WO2014142887A1 - Low crystallinity susceptor films - Google Patents
Low crystallinity susceptor films Download PDFInfo
- Publication number
- WO2014142887A1 WO2014142887A1 PCT/US2013/031420 US2013031420W WO2014142887A1 WO 2014142887 A1 WO2014142887 A1 WO 2014142887A1 US 2013031420 W US2013031420 W US 2013031420W WO 2014142887 A1 WO2014142887 A1 WO 2014142887A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- microwave energy
- susceptor
- films
- energy interactive
- polymer film
- Prior art date
Links
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- FDSYTWVNUJTPMA-UHFFFAOYSA-N 2-[3,9-bis(carboxymethyl)-3,6,9,15-tetrazabicyclo[9.3.1]pentadeca-1(15),11,13-trien-6-yl]acetic acid Chemical compound C1N(CC(O)=O)CCN(CC(=O)O)CCN(CC(O)=O)CC2=CC=CC1=N2 FDSYTWVNUJTPMA-UHFFFAOYSA-N 0.000 description 1
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- OKTJSMMVPCPJKN-UHFFFAOYSA-N Carbon Chemical compound [C] OKTJSMMVPCPJKN-UHFFFAOYSA-N 0.000 description 1
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- 241001315609 Pittosporum crassifolium Species 0.000 description 1
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- KFGCQOLKVDHXJP-UHFFFAOYSA-N benzene-1,3-dicarboxylic acid;2-(2-hydroxyethoxy)ethanol Chemical compound OCCOCCO.OC(=O)C1=CC=CC(C(O)=O)=C1 KFGCQOLKVDHXJP-UHFFFAOYSA-N 0.000 description 1
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- VNGOYPQMJFJDLV-UHFFFAOYSA-N dimethyl benzene-1,3-dicarboxylate Chemical compound COC(=O)C1=CC=CC(C(=O)OC)=C1 VNGOYPQMJFJDLV-UHFFFAOYSA-N 0.000 description 1
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- AMGQUBHHOARCQH-UHFFFAOYSA-N indium;oxotin Chemical group [In].[Sn]=O AMGQUBHHOARCQH-UHFFFAOYSA-N 0.000 description 1
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- OJURWUUOVGOHJZ-UHFFFAOYSA-N methyl 2-[(2-acetyloxyphenyl)methyl-[2-[(2-acetyloxyphenyl)methyl-(2-methoxy-2-oxoethyl)amino]ethyl]amino]acetate Chemical compound C=1C=CC=C(OC(C)=O)C=1CN(CC(=O)OC)CCN(CC(=O)OC)CC1=CC=CC=C1OC(C)=O OJURWUUOVGOHJZ-UHFFFAOYSA-N 0.000 description 1
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- GUCVJGMIXFAOAE-UHFFFAOYSA-N niobium atom Chemical compound [Nb] GUCVJGMIXFAOAE-UHFFFAOYSA-N 0.000 description 1
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- KKEYFWRCBNTPAC-UHFFFAOYSA-L terephthalate(2-) Chemical compound [O-]C(=O)C1=CC=C(C([O-])=O)C=C1 KKEYFWRCBNTPAC-UHFFFAOYSA-L 0.000 description 1
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- WFKWXMTUELFFGS-UHFFFAOYSA-N tungsten Chemical compound [W] WFKWXMTUELFFGS-UHFFFAOYSA-N 0.000 description 1
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- 239000010937 tungsten Substances 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/64—Heating using microwaves
- H05B6/647—Aspects related to microwave heating combined with other heating techniques
- H05B6/6491—Aspects related to microwave heating combined with other heating techniques combined with the use of susceptors
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
- B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
- B65D81/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
- B65D81/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within the package
- B65D81/3446—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within the package specially adapted to be heated by microwaves
-
- H—ELECTRICITY
- H05—ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
- H05B—ELECTRIC HEATING; ELECTRIC LIGHT SOURCES NOT OTHERWISE PROVIDED FOR; CIRCUIT ARRANGEMENTS FOR ELECTRIC LIGHT SOURCES, IN GENERAL
- H05B6/00—Heating by electric, magnetic or electromagnetic fields
- H05B6/64—Heating using microwaves
- H05B6/6408—Supports or covers specially adapted for use in microwave heating apparatus
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
- B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
- B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
- B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
- B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
- B65D2581/3471—Microwave reactive substances present in the packaging material
- B65D2581/3472—Aluminium or compounds thereof
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B65—CONVEYING; PACKING; STORING; HANDLING THIN OR FILAMENTARY MATERIAL
- B65D—CONTAINERS FOR STORAGE OR TRANSPORT OF ARTICLES OR MATERIALS, e.g. BAGS, BARRELS, BOTTLES, BOXES, CANS, CARTONS, CRATES, DRUMS, JARS, TANKS, HOPPERS, FORWARDING CONTAINERS; ACCESSORIES, CLOSURES, OR FITTINGS THEREFOR; PACKAGING ELEMENTS; PACKAGES
- B65D2581/00—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents
- B65D2581/34—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within
- B65D2581/3437—Containers, packaging elements, or packages, for contents presenting particular transport or storage problems, or adapted to be used for non-packaging purposes after removal of contents for packaging foodstuffs or other articles intended to be cooked or heated within specially adapted to be heated by microwaves
- B65D2581/3486—Dielectric characteristics of microwave reactive packaging
- B65D2581/3494—Microwave susceptor
Definitions
- a susceptor is a thin layer of microwave energy interactive material that tends to absorb at least a portion of impinging microwave energy and convert it to thermal energy (i.e., heat) through resistive losses in the layer of microwave energy interactive material. The remainder of the microwave energy is either reflected by or transmitted through the susceptor.
- Typical susceptors comprise aluminum, generally less than about 500 angstroms in thickness, for example, from about 60 to about 100 angstroms in thickness, and having an optical density of from about 0.15 to about 0.35, for example, about 0.17 to about 0.28.
- the layer of microwave energy interactive material (i.e., susceptor) 102 is typically supported on a polymer film 104 to define a susceptor film 106.
- the polymer film comprises biaxially oriented, heat set polyethylene terephthalate, but other films may be suitable.
- the susceptor film is typically joined (e.g., laminated) to a support layer 108, for example, paper or paperboard, using an adhesive or otherwise, to impart dimensional stability to the susceptor film and to protect the layer of metal from being damaged.
- the resulting structure 110 may be referred to as a ''susceptor structure".
- the first commercial microwave susceptor films, and subsequently introduced and commercially used susceptor packages have relied upon the use of highly oriented, highly crystallized, biaxially oriented and heat set films produced from polyethylene terephthalate polymer, or PET.
- PET will refer to such biaxially oriented films, unless specified otherwise.
- films are highly oriented, that is, the degree of stretch during the orienting process is from about 3.5:1 to about 4: 1 in the machine direction (MD) and from about 3.5: 1 to about 4: 1 in the cross-machine direction (CD).
- Biaxially oriented PET films made from this polymer are commonly used in a wide variety of packaging and non-packaging uses where combinations of some or all of clarity, gloss, smoothness, a good combination of moisture vapor and oxygen barrier, good mechanical strength and modest dimensional heat stability are useful.
- Commercially available films that are used in standard susceptor structures typically comprise films of this general description, whose properties have generally been optimized for high volume applications other than for use in microwave susceptors.
- microwave susceptor packages There are several key elements of this standard construction that may limit the heating performance of these films, and hence of microwave susceptor packages, packaging components or composite susceptor and field modification or shielding packages or components.
- microwave susceptor packaging of the phenomenon of self-limiting heating, the visual evidence of which is commonly referred to as crazing.
- crazing During heating induced in the susceptor layer itself resulting from interaction of the susceptor material with either or both the electrical or magnetic components of the electromagnetic microwave energy, the temperature of the susceptor substrate film is raised.
- the network of intersecting lines subdivides the plane of the susceptor into progressively smaller conductive islands.
- the overall reflectance of the susceptor decreases the overall transmission of the susceptor increases, and the amount of energy converted by the susceptor into sensible heat decreases.
