HK1053281B - Heat-shrinkable, heat-sealable polyester film for packaging - Google Patents

Description

Heat-shrinkable, heat-sealable polyester film for packaging

Technical Field

The present invention relates generally to a particular type of polyester shrink film. More particularly, the present invention relates to a heat-shrinkable polyethylene terephthalate film that is coated and/or laminated to other films with a solvent-based heat seal coating for packaging, such as a bag or lidded tray.

Discussion of the related Art

Background

The use of heat shrinkable thermoplastic films is well known to the packaging industry. For example, poultry products are typically sealed in bags made from these films and heated, causing the bag to shrink until it fits snugly around the product. One such bag is a single layer of polyester film. These bags provide strength and protection by adhering tightly to the article, although they have the disadvantage that they must be sealed with an adhesive, since polyester is not heat sealable except at very high temperatures. Bags sealed with adhesive are generally not as strong in the sealed area as heat sealed bags and cannot be closed at the open end with existing heat sealing equipment.

Coextruded films (e.g., polyolefins) are used to produce heat shrinkable bags that can be economically produced on existing heat sealing equipment because they are heat sealable. They remain in good physical contact with the packaged product after heat shrinking, thereby retaining the juices in the packaged meat, but do not perform as well as laminated shrink bags. However, the mechanical properties of coextruded films are not very good, such as tensile strength and modulus, and therefore bags made from these films are more prone to tearing or physical damage during handling. Moreover, they generally do not have high temperature resistance, which limits their use for cooking with a package.

U.S. patent 4,971,845 discloses an oriented heat-seal, heat-shrinkable adhesive laminate film, in one instance, comprising a polyester film layer and a polyolefin film layer, wherein the film layers have similar shrink characteristics. One example discussed combines layers having "approximately the same shrink characteristics", i.e., a shrinkage of about 50%.

There is a need for thermoplastic films that have high strength, are heat shrinkable and heat sealable, and are resistant to high temperatures.

Summary of The Invention

The present invention provides a heat-shrinkable film for packaging, comprising:

(a) a heat-shrinkable film comprising a polymer comprising at least 80% by weight of a polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of 5% to 55%, said film having an outer surface and an inner surface; and

(b) a solvent-based, heat-seal coating on at least one of the outer surface and the inner surface.

The present invention also provides a method of packaging an article comprising the steps of:

(a) wrapping an article in a shrink film comprising a polymer comprising at least 80% by weight of a polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of 5% to 55%, said film having an outer surface and an inner surface, said inner surface being coated with a solvent-based heat seal coating;

(b) heat sealing the desired edges to form a bag;

(c) sealing the bag with the article therein; and

(d) shrinking the bag by heating.

The present invention also provides a tamper-resistant packaging container capable of venting steam, comprising in combination:

a) an open rigid or semi-rigid container having a bottom portion and a sidewall extending upwardly from the bottom, the sidewall having an outer surface;

b) a heat-shrinkable film having an inner surface and a solvent-based heat-seal coating applied to such inner surface, said inner surface of said film being in intimate sealing contact with at least a portion of the outer surface of the sidewall, wherein the heat-shrinkable film comprises:

i) a polymer comprising at least 80% by weight of a polyethylene terephthalate polymer; and

ii) wherein the film is biaxially oriented in the range of 5% to 30%.

The present invention comprises a tamper resistant heat shrinkable lidding film having an inner surface for covering a rigid or semi-rigid container having an outer surface, said film comprising at least 80% by weight of a polyethylene terephthalate homopolymer or copolymer and having a plurality of edges and a thickness of 12 to 75 microns, and being biaxially oriented in the range of 5% to 30% shrinkage factor; the inner surface comprising a solvent-based heat seal coating thereon, wherein:

a) said film being in intimate contact with at least a portion of the outer surface of said container;

b) the film maintains tamper-resistant engagement with the container when the edges of the film are heated, hermetically closing the container, but allowing air and moisture to pass into and out of the lidding film when heated.

The present invention also provides an improved process for laminating PET film with other films comprising the steps of:

(a) applying a solvent-based laminating adhesive to the PET film;

(b) heating below a shrinkage temperature to dry the adhesive;

(c) the film is laminated to other films without the need for additional adhesives.

According to another embodiment of the heat-shrinkable film of the present invention, the heat-shrinkable film of the present invention further comprises a metallized layer under the heat-seal coating layer.

According to another embodiment of the heat-shrinkable film of the present invention, the film is laminated to one or more other layers to form a laminated film.

Another aspect of the present invention relates to a bag formed from the heat-shrinkable film of the present invention.

Another aspect of the present invention relates to a package formed from the heat-shrinkable film of the present invention.

According to another embodiment of the package according to the present invention, the heat-shrinkable film further comprises a metallized layer on the inner surface, and a heat-seal coating on the metallized layer, and wherein the package exhibits a pearlescent appearance after shrinking while maintaining functional barrier properties against oxygen and moisture.

Brief Description of Drawings

FIG. 1 is a perspective view of a container made according to the present invention prior to shrinking the sides of the film lid.

Fig. 2 is a perspective view of a container made according to the present invention after shrinking the sides of the film lid and before shrinking the top of the lid.

Fig. 3 is a perspective view of a sealed container prepared using the present invention.

Fig. 4 and 5 are schematic cross-sectional views of the apparatus according to the invention.

