extruder, melted together with the dried PET pellets, mixed and extruded from a slot die to form a flat molten film. The molten film was cast on a cooling drum, quenched to a primarily amorphous solid state, transported via a roller, and the film was wound on a roll for further processing. The film thickness was about 0.0008 inch or about 80 gauge. It is noted that thicker or thinner films can be produced by varying the discharge rate of the extruder and the surface speed of the cooling drum. The process using SML Machinesellschaft mbH was similar.

  Samples were sampled with a Perkin-Elmer Differential Scanning Calorimeter (DSC-7) (Perkin-Elmer, Inc., Waltham, Mass.) By purging with nitrogen to prevent degradation and heating at 10 ° C./min. DSC data was obtained. Values were measured for samples heated to 300 ° C. and cooled to 40 ° C. The results are shown in Table 7. It is important to note that the DSC data was taken from the initial heating of the test sample. Thus, the values reflect the specific thermal history that each sample has undergone during processing relative to the effect of any orientation after extrusion and the crystallinity of the sample. The negative enthalpy change with crystallization is proportional to the amount of amorphous polymer present in the sample. The positive enthalpy change with melting is a measure of the degree of crystallization experienced by the sample during the DSC measurement. If the absolute values of these enthalpy values are closer, the sample is more amorphous. Therefore, Sample 5-1 which is a highly oriented film has a very high level of orientation and crystallinity, and the cast APET film 5-3-5-15 has a low level of crystallinity. It is confirmed by the value. The slightly larger enthalpy differences noted for Samples 5-6 to 5-15 reflect the impact of existing non-PET reinforcing additives, but also the low crystallinity levels of these films. Show.

  The apparent surface roughness (PEL) of each film was evaluated before and after treatment. An image of the film surface was obtained by an atomic force microscope (AFM) with a full scale of 0 to 100 nm. Using an image analysis system developed by Integrated Paper Services (IPS), Appleton, WI, gray level histograms were generated with 0-256 unit full scale light and dark gray scales. A binary image was produced with a gray scale 120. This is equivalent to a plane intersecting the Z direction of the AFM image at a height of 120/256 * 100 nm = 46.9 nm, ie 469 angstroms. The perimeter of the detected area was measured and normalized by the linear dimension of the image to determine the dimensionless ratio, ie perimeter / edge length, ie PEL. A large PEL value indicates a large surface roughness. In general, PEL data indicates that lower PEL levels (smooth film surface) are associated with greater calorimetric and browning results.

  The peak load before break was measured according to TAPPI T-494 om-01. The value shows that the strength of the film could be increased by the reinforcing additive in Samples 5-6 to 5-15. This has been confirmed in tests on commercial production equipment where films modified with reinforcing additives can be processed without difficulty. On the other hand, the non-modified film of the type represented by Samples 5-3 to 5-5 was more fragile in the conversion operation, required adjustment to normal process parameters such as tension, and the conversion efficiency was low.

  The haze of each polymer film was measured according to ASTM D1003 using a BYK Gardner Haze-Gard plus 4725 haze meter (BYK-Gardner, USA, Columbia, MD). In all cases, the incorporation of reinforcing additives increased the haze of the film. In some examples, the most preferred additives are those that exhibit a low haze level while providing a desirable increase in strength for processing and advantageously resulting in increased heating performance when manufacturing susceptor films and structures. It is.

  Next, each susceptor structure was evaluated by the calorimetric test described in Example 1. The results are shown in Table 7. Here, ΔT is the temperature rise of the sample, and ΔΔT is the difference between the temperature rise of the sample and the temperature rise of the control sample (structure 5-1, standard biaxial orientation, heat-fixed PET film).

  In addition, 104 was used as the RGB (red / green / blue) setpoint (RGB = 104/60/25, which visually corresponds to the brown hue associated with generally browned crispy pizza crust). Except for the above, each structure was evaluated by the food browning test described in Example 1. The maximum pixel selection tolerance was selected as the best representation of the visual browning assessment performed by an experienced observer on the pizza crust. In this case, it is necessary (as in Example 1) to use the multiple average levels of the brightness values of this RGB hue setpoint to quantify browning in a way that can accurately identify the browning performance of the test variable. It was determined that there was no. In addition, Kraft DiGiorno microwaveable pizza was used. The number of pixels with that hue was recorded, thereby indicating that a large number of pixels was more browning.