- this self-limiting behavior occurs prematurely (i.e., too early in the heating cycle), the susceptor may not be able to generate the necessary amount of heat for a particular food heating application.
- this self-limiting behavior may be advantageous where runaway (i.e., uncontrolled) heating of the susceptor might otherwise cause excessive charring or scorching of the adjacent food item and/or any supporting structures or substrates, for example, paper or paperboard.
- runaway i.e., uncontrolled
- the temperature at which crazing occurs can only be slightly controlled, for example, by modifying the thickness of the metal layer, the type and amount of adhesive, and the uniformity of the adhesive application. Delaying the onset of crazing in susceptor structures has been the subject of significant efforts, but little or no meaningful improvements have been achieved using traditional thinking. However, significant and previously unanticipated changes to the base polymeric materials used and/or the way they are processed into films are shown in this invention to result in higher temperature or delayed onset of crazing and controllably higher microwave heating potential.
- This disclosure is directed generally to a polymer film (or film) for use as a base film or substrate in a susceptor film, a method of making such a polymer film, and a susceptor film including the polymer film.
- the susceptor film may be joined to a support layer to form a susceptor structure.
- the susceptor film and/or susceptor structure may be used to form countless microwave energy interactive structures, microwave heating packages, or other microwave energy interactive constructs.
- 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 present susceptor structure may provide a noticeable improvement in browning and/or crisping of a food item heated using the susceptor structure.
- the polymer film may be unoriented or oriented to varying degrees. Where the film is oriented, the orientation and heat setting conditions may be customized to provide a desired level of crystallinity, residual orientation, and therefore, desired heating performance for a particular susceptor film application.
- the film may be characterized as having one or more of the following: a refractive index (n z ) of less than about 1 .64; a birefringence (n z -n x ) of less than about 0.15; and a crystallinity of less than about 50% (where all crystallinity values herein are derived from density measurements, as described below).
- a refractive index (n z ) of less than about 1 .64
- n z -n x a birefringence
- crystallinity less than about 50% (where all crystallinity values herein are derived from density measurements, as described below).
- biaxially oriented homopolymer films typically used in susceptors may have a refractive index (n z ) of from about 1.6447 to about 1.6639, and a birefringence (n z -n x ) of from about 0.1500 to about 0.1700.
- the polymer film may comprise amorphous polyethylene terephthalate (APET), amorphous nylon, various copolyesters, or any combination thereof.
- APET amorphous polyethylene terephthalate
- amorphous nylon amorphous nylon
- various copolyesters or any combination thereof.
- one or more additives may be incorporated into the polymer film to enhance the strength and/or processability of the polymer film.
- the strength and/or processability of the polymer film may be enhanced by using a multilayer polymer film, where one or more of such layers provide the desired level of robustness for the polymer film.
- the multilayer film may feature enhanced tear strength, toughness, and improved dimensional tolerance so that the film may be processed (e.g., metallized, chemically etched, laminated, and/or printed) and converted into various susceptor structures and/or packages using high speed converting operations.
- the multilayer film may have barrier characteristics that may render the polymer film suitable for numerous applications, for example, for packages for refrigerated microwavable food items that require an extended shelf life.
- FIG. 1 is a schematic cross-sectional view of an exemplary microwave energy interactive structure
- FIG. 2 presents the refractive index (n z ) and rise in temperature ( ⁇ ) in °C for various exemplary susceptor base films and susceptor structures;
- FIG. 3 presents the relative browning reaction rate and pizza browning pixel count for various exemplary susceptor structures:
- FIG. 4 presents the refractive index (n 2 ) and rise in temperature ( ⁇ ) in °C for various exemplary susceptor base films and susceptor structures;
- FIG. 5 presents the relative browning reaction rate and pizza browning pixel count for various exemplary susceptor structures
- FIG. 6 presents the dynamic dimensional temperature response for susceptor base film 6-
- FIG. 7 presents the dynamic dimensional temperature response for susceptor base film 1 - 6;
- FIG. 8 presents the dynamic dimensional temperature response for susceptor base film 7- i ;
- FIG. 9 presents the dynamic dimensional temperature response for susceptor base film 6-
- FIG. 10 presents the dynamic dimensional temperature response for susceptor base film
- FIG. 11 presents the dynamic dimensional temperature response for susceptor base film
- This disclosure is directed generally to polymer films (or films) for use as a base film or substrate in susceptor films, a method of making such polymer films, and susceptor films and structures including the polymer film.
- the present inventors have determined that low crystallinity films made in a variety of ways may be superior base films when compared to their highly crystalline counterparts. Specifically, it has been discovered that lower crystallinity and lower residua! orientation levels generally correspond to higher heating capability in the resulting susceptor film and/or susceptor structure, even where the base film has a high absolute orientation level similar to a conventional base film. This presents a significant departure from the conventional use of highly oriented, highly crystalline susceptor films. While some attempts to understand the self-limiting behavior of susceptors have been made, it is believed that the relationship between the crystallinity and dynamic dimensional temperature response characteristics of oriented films used for microwave susceptor films and the resulting susceptor performance has generally not been explored or appreciated by others.
- the crystallinit values disclosed herein are derived from density measurements unless otherwise noted.
- Some methods e.g., density or differential scanning calorimetry (DSC)
- DSC differential scanning calorimetry
- Some methods are considered to be indirect methods because they involve the use of physical constants in calculations to arrive at a crystallinity value
- other methods e.g., x-ray diffraction or similar techniques
- DSC differential scanning calorimetry
- Equation ( 1 ) the percent crystallinity values presented herein were calculated using Equation ( 1 ), in which p s is the measured density of the sample, p a is the density of 0% crystalline material, and p c is the density of 100% crystalline material:
- the refractive index and/or the birefringence of the polymer film may be more indicative of performance than the criteria set forth in the prior art.
- the refractive index of a material is the ratio of the velocity of light in a vacuum to the velocity of light in that material. By polarizing light in a particular direction of a material, one can measure the refractive index in that direction. For amorphous materials with no molecular ordering, a single value can be used to optically define that material.
- the absolute value of refractive index increases with increasing crystallinity and orientation, and the different values of refractive indexes that will be measured in different directions may be used to characterize anisotropy of a structure such as a film.
- the difference between refractive indexes in two directions of a polymer film is defined as the birefringence between those two directions and can be used to understand differences in orientation between these directions.
- birefringence may be a positive or negative number depending the morphology of a particular sample and the directions chosen for the refractive indexes; higher absolute values of birefringence are widely acknowledged to be associated with greater differences in orientation in those two directions.
- Some polymer films useful for forming susceptor structures according to the disclosure may generally have a refractive index (n z ) (where n z refers to the refractive index in the machine direction of the film) of less than about 1.64, for example, from about 1.57 to about 1.62.
- the polymer film may have a refractive index (n 2 ) of 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.
- the polymer films may generally have a birefringence (n z -n x ) (where n x refers to the refractive index in the thickness of the film) of less than about 0.15 (including negative birefringence values), hi each of various independent examples, the birefringence n z -n x ) may be less than about 0.14, less than about 0.13, less than about 0.12, less than about 0.1 1 , less than about 0.10.
- prior art films have typically been characterized based on their static shrink properties (e.g., ' 1 % shrink at 1 0°C for 30 min' ).
- this traditional performance definition may be inadequate and misleading, and that specifying and producing susceptor substrate films using this definition as a primary criteria for selecting materials and processes has possibly limited the development of superior performing microwave susceptor substrate films, susceptor components, susceptor packages and susceptor package/field modification or shielding packages and components.
- the present inventors have discovered that to realistically characterize the microwave heating behavior of susceptor structures, one must understand the dimensional response of susceptor substrate films to dynamic temperature exposure, which is more representative of the actual conditions experienced during microwave heating using susceptor structures. This understanding has been discovered to be particularly useful for designing superior susceptor substrate films and useful microwave heating structures for food applications or other industrial uses.
- the inventors have measured the dynamic dimensional temperature response of example susceptor structures produced from 'heat stabilized PET' claimed to offer improved susceptor performance and show it to be inferior in stability at useful microwave heating conditions compared to those of the present disclosure.