Detailed description of the preferred embodiments

The present invention relates to a particular type of polyethylene terephthalate ("PET") shrink film, and methods related thereto, which provide the advantages of packaging for non-culinary uses and uses for culinary purposes therein. The present invention is based on polyethylene terephthalate polymer (homopolymer or copolymer) films that are heat shrinkable and heat sealable, are typically of thin gauge and have low shrinkage. The present inventors have discovered a method of pre-applying a solvent-based heat seal coating to a film during its manufacture, thus avoiding the need for post-application of adhesives (which is typically required for applying adhesives to films with low shrink temperatures).

This is a significant advance over the prior art, reducing the time and cost of manufacturing the film, and providing new uses for the film. Conventional post-applied adhesives must be aligned with the area to be sealed and cannot be intertwined with each other. They typically take 14 days to cure in air, limiting their use. They are generally limited to the manufacture of bag trays (bagstock) in which the film is sealed to itself. The present invention allows the sealant to be applied over the entire web and therefore does not limit the sealing area or structure in which it is used.

In addition to the ability to apply solvent-based heat seal coatings to base films, the present invention also provides techniques for applying metallized coatings to base films, and between the film and any solvent-based heat seal coatings, as well as techniques for applying antifogging agents and other agents.

In addition, the base film (with or without a solvent-based heat seal coating) is optionally metallized and/or coated with an antifog agent, which can be laminated to other polymeric films to provide unique options and properties, particularly for applications in packaging applications, such as single bag systems for packaging meat instead of current two-bag packaging. The additional laminate may provide greater strength, barrier properties, printed surfaces, sealing characteristics, and decorative visual features when a metallization process is used.

Heat-shrinkable, heat-sealable base film

The heat-shrinkable, heat-sealable PET shrink film of the present invention, the "base film", comprises at least about 80% by weight PET, more preferably at least 90% by weight PET. The PET may be a homopolymer or copolymer of PET. By PET homopolymer is meant a polymer formed substantially from the polymerization of ethylene glycol and terephthalic acid, or alternatively, from its ester forming equivalent (e.g., any reactants that can polymerize to ultimately provide a polyethylene terephthalate polymer). By copolymer of PET is meant any polymer comprising (or derived from) at least about 50 mole percent ethylene terephthalate, the remainder of the polymer being formed from monomers other than terephthalic acid and ethylene glycol (or their ester-forming equivalents).

The PET base film of the present invention is further defined as:

1. biaxially oriented in the range of about 5% to 55% shrinkage factor, more preferably in the range of 10% to 30% shrinkage factor; and

2. for cap applications, the thickness is 12-75 microns (more preferably 12-20 microns).

Suitable polyethylene terephthalate shrink films are available from e.i. du Pont DE nemours and Company, Wilmington, DE under the trademarks Mylar and Mellinex.

The shrink films of the present invention are beneficial in packaging applications for a number of reasons relative to conventional shrink films. The PET shrink film is tough relative to many conventional shrink films, and it has been found that a smaller amount of orientation of the film ("low shrink") can better accommodate the protrusions, i.e., not shrink to such an extent as to create a risk of puncturing the film or to crush the contents of the protrusions, and not shrink to such an extent as to form an unsightly mass of shrink film around the protrusions. As a lidding film, this thin gauge of shrink film (combined with low shrinkage) causes the lidding film to tear when the package is opened, creating tamper evidence. Conventional shrink films typically shrink to a significantly greater film thickness to provide a cap that can be pulled off without tearing, leaving no evidence of tampering.

The base film of the present invention forms a tight, hermetic seal due to the presence of the adhesive coating when previously shrunk into a package. However, due to the thin gauge and low shrinkage, the seal itself can still outgas when heated in an oven or the like. Conventional shrink films either form a tight laminate seal or are inherently vented, but these two are generally not possible. By means of the invention, the cross-over of the outer wall, the outer lid rim, the top of the lid and the compartment can be sealed against leakage from each other. Another advantage is that this structure can be used for Modified Atmospheric Packaging (MAP).

The lower shrink force films of the present invention also provide superior appearance and are generally more economical to use than conventional (higher gauge, higher shrink) shrink films, particularly for closure applications, because they minimize the amount of material required to cover the product and container used.

The heat-shrinkable PET base film has other additional advantages. It may be surface printed or printed by itself or capture on lamination (trap). For the material in which the cooking process is carried out, it may be laminated with a variable shrinkage of 5-45%. Both of the laminate type and the heat seal type can be used as the heat-shrinkable type lidded tray. Both of these versions can be used to package substances other than food.

Application of heat-seal coating

The applicant has found a method of imparting adhesive to a base film which avoids time consuming or difficult post-processing procedures. Surprisingly, the present inventors used solvent-based heat seal coatings, which are generally considered impossible to impart a low shrink onset temperature to the base film, typically about 80 ℃. The heat sealer material itself is preferably a copolyester or ethylene vinyl acetate copolymer (EVA), more preferably an amorphous copolyester. It is contemplated that other sealants known to those skilled in the art may also be used.

More preferred copolyester heat sealers are derived from at least the following: about 10-60 mole% terephthalic acid ("monomer a"); about 10-60 mole% ethylene glycol ("monomer B"); and about 5-60 mole% of a third party monomer which is a secondary diacid ("monomer C") and/or a secondary diol ("monomer D").