It is noted that before evaluating structure 5-1 (control), the unheated pizza crust was measured to determine 24313 pixels, which is the number of baseline pixels with colors associated with RGB values of 104/60/25. The This baseline value was used to calculate the results shown in Table 7. Where ΔUB is the number of pixels minus the unbrown crust baseline value (24313) from a given sample,
Δ% Imp is the rate of improvement relative to the result obtained for the control sample (structure 5-1).

  The calorimetric results and the pizza browning results both show a marked increase compared to the control for all unoriented, low crystallinity films, with or without the incorporation of additives. Visual observation of the cooked pizza crust confirmed the desired level of browning much higher than that obtained in a standard control sample made from highly biaxially oriented high crystallinity films. Thus, the incorporation of reinforcing additives into microwave susceptor films and structures can improve film robustness without performance degradation.

Then, using the simplified Arrhenius relationship for the browning kinetics of roughly doubling the browning reaction rate for each 10 ° C. temperature increase, the relative browning reaction rate (RBRR) is As calculated.
Relative browning reaction rate = 2 ^(ΔT10)

  Various Pure-Stat base films were uniaxially oriented and used to prepare susceptor structures for evaluation. Mylar® 800 biaxially oriented PET (DuPont Teijan ™ Films, Hopewell, Va.) Was also evaluated as a control material.

  Structure 6-1 was manufactured for commercial use, including the commercially available metallized 48 gauge standard biaxially oriented PET film described in the previous paragraph, laminated to paperboard as described in Example 4. Was a susceptor structure. Structure 6-2 was the same as Structure 6-1 except that it was laminated by hand.

  Samples 6-3 to 6-17 were subjected to fixing using a Bruckner Karo IV Laboratory Stretching Machine (laboratory stretcher) (Bruckner Machinechinbau GmbH & Co. KG. Siegsdorf, Germany) with the temperature fixed in Table 8 Uniaxially oriented in the machine direction at a temperature and a draw ratio of about 1.5: 1. The oriented films were then evaluated for various properties as shown in Table 8. Since the film containing the white pigment contains the pigment, the crystallinity cannot be measured by density, refractive index or birefringence, but the white samples 6-7 and 6-10 and the transparent sample 6-9 Comparison of ΔH values of crystallization exotherm and melt endotherm of white sample 6-10 oriented and heat-fixed under the same conditions as transparent sample 6-9 has the same level of crystallinity and residual orientation Is shown.

As shown in FIG. 2, a moderate uniaxial orientation could produce a film with improved susceptor heating performance compared to the control susceptor film. The refractive index values (n _z ) of these samples are essentially amorphous in nature, even after they are moderately oriented, with moderate crystallinity increases for some samples. It shows that it accompanies. The much higher value of ( _nz ) for the control is consistent with a highly crystalline film with a high degree of residual orientation.

  The film was then metallized and manually laminated to 14 pt paperboard as described in connection with Example 4 to form various susceptor structures.

  The sample sheet oriented with a laboratory stretching machine was then taped to a 48 gauge standard biaxially oriented PET full size roll, and the commercial parameters (target optical density = 0.20 normal parameter range). The film was metallized by running the roll through an aluminum metallizer in the inner). The metallized sheet sample was removed and then laminated by hand to form a susceptor structure. Hand laminated film samples were prepared using a No. 5 to apply Royal Hydra Fast-en Bond-Plus 20123 adhesive (Royal Adhesives and Sealants, South Bend, IN). A Chemistry Laboratories Laminator (Chemimments, Inc., Fair), laminated to 0.014 inch uncoated SBS paperboard (International Paper, Memphis, TN) using a 10 Mayer rod and set to a nip load of 50 psi and a speed of 4. And laminated.

  Each susceptor structure was then evaluated by calorimetry as described in Example 1. The results are shown in Table 8. Here, ΔT is the temperature rise of the sample, and ΔΔT is the difference between the temperature rise of the sample and the temperature rise of the control sample (structure 6-2, standard biaxial orientation, heat-fixed PET film).

