US 3865302 A
A container is provided consisting of a polyethylene terephthalate tray with a polyethylene terephthalate lid bonded, by a novel adhesive, to an upper surface of the tray. The adhesive is coated only onto this upper surface, which surface is in the form of a peripheral flange. The adhesive is free from flow at cooking temperatures of up to about 400 DEG F. and the bond strength at these cooking temperatures is sufficient to prevent leakage from the tray, yet, nonetheless, permit peeling of the lid from the tray without tearing, after cooking.
Claims available in
Description (OCR text may contain errors)
United States Patent [191 Kane 1 1 CONTAINER FOR COOKING FOOD THEREIN  Inventor: William P. Kane, Bon Air, Va.
 Assignee: E. I. du Pont de Nemours and Company, Wilmington, Del.
 Filed: Nov. 10, 1972 [211 Appl. No.: 305,250
[52} US. Cl 229/43, 156/69, 156/334, 229/35 R, 229/48 T, 426/106, 426/396,
 Int. Cl. A47j 27/00, B65b 7/28, 865d 1/O0, B65d 5/64, 1365b 29/08  Field of Search ..426/106,113,l14,123. 426/396, 410, 412, 413, 415; 206/46 F;
T; 229/35 R, 43, 48 T  References Cited UNITED STATES PATENTS 2,650,213 8/1953 Hofrichter, Jrv 161/231 X 2,834,686 5/1958 Reuman 426/415 X 1 1 Feb, 11,1975
3,189,573 6/1965 Oken 260/28.5 AV 3,192,091 6/1965 Hey et a1. 156/69 3,215,657 11/1965 Beresniewkz et a1. 260/28.5 AV 3,414,647 12/1968 Saunders et a1. 264/235 3,455,720 7/1969 Davies et a1 260/75 T 3,496,143 2/1970 Sig'gel et a1 260/75 T 3,505,259 4/1970 Hallis, Jr. 260/28.5 AV 3,719,625 3/1973 Parker 260/28.5 AV
Primary Examiner-Frank W. Lutter Assistant ExaminerSteven L. Weinstein  ABSTRACT A container is provided consisting of a polyethylene terephthalate tray with a polyethylene terephthalate lid bonded, by a novel adhesive, to an upper surface of the tray. The adhesive is coated only onto this upper surface, which surface is in the form of a peripheral flange. The adhesive is free from flow at cooking temperatures of up to about 400F. and the bond strength at these cooking temperatures is sufficient to prevent leakage from the tray, yet, nonetheless, permit peeling of the lid from the tray without tearing, after cooking.
3 Claims, 6 Drawing Figures PATENTED 1 $865,302
SHEEI 1 OF 2 PATENTEU 1 H975 3,885,302
SHEET 2 OF 2 SURFACE FREE ENERGY (DYNES) I 50 I00 ETHYLENE GUPOLYNER FIG-5 SURFACE FREE ENERGY (UYNES) RELATIVE PEEL STRENGTH (ARBITRARY UNITS) CONTAINER FOR COOKING FOOD TI-IEREIN BACKGROUND OF THE INVENTION 1. Field of the Invention This invention is a container' consisting ofa tray having a lid secured to a flange of the tray by a novel adhesive.
This invention further is a method of using this novel container, both the lid and tray of which are polyethylene terephthalate, with an adherent bond while foods are heated at temperatures up to about 400F. and, after cooking and cooling, the lid of which may be peeled with ease from the tray to permit access to the cooked contents therein.
2. Description of the Prior Art US. Pat. No. 3,547,388 to Hernnes is exemplary of the prior art. Such patent discloses a food package including a one piece container and a two-layer laminated film cover joined to a flange thereof. The upper layer of the cover film laminate has properties similar to the material from which the container body is made while the lower layer of the film cover laminate permits bonding, but easy separation from the container body.
Other patents of the prior art of interest to the container and method of using such container (including cooking in it and opening of it, after cooking) also of this invention include Pat. Nos.: 3,547,338; 3,491,935; 3,616,898; 3,202,271; 3,615,707; 3,655,503; 3,656,210; 3,530,917; 3,561,668.; and 3,498,525.