Examples of monomer C include: succinic acid, adipic acid, azelaic acid, sebacic acid, 1, 10-decanedicarboxylic acid, phthalic acid, isophthalic acid, dodecanedioic acid, and the like. Examples of the monomer D include: methoxy polyalkylene glycol, neopentyl glycol, propylene glycol, 1, 4-butanediol, trimethylene glycol, 1, 6-hexanediol, 1, 4-butanediol, diethylene isophthalic acid.

The adhesive is applied in the form of a solution. Key factors in providing a solvent-based heat seal coating of a heat-shrinkable PET base film are a) proper selection of the solvent(s) of the adhesive (e.g., tetrahydrofuran is preferred) so that drying can be accomplished at temperatures below 80 ℃, and (2) the use of lower winding tensions during processing than typical films.

The solvent-based heat seal coating may be applied to one side (or both sides) of the heat-shrinkable PET substrate by any of a number of coating techniques well known to those skilled in the art. For example, it can be applied by roll coating (coating roll), spray coating, gravure coating, slot die coating, and preferably roll coating or gravure coating using a solution coating method.

In particular, the conditions given below are important since the film is very unstable and shrinks prematurely when exposed to high temperatures and stretches when wound too tightly.

● drying temperature (. degree. C.): about 70-75

● film winding tension (pli): about-0.25-1.0 (288-1153g/cm)

● coating/winding speed (ypm): about 100-

● coating weight (grams per square meter): about 1.0 to 10

The resulting heat-sealable, heat-shrinkable PET film exhibits a heat-seal strength of greater than 260g/in (102g/cm), from about 1 to 10g/cm²Preferably about 1.5 to 2.5g/cm²And film shrinkage at 100 ℃ of about 5% to 55%, preferably 10% to 30%.

Application of metallized layers and other additives

The above-described solvent-based heat seal coating provides a heat sealable surface for the heat shrinkable PET base film. The additional metallization layer may provide an oxygen and moisture barrier as well as decorative features to the film.

The heat-shrinkable PET base film has an outer surface and an inner surface, and may be coated with a metal layer on one or both surfaces. A solvent-based heat seal coating may be applied over the metal layer. The heat-shrinkable PET base film may be heat seal coated on both sides or aluminized on both sides in any combination.

The metal used may vary, although aluminum is preferred. The metal layer may be applied to the heat-shrinkable PET base film by known methods, such as standard vacuum deposition.

For the aluminised layer, the key conditions are:

● optical density (heavy metal deposition): about 0.75 to about 4; preferably 2.6-3.0

● optical density (light metal deposition): about 0.25 +/-10%.

Optical density was measured using a digital display transmission densitometer in the range of 0-4.0 equipped with four selectable filters for color and visible light density measurements. Measurement shows that: american Standard (ANSI) opal glass, diffuse visible transmission density.

In addition to one or more metallized layers, other components may be applied to the base film to achieve other desired properties. For example, antifog or anti-slip agents may be incorporated into the solvent-based heat seal coating prior to application to the shrink film to achieve antifog or anti-slip properties. Other additives commonly used in the industry can be introduced in a similar manner.

Lamination with other films

The heat-shrinkable PET base film may also be adhesively laminated to other materials to enhance the overall structure performance, depending on packaging requirements. The shrinkable substrate used may be plain uncoated, solvent-based heat seal coated, and/or metallized. Other materials for the laminate will vary greatly depending on the properties sought, but include nylon, polypropylene, polyethylene, ionomers, Acrylates (ACR), Ethylene Vinyl Acetate (EVA), polyethylene terephthalate (PET), polystyrene, ethylene vinyl alcohol (EVOH), polyvinylidene chloride (PVDC), and combinations thereof.

For example, an uncoated base film may be laminated to an ionomer film, as well as a one-sided metallized version. Typically, PET base films have shrinkage of about 20% in both MD/TD directions. The two-part polyester polyurethane adhesive can be applied to heat-shrinkable PET as a laminating adhesive by a gravure roll. The laminating adhesive is applied to the web from solution.

The shrinkage of the PET base film and the other film to be laminated need not be similar, and in fact, a shrinkage difference of about 5% or significantly more does not cause any problem. The amount of shrinkage is determined by the heat-shrinkable PET substrate, but is of importance exactly as it is determined by the thickness and rigidity of the second web. One example is that when adhesively laminated to a thicker or stiffer substrate, the shrinkage of the PET substrate is required to be higher in order to obtain the desired shrinkage from the laminate. It is also important to select a solvent-based laminating adhesive, wherein the diluent solvent used, as well as the adhesive, can be dried at less than 80 ℃. This is due to the heat sensitivity of heat-shrinkable PET to prevent premature shrinkage of the film prior to lamination to the second web.

It is recommended to treat both web surfaces in contact with the applied adhesive with a corona process. If no heat seal agent is applied to the base film, a secondary structure such as an ionomer or multilayer coextruded film may be used as the heat seal layer of the resulting laminate, also providing additional puncture resistance and formability. Such laminated films have unique utility in packaging, such as providing the ability to package single-pouch systems of meat (previously used two-pouch systems). Such two-bag systems typically use olefin or nylon bags to package the meat or poultry to keep the juice adjacent to the product, and then place the package in a second non-shrink metallized bag for protection and decoration with a barrier layer.

The present invention provides a single bag system capable of retaining juice and providing barrier and decorative properties and provides greater economic benefits due to the reduction in materials and effort of the two-bag process. Metallized varieties provide a high oxygen/water barrier and decorative features when formed into bags or lidded trays. The decorative feature is "pearl-like" which results from the shrinkage deformation of the metal layer as it passes through the package, but still maintains a good barrier layer.