  Structure 6-2 showed a temperature rise very similar to the commercial control structure 6-1. Thus, structure 6-2 is considered a legitimate representative of a commercially available susceptor structure. Some uniaxially oriented laminate susceptor samples (6-13 and 6-15) showed poor heating performance compared to the control sample. As noted in Table 8, these samples are oriented at the lowest temperature so that they have the highest stretch stress, the lowest exposed annealing temperature, and the least strain relaxation. Corresponding samples 6-14 and 6-16 were oriented at the same temperature as 6-13 and 6-15, respectively, but were exposed to higher annealing temperatures and increased significantly compared to the control sample The heating ability was shown. Samples 6-5, 6-10 and 6-11 show the maximum reached ΔT and RBRR, reinforcing the utility of the present invention to combine moderate orientation and heat setting to mitigate orientation stress. . These comparisons clearly show that the present disclosure can create excellent susceptor structures from a wide range of materials and processing methods, but not all combinations yield structures that function similarly. It also indicates that care must be taken when selecting specific process conditions. The advantage of this embodiment is the possibility to adjust the heating capacity of the resulting susceptor structure by a relatively simple modification of the orientation conditions for uniaxial orientation.

  As described above (Example 1), all samples had an initial temperature of about 22 ° C., so the absolute temperature reached by each sample was about 22 ° C. higher than the reported ΔT. In particular, each of the samples (even those with relatively poor performance) exhibited a temperature increase to an absolute temperature exceeding the 150 ° C. shrinkage performance standard commonly used as a film. Furthermore, it is noted that a temperature of 150 ° C. is lower than the temperature at which the Maillard browning reaction is reasonably expected to occur at the rate of interest for heating microwave treatable foods. This further emphasizes that conventional means of characterizing films used as susceptor base films are inadequate.

  The relative browning reaction rate (RBRR) was then calculated as described in Example 4. Furthermore, each structure was evaluated by the browning test of pizza described in Example 4. The browning test results are shown in FIG. Given the pizza recipe variation and the resulting different temperature rise kinetics, the relative browning rate based on ΔΔT calorimetry normalized to the ΔT of the control material provides a reasonable prediction of the number of browned pixels. Yes, and confirmed by visual inspection of pizza.

  Various films were biaxially oriented and used to prepare susceptor structures for evaluation. Homopolymer (HP) films, copolymer (CP) films, and coextruded (CX) films were evaluated. Films for orientation were produced by Pacur (PACUR, Oshkosh, WI) using a standard sheet extrusion apparatus. Homopolymer films were made from PQB 15-093 homopolymer PET resin supplied by Polyquest (Polyquest, Inc., Wilmington, NC). The copolymer film was made from SKYPET-BR 8040 copolyester (SKC Chemicals, Seoul, South Korea). The co-extruded film is an A / B / A three-layer co-extruded structure in which 70% homopolymer core (B) layer is wrapped with 2-15% copolyester skin (A) layer using the above-mentioned resin. there were.

  The film was oriented using a Karo series laboratory stretcher under the conditions described in Table 9. The sample sheet oriented with a laboratory stretching machine was then taped to a 48 gauge standard biaxially oriented PET full size roll, and the commercial parameters (target optical density = 0.20 normal parameter range). The film was metallized by running the roll through an aluminum metallizer in the inner). The metallized sheet sample was removed and then laminated by hand to form a susceptor structure. Hand laminated film samples were prepared using a No. 5 to apply Royal Hydra Fast-en Bond-Plus 20123 adhesive (Royal Adhesives and Sealants, South Bend, IN). Using a 10 Mayer rod, a Chemistry Laboratories Laminator (Chemimments, Inc., Fai), laminated to 0.012 inch uncoated SBS paperboard (International Paper, Memphis, TN) and set to a nip load of 50 psi and a speed of 4. And laminated.

  Refractive index and birefringence were obtained for various samples. As shown in FIG. 4, since the residual orientation and crystallinity are low, a novel and superior susceptor heating method can be obtained as compared with a commercially available standard highly oriented homopolymer PET film.

  Each susceptor structure was evaluated by the calorimetric test described in Example 1 and the pizza browning test described in Example 4, and the relative browning reaction rate (RBRR) was calculated as described in Example 5. . The results are shown in Table 9 and FIG. Among these, sample / susceptor structures 6-1 and 6-2 (from Example 6) are included for comparison.