None of these patents show the thermally stable container of the instant invention in which only the upper surface of the flange of the tray is coated with an adhesivc, to minimize the exposure of the adhesive to the contents in the tray, and in which food may be cooked or reheated at normal cooking temperatures up to about 400 F. and the lid of which may be peeled easily from the tray, after cooking.
SUMMARY OF THE INVENTION According to the present invention, containers are provided for the packaging of food and for use in the subsequent cooking and reheating of the food in preparation for human consumption.
The invention comprises a versatile container consisting of a thermally stable tray and a lid secured to a flange of the tray by an adhesive on the flange which provides a secure seal at heating temperatures and a seal peelable after heating and cooling to room temperature.
Trays of various materials have been widely employed for packaging and direct consumption therefrom of preprocessed foods. Limitations of materials from which trays were formed as well as limitations in other components of the package have resulted in restrictions in use of such packages.
Previously it has been the practice to form trays of heat scalable polymers, such as vinyls and oleflns, to facilitate securing lids thereon, but these in general do not possess the thermal stability required for reheating foods. Formed trays of aluminum foil have been employed, but these lack versatility in that they are unsuitable for use in microwave ovens.
The requirements for a satisfactory, versatile tray to which the present invention is directed are that it should have a closure, i.e., a lid, that is bonded to the tray body so that the bond does not rupture during reheating the container and its contents up to about 400F., and yet with the sealed lid readily peelable to gain access to the contents after suchheating and cooling to temperatures suitable for direct consumption, i.e., to F. Further, the combination should have thermal stability in that the tray or lid should not embrittle or be prone to cracking upon heating. Likewise, it should not become distorted upon heating. F inally, the lid should be adhered in a manner to minimize exposure of the contents by the adhesive.
The present invention provides a tray and lid combination capable of being sealed with an adhesive critically located and having these particular characteristics.
The container is formed from a tray of polyethylene terephthalate to which is secured or bonded a polyethylene terephthalate lid. This bonding of the lid to the tray is accomplished by use of a novel adhesive that is coated only onto a flange of the tray. By doing this, undue exposure of the adhesive to the contents of the tray is precluded or prevented. The food in the tray may be reheated or heated to temperatures up to about 400F. without having any problems as far as the bond is concerned; it remains a tight seal. After cooking, this bond is easily broken when the lid is peeled from the tray.
In short, then, this invention is a container in the form of a lidded tray that can be effectively used for cooking, or reheating at oven temperatures and which after cooking and cooling may be easily opened, by peeling the lid from the tray to reach the food contained therein, without damage to the tray or contents.
More specifically, these requirements are provided by a container comprising a tray of polyethylene terephthalate and a lid of polyethylene terephthalate bonded thereto with an adhesive applied only to an upper surface of the tray, the adhesive being free from flow at cooking temperatures up to about 400F. and having a bond strength at such cooking temperatures sufficient to prevent leakage from the tray and the lid being peelable from the tray without tearing after heating at such cooking temperatures and cooling to room temperatures. The container as above described wherein the bond strength is greater than 100 gramslinch at 400F. and less than 500 grams/inch at room temperature, and also wherein the polyethylene terephthalate in the tray has an intrinsic viscosity of at least 0.65 in a 60/40 mixture of tetrachloroethane/phenol at 25C also provides these requirements.
The invention also comprehends a container comprising: a dish-like tray of polyethylene terephthalate having an intrinsic viscosity of at least 0.65 in 60/40 tetrachloroethane/phenol, the tray having abottom part and a body part and an outwardly projecting peripheral flange having an upper surface on the upper extremity of the body part; an adhesive only on the upper surface ofthe flange, the adhesive having a bond strength at least 100 g./in. at 400F. and less than 500 g./in. at 70F, a melt flow of less than 0.2 grams at 320F. (C) for 10 minutes and a surface free energy of less than 30 dynes; and, a substantially flat cover having its lower surface substantially coextensive with and bonded to the flange by the adhesive.
Also comprehended in the foregoing structures is the aforesaid polyethylene terephthalate containing up to 1.0 percent talc and up to 2 percent TiO The specific combination of adhesive used in sealing the lid to the tray, the tray and the location of the adhesive of the present invention permits these strong bonds at higher cooking temperatures which are, nonetheless, easily peelable after cooking, and do not contaminate the contents. The adhesive is coated only onto the flange of the tray, thus sealing of the lid is simplified, and there is a minimum of contact potential with the contents. For example, if the adhesive is coated onto only selected parts of the lid, registration problems may be presented in making certain that none of it is applied so as to overlie the food when the lid is placed on the tray.