The laminating adhesive may be applied by any of the well known coating techniques described above, preferably by roll or gravure coating using a solution coating process. The types of adhesives used to make the laminates herein are those commonly used in the industry, but due to the sensitivity of the shrinkable substrate to heat, the correct combination of adhesive and solvent selection is critical.

In addition, similar to the application of solvent-based heat-seal adhesive coatings, important operating conditions for the application process of the urethane adhesive coating to provide a heat-shrinkable base film are a) proper selection of solvent (e.g., isopropanol or ethyl acetate) to accomplish drying below 80 ℃, and b) use of a heated nip roll to bring together the heat-shrinkable web and the second ionomer web, activating the adhesive at below 80 ℃ without inducing premature shrinkage of the heat-shrinkable PET substrate. The same requirements must be followed when laminating heat-shrinkable PET substrates to other substrates or films.

The resulting laminated structure had the general properties of:

● bond strength between laminated film layers: > 300 g/inch (118g/cm)

● film shrinkage (%) at 100 ℃: 5-45, with 10-30 being preferred

● barrier Property before shrinkage OTR cc/100in²24 hr: 0.10/after shrinkage of 0.2-0.5 (in cc/100 Gm)²24hr means 0.645/after shrinkage 1.29-3.225)

● barrier property before shrinking of metal variety of laminate, WVTR gm/100in²24 hr: 0.05/after shrinkage of 0.25-0.35 (in cc/100 cm)²24hr means 0.3225/after shrinkage 1.61-2.26)

Note that: the effect on barrier properties is determined by how much the metal is deformed by the shrinking step. Even when the full 20% shrinkage occurs in the machine/cross direction, the barrier layer is still functional and never exceeds 0.5 OTR/WVTR.

Application in packaging in general

In use, the film can be used in a myriad of applications. For capping, the film is cut to size and covered on a tray to be capped. The film is then held in place by mechanical means, vacuum, or the like. Heat is then applied to the sides of the package, causing the film to shrink around the perimeter of the package. The film may then be shrunk along its central portion to further tension the film and eliminate wrinkles and the like.

For other applications, the film may be pre-sealed to form an open pouch, which may then be filled with contents on an in-line packaging machine. The bag is then sealed and heat shrunk around the contents. Such bags are heatable and can become self-venting once the internal temperature and pressure reach the softening point of the sealant.

For capping, the films of the present invention are well suited because the films are substantially non-elastomeric once shrunk. The membrane may also be hermetically sealed to the container. This is important for improved atmospheric packaging (MAP) and the need for a hermetic seal to the tray and between the chambers. This prevents overflow during processing and dispensing. In addition, the package is designed to become self-venting once the internal temperature and pressure reach the softening point of the sealant. The sealant is intentionally designed to fail under these conditions. Even after the sealant fails, the package remains visibly tamped due to the lid-to-container configuration.

When covering disposable containers, in particular trays made of crystalline pet (cpet), amorphous pet (apet), paper, aluminium, polypropylene (PP), Polyethylene (PE), polyvinyl chloride (PVC), polyvinylidene chloride (PVDC) or Polystyrene (PS), it is recommended to use a heat-shrinkable, heat-sealable base film.

The films of the present invention also provide less shrinkage relative to conventional shrink films and still provide the sealing benefits of shrink films. This makes the film easy and simple to use.

Use of films for capping applications

The invention is also applicable to the sealing/capping of food containers, including rigid (reusable) and semi-rigid (disposable) containers.

The preferred container structure includes a wall extending outwardly from the center of the tray. The design of the walls can vary widely, but when the film on the top of the container shrinks (first along the sides and then at the top) it should be sufficient to hold the heat shrinkable film on the sides of the container.

The shrink films of the present invention are preferably oriented in a range of about 5% to about 30%, more preferably about 10% to about 20%, in each direction. Many commercially available heat-shrinkable polyethylene terephthalate films have a degree of orientation of about 40 to 50% or more, and such shrink films are not suitable for use in the present invention for capping. A lower shrinkage rate is preferred because higher shrinkage rates deform less rigid materials when applied to a dish and during reheating and/or cooking cycles.

The thickness of the coverlay film is preferably about 12-75 microns, more preferably 12-20 microns. Suitable polyethylene terephthalate shrink films are available under the trademark Mylar _ LD from e.i.du Pont de Nemours and Company for capping applications.

In use, a heat shrinkable film is placed on top of the container. Sufficient coverage or overlap of the heat shrinkable film should be provided so that the film is secured below the upper surface of the container after shrinking. The maximum overlap requirement is not critical as long as the overlap does not significantly exceed the depth of the sides of the container after shrinkage. The dimensions of the film before shrinking can be adjusted according to the known shrink characteristics of the film. Generally, a final overlap of about 1-7 centimeters is satisfactory.

Preferably, the film is then constrained to the container. The film may be constrained to the top alone, such as with a platen, or to the sides, or to both the top and the sides. The constraining mechanism can vary widely and can include, for example, positive air pressure, vacuum, robotic arm, or pad. It has been found that a rubber gasket, in particular silicone rubber, is particularly satisfactory for the constraining side. The liner may be secured to the inside of a frame or panel in which an aperture is formed which may pass over the upper surface of the container.

The film is heated to a temperature above the shrink/seal temperature and below the melting temperature of the heat shrinkable film while the film is restrained on the container. The heating means may vary widely and may comprise, for example, heated fluid or radiant heat. The film is preferably heated by a heated fluid. For example, the fluid may be water or gas, with air being preferred for convenience and economy. In any case, the fluid should be heated above the shrink temperature of the heat shrinkable film.