- the typical method of characterizing heating potential by long term exposure to a temperature that is at best at the low end of temperatures of interest for microwave susceptor heating is inconsistent with developing an understanding of the actual heating performance achieved during the actual event of interest, the heating of microwave foods in microwave ovens, which is a dynamic process in which it is very often desired that susceptor package components reach temperatures significantly in excess of the typical 150°C static test condition.
- susceptor structures formed from the base films 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, for example, such that a discernible difference in browning and/or crisping of food items may be observed.
- the susceptor film may comprise a minimally oriented film.
- Minimally oriented films may be unoriented (i.e., non-oriented) or slightly oriented (i.e., from greater than 0% to about 20% orientation, as will be discussed below).
- Unoriented polymer films are films that are not subjected to stretching in either or both the machine direction (MD) and cross directions (CD) at temperatures below the melting point of the polymer. Unoriented polymer films can be quenched rapidly, which results in a low crystallinity, for example, less than about 25%, which may generally be attributed to the residual melt orientation associated with drawing down the melt to the desired final film thickness.
- minimally oriented films according to the present disclosure may be characterized as having one or more of the following:
- n 2 a refractive index (n 2 ) of less than about 1.59, for example, from about 1.57 to about 1.58, or from about 1.5727 to about 1.58, for example, about 1.575 (see, e.g., Polymer Handbook, J. Brandrup, E. H. Immergut, and E. A. Gru!ke, 4th ed., John Wiley & Sons, Inc., 1999, ISBN 0- 471 -16628-6), or from about 1.5723 to about 1 .5727, for example, about 1.5725;
- the film may have a crystallinity of about 4%.
- other crystallinities, refractive indexes, birefringences, and ranges of each may be suitable.
- the film may comprise amorphous PET (APET), for example, APET film commercially available from Pure-Stat Technologies, Inc. (Lewiston. Maine).
- APET amorphous PET
- other suitable APET films and/or other polymer films may be used.
- susceptor films including lower orientation levels may tend to resist crazing to a greater extent than conventional, highly oriented, highly crystalline biaxially oriented PET films. While not wishing to be bound by theory, it is believed that high residual shrink forces may have a significant role in the onset and propagation of crazing of susceptor structures. Since less oriented films exhibit much lower heat induced dimensional shrinkage forces than highly oriented films, minimally oriented films may tend to resist crazing more than highly oriented polymer films.
- a minimally oriented film with inherently low, very low, or even no slirinkage forces may tend to resist crazing to a greater extent than a conventional, highly oriented film with inherently high shrink forces, for example, a highly oriented PET. This is believed to be a clear departure from the conventional approach to designing susceptor films as taught in the art.
- the crystallinity of the minimally oriented polymer film may increase during the heating cycle, thereby rendering the polymer more resistant to heat, and therefore, more heat stable. As a result, the stability of the susceptor film may increase during the heating cycle.
- the susceptor film may comprise a moderately oriented polymer film, that is, a polymer film that has been subject to an orientation process by which at least one dimension of the film is increased from 20% to about 200%, for example, from about 20% to about 150%, for example, from about 30% to about 70%, for example, about 50%.
- a moderately oriented polymer film that is, a polymer film that has been subject to an orientation process by which at least one dimension of the film is increased from 20% to about 200%, for example, from about 20% to about 150%, for example, from about 30% to about 70%, for example, about 50%.
- Such an orientation process may be useful where the unoriented film lacks sufficient strength for
- moderately oriented films according to the present disclosure may be characterized as having one or more of the following:
- n z a refractive index (n z ) of less than about 1.62, less than about 1.61. less than about 1.60, or less than about 1.59, for example, from about 1.57 to about 1.59, for example, from about 1.5733 to about 1.5848, for example, about 1.5791 ;
- n z -n x a birefringence (n z -n x ) of less than about 0.05, less than about 0.035, less than about 0.01 , or less than about 0.0024, for example, from about -0.013 to about 0.0024, for example, about -0.0029;
- the crystallinity of the film may be about 4%. However, other crystallinities. refractive indexes, birefringences, and ranges of each may be suitable.
- the orientation may be biaxial or bidirectional (i.e., in both the machine direction (MD) and cross-machine direction (CD)), or may be uniaxial or unidirectional (i.e., in either the MD or CD).
- the present inventors have determined that under certain process conditions, strength may be imparted to the film without driving the level of crystallinity to the levels observed in conventional biaxially oriented PET.
- the conditions may be selected to minimize the strain induced crystallization and thermal induced crystallization commonly associated with typical orienting processes.
- the orientation process conditions may ⁇ be customized to provide a desired level of crystallinity, and therefore, desired heating performance for a particular susceptor film application.
- films that were stretched at temperatures of at least about 105°C and annealed at a temperature of at least about 170°C achieved a significant improvement in strength without sacrificing the heating performance of an unoriented film, as will be discussed further below in connection with the examples.
- these temperatures are exemplary only and were obtained on laboratory scale equipment (see the Examples), and such temperatures may or may not apply on commercial scale equipment for a given polymer system.
- the present inventors have also discovered that if the polymer is chosen properly, higher levels of orientation may still result in relatively low crystallinity, and therefore, may produce susceptor films having a higher heating potential than conventional films.
- copolyesters having a melting point at or below that of standard PET homopolymer can produce susceptor films that have excellent heating characteristics. While not wishing to be bound by theory, it is believed that steric hindrance considerations of copolyesters of interest in this disclosure retard crystallinity development and even highly oriented films made from these materials are typically incapable of reaching absolute crystallinity levels that are commonly achieved with PET homopolymers.
- films produced from copolyesters of the type disclosed herein, whether unoriented. slightly oriented, moderately oriented, or highly oriented have lower refractive indexes and birefringence values than typical in biaxiallv oriented standard and heat stabilized standard PET films.
- highly oriented films according to the present disclosure may be characterized as having one or more of the following:
- n z a refractive index (n z ) of less than about 1.64, for example, from 1.56 to about 1.63, for example, from about 1.58 to about 1.61 , for example, from about 1.5769 to about 1.6124, for example, about 1.5920;
- n z -n x a birefringence (n z -n x ) of less than about 0.15, less than about 0.14, less than about 0.125, or less than about 0.1 1 , for example, from about 0.0030 to about 0.10, for example, from about 0.0029 to about 0.1022, for example, about 0.0437; and
- a crystallinity of greater than 0% for example, 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%, less than about 17%, less than about 15%, less than about 12%, or less than about 10%.
- the crystallinity may be about 7%.
- other crystal Unities, refractive indexes, birefringences, and ranges of each may be suitable.
- the copolyester may generally have a melting point of less than about 260°C, for example, from about 200°C to about 260°C, for example, from about 220°C to about 260°C. In some examples, the copolyester may have a melting point of less than about 250°C, for example, from about 200°C to about 250°C for example, from about 220°C to about 250°C.
- the melting point may be similar to or lower than the melting point of standard PET polymers, which have peak crystalline melt points when determined by second DSC heating in the range of 250°C to 260°C, although some references report PET homopolymer melt points as high as 265°C (see “Polymer Chemistry, An Introduction 3 rd Edition” by Malcolm Stevens published 1999 by Oxford University Press, p. 344).
- the copolyester also may generally be resistant to the development of the very high (>50%) levels of crystallinity commonly associated with standard biaxially oriented PET or heat stabilized PET films, hi some examples, the base film may have a crystallinity of less than about 37%, despite having undergone high degrees of biaxial orientation (similar to stretching ratios common for biaxially oriented standard or heat stabilized PET polymer films). It is anticipated that similarly low or even lower crystallinity values would be observed for unoriented, slightly oriented, or moderately oriented films produced from copo!yesters.
- SKYPET-BR SK Chemicals of Seoul. Korea
- SKYPET-BR has a melting point of 236+/- 2°C, well below the typical melting point of standard PET polymer.
- An alternative polymer with similar properties is Eastman PET 9921 produced by Eastman Chemical Company. Kingsport, TN.