To gain access to the contents of the container the lid is peeled from the upper margin of the container thereby exposing the container contents.
In containers in which components are hermetically sealed to one another, there have been several approaches in facilitating the opening of the container and thus exposing the container contents. For example, the film covering or lid has been made from a different plastic material than that from which the container is made. By selecting two such different plastic materials, a weld or bond can be obtained between the film covering and container body which is also properly hermetically sealed, and further permits easy removal of the film covering from the container body.
Such prior art containers have the disadvantage, however, that the container body or covering film, in most cases the covering film, consists of a plastic material having a low softening range or melting point, and as such they are not well suited for the packaging of food substances which are to be externally heated, such as in a hot air chamber, whether it be for the purpose of sterilization or whether it be for the purpose of heating the contents of the package prior to consuming the same. It has generally been the experience with such prior art containers that the part which is formed or made from the plastic having the low temperature stability softens during heating and deforms, thus making the container potentially undesirable from a sanitary, as a serving tray for edible materials, and/or aesthetic viewpoint.
Accordingly, the instant invention provides a novel container which exhibits unique packaging versatility and which overcomes disadvantages, such as loss of bondng strength at high cooking temperatures or lack of ease of opening after cooking or which minimizes the exposure of the contents to bonding adhesives, of currently used containers. In its broadest terms, this invention is a disposable container which comprises a lid of polyethylene terephthalate bonded to a tray of the same material by a novel adhesive critically positioned only on a flange of the tray. This container, and method of using it, will now be described in greater detail.
BRIEF DESCRIPTION OF THE DRAWING FIG. I is a side elevational view, with parts broken away for clarity, ofa container of this invention consisting of a tray having a lid secured to an upper flange thereof by a novel adhesive.
FIG. 2 is an enlarged cross-sectional view of parts of the container of FIG. 1.
FIG. 3 is a perspective, exploded, view of the container of FIG. 1 showing the adhesive only on the flange of the tray ready to bond the lid thereto.
FIGS. 4 and 5 are graphs showing relationships of surface free energy (in dynes) to the ethylene copolymer component of the adhesive and the relative peel strength to this surface free energy.
FIG. 6 is a schematic illustration of an apparatus for applying an adhesive to a flange of a tray,
DESCRIPTION OF THE PREFERRED EMBODIMENT The invention is a container of a thermoplastic material (polyethylene terephthalate) which may be tightly sealed and which, nevertheless, may be opened easily.
In recent years various products of perishable and durable nature have been packaged in containers formed from plastic materials and particularly thermoplastic materials which can be heat sealed. One type of container employed for packaging such items, such as frozen foods and the like, is a tray container which can be made from heat-scalable thermoplastic materials such as polyethylene, polypropylene, polyvinyl chloride and the like. The tray is provided with a lid formed of a heat-scalable thermoplastic material generally the same material as in the tray itself. Sealing of the container is conveniently accomplished by fusing the lid and tray together by means of heat and pressure after the contents of the container are placed therewithin. This type of container, after being properly heatsealed, is substantially impervious to vapors, moisture and satisfactorily protects the contents thereof against contamination from external sources. These containers can be produced relatively inexpensively in a variety of shapes and sizes but generally are unsuitable for certain uses when being subjected to temperatures of up to 400F. whereby the contents can be heated and cooked or sterilized as is sometimes desired when certain food items are packaged.
With the thermoplastic materials used to form the containers, it is extremely difficult to peel the lid from the tray when attempting to remove the cover or lid from the tray to reach the contents. Moreover, with a tight bond or fusion between the lid and the tray it is virtually impossible to initiate a tear therebetween. The difficulties in opening these containers are further increased when the containers are subjected to heat as when the contents are heated while in the container.
The container of this invention solves these and other problems.
Referring to the drawings and FIG. I in particular, there is shown a typical novel container C of the present invention.
Such container C is formed of polyethylene terephthalate and consists of a tray 10 having a lidll secured to it by a novel adhesive 13.