Preferably, the film is heat shrunk along the sides of the container before the film is heat shrunk/sealed along the top of the container. In this way, the film along the sides will shrink and form an airtight seal around the rim of the container. The hot fluid temperature needs to be above the shrink temperature of the film and above the softening point of the sealant so that the film shrinks and the heat sealant is activated and stretched and closed against the exterior of the container to form a hermetic seal. Any exterior as well as the top can be hermetically sealed, if desired.

The top of the film can then be heat shrunk to remove any wrinkles and create a clean, taut lid surface along the top of the container. This sequential shrinking of the film can be achieved by shielding the upper surface of the container with a platen, while exposing the sides to heated fluid or shrink tunnel; the platen may then be removed and the top of the film then heat shrunk.

The heated fluid may be generated by some conventional or non-conventional method. For example, the film may be heated using a conventional air heating gun, and the air temperature may be controlled by Resistance Temperature Detectors (RTDs) incorporated into the air heating gun. The heated air may be directed along the sides of the container using circumferential or peripheral ducts. The heated air flow may also serve to restrain the film by impinging on the film along the sides of the container. In such embodiments, the peripheral tubing may be adapted to the shape of the container and adapted to accommodate the air heating gun.

The heating mechanism can vary widely depending on the size of the container and the desired speed of operation. It has been found that a 200 volt, 2000 watt air heating gun can produce sufficient heated fluid flow for an average size food container packaged in accordance with the present invention. The air heating gun can be controlled by a PID microprocessor temperature controller and RTD (resistance temperature detector) sensing device. The RTD changes resistance with temperature changes and therefore instructs the temperature controller to adjust the current as needed to maintain a constant temperature.

The exact position of the nozzle for the heated air can be adjusted to suit the particular configuration of the container being covered. For example, the hot air pipe may be above the top of the container with the nozzle directed to a point below the top of the container, thereby shrinking the film covered and restrained along the sides of the container.

The temperature used to heat-shrink the material on top can also vary widely, being higher than the shrink temperature of the film (as it shrinks along the sides of the package), but lower than its melting temperature. Fluid temperatures above the melting temperature of the film and heat seal coating can be used to shrink and seal the film to the container without melting as long as the exposure time is properly adjusted. Similarly, the shrinking of the top of the film can also be achieved with an air heating gun, which mounts a nozzle adapted to the size and shape of the packages to be treated.

The heat shrinkable film used is typically cut to overlap the top of the container to be capped and to have a dimension of at least about 1cm on each side of the container after shrinking. Depending on the shrink sequence and the particular equipment used, the heat-shrinkable/heat-sealable film can be cut to size prior to, simultaneously with, or after heat-shrinking/heat-sealing.

The invention may be further understood by reference to the accompanying drawings, in which figure 1 is a perspective view of a container 1 having a side wall 2. In the embodiment shown in the figure, the side wall 2 has an outwardly turned edge 3. While the illustrated edged tray is preferred, it should be understood that the present invention is not limited to the use of such a tray. For example, the container may be subdivided into individual food portions. The heat-shrinkable, heat-sealable film 4 is cut to a size larger than the outer dimensions of the top of the container. The film is held along the container sides 5 below the top of the container by a frame 6 having an inner rim 7 to which a liner 8 is attached. The inner edge of the liner is slightly smaller than the outer periphery of the container top so that the frame with the liner is mounted on the container top. A heated fluid, such as air, may be directed to the edges of the film below the top periphery of the container, thereby shrinking/sealing the film and securing the film to the container.

Figure 2 shows the container after the heat shrink film side, where the shrink film is sealed to the side wall 2 of the container. At this point, the heated fluid may be applied to the film on top of the container, shrinking the portion and creating a hermetically sealed package as shown in FIG. 3. With sequential shrinking of the film, the restraining means are typically removed prior to shrinking the film on top of the container.

While it is possible to shrink wrap the surrounding film in sequence, as described above, it is also possible to shrink the entire film simultaneously with a platen of heated fluid of appropriate size.

Fig. 4 and 5 are schematic illustrations of exemplary devices that may be used in accordance with the present invention.

In fig. 4, a tray 11 placed on a conveyor belt 12 is covered with a film 13 supplied by rollers 14. The film is held in place on top of each container by a press plate 15 and the edges of the film are cut by a die cutter 16. The sides of the film are then shrunk with hot air passing through an air heating gun 17. The tray is then moved into the collapsing channel 18, where the top of the container is collapsed.

Another device that can be used according to the invention is shown in fig. 5. Here, the tray 11 on the conveyor belt 12 is similarly covered with a film 13 supplied by rollers 14. The edges of the film, however, are held in place around the top of the container by a tucking device 51 and die cut by a blade 52. This constrains the sides and the trays with the tucker or constraining means to move into the retraction channel 53 where they are held in place by vacuum holding means 54 applied to the bottom of each container. In this apparatus, heat shrinkable/heat sealable films on the sides and top of the container are simultaneously shrunk/sealed in a shrink tunnel. The discarded heat-shrinkable, heat-sealable film is rewound on a roll 55.

The heat-shrunk, heat-sealed polymeric film can be easily removed from the sides and top of the container by removing or tearing off the top with exposed edges. Suitable cuts, tear-off remainders or peel-off remainders may also be provided as desired.