- CHDM 1 ,4-cyclohexanedimethanol
- CHDM 1 ,4-cyclohexanedimethanol
- polyethylene terephthalate copolyesters diethylene glycol-isophthalate modified
- diethylene glycol-isophthalate modified diethylene glycol-isophthalate modified
- the resulting polymers may vary in terms of modification or co-polymerization levels, y ielding different properties, which can be exploited in the context of modifying performance of substrate susceptor films produced from these polymers.
- the present inventors have observed that higher orientation temperatures (all else being equal) may lead to higher heating capability. While not wishing to be bound by theory, it is believed that this is the result of higher polymer chain mobility at higher temperatures and resulting greater ease of the chains slipping past one another during orientation. This reduces the stress required to achieve a given degree of stretching, and can act to reduce strain induced crystallinity, the minimization of which is believed to be advantageous.
- Two rotational isomers of polyester exist, gauche and trans.
- Gauche isomers can be generally characterized as in a relaxed state, while trans isomers are in an extended, higher energy state.
- the gauche isomer is only found in the amorphous domains or regions of the structure, while trans can exist in both crystalline and amorphous regions. Only trans exists in crystalline regions.
- copolyesters described above may likewise be used to produce unoriented films, slightly oriented films, and moderately oriented films.
- the copolyester can be used either in films of homogenous structure made solely of the copolymers or in coextrusions combining discrete layers of copolymer and homopolymer.
- Blends of co- and homopolymer polyester may also be used advantageously compared to 100% homopolymer structures. Incorporation of copolyester. even with homopolymer present, serves to result in films with properties as susceptor base films superior to 100% homopolymer comprised base films.
- copolyester is used in a moderately oriented film, alone or in combination with one or more other polymers, a greater degree of orientation may be used without driving the level of crystallinity above 50%, as compared with susceptor base films comprising only PET homopolymer.
- films including copolyester and PET homopolymer may be characterized as having one or more of the following:
- n z a refractive index (n z ) of less than about 1.64, for example, from about 1.60 to about 1 .63 or from about 1.5975 to about 1.6280, for example, about 1.6123;
- crystallinities, refractive indexes, birefringences, and ranges of each may be suitable. It is also contemplated that other polymers may be used alone or any combination to form various susceptor base films having the properties described herein in connection with the present base films.
- the kinetics of crystallization of the polymer film may be manipulated to achieve the desired level of crystallinity at various points in the heating cycle, with time, temperature, and the use of nucleating agents being variables that may be adjusted as needed to attain the desired susceptor film performance.
- time, temperature, and the use of nucleating agents being variables that may be adjusted as needed to attain the desired susceptor film performance.
- the susceptor film may be intended to be used more than once. In such instances, the crystallinity of the polymer film may be higher upon the second use and any subsequent use.
- the polymer film may be formed in any suitable manner.
- the polymer film substrate may be a water quenched film, a cast film, or any other type of polymer film that is fonned using a rapid quenching process.
- numerous other processes and systems may be used. When such films do not undergo a conventional post-extrusion orientation process, it will be appreciated that, in some instances, the film may be difficult to handle and/or convert into a susceptor structure.
- the film may be subject to a minimal orienting process to orient (i.e., stretch) the film slightly (e.g., up to 20%, for example, from about 5% to 20%) to improve processability of the film. Since such orienting is relatively minor as compared with standard highly oriented films that are stretched about 350-450% in each direction, such slightly oriented films may be considered herein to be substantially unoriented.
- the crystallinity of minimally oriented films can be controllably increased through post-extrusion heat treatment or conditioning.
- Crystallization kinetic modifying additives may also be used, as described above.
- additives may be incorporated into the film to modify its properties to facilitate processing or to provide more robust microwave heating performance.
- a strength enhancing additive e.g., a polymer
- additives may be suitable include an ethylene methyl aery late copolymer, an ethylene-octene copolymer, or any other suitable polymer or material that improves the strength and/or processabiliry of the polymer film.
- Other additives providing different functions or benefits may also be used.
- additives may be added in any suitable amount, for example, up to about 1 5% 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 in any other suitable amount.
- the additives may be used in an amount of from about 1 % to about 10%, from about 2% to about 8%, or from 3% to about 5% by weight of the polymer film, or in any suitable amount or range of amounts.
- any of the susceptor base films may comprise a multilayer film including at least two distinct layers, each of which may comprise one or more polymers and, optionally, one or more additives.
- the layers may be coextruded or may be formed separately and joined to one another using an adhesive, a tie layer, thermal bonding, or using any other suitable technique.
- Other suitable techniques may include extrusion coating and coextrusion coating.
- each layer of the multilayer film may be a rapidly quenched film, i.e., a film formed under conditions that provide very fast freezing of the polymer melt after it has exited the opening of the extrusion die.
- This rapid freezing and further lowering of the temperature of the solidified polymer film minimizes the development of crystalline micro or macro structures.
- the susceptor film is capable of achieving liigher temperatures and heat tlux during microwave heating, as compared with conventional susceptors made from bia ially oriented polyethylene terephthalate.
- ethylene vinyl alcohol may be used to impart oxygen barrier properties.
- Polypropylene may be used to impart water vapor barrier properties. Such properties may render the film useful for controlled or modified atmosphere packaging, and in particular, for chilled or shelf stable foods, where higher oxygen and moisture barriers are typically required than for frozen foods. Numerous other possibilities are contemplated.
- 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/regrind of all layers/tie layer/EVOH/tie layer/APET; (j) APET/tie layer/EVOH/tie layer/amorphous nylon 6 or nylon 6,6; (k) APET/t
- the olefin layer may comprise any suitable polyolefin, for example, low density polyethylene (LDPE), linear low densit polyethylene (LLDPE), medium density polyethylene (MDPE). high density polyethylene (HDPE), polypropylene (PP), copolymers of any of such polymers, and/or metallocene catalyzed versions of these polymers or copolymers.
- LDPE low density polyethylene
- LLDPE linear low densit polyethylene
- MDPE medium density polyethylene
- HDPE high density polyethylene
- PP polypropylene
- copolymers of any of such polymers and/or metallocene catalyzed versions of these polymers or copolymers.
- the regrind layer may include the film edge scrap and any other recyclable material, according to conventional practice. Any of the various other examples (exa-h or /-/) or other films contemplated by this disclosure may contain such a regrind layer. In some cases, regrind layers may require a tie layer to bond them satisfactorily to the adjacent film layers.
- the tie layer may comprise any suitable material that provides the desired level of adhesion between the adjacent layers.
- the tie layer may comprise Bynel® from
- DuPont Bynel 21 E781 is part of the Bynel 2100 Series of anhydride modified ethylene acrylate resins that are most often used to adhere to PET, nylon, EVOH, polyethylene (PE), PP, and ethylene copolymers.
- Plexar® PX 1007 is one of a class of ethylene vinyl acetate copolymers that can be used to bond a similar range of materials as the Bynel resin mentioned previously.
- Exxlor® grades may be used to enhance the impact performance of various nylon polymers.
- the tie layers and other resins may be selected for their prior sanctioned use in high temperature films for applications such as retort pouches, where minimal resin extractabies into food are allowed.
- amorphous nylon 6 or nylon 6,6 could be substituted for APET in any of the above multilayer film structures or any other structure within the scope of the disclosure. Countless other structures are contemplated.
- Numerous techniques may be used to form a multilayer film. While film casting is a commonly used rapid quench film production technique, adaptations of the air-cooled blown film process may also create quench rates suitable for the creation of the multilayer films of this disclosure.
- the use of chilled air applied to the outside of the blown film "bubble" can increase the quench rate compared to the use of room temperature air directed only on the exterior surface of the bubble. Additionally, the use of chilled air exchange for internal bubble cooling can boost output rates.
- TWQ tubular water quench process
- TWQ entails the direct contact of cooling water with the exterior of the polymer bubble, which results in extremely high heat transfer rates and very rapid quenching of the extruded polymer film.
- Some TWQ processes combine direct water contact with the exterior of the bubble with an internal mandrel for support and further cooling.