The tray 10 includes a bottom part 14 with a body part or sides 15 connected thereto and further includes a peripheral flange 16 having an upper surface 17 on the upper extremity of the body part 15.
With reference to the drawings, the tray 10 is shown to be dish-like in shape but can be of any desired shape such as circular, square, triangular and so forth. Tray 10 is formed by conventional operations from a thermoplastic material, polyethylene terephthalate.
The lid 11 is made of the same thermoplastic material (polyethylene terephthalate) as the tray 10 may be provided with a pull-tab which preferably extends beyond the marginal edge of the associated flange 14. One or more such pull-tabs can be provided for peeling the lid from the tray.
After placing the contents in the tray 10, the cover or lid 11 is placed thereover with its peripheral edges substantially coincident with those of flanges 16. The cover 11 is sealed to flange 16 by the adhesive 13 by means of heat and pressure in a conventional manner to form a seal extending the entire length of the flange 16. The line of heat seal is disposed in parallel relationship to the annular surface and toward the peripheral edge of the flange.
In the embodiment illustrated in the drawing. the container C includes the dish-like tray having the bottom part 14 and body part 15 which is circumferentially continuous and which extends upwardly (and outwardly if nesting between containers is desired) from the bottom and terminates in the radially outwardly directed circumferentially continuous flange 16 at the open upper end or upper margin of the container. The container further includes the covering film or lid 11 bonded to the radially outwardly directed flange of the container body.
The container body or tray 10 is thermoformed into a one-piece shape as illustrated from a plastic material having a predetermined strength and rigidity at elevated temperatures. A material which can be subjected to elevated temperatures in a hot air oven is polyethylene terephthalate which does not distort easily at elevated temperatures and also retains its strength and rigidity at such temperatures. The lid is also made from a plastic material having properties similar to the container body or tray such as polyethylene terephthalate. The lid so made permits bonding to the radially outwardly directed flange of the tray, but permits easy separation therefrom.
The container body or tray 10 is thermoformed in a one-piece shape, as illustrated, by conventional forming apparatus, in particular apparatus having a heated mold and ram to enhance crystallization of the formed structure. Starting with unoriented, amorphous polyethylene terephthalate film (lV=0.65 or greater in 60/40 TCE/phenol) with a nominal thickness of 15 mils (0.015 inch) containing 0.5 to L5 percent titanium dioxide (TiO and 0.25 to 1 percent talc, the former as an opacifier and the latter to enhance the rate of crystallization, the film is preheated from about 80C. (176F.) to about 150C. (302F.), preferably l00-l 10C. for not more than 30 seconds. (Preheating should not exceed about 30 seconds or crystallization may occur, making forming impossible.) The mold and the ram, (preferably a ram is employed) is heated to l30-l85C. (266465F.). preferably l50-l55C'. The residence time in the mold may be up to 30 seconds; however, with the upper range of talc, residence time as low as 6-7 seconds are sufficient to form and crystallize the tray, and are preferred. Crystallization may continue momentarily after the tray is removed from the mold.
Due to the ready separability and bonding characteristics of the lid 11 relative to the radially outwardly directed flange 16 of the tray 10 due to the adhesive 13, the lid 11 may be easily stripped or peeled from the tray 10, through the use of suitable finger tabs or the like (not shown), as a result of the bond strength which is provided between the lid and the flange ofthe tray. The high temperature stability characteristics of the tray 10 as well as lid, provides an overall container which can be subjected to elevated temperatures as in a hot air oven without incurring any of the disadvantages previously mentioned due to this bond.
An adhesive of this invention may be formed from a composition consisting essentially of (a) from 60 to percent by weight of an ethylene copolymer" of the type defined and disclosed in U.S. Pat. No. 3,2l5,657 to Beresniewicz et al. and (b) from 40 to 30 percent of a petroleum wax.
Minor amounts of other materials such as pigments, dyes, anti-oxidants and the like may also be present, but, in general. the ethylene copolymer and petroleum wax should constitute at least percent of the total composition weight.
The critical aspect of this is the percentage of ethylene copolymer and paraffin wax or petroleum wax in the composition. If the wax content is by weight less than 30 percent and more than 40 percent. marginally effective seals, particularly as to peelability, are produced by the adhesive between the polyethylene terephthalate lid and tray after heating for 1 hour at 400C, for example. The key to acceptability is a combination of melt flow at l60C./l0 minute period in an oven, surface free energy of less than 28 dynes. The flow is 0 grams to 0.2 grams at 160C. for l0 minutes (ASTM 1238).