While the lid prevents tamping of the food and hermetic sealing over and between the top chamber, it also prevents leakage from one chamber to the other during transport and display of the article. Also, upon cooking or reheating, the design is such that the sealant fails and the package itself becomes self-venting or otherwise prevents escape of the expanding gases therein.

The invention is further illustrated by the following examples.

Examples 1 to 2

The heat-shrinkable, heat-sealable polyethylene terephthalate film having a thickness of about 19 microns and shrinkage rates of 20% and 9% was used to cap a linearly formed food container having approximate dimensions of 6 ". times.8" ". times.2" (15.9 cm. times.21.6 cm. times.5.1 cm). Each film was cut to size on each side of the container at each dimension of the top surface, taking into account shrinkage. The membrane is held in place under the rim by a frame lined on the inner surface of the frame.

The tray contains a wooden block with two nails protruding about 2.6 cm above the edge of the tray to simulate a sharp-edged wrapper. The first tray was covered with a biaxially oriented shrink film of about 20% and the second tray was covered with a biaxially oriented film of about 9%. The film is first shrunk around the sides by air applied through a nozzle, heated to a temperature above the shrink temperature of the film and below its melting temperature, and then the constraining liner is removed. The film on top of the container is then retracted with the same air supply of the continuous path of the nozzle over the top of the film. After placing the lidding film and placing in a 350 f (177 c) oven for about 20 minutes, no film perforation occurred, which remained hermetically sealed over the entire tray edge.

Examples 3 to 5

EXAMPLE 3 bag to be cooked therein

The copolyester coated heat sealable/heat shrinkable substrate can be formed into a bag stock for cooking or reheating various meats and/or vegetables in a microwave oven or conventional oven. Both the heat-shrinkable PET base film and the sealant are FDA approved for use up to 204 ℃. For example, bags using the base film can be formed using known bag making techniques. The copolyester sealant is heat sealed to itself by heat sealing or to a common PET substrate as desired.

In the test case, heat sealing of the sealant and the sealant was performed using a 1-inch (2.54cm) heat sealing bar to form a three-side sealed, one-side open bag. Two fully seasoned, entire 6 pound chickens were then individually placed in separate bags and heat sealed to close the bags in the same manner as the other three side seals. The sealing temperature was 150 ℃ and 30psi (20.7N/cm)²) And the pressing time is 0.5 second. The recommended temperature range for sealing such copolyester sealants may range from 93 to 204 deg.c depending on the equipment and sealing method used. One of the sealed bags was placed in an electric furnace and the other in a gas furnace. Both stoves were set to a cooking temperature of 177 ℃.

The bag is free of vents as recommended by other bags cooked therein. These types of bags are typically made of uncoated nylon or polyester and also require a closure clip or strap. When the inner package begins to reach a high temperature, the bag begins to expand due to the steam pressure generated in the bag. When the internal temperature reaches the softening temperature of the sealant (which keeps the bags heat sealed together), the seal will fail at its weakest point and a self venting process can occur. The benefits of self-venting are 1) oxygen exchange is possible, thus helping to promote the desired browning effect on the package; 2) it also maintains a closed moisture, allowing the food to cook in its own juice, maintaining its moisture and flavor.

After two hours, the two bags were removed. The result was that both chickens were very moist and fragrant and, when removed from the bag, the chicken had deboned.

EXAMPLE 4 Container/capping application

The heat-seal/heat-shrinkable PET film in the new invention can be used in the same manner and provides all the benefits described in us patent 5,873,218. The main difference is that the outer lid of the container can be hermetically heat sealed with a copolyester sealant in addition to shrinking around and under the lid itself. Another benefit over the prior invention is that containers separated by bridges forming separate chambers can be completely sealed along those bridging regions, thereby eliminating not only leakage along the outer edges, but also leakage from one chamber to another during dispensing.

For example, the film of the invention is pre-cut to size, in proportion to the shrinkage rate, (covering the opening and) covering the outer wall of the container. The film is then put in place as described in the referenced invention. The sides of the container are then heated, causing the film to shrink around the perimeter of the tray. This securely fixes the film to the container and also activates the heat seal layer to form a hermetic seal of the outward lid. The method is also applicable to containers with sloping sidewalls without external lids. By heating the film to the point where the sealant melts but below the melting point of the film, in combination with the pressure created by the film shrinking, is sufficient to bond the film to the container. The disclosed packaging structure types can be used to produce a tamper evident cooked or reheated product, a dual heat resistant self venting package or a product requiring tamper evident features that do not imply exposure to higher temperatures.

Example 5 lamination

The laminated product of the heat-shrinkable PET base film of the present invention can be formed into a bag material as described above. In the case where shrinkage, perforation, flex-crack resistance, metallization for barrier or decorative purposes, and heat resistance may be desired, any number of combinations from the materials mentioned may be made based on the application requirements. For example, the following bags are prepared in a stacked configuration from the outside to the inside:

heat shrinkable (0.5 mil) (20% MD/TD) PET substrate/aluminum/adhesive/ionomer sealant layer (2 mil).

The bags were made by taking a length of film and folding the sealant layers over each other, heat sealing them to create a two-sided seal, leaving the top open. Two cooked meats are packaged in two separate bags. A turkey breast and a ham weighing about 4-6 pounds (1.8-2.7kg) were used in the test. Due to shrinkage factors, the bag must be made larger than the wrapper to compensate, but not so large that the bag cannot be tightly shrunk onto the wrapper. Each package was individually placed in a vacuum chamber to evacuate oxygen and sealed. The package is then removed and exposed to hot air above the shrink temperature but below the melting point of the film. The film shrinks tightly, overlapping around the package and gaining the physical signs of the package.