  Structure 6-2 (commercially available control laminated by hand of Example 6) showed a temperature increase of 146.7 ° C. This closely corresponds to structure 7-1 which showed a temperature increase of 145.5 ° C. (which was oriented by a laboratory scale stretcher). Therefore, for the purposes of this discussion, structure 7-1 represents a reasonable approximation of commercially available susceptor film material. Structure 7-2, similar to structure 7-1 (except that the orientation level was 3.8 × 3.8) showed only a 134.7 ° C. temperature increase. Therefore, the homopolymer PET film-based susceptor stretched in this laboratory had a decrease in performance at a low orientation level.

  Structure 7-17, which includes a low melting copolyester (all other conditions are the same as structure 7-2), shows a temperature increase of 154.9 ° C., which is a commercial control (structure 6-2). Higher than any of the structures made from the containing homopolymer base film. The structure 7-18 having a high degree of orientation exhibited a temperature increase of 158.8 ° C. In contrast, Structure 7-21, which has a low orientation temperature and a low degree of orientation, showed a temperature increase of only 125.4 ° C. While this performance is unacceptable for most microwave heating applications, it is noted that there are other applications where low heating capabilities are desirable. Finally, in structure 7-19 fabricated at a higher annealing (ie heat setting) temperature than structure 7-21, the temperature rise was 144.2 ° C. The corresponding value for a homopolymer susceptor base film made under the same conditions was 138.2 ° C. (Structure 7-6).

  In addition, the increase in orientation temperature from 120 ° C. to 125 ° C. increased the ΔT by about 4 ° C. (structures 7-17 and 7-18) for the copolymer (change from 3.8 × 3.8 to 4 × 4). However, for homopolymers (4x4 at any temperature), the same increase in orientation temperature results in a ΔT increase of only about 2.3 ° C (see structures 7-1 and 7-3) )

  The co-extruded structure appears to be somewhat susceptible to process conditions, but still provides excellent calorimetry and pizza browning results when high stretching and annealing temperatures are employed. is there. In fact, the best pizza browning was achieved in this set of samples by coextruded sample 7-26. Without wishing to be bound by theory, the presence of both homopolymer PET and copolyester layers in the structure provides some “mixed mode” of orientation, crystallization, and relaxation, A somewhat narrow window of process operating conditions is expected. However, since it has been shown that this film can be used to create superior heated susceptors, they can be reduced in cost compared to 100% copolyester films and are commercially useful, Other useful functionalities are provided, such as when a surface copolyester layer is used as a heat seal material.

  FIG. 5 plots the relationship between the relative browning rate and the number of pixels based on calorimetry obtained in an actual pizza cooking test. These data confirm that there is a clear positive correlation between calorimetric results and actual food browning results within the constraints of pizza variability and dynamic cooking conditions.

To demonstrate that the dynamic dimensional temperature response of the disclosed film is significantly different compared to standard susceptor films of the prior art, Perkin-Elmer (Perkin-Elmer Inc., Waltham, Mass.) DMA 7e. Was used to measure the change in sample length as a function of temperature. The instrument was used in thermomechanical analysis mode with constant force. The sample was heated from 40 ° C. to 220 ° C. at 2.5 ° C./min under a helium purge under a constant static force of 10 mN. The sample was cut into a width of 3.2 mm, the gauge length was about 10 mm with a thickness corresponding to the film to be measured, and an elongation analysis measurement system was used. An ice / water bath was used to assist with furnace temperature control. Measurements were made on film strips cut in the machine direction (MD) or transverse (transverse) direction (CD). Examples of dimensional changes recorded while heating a film sample are shown in FIGS. Values were calculated as percent change compared to the original sample length before heating as follows.
% Change in sample length = instant length during heating / original length × (100)

  The characteristics of this test method that affect the displayed results are important for the correct interpretation of the data. In order to ensure accuracy in the instantaneous length measurement used to calculate the rate of change of the sample length, very little tension is applied to both ends of the sample. By overcoming this slight tension, the sample exhibits a rate of change value (shrinkage) of less than 100 and shortens as the temperature reaches the temperature at which the remaining shrinkage energy is released (shrinkage). Samples that exhibit elongation (values greater than 100%) in the curve obtained by this test should not be considered to have intrinsic elongation or elongation properties with increasing temperature. Films above Tg that have little or no dimensional change tendency at a particular temperature tend to be affected by slight stretching due to sample holding tension at that temperature, and the curve is subject to conditions under no tension loading. Will be misinterpreted as stretching or stretching occurs. Highly oriented and / or highly crystalline films are expected to have less resistance to residual stretch in the rubbery state and more resistant to the appearance of stretch and misstretching than films with lower levels of orientation and crystallinity .