- Another TWQ process may solely utilize direct water contact on the external surface of the bubble, sometimes supplemented with chilled air exchange in the interior of the bubble. In some circumstances, the latter TWQ process may be more advantageous to use because equipment without internal mandrels is less costly to build and operate and provides more flexibility in film width changes.
- Such TWQ extrusion lines are available, for example, from Brampton Engineering of Canada under the trade name AquaFrost® systems. However, numerous other processes and systems may be used.
- the basis weight and/or caliper of the polymer film may vary for each application.
- the film may be from about 12 to about 50 microns thick, for example, from about 12 to about 35 microns thick, for example, about 12 to 20 microns thick.
- other calipers are contemplated.
- a layer of microwave energy interactive material may be deposited on one or both sides of the polymer film to form a susceptor film.
- the microwave energy interactive material may comprise an electroconductive or semiconductive material, for example, a vacuum deposited metal or metal alloy, or a metallic ink, an organic ink, an inorganic ink, a metallic paste, an organic paste, an inorganic paste, or any combination thereof.
- metals and metal alloys that may be suitable include, but are not limited to. aluminum, chromium, copper, inconel alloys (nickel-cliromium-molybdenum alloy with niobium), iron, magnesium, nickel, stainless steel, tin, titanium, tungsten, and any combination or alloy thereof.
- the microwave energy interactive material may comprise a metal oxide, for example, oxides of aluminum, iron, and tin, optionally used in conjunction with an electrically conductive material.
- a metal oxide for example, oxides of aluminum, iron, and tin
- ITO indium tin oxide
- the microwave energy interactive material may comprise a suitable electroconductive, semiconductive, or non-conductive artificial dielectric or ferroelectric.
- Artificial dielectrics comprise conductive, subdivided material in a polymeric or other suitable matrix or binder, and may include flakes of an electroconductive metal, for example, aluminum.
- the microwave energy interactive material may be carbon-based, for example, as disclosed in U.S. Patent Nos. 4,943,456, 5,002,826, 5, 1 18,747, and 5,410,135.
- the microwave energy interactive material may interact with the magnetic portion of the electromagnetic energy in the microwave oven. Correctly chosen materials of this type can self-limit based on the loss of interaction when the Curie temperature of the material is reached.
- An example of such an interactive coating is described in U.S. Patent No. 4,283,427.
- microwave energy interactive materials capable of being combined with the films of the present invention to create microwave susceptor structures will represent other embodiments of this invention.
- the susceptor film may then be laminated or otherwise joined to another material to produce a susceptor structure or package.
- the susceptor film may be laminated or otherwise joined to paper or paperboard to make a susceptor structure having a higher thermal flux output than conventional paper or paperboard based susceptor structures.
- the paper may have a basis weight of from about 15 to about 60 lb/ream (lb/3000 sq. ft. ), for example, from about 20 to about 40 lb/ream, for example, about 25 lb/ream.
- the paperboard may have a basis weight of from about 60 to about 330 lb/ream, for example, from about 80 to about 140 lb/ream.
- the paperboard generally may have a thickness of from about 6 to about 30 mils, for example, from about 12 to about 28 mils. In one particular example, the paperboard has a thickness of about 14 mils (0.014 inches).
- Any suitable paperboard may be used, for example, a solid bleached sulfate board, for example, Fortress® board, commercially available from International Paper Company, Memphis, TN, or solid unbleached sulfate board, such as SUS® board, commercially available from Graphic Packaging International.
- the susceptor film may be laminated or otherwise joined to another polymer film. It is contemplated that the polymer film would exhibit little or no slirink, similar to its base film counterpart, such that the performance attributes of the susceptor film are not adversely affected. It is also contemplated that such polymer films may be clear, translucent, or opaque, as needed for a particular application. It is further contemplated that the laminated (or otherwise joined) structures may be capable of being thermoformable. It is anticipated that shallow draw- shapes could preserve susceptor functionalit in all but the highest stretch areas during thermoforming, and one could advantageously use die and or plug design to tailor local stretch ratios to customize degree of susceptor functionality.
- the susceptor base film may undergo one or more treatments to modify the surface prior to depositing the microwave energy interactive material onto the polymer film.
- the polymer film may undergo a plasma treatment to modify the roughness of the surface of the polymer film. While not wishing to be bound by theory, it is believed that such surface treatments may provide a more uniform surface for receiving the microwave energy interactive material, which in turn, may increase the heat flux and maximum temperature of the resulting susceptor structure.
- Such treatments are discussed in U.S. Patent Application Publication No. 2010/0213 192A 1 and U.S. Patent Application No. 13/804,673, filed March 14, 2013, both of which are incorporated by reference herein in its entirety.
- the susceptor film may be used in conjunction with other microwave energy interactive elements and/or structures. Structures including multiple susceptor layers are also contemplated. It will be appreciated that the use of the present susceptor film and/or structure with such elements and/or structures may provide enhanced results as compared with a conventional susceptor.
- the susceptor film may be used with a foil or high optical density evaporated material having a thickness sufficient to reflect a substantial portion of impinging microwave energy.
- a foil or high optical density evaporated material having a thickness sufficient to reflect a substantial portion of impinging microwave energy.
- Such elements typically are formed from a conductive, reflective metal or metal alloy, for example, aluminum, copper, or stainless steel, in the form of a solid patch generally having a thickness of from about 0.000285 inches to about 0.005 inches, for example, from about 0.0003 inches to about 0.003 inches. Other such elements may have a thickness of from about 0.00035 inches to about 0.002 inches, for example, 0.0016 inches.
- microwave energy reflecting (or reflective) elements may be used as shielding elements where the food item is prone to scorching or drying out during heating.
- smaller microwave energy reflecting elements may be used to diffuse or lessen the intensity of microwave energy.
- One example of a material utilizing such microwave energy reflecting elements is commercially available from Graphic Packaging International, Inc. (Marietta, GA) under the trade name MicroRite® packaging material.
- a plurality of microwave energy reflecting elements may be arranged to form a microwave energy distributing element to direct microwave energy to specific areas of the food item. If desired, the loops may be of a length that causes microwave energy to resonate, thereby enhancing the distribution effect. Examples of microwave energy distributing elements are described in U.S. Patent Nos. 6,204,492, 6,433,322, 6,552,315, and 6,677,563, each of which is incorporated by reference in its entirety.
- the susceptor film and/or structure may be used with or may be used to form a microwave energy interactive insulating material.
- a microwave energy interactive insulating material examples include U.S. Patent No. 7,019,271 , U.S. Patent No. 7.351 ,942, and U.S. Patent Application Publication No. 2008/0078759 A l , published April 3. 2008, each of which is incorporated by reference herein in its entirety.
- any of the numerous microwave energy interactive elements described herein or contemplated hereby may be substantially continuous, that is. without substantial breaks or interruptions, or may be discontinuous, for example, by including one or more breaks or apertures that transmit microwave energy.
- the breaks or apertures may extend through the entire structure, or only through one or more layers.
- the number, shape, size, and positioning of such breaks or apertures may var for a particular application depending on the ty pe of construct being formed, the food item to be heated therein or thereon, the desired degree of heating, browning, and/or crisping, whether direct exposure to microwave energy is needed or desired to attain uniform heating of the food item, the need for regulating the change in temperature of the food item through direct heating, and whether and to what extent there is a need for venting.
- a microwave energy interactive element may include one or more transparent areas to effect dielectric heating of the food item.
- the microwave energy interactive element comprises a susceptor
- such apertures decrease the total microwave energy interactive area, and therefore, decrease the amount of microwave energy interactive material available for heating, browning, and/or crisping the surface of the food item.
- the relative amounts of microwave energy interactive areas and microwave energy transparent areas must be balanced to attain the desired overall heating characteristics for the particular food item.
- one or more portions of a susceptor may be designed to be microwave energy inactive to ensure that the microwave energy is focused efficiently on the areas to be heated, browned, and/or crisped, rather than being lost to portions of the food item not intended to be browned and/or crisped or to heating the environment. Additionally or alternatively, it may be beneficial to create one or more discontinuities or inactive regions to prevent overheating or charring of the food item and/or the construct including the susceptor.