The advantages of this structure and method are 1) metallization not only to increase shelf life protection but also to provide a decorative function, 2) coefficient of shrinkage to eliminate exposed edges by allowing the wrap to overlap and to show surface signs of the packaged article. By eliminating exposed edges, the chance of puncturing other packages is also reduced. 3) The decorative function is improved by the shrinkage. As the film structure shrinks, the metal deforms and takes on a pearl-like effect. This effect can be controlled by the shrinkage rate determined by the structure.

Examples 6 to 13-Heat seal Strength

A heat-shrinkable copolyester film (50 gauge Mylar (r) film available from DuPont, Wilmington DE) was coated with a solvent-based amorphous copolyester adhesive. The coated film was sealed to itself and to a semi-rigid substrate (i.e., a tray made of PVC, amorphous PET, and crystalline PET) at both 250 deg.C (121 deg.C) and 300 deg.C (149 deg.C). Heat seal strength was measured for each heat seal and after 3 days of aging at 32 ℃ and 3 days of aging at 0 ℃ (-18 ℃).

Similarly, the heat shrinkable copolyester film is first metallized and then coated with a solvent based amorphous copolyester adhesive. The resulting film was then sealed to itself and to a semi-rigid substrate (i.e., a tray made of PVC, amorphous PET, and crystalline PET) at two temperatures, 250 deg.C (121 deg.C) and 300 deg.C (149 deg.C). Heat seal strength was measured for each heat seal and heat seal strength was measured on 3 days at 32 ℃ aging (0 ℃) and 3 days at 0 ℃ (-18 ℃).

The heat seal strength was measured in triplicate on a 1 inch (2.54cm) wide sample. Seal strength was measured by peeling the seal apart in the Machine Direction (MD) using an Instron (Instron tester). The heat seal strength is the maximum force required to cause seal failure, is read from the appropriate scale of the tester, and is measured in grams per inch width (g/2.54 cm).

TABLE 1 Heat seal Strength

The method and the conditions are as follows: CR-188 at 12 "/min/0.50 second nip/20 psi 1" tape

(30.5cm/min/0.50 sec press/13.8N/cm²) (2.54cm)

72A/50% R.H. aging

Coated with amorphous copolyester Sealing for 24 hours Seal at 32_ Weathering for 3 days

Temperature of250F failure 300F failure

Examples 6-987 Peel 950 Peel 647 Peel/tear 792P/T

Coated and coated

Example 7-coated peeling/tearing 671 peeling from PVC 616 disruption 1237 disruption

Example 8-coated Peel off from APET 664 788 Peel off 460 Peel/tear 926 Damage

Example 9-coated Peel 568 Peel/tear 569 Peel/tear 605P/T from CPET 477

Amorphous copolyester coating on metal

Temperature 250F failure 300F failure

Examples 10-797 Peel/tear 848 Peel/tear 456 disruption 906P/T

Coated and coated

Example 11-coated PVC 446 Peel/tear 510 Peel/tear 702 Peel/tear 862P/T

Example 12 coated and APET 565 peels 844 peels 685 peels 980P/T peels

Example 13-coated with CPET 397 peel-585 peel-690 peel-871 peel

Interpretation of failure modes

Stripping: the film clearly peeled off from the substrate

Peel/tear (P/T): the film being partly peeled off and then torn

And (3) destruction: the seal strength is greater than the film strength, resulting in orderly failure of the seal interface

Examples 14-23 Properties of laminated Heat-shrinkable PET base film/ionomer film

A heat-shrinkable, heat-sealable PET base film (Mylar (r) film having a shrinkage of about 20%) was laminated to a 2mil (0.005cm) ionomer film (Surlyn 1601 ionomer film, available from DuPont, Wilmington DE) using a two-part polyester polyurethane adhesive. The bond strength was determined using a method similar to the heat seal strength determination described above. Provided that the sample was a 1 inch (2.54cm) tape aged at 72 deg.C (22.2 deg.C) and 50% relative humidity for one week using an Instron at a speed of 12 inches per minute.

Table 2: bond strength of laminate

Common thermal shrinkage type PET base material

Sample numbering	Grams per inch (g/2.54cm)	Fail to work
			1415161718	458470448450449	Easy to tear (Zipper) peel
Average	455

Metallized heat shrinkable substrate

Sample numbering	Grams per inch (g/2.54cm)	Fail to work
			1920212223	17421015822137	Peeling and peeling
Average	160

Note that: failure is the result of metal to substrate adhesion

Examples 24 to 31: shrinkage measurement of films and laminates

Shrinkage was measured for various film and laminate samples. Three measurements were made for each specific sample. The experimental procedure generally involved placing a 5 inch by 5 inch (12.7cm by 12.7cm) sample in a boiling water heating bath. The sample was held between clamped screens and left in the bath for 5 seconds. The length of the cooled sample was measured in the Machine Direction (MD) and Transverse Direction (TD) and the shrinkage was calculated as specified.

These data clearly show that heat shrinkable polyester films control the shrinkage of the laminate and the coated film.