  Films with dynamic dimensional temperature response curves are listed in Table 10. Several embodiments of the present disclosure, which represent three types of films representing standard PET homopolymer films of conventional high orientation, high crystallinity, high refractive index and high birefringence, all exhibiting excellent susceptor performance Are compared with three types of films.

  Samples 6-2, 1-6, and 7-1 are all common commercial susceptor base films (Sample 6-2), prior art heat stabilization films (1-6), and Sample 6-2 It is a homopolymer PET highly oriented film representing a film (sample 7-1) stretched in the laboratory for the purpose of simulating similar film properties and performance. Samples 6-9, 6-11, and 7-8 are all samples oriented with the experimental stretching machine of the present disclosure. Samples 6-9 and 6-11 are moderately uniaxially oriented films of the present disclosure made from homopolymer PET, and Sample 6-11 was exposed to a higher heat setting temperature than Sample 6-9. Samples 7-8 are highly oriented films of the present disclosure made from a copolyester having a lower melting point than typical homopolymer PET.

  Figures 6-11 show the temperature response of the dynamic dimensions of these films. Each figure shows two measurements on the MD and CD of each film. It is first noticed that in all six samples, a response begins to increase in temperature at about 80 ° C., which is close to the Tg of PET. This is to be expected as a polymer transition from the glassy state to the rubbery state. In particular, as the temperature of Samples 6-2, 1-6, and 7-1 continued to rise, significant shrinkage occurred in MD for all samples and in CD for samples other than Sample 1-6, and particularly good susceptor performance. At temperatures above 150 ° C., the static exposure temperature for shrinkage measurements that are typically referred to as representing critical judgment material, shrinkage continues to increase with increasing temperature.

  The dynamic dimensional temperature response of these films represents shrinkage behavior resulting in typical crazing and heating capability limitations in laminated susceptor structures. Calorimetric data for these three samples shows this limit in heating. Samples 1-6 appear to stretch slightly to CD, possibly as a result of the somewhat enhanced thermal stability in this direction compared to MD, and the misextension indications discussed above. MD shrinks significantly at the desired temperature commonly encountered in susceptor heating. Sample 1-6 exhibits a somewhat increased heating capacity compared to Samples 6-2 and 7-1, but is still inferior to susceptors made in accordance with the present disclosure. The large shrinkage seen in Sample 7-1 compared to Sample 6-2 is expected to be due to the difference between the commercial machine and the experimental stretcher, but meaningfully, the two films Susceptors made from the above function in exactly the same way in heating and cooking tests. This confirms that if the crazing threshold is exceeded, the susceptor performance is lost and becomes virtually unrecoverable.

  Samples 6-9, 6-11, and 7-8 are all experimental films of the present disclosure that have better heating capabilities than the three standard films, but have completely different responses to temperature increases. Demonstrating that standard films do not shrink in either MD or CD over the same temperature range in which they shrink. Similarly, all of the superior performance susceptors of this disclosure tested according to this method (including, for example, Samples 5-6, 5-7, 5-8, 5-10, 5-12, and 5-14) Similar behavior was exhibited. Even taking into account false stretch indications for the use of susceptors, these films are considered dimensionally stable and have excellent resistance to crazing. Calorimetry and browning tests confirm this excellent performance.

  The large difference in dynamic dimensional temperature response is consistent with the low residual orientation maintained as evidenced by the low level of refractive index and birefringence and low crystallinity in the susceptor base film. They demonstrate the novelty of the various aspects of the invention and strong evidence that the invention can be implemented in a very useful manner.

  Although the invention herein has been described in detail in connection with specific aspects and embodiments, the detailed description is merely illustrative and exemplary of the invention and provides a thorough and possible disclosure of the invention. However, it should be understood that the present invention is performed only for the purpose of explaining the best mode for carrying out the present invention, which is known to the inventors at the time of the present invention. The detailed description set forth herein is for purposes of illustration only and excludes any limitation of the invention or any other embodiment, adaptation, variation, modification, and equivalent arrangement of the invention. It is not intended and should not be interpreted as such. All references to direction (eg, top, bottom, top, bottom, left, right, left, right, top, bottom, top, bottom, vertical, horizontal, clockwise, and counterclockwise) It is used for identification purposes only to aid the reader's understanding of various embodiments of the invention, and is not intended to limit the position, orientation or use of the invention, unless specifically stated in the claims. Absent. Merged references (eg merged, attached, linked, combined, etc.) are to be interpreted broadly and may include intermediate members between element combinations and relative movement between elements. Thus, a merged reference does not necessarily mean that two elements are joined directly and in a fixed relationship to each other. Further, the various elements discussed in relation to the various embodiments can be interchanged to create entirely new embodiments that fall within the scope of the present invention.