- a susceptor may incorporate one or more "fuse" elements that limit the propagation of cracks in the susceptor. and thereby control overheating, in areas of the susceptor where heat transfer to the food is low and the susceptor might tend to become too hot.
- the size and shape of the fuses may be varied as needed. Examples of susceptors including such fuses are provided, for example, in U.S. Patent No. 5,412, 187, U.S. Patent No. 5,530,231 , U.S. Patent Application Publication No. 2008/0035634A 1 , published February 14, 2008, and PCT Application Publication No. WO 2007/127371 , published November 8, 2007, each of which is incorporated by reference herein in its entirety.
- any of such discontinuities or apertures in a susceptor may comprise a physical aperture or void in one or inore layers or materials used to form the structure or construct, or may be a non-physical "aperture' " .
- a non-physical aperture is a microwave energy transparent area that allows microwave energy to pass through the structure without an actual void or hole cut through the structure. Such areas may be fonried by simply not applying microwave energy interactive material to the particular area, by removing microwave energy interactive material from the particular area, or by mechanically deactivating the particular area (rendering the area electrically discontinuous).
- the areas may be formed by chemically deactivating the microwave energy interactive material in the particular area, thereby transforming the microwave energy interactive material in the area into a substance that is transparent to microwave energy (i.e., so that the microwave energy transparent or inactive area comprises the microwave energy- interactive material in an inactivated condition).
- a physical aperture also provides a venting function to allow steam or other vapors or liquid released from the food item to be carried away from the food item.
- a calorimetry test was conducted to determine the thermal flux produced by and maximum temperature reached by various susceptor structures.
- Various polymer films were used to form the susceptor structures, as set forth in Table 1.
- the polymer films included DuPont Mylar® 800C biaxiaily oriented PET (DuPont Teijin FilmsTM, Hopewell, VA), Pure-Stat APET (Pure-Stat Technologies, Inc., Lewiston, Maine), DuPont HS2 biaxiaily oriented PET (DuPont Teijin FilmsTM, Hopewell, VA), and Toray Lumirror® F65 biaxiaily oriented PET (Toray Films USA, Kingstown, RJ). Physical properties of the raw films (some of which were obtained from the manufacturer data sheets) are set forth in Table 1. Table 1
- control films (samples 1 - 1 , 1 -6, and 1 -7) were determined to have a percent crystallinity of 52-55%, which is consistent with information provided by manufacturers of 5 BOPET films commonly used to form susceptor films, as set forth in Table 2.
- Refractive index measurements were taken using a Metricon 2010 Prism Coupler (Metricon Corporation, Pennington, NJ) at 633 nm. Results for the films are listed as (n z ) (machine direction, MD), (n y ) (cross or transverse direction, CD), and (n x ) (thickness) directions.
- the birefringence (n z -n x ) was calculated from the refractive indexes and, throughout this specification, represents the difference in refractive indexes in the MD and thickness of the films. All these films were without added colorants or pigmentation, and thus were clear.
- Samples 1-1 and 1 -6 made using commercial standard and heat stabilized PET homopolymer films, respectively, exhibited high refractive indexes in both MD and CD (n z and n y , respectively ) that are characteristic of the highly oriented, highly crystalline films of the prior art.
- the superior performing cast film based structures samples 1-3 and 1-4 exhibit much lower (n z ) and (n y ) values that in fact are quite close to reported values for amorphous homopolymer PET polymer.
- Combining this data with crystallinity >50% for samples l - I and 1-6 and only about 5% for samples 1-3 and 1 -4 confirms that these samples represent largely amorphous films with little if any residual orientation.
- the differences between (n z ) and (n y ) values in samples I - I and 1 -6 represent small differences in MD and CD orientation or heat setting, but these are within the range of what one could expect to encounter with films possessing reasonably balanced MD/CD orientation.
- the calorimetry data was collected using a FISO MWS Microwave Work Station fiber optic temperature sensing device (FISO, Quebec, Canada) with eight (8) channels mounted onto a Panasonic 1300 watt consumer microwave oven model NN-S760WA.
- a sample having a diameter of about 5 in. was positioned between two circular Pyrex® plates, each having a thickness of about 0.25 in. and a diameter of about 5 in.
- An about 250 g water load in a plastic bowl resting on an about 1 in. thick expanded polystyrene insulating sheet was placed above the plates (so that radiant heat from the water did not affect the plates).
- the bottom plate was raised about 1 in. above the glass turntable using three substantially triangular ceramic stands.
- Thermo- optic probes were affixed to the top surface of the top plate to measure the surface temperature of the plate. After heating the sample at full power for about 5 minutes in an about 1300W microwave oven, the average maximum temperature rise from initial ambient temperature in degrees C of the top plate surface was recorded. (Finite element analysis modeling of the calorimetry test method has shown that the average maximum temperature rise is proportional to the thermal flux generated by the susceptor structure.)
- the conductivity ⁇ (mmho/sq) of each sample was measured using a Delcom 717 conductance monitor (Delcom Instruments, Inc., Prescott, WI) prior to conducting the calorimetry test, with five data points being collected and averaged.
- susceptor structures 1 -3 and 1-4 provided the most heating power and no visible crazing was evident, while structure 1 -1 exhibited a lower heating power than structures 1- 3 and 1 -4 and exhibited the expected amount of crazing for standard commercial susceptors.
- Structure 1 -6 had somewhat less crazing than structure 1 - 1 and provided a moderate heating power.
- Structure 1-7 was intermediate to 1 -1 and 1 -6 in both heating power and crazing.
- structure 1 -5 which had already been heated once, exhibited a greater power output than structure 1 - 1. Although no visible crazing was observed, the sample still exhibited some degree of self-limiting behavior (as evidenced by ATmax).
- this self-limiting behavior is at least partially the result of a change in densit of the polymer film during the microwave heating cycle.
- the density of a polymer film may decrease as the polymer film heats.
- crystallinity and an accompanying increase in density it is believed that the magnitude of this increase in density exceeds the magnitude of the initial density decrease, such that there is an overall increase in density during the heating cycle.
- this increase in density may cause disruptions or microcrazing in the susceptor structure that create electrical discontinuities on an atomic scale.
- the microwave reflection, absorption, and transmission (RAT) properties of a conventional susceptor structure were compared with an experimental susceptor structure (structure 1-3) using the calorimetry test described in Example 1 with various heating times.
- Each sample evaluated for low power RAT was placed into an HP8753A Network Analyzer. The output is used to calculate the reflection (R), absorption (A), and transmission (T) (collectively "RAT") characteristics of the sample.
- a merit factor was also calculated at each heating time, where:
- the merit factor is a useful measure of the ability of a microwave susceptor structure to resist the development of high transmission fraction, which detracts from good browning performance.
- a high merit factor maintained during a cook cycle indicates a susceptor retains the ability to both convert microwave energy to sensible heat to create effective surface browning and to reflect energy so as to avoid excessive direct microwave heating of the interior of the food product.
- a new parameter, craze perimeter divided by field area (P/A, mm/mm 2 ), was determined for some heating times of structure 1 - 1.
- the perimeter length of each craze of each sample was measured under magnification using image analysis to examine the respective samples after heating.
- the total craze perimeter was divided by the filed area to arrive at P/A. The results are presented in Tables 4 and 5.
- structure 1 -3 provided greater heating than structure 1 -1.
- Susceptor structures with larger merit factors generally exhibit greater food surface browning and crisping because they limit the amount of direct microwave heating of the food while maximizing the susceptor absorbance. Therefore, as a practical matter, a structure using a low crystallinity polymer film may be able to advantageously provide a greater level of surface browning and/or crisping while minimizing dielectric heating of the food item.
- the films were then metallized with aluminum and joined to 14 pt (0.014 inches thick) Fortress® board (International Paper Company, Memphis, TN) using a substantially continuous layer of from about 1 to about 2 lb/ream (as needed) Royal Hydra Fast-en® 20123 adhesive (Royal Adhesives, South Bend, IN) to form various susceptor structures.