An important consideration, as previously mentioned, is the behavior of the polymer from which the trays are formed. In the forming process the body of the tray is oriented and crystallized, and as such is not sub ject to cracking and exhibits a minimum of dimensional change in the heating-cooling sequence incidental to food preparation or instrument sterilization. The flange, on the other hand, receives slight, if any orientation in the forming process, but crystallizes. Accordingly, if the intrinsic viscosity of the polyethylene terephthalate in the structure is of the range customarily used for films and fibers (0.55-0.60 in 60 parts TCE/40 parts phenol), the unoriented crystallized flange can become brittle and crack, causing leakage and other failure of the bond. The polyethylene terephthalate in the trays, it has been found, with an intrinsic viscosity of at least 0.65, is free from such embrittlement. As is known to those skilled in the art, the polymer from which the film for the trays is made, should have a viscosity slightly higher than this level to compensate for losses by depolymerization in processing. The extent of such loss will, of course, depend upon process conditions, such as the moisture content of the starting polymer.
The adhesives effective in the present invention are those which bond the polyethylene terephthalate without special treatment of this substrate. Particularly useful are wax-ethylene terpolymer compositions described in U.S. Pat. No. 3,2l5,657. The terpolymer component provides the effective bonding agent and the wax acts as a diluent to control the peel strength and the physical properties (e.g., melt index) of the composition.
To determine effective compositions an acid terpolymer resin sold under the trademark Elvax 4355 an interpolymer of ethylene, vinyl acetate and an organic acid was used at various ratios in a composition with a paraffin wax having a C. melting point. The undiluted terpolymer had a melt index of 6.0 g./l0 min. (ASTM 1238), typically 25 percent vinyl acetate and an acid number of 6. The melt flow (index) is a function of the percentage terpolymer.
Effective adhesives may be found by determination of the surface free energy, a function of the contact angle of the adhesive on the polyethylene terephthalate substrate. FIG. 4 illustrates this surface free energy, as a function of composition. Peel strength can be correlated with surface free energy, as shown in FIG. 5. lnspection of these two figures shows that at about 75 percent Elvax 4355 the rapid rise in surface free energy, with the corresponding unwanted increase in peel strength, suggests that a surface free energy of about 30 dynes is an upper limit.
The adhesive is applied to the rim of the tray, rather than the lid for significant reasons. First, due to health standards in trays intended for foodstuffs, as most of them are, it is essential to present the minimum possible Contact between the adhesive and the contents of the tray. Accordingly, with a lid free of adhesive except where bonded to the tray at the flange, the only contact of the contents and the adhesive in lidded tray of this. invention is at the verythin edge of the seal. Applying.
the adhesive on the lid, even if only annularly at the edge of the lid, would require a relatively broad band of the adhesive material to assure contact with the flange, and would present considerable mechanical difficulty and expensive machinery for registering the adhesive with the flange on the tray. 1
Similarly, lids with an adhesive laminate or fully coated with adhesive to minimize the problem of registration, could present an intolerable exposure of the adhesive to the contents. This could cause difficulty not only from the possible contamination of the contents with toxic materials, but also may cause adhesion of the contents, e.g., surgical instruments subsequent to heating, as for sterilization.
Apparatus for application of the adhesive to the rim of the tray is schematically illustrated in FIG. 6. Conveyor 20 driven by rolls 21 and 22 engages trays l0, either singly or in groups of two or more and moves them in kiss contact with applicator roll 23. Adhesive, as described, is applied to roll 23 by hopper-25 with the coating thickness controlled by doctor blade 26. The adhesive may be applied as a melt, in which case roll 23 is heated to the softening temperature of the adhesive, or quite effectively, it may be applied from a toluene solution, in which case applicator roll 23 is at ambient temperatures. With the solvent applied adhesive it is desirable to employ a heated drying tunnel to remove the solvent from the adhesive.
Specifically, the instant invention in one embodiment provides a container for baking and cooking which comprises:
a substantially flat bottom part;
at least four upstanding sides connected to the flat bottom part; I
the container being formed of substantially amorphous polyethylene terephthalate; and
a lid of polyethylene terephthalate secured thereto.