Claims (22)

A polymer film having a crystallinity of about 1% to about 50% before being exposed to microwave energy, wherein the crystallinity of the polymer film is determined by density measurement;
A layer of microwave energy interactive material on the polymer film, the layer comprising a layer of microwave energy interactive material effective to convert at least a portion of incident microwave energy into thermal energy. Wave energy interaction structure.   The microwave energy interactive structure of claim 1, wherein the polymer film has a crystallinity of about 1% to about 25% before being exposed to microwave energy.   The microwave energy interactive structure of claim 1, wherein the polymer film has a crystallinity of about 1% to about 10% before being exposed to microwave energy.   The microwave energy interactive structure of claim 1, wherein the polymer film has a crystallinity of about 1% to about 7% before being exposed to microwave energy.   The microwave energy interactive structure of claim 1, wherein the polymer film has a crystallinity of about 4% before being exposed to microwave energy.   The microwave energy interactive structure of claim 1, wherein the polymer film comprises polyethylene terephthalate.   The microwave energy interactive structure of claim 1, wherein the polymer film comprises nylon.   The microwave energy interactive structure of claim 1, wherein the polymer film is non-oriented.   The microwave energy interactive structure of claim 1, wherein the polymer film is substantially unoriented.   The microwave energy interactive structure according to claim 1, further comprising an additive for improving the strength of the polymer film.   The microwave energy interactive structure of claim 10, wherein the additive is present in an amount up to about 10% by weight of the polymer film.   The microwave energy interactive structure of claim 10, wherein the additive is present in an amount up to about 5% by weight of the polymer film.   The microwave energy interactive structure of claim 10, wherein the additive comprises at least one of an ethylene methyl acrylate copolymer and an ethylene-octene copolymer.   The microwave energy interactive structure of claim 1, wherein the polymer film is a multilayer polymer film. The multilayer film is
A layer comprising polyethylene terephthalate;
15. The microwave energy interactive structure of claim 14, comprising at least one of a nylon layer, a nylon 6,6 layer, an olefin layer, and an ethylene vinyl alcohol layer. The multilayer film is
A layer comprising at least one of nylon and nylon 6,6;
The microwave energy interactive structure of claim 14, comprising at least one of a layer of polyethylene terephthalate, a layer of olefin, and a layer of ethylene vinyl alcohol.   17. A support layer bonded to the layer of microwave energy interactive material, further comprising a layer of the microwave energy interactive material disposed between the polymer film and the support layer. The microwave energy interaction structure according to any one of the above.   The microwave energy interactive structure of claim 17, wherein the support layer comprises paper, paperboard, or any combination thereof.   The microwave energy interactive structure of claim 17, wherein the support layer is bonded to the layer of microwave energy interactive material with an adhesive.   The microwave energy interactive structure of claim 1, comprising at least a portion of a microwave heating structure for heating, browning, and / or improving crispy food in a microwave oven. A polymer film having a crystallinity of about 1% to about 50% before being exposed to microwave energy;
A layer of microwave energy interactive material on the polymer film, the microwave energy interactive material acting to make the microwave energy interactive structure effective to absorb a portion of the microwave energy. A microwave energy interactive structure comprising a layer,
A microwave energy interactive structure that is effective to absorb at least 40% of incident microwave energy after 60 seconds of exposure to microwave energy. A polymer film having a crystallinity of about 1% to about 50% before being exposed to microwave energy;
A microwave energy interactive material layer on the polymer film, wherein the microwave energy interactive material layer is effective to convert at least a portion of the incident microwave energy into thermal energy A microwave energy interactive structure comprising a layer of interactive material comprising:
A microwave energy interaction structure, wherein the microwave energy interaction structure exhibits a ratio of crazing perimeter length to region area of less than 1 after exposing the microwave energy interaction structure to microwave energy for 60 seconds.

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