- Various strength enhancing additives were also evaluated, including OptemaTM TC 120 and OptemaTM TC 220 ExCo (ethylene methyl acrylate copolymer resins, ExxonMobil Chemical), Sukano im F535 (ethylene methyl acrylate copolymer resin, Sukano Polymers Corporation, Duncan, SC), EngageTM 8401 (ethylene-octene copolymer, Dow Plastics), and Americhem 60461 - CD1 (composition unknown) (Americhem Cuyahoga Falls, OH).
- the process for forming the APET film used by Pure-Stat Technologies, Inc. was as follows.
- Traytuf® 9506 PET resin pellets (M&G Polymers USA, LLC, Houston, TX) were desiccant dried and conveyed to a cast film line extruder hopper.
- the additive pellets were metered into the extruder throat, combined with the dry PET pellets, melted, mixed, and extruded through a slot die to form a flat molten film.
- the molten film was cast onto a cooling drum, rapidly quenched into a largely amorphous solid state, and conveyed over rollers to a windup where the film was wound into a roll for further processing.
- the film was about 0.0008 inches or about 80 gauge in thickness. It will be noted that thicker or thinner films can be produced by varying the extruder output and cooling drum surface speed. The process used by SML Maschinen GmbH mbH was similar.
- DSC data was obtained for each film sample by heating the sample in a Perkin-Elmer differential scanning calorimeter (DSC-7) (Perkin-Elmer, Inc., Waltham, MA) at 10°C/minute. with a nitrogen purge to prevent degradation. Values were measured for samples heated to 300°C and cooled to 40°C. The results are presented in Table 7. It is important to note that the DSC data was taken from an initial heating of the test specimens. Therefore, the values reflect the impact of any post-extrusion orientation and the specific thermal heat history each specimen experienced due to processing on the crystallinity of the specimen. The negative enthalpy change associated with crystallization is proportional to the amount of non-crystalline polymer present in the specimen.
- the positive enthalpy change associated with melting is a measure of the degree of crystallinity attained by the specimen during the DSC measurement. The more equal the absolute values of these enthalpy values the more amorphous the specimen. Therefore, the values confirm that the highly oriented film, sample 5-1 , possessed very high levels of orientation and crystallinity and the cast APET films 5-3 through 5- 15, films possessed low levels of cry stallinity. The somewhat larger differences in enthalpy noted for samples 5-6 through 5- 15 reflect the impact of the non-PET strengthening additives present, but still are indicative of low levels of crystallinity in these films.
- AFM atomic force microscopy
- the perimeter of the detected region was measured and normalized by the linear size of the image to form a dimensionless ratio, perimeter divided by edge length, or PEL, with greater PEL values indicating a rougher surface.
- PEL perimeter divided by edge length
- the PEL data indicate that lower PEL levels (smoother film surface) are associated higher calorimetry and browning results. Peak load before break was measured according to TAPPI T-494 om-01. The values indicate that the strengthening additives in samples 5-6 through 5-15 were successful in increasing the robustness of the films. This was borne out in trials on commercial production equipment, where strengthening additive modified films processed without difficulties, while unmodified films of the type represented by samples 5-3 through 5-5 were more fragile in converting operations, and required adjustments to normal process parameters such as tension, and were converted less efficiently.
- the haze of each polymer film was measured according to ASTM D 1003 using a BYK Gardner Haze-Gard plus 4725 haze meter (BYK-Gardner, USA, Columbia, MD).
- the incorporation of strengthening additives increased the haze of the films.
- the most preferable additives may be those which exhibit lower levels of haze while providing the desired increase in strength for processing, and result in beneficially increased heating performance when made into susceptor films and structures.
- AUB is the number of pixels for a given sample minus the baseline value for an unbrowned crust (24313);
- A% Imp is the percent improvement over the results obtained by the control sample (structure 5- 1 ).
- the calorimetr results and pizza browning results both show significant increases over control for all the unoriented, low crystallinity films, whether they incorporated additives or not.
- Visual observations of the cooked pizza crusts confirmed much more desirable levels of browning than were achieved with the standard control sample made from highly biaxially oriented, high crystallinity film.
- strengthening additives can improve film robustness with no detriment to performance when incorporated into microwave susceptor films and structures.
- Structure 6-1 was a commercially produced susceptor structure comprising the commercially metallized, 48 gauge standard biaxially oriented PET film described in the previous paragraph laminated to paperboard, as described in Example 4.
- Structure 6-2 was the same as structure 6-1, except that structure 6-2 was hand laminated.
- Samples 6-3 through 6- 17 were unixially oriented in the machine direction on a Bruckner Karo IV Laboratory Stretching Machine (lab stretcher) (Bruckner Maschinenbau GmbH & Co. KG. Siegsdorf, Germany) using the orientation and heat setting temperatures set forth in Table 8 and a dra ratio of about 1.5: 1. The oriented films were then evaluated for various properties, as indicated in Table 8.
- White pigmented samples do not lend themselves to crystallinity deter ination by density or refractive index or birefringence due to the inclusion of pigment, but a comparison of the ⁇ values for crystallization exotherm and melting endothemn for white samples 6-7 and 6-10 with clear sample 6-9 indicates white sample 6- 10, oriented and heat set at the same conditions as clear sample 6-9 have similar levels of crystallinity and residual orientation.
- moderate uniaxial orientation was capable of creating films with improved susceptor heating performance compared to control susceptor films.
- Refractive index values (n z ) for these samples indicate them to be largely amorphous in nature, even after moderate orientation, with some modest increase in crystallinity for several of the samples.
- the much higher value for (n z ) for the control is consistent with highly crystalline film with high residual orientation.
- the films were then metallized and hand laminated to 14 pt paperboard, as described in connection with Example 4 to form various susceptor structures.
- the metallized sheet samples were removed, and then hand laminated to form susceptor structures.
- Hand laminated film samples were laminated to 0.014" uncoated SBS paperboard (International Paper, Memphis, TN) using a No.
- Structure 6-2 exhibited a rise in temperature very similar to that of the commercial control structure 6-1 . Accordingly, structure 6-2 may be considered to be a reasonable representation of commercially available susceptor structures.
- Several uniaxially oriented laminated susceptor samples (6-13 and 6-15) demonstrated lower heating performance compared to the control sample. As will be noted in Table 8, these samples were oriented at the lowest temperatures, resulting in the highest stretching stress, and received the lowest annealing temperature exposure, minimizing relaxation of that strain.
- Samples 6-5, 6- 10 and 6-1 1 demonstrated the highest achieved ATs and BRR's, reinforcing the utility of the present invention of combining modest orientation with heat setting for relaxation of the orientation stress.
- 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.
- the films for orienting were produced by Pacur (PACUR. Oshkosh. WI) on standard sheet extrusion equipment.
- Homopolymer films were produced from PQB 15-093 homopolymer PET resin supplied by Polyquest (Polyquest, Inc., Wilmington, NC), the copolymer films were produced from SKYPET-BR 8040 copolyester (S C Chemicals, Seoul.
- coextruded films were an A/B/A three layer coextruded structure with 2- 1 5% copolyester skin (A) layers encapsulating a 70% homopolymer core (B) layer using the resins described above.
- the films were oriented using a using a Karo series lab stretching machine with the conditions set forth in Table 9.
- the metallized sheet samples were removed, and then hand laminated to form susceptor structures.
- Hand laminated film samples were laminated to 0.012" uncoated SBS paperboard (International Paper, Memphis. TN) using a No.
- Example 5 Each susceptor structure was evaluated using the calorimetry test described in Example 1 and pizza browning test described in Example 4, and a relative browning reaction rate (RBRR) was calculated as described in Example 5. The results are presented in Table 9 and FIG. 5, in which samples/susceptor structures 6- 1 and 6-2 (from Example 6) are included for comparison.
- Structure 6-2 (the hand laminated commercial control from Example 6) exhibited a temperature rise of 146.7°C. This corresponds closely to structure 7- 1 (which was oriented on the lab scale stretching machine), which exhibited a temperature rise of 145.5°C. Accordingly, for purposes of this discussion, structure 7- 1 represents a reasonable simulation of a commercial susceptor film material. Structure 7-2, which was similar to structure 7- 1 (except that the orientation level was 3.8 x 3.8). exhibited a rise in temperature of only 134.7°C. Thus, there was a decrease in performance at the lower orientation level for this lab stretched homopolymer PET film based susceptor.