In another embodiment, this invention is a container for baking and cooking which comprises:
a dish-like tray of polyethylene terephthalate, the
tray having a bottom part and a body part; an outwardly projecting peripheral flange having an upper surface on the upper extremity, of the body part;
a substantially flat cover part having lowersurface parts substantially coextensive with the flange and in abutting contact-therewith; I
a thin coating of a heat-activated adhesive only on the upper surface of the flange;
the adhesive having a melt flow of less than 0.2 g. in
10 min. at 160C. and'a surface free energy of less than 30 dynes. g
And, a method is provided for cooking in these novel containers, which cooking method includes the steps of: forming a container formed from a tray and lid both of polyethylene terephthalate and bonded thereto by an adhesive by the methods previously described; placing food in the container; and, cooking the food at a temperature of up to about 400F.
This invention further is a method of sealing a polyethylene terephthalate lid to the flange of a polyethylene terephthalate tray to provide'a critical bond between the lid and tray. This bond or seal is such that during cooking at temperatures up toat least 400F. the container remains hermetically sealed and, after cooking, the lid may be easily peeled from the tray. The traymay be suitably formed by heating the material from which the tray is formed (i.e., the container or tray blank) and by pressing the heated material to form such'tray with its bottom part and body part with the flange connected thereto. The specific type of polyethylene terephthalate material used in the tray is critical.
ln forming the tray, the portions of it except the flange may be oriented. Due to lack of orientation the flange, unless it'has specific properties, tends to crack. Such properties for the polyethylene terephthalate material which prevents this cracking include an intrinsic viscosity of at least 0.65.
The intrinsic viscosity of the polyethylene terephthalate of the tray of this invention is critical to the practice of such invention. lt has been discovered that such intrinsic viscosity must be above 0.65, for reasons further to be explained.
The intrinsic viscosity of polyethylene terephthalate of the present invention is determined in a trifluoroacetic acid-methylene chloride solvent system, since dissolution times are prohibitively long in the tetrachloroethane-phenol solvent system used for conventional polyethylene terephthalate. For this determination the relative viscosity (nr) of the present polymer is determined in a 1 percent solution at 30C. in a solvent comprising 25 parts by weight trifluoroacetic acid (TFA) and parts by weight methylene chloride (Ch Cl The relative viscosity in this solvent (relative viscosity is the flow time of the solution through a capillary viscometer divided by the flow time'of the solvent) is converted to the relative viscosity which would be obtained in a conventional solvent of 0.6 parts by weight 1,1,2,2-tetrachloroethane (TCE) and 1 part by weight phenol at 25C. Using the empirical equation nr(TCE-phenol) 1.0324 nr (TFA-CH Cl 0.19, the relative viscosity in TCE-phenol is calculated. From this relative viscosity the intrinsic viscosity is determined from the experimentally constructed table:
Continued INTRINSIC VS. RELATIVE VISCOSITY POLYETHYLENE TEREPHTHALATE IN TCE/PHENOL (This table is extrapolated for relative viscosities above 2.42. intrinsic greater than I .00).
Alternatively, intrinsic viscosity can be determined in a solution of polyethylene terephthalate in a trifluoroacetic-acid-methylene chloride solvent and converted to the value in tetrachIoroethylene-phenol by an empirical equation obtained from multi-point viscosity determinations of film of the priorart and film of the present invention:
in which  =the intrinsic viscosity of polyethylene terephthalate in a solvent comprising 40 parts by weight tetrachloroethane and 60 parts by weight phenol; lnh ==the intrinsic viscosity of polyethylene terephthalate in a solvent comprising 25 parts by volume trifluoroacetic acid, 75 parts by volume methylene chloride.
The polyethylene terephthalate material of this invention is formed from a polymer formed by the condensation reaction of ethyl glycol and terephthalic acid. Particulars of this material are shown and described in US. Pat. No. 2,465,319 to Whinfield and Dickson. More specifically, polyethylene terephthalate useful in preparing the structure of this invention includes (a) polymers wherein at least about 97 percent of the polymer contains the repeating ethylene terephthalate units of the formula:
o -cn,cmo (LGJL with the remainder being minor amounts of esterforming components.