- Structure 7-17 which comprised the lower melting point copolyester (with all other conditions being the same as structure 7-2), exhibited a rise in temperature of 154.9°C, which was greater than any of the structures made from a homopolymer base film, including the commercial control (structure 6-2).
- Structure 7-18. which had an increased degree of orientation, exhibited an increase in temperature of 158.8°C.
- structure 7-21 which had a lower orientation temperature and a lower degree of orientation, showed only a temperature increase of 125.4°C. It will be noted that while this performance may not be acceptable for most microwave heating applications, there are other applications in which lower heating capacity may be desirable.
- structure 7- 19 which was made with a greater annealing (i.e., heat set) temperature than structure 7-21 , the rise in temperature was 144.2°C. It will be noted that the corresponding value for a homopolymer susceptor base film made under the same conditions was 138.2°C (structure 7- 6).
- Coextruded structures appear to be somewhat more sensitive to process conditions, but are still capable of superior calorimetry and pizza browning results when higher stretching temperatures and higher annealing temperatures are employed. In fact, the best pizza browning of this set of samples was achieved by coextruded sample 7-26. Without wishing to be bound by theory it is believed that the presence of both homopolymer PET and copolyester layers of the structure results in something of a 'mixed mode' for orientation, crystallization and relaxation and may result in a somewhat narrower window of process operating conditions.
- FIG. 5 plots calorimetr based relative browning rate vs. pixel count from actual pizza cooking tests; these data confirm that within the limitations of pizza variability and cooking dynamics a clear positive correlation exists between the calorimetry results and actual food browning results.
- Table 9
- films with little or no tendency for dimensional change at a particular temperature will tend to be susceptible to slight stretching by the sample holding tension at that temperature and the curve will erroneously imply that growth or expansion would occur under conditions of no tensional load.
- Table 10 identifies the films for which dynamic dimensional temperature response curves are shown; three films representing conventional highly oriented, highly crystalline, high refractive index and birefringence standard PET homopolymer films are compared with three films representing several embodiments of the present disclosure, all of which demonstrate superior susceptor performance. Table 10
- Samples 6-2, 1 -6, and 7- 1 are all homopolymer PET highly oriented films representing a common commercial susceptor base film (sample 6-2), a heat stabilized film of the prior art (1 -6) and a laboratory stretched film targeted to simulate the properties and performance of a film similar to sample 6-2 (sample 7-1 ).
- Samples 6-9, 6-1 1 , and 7-8 are all lab stretching machine oriented samples of the present disclosure.
- Samples 6-9 and 6-1 1 are moderately uniaxially oriented films of the present disclosure made from homopolymer PET, with sample 6-1 1 exposed to a higher heat set temperature than sample 6-9; sample 7-8 is a highly oriented film of the present disclosure made from a copolyester with a melting point below that of typical homopolymer PET.
- FIGS. 6-11 trace the dynamic dimensional temperature responses of these films. Duplicate runs for each film in MD and CD are shown in each figure. Of immediate note is that all 6 samples begin with some response to increased temperature at about 80°C, roughly Tg for PET. This is to be expected as the polymer transitions from the glassy state to the rubbery state. Notably, as samples 6-2, 1 -6, and 7- 1 continue to increase in temperature, meaningful shrinkage occurs in the MD for all samples and CD for all but sample 1 -6, and continues to increase with increased temperature, especially at temperatures above 150°C, the static exposure temperature for shrinkage measurements typically referenced as representing a good predictor of susceptor performance.
- sample 1 -6 appears to grow slightly in the CD, which is likely the result of somewhat enhanced thermal stability in that direction compared to the MD and the false expansion indication discussed above, but shrinks significantly in the MD at temperatures commonly encountered and desired in susceptor heating. While sample 1 -6 shows some increased heating capability compared to samples 6-2 and 7-1 , it is still inferior to susceptors made according to this disclosure.
- sample 7-1 The greater magnitude of shrinkage seen in sample 7-1 compared to sample 6-2 is expected to result from differences between commercial machine and iab stretching equipment, but significantly, susceptors made from the two films perform quite similarly in heating and cook tests. This confirms that past a threshold of craze initiation, susceptor performance is lost in an essentially irretrievable manner.
- Samples 6-9, 6-1 1 , and 7-8 all of which are experimental films of the present disclosure that provide superior heating to the three standard type films, reacted quite differently to increased temperature, evidencing no shrinkage in either MD or CD over the same temperature range where the standard films shrink.
- all of the superior performing susceptors of this disclosure that were tested according to this method (including, for example, samples 5-6, 5-7, 5-8. 5-10, 5- 12, and 5- 14), exhibited similar behavior.
- these films may be considered to be dimensionally stable and should resist crazing onset in a superior fashion; calorimetry and browning tests confirm this superior performance.
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Abstract
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BR112015020334A BR112015020334A2 (en) | 2013-03-14 | 2013-03-14 | microwave energy interactive structure |
MX2015012426A MX2015012426A (en) | 2013-03-14 | 2013-03-14 | Low crystallinity susceptor films. |
CA2900458A CA2900458A1 (en) | 2013-03-14 | 2013-03-14 | Low crystallinity susceptor films |
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IN169503B (en) * | 1985-03-11 | 1991-10-26 | Goodyear Tire & Rubber | |
US4894503A (en) * | 1987-10-23 | 1990-01-16 | The Pillsbury Company | Packages materials for shielded food containers used in microwave ovens |
JP2931911B2 (en) * | 1990-01-09 | 1999-08-09 | 共同印刷株式会社 | Heat resistant double container and method for producing the same |
GB0101994D0 (en) * | 2001-01-25 | 2001-03-14 | Dupont Teijin Films Us Ltd | Process for the production of coated polumeric film |
CA2749377C (en) * | 2009-02-23 | 2014-07-29 | Graphic Packaging International, Inc. | Low crystallinity susceptor films |
EP2937378A1 (en) * | 2009-07-30 | 2015-10-28 | Graphic Packaging International, Inc. | Low crystallinity susceptor films |
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2013
- 2013-03-14 JP JP2016500043A patent/JP2016520948A/en active Pending
- 2013-03-14 CA CA2900458A patent/CA2900458A1/en not_active Abandoned
- 2013-03-14 BR BR112015020334A patent/BR112015020334A2/en not_active IP Right Cessation
- 2013-03-14 EP EP13877886.5A patent/EP2974528A4/en not_active Withdrawn
- 2013-03-14 MX MX2015012426A patent/MX2015012426A/en unknown
- 2013-03-14 WO PCT/US2013/031420 patent/WO2014142887A1/en active Application Filing
Patent Citations (5)
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US4866235A (en) * | 1989-01-24 | 1989-09-12 | The Boc Group, Inc. | Microwavable containers useful for controlled heating |
US5039001A (en) * | 1990-06-18 | 1991-08-13 | Kraft General Foods, Inc. | Microwavable package and process |
US5405663A (en) * | 1991-11-12 | 1995-04-11 | Hunt-Wesson, Inc. | Microwave package laminate with extrusion bonded susceptor |
US20060289521A1 (en) * | 2005-04-14 | 2006-12-28 | Reinhard Bohme | Thermally activatable microwave interactive materials |
US20070062936A1 (en) * | 2005-08-29 | 2007-03-22 | Young James C | Microwave susceptors incorporating expandable polymeric particles |
Non-Patent Citations (1)
Title |
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See also references of EP2974528A4 * |
Also Published As
Publication number | Publication date |
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MX2015012426A (en) | 2016-01-12 |
EP2974528A4 (en) | 2016-11-02 |
BR112015020334A2 (en) | 2018-05-15 |
EP2974528A1 (en) | 2016-01-20 |
CA2900458A1 (en) | 2014-09-18 |
JP2016520948A (en) | 2016-07-14 |
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