The polyethylene terephthalate material of this invention also includes blends of polyethylene terephthalate with a different polymer compatible therewith wherein the blend contains at least 70 percent polyethylene terephthalate, as defined above.
This material does not get brittle with age, it has a long shelf life; it provides a permanent, nonyellowing, dimensionally stable base and has excellent resistance to grease, oxygen and moisture. Further, it is resistant to initial tear and scratching and can withstand heavy impact and prolonged flexing. These and other properties are of great value in packaging applications.
This invention, then, is a container comprising a tray of polyethylene terephthalate and a lid of polyethylene terephthalate bonded thereto with an adhesive applied only to an upper surface of the tray, the adhesive being free from flow at cooking temperatures up to about 400F. and having a bond strength at such cooking temperatures sufficient to prevent leakage from the tray and the lid being peelable from the tray without tearing after cooking at such cooking temperatures.
The bond strength is greater than 100 grams/inch at 400F. and less than 500 grams/inch at room temperature.
The container of this invention comprises a dish-like tray of polyethylene terephthalate. the tray having a bottom part and a body part and an outwardly projecting peripheral flange having an upper surface on the upper extremity of the body part; an adhesive only on the upper surface of the flange. the adhesive having a melt flow of less than 0.2 grams at 320F. (160C) for 10 minutes and a surface free energy of less than 30 dynes; and, a substantially flat cover part having lower surface parts substantially coextensive with and bonded to the flange by the adhesive.
A method of making this container includes the steps of: forming a tray having a bottom part and a body part with a flange having an upper surface connected to such body part, the tray being of polyethylene terephthalate; coating an adhesive onto the upper surface only of the flange of the tray; placing a lid of polyethylene terephthalate having lower outer surface parts in abutting contact with the adhesive and sealing the abutting surface portions of the lid and tray together.
The adhesive is a composition consisting essentially of (a) 30 percent to 40 percent by weight of a petroleum wax and (b) percent to 60 percent by weight of an ethylene copolymer.
The wax and ethylene copolymer together constitute at least percent of the total weight of such composition.
The ethylene copolymer is of an acid terpolymer resin consisting of an interpolymer of ethylene, vinyl acetate, and an organic acid.
The adhesive is a composition consisting essentially of (a) at least 30 percent by weight of a petroleum wax and (b) an ethylene copolymer and wherein such wax and ethylene copolymer constitute at least 80 percent of the total weight of the composition.
This invention further is a method including the steps of: placing food in a tray formed of polyethylene terephthalate; coating an adhesive only onto an upper surface of such tray; bonding a lid formed of polyethylene terephthalate to the upper surface of such tray by means of such adhesive; cooking the food contained in the tray at a temperature of up to about 400F.; and peeling the lid from the tray after cooking.
The bond between the tray and lid is hermetically tight when cooking the food contained in the tray at a temperature of up to about 400F. and wherein the bond between the tray and lid is of peelable strength after cooking at up to about such 400F. temperature and at room temperature whereby the lid may be peeled from the tray without tearing.
1. A container for cooking food including:
the combination of a dish-like tray of polyethylene terephthalate, the tray having a bottom part and a body part and an outwardly projecting peripheral flange having an upper surface on the upper extremity of the body part;
an adhesive of an ethylene copolymer and petroleum wax having a composition consisting essentially of (a) 30 percent to 40 percent by weight of petroleum wax and (b) 70 percent to 60 percent by weight of such ethylene copolymer, positioned only on the upper surface of the flange. the adhesive having a melt flow of less than 0.2 grams at 320F. (160C) for 10 minutes and a surface free energy of less than 30 dynes; and
substantially flat cover part of polyethylene terephthalate having lower surface parts adapted to be bonded to the flange by the adhesive, wherein said adhesive is free from flow at cooking temperatures up to about 400F and said combination exhibits a 12 bond strength greater than 100 grams/inch at 400Fand less than 500 grams/inch at room temperature when said cover part is bonded to said flange.
2. The container of claim 1 wherein the polyethylene terephthalate in the tray has an intrinsic viscosity of at least 0.7.
3. The container of claim 1 wherein the ethylene copolymer is of an acid terpolymer resin consisting of an interpolymer of ethylene, vinyl acetate, and an organic acid.