Table 3: dimensional change: shrinkage rate

Rate of change%

24)0.5mil Heat-shrinkable PET (20% MD/TD)/Metal/adhesive/2 mil ionomer MD-21.5

25)0.5mil Heat-shrinkable PET (20% MD/TD)/Metal/adhesive/2 mil ionomer TD-17.5

26)0.65mi Heat-shrinkable PET (45% MD/TD)/adhesive/2 mil ionomer MD-40.5

27)0.65mil Heat-shrinkable PET (45% MD/TD)/adhesive/2 mil ionomer TD-38.5

28)0.50mil Heat-shrinkable PET (20% MD/TD)/copolyester coated MD-18.85

29)0.50mil Heat-shrinkable PET (20% MD/TD)/copolyester coated TD-19

30)0.50mil Heat-shrinkable PET (20% MD/TD)/Metal/copolyester coated MD-18.2

31)0.50mil Heat-shrinkable PET (20% MD/TD)/Metal/copolyester coated

TD -19.5

Examples 32 to 33: seepage prevention data

The barrier properties of heat shrinkable, heat sealable, metallized films and their laminates with ionomer films were tested using the following experimental methods:

ASTM D3985, oxygen test method at 72 ° F (22.2 ℃); and

ASTM F1249, conducted at 38 ℃, 99% relative humidity, Water vapor test method.

Table 4: seepage prevention data

cc/100in2/24 g/100in2/24

Hour(s)^*Hour(s)^**

32) Heat-shrinkable typeOTR H₂O

Of the metallisation

0.01750.0777% before 20% MD/TD shrink

0.110.315 after shrinkage

33) Heat-shrinkable metallized laminated

0.1150.0777% before 20% MD/TD shrink

0.4340.296 after shrinkage

^*Equivalent to cc/15.5cm²/24hrs

^**Equivalent to cc/15.5cm²/24hrs

Claims (10)

1. A heat-shrinkable film (4) for packaging, comprising:

(a) a heat-shrinkable film comprising a polymer comprising at least 80% by weight of a polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of 5% to 55%, said film having an outer surface and an inner surface; and

(b) a heat seal coating selected from the group consisting of ethylene vinyl acetate copolymer, polyethylene terephthalate copolymer, and combinations thereof, applied in solution on at least one of the outer surface or the inner surface.

2. The heat-shrinkable film of claim 1 wherein the heat-shrinkable film further comprises a metallized layer under the heat-seal coating.

3. The heat-shrinkable film of claim 1 wherein said film is laminated to one or more other layers to form a laminated film.

4. A bag formed from the heat-shrinkable film of claim 1.

5. A packaging article formed from the heat-shrinkable film of claim 1.

6. The packaging article of claim 5, wherein the heat-shrinkable film further comprises a metallized layer on the inner surface, and a heat-seal coating on the metallized layer, and wherein the packaging article exhibits a pearlescent appearance after shrinking while maintaining functional barrier properties to oxygen and moisture.

7. A method of packaging an article comprising the steps of:

(a) wrapping the article in a shrink film comprising a polymer comprising at least 80% by weight polyethylene terephthalate polymer, wherein said film is biaxially oriented in the range of 5% to 55%, said film having an outer surface and an inner surface, said inner surface having applied thereto a heat seal coating selected from the group consisting of ethylene vinyl acetate copolymer, polyethylene terephthalate copolymer, and combinations thereof, applied in solution;

(b) heat-sealing the desired edges to form a pouch;

(c) sealing the bag with the article therein; and

(d) heat shrinking the bag.

8. A tamper-resistant packaging container capable of venting steam, comprising in combination:

(a) an open rigid or semi-rigid container having a bottom portion and a sidewall extending upwardly therefrom, said sidewall having an outer surface;

(b) a heat-shrinkable film having an inner surface and a heat-seal coating applied in solution on the inner surface, the heat-seal coating being selected from the group consisting of ethylene vinyl acetate copolymer, polyethylene terephthalate copolymer, and combinations thereof; said inner surface of said film being in intimate sealing contact with at least a portion of an outer surface of said sidewall, wherein said heat-shrinkable film comprises:

i) a polymer comprising at least 80% by weight of a polyethylene terephthalate polymer; and

ii) wherein said film is biaxially oriented in the range of 5% to 30%.

9. A tamper-resistant heat-shrinkable lidding film having an inner surface for covering a rigid or semi-rigid container having an outer surface, said film comprising at least 80% by weight of a polyethylene terephthalate homopolymer or copolymer, having a plurality of edges and a thickness of 12-75 microns, biaxially oriented in the range of 5% -30% shrinkage factor; and said inner surface comprising a heat seal coating applied thereto in solution form selected from the group consisting of ethylene vinyl acetate copolymer, polyethylene terephthalate copolymer, and combinations thereof, wherein:

a) said film being in intimate contact with at least a portion of the outer surface of said container;

b) when the sides of the film are heated, the film remains in tamper-resistant engagement with the container, hermetically sealing the container, but allowing air and moisture to enter and exit the lidding film upon heating.

10. A heat-shrinkable film laminate for packaging comprising a plurality of film layers, wherein:

(a) at least one film layer of the laminate comprises a polymeric film comprising at least 80% by weight of a polyethylene terephthalate polymer, wherein the film is biaxially oriented in the range of 5% to 55%; said film having an outer surface and an inner surface, at least one of said outer and inner surfaces having a heat seal coating applied thereto, said heat seal coating being selected from the group consisting of ethylene vinyl acetate copolymer, polyethylene terephthalate copolymer, and combinations thereof, applied in solution;

(b) a solvent-based laminating adhesive between the film layers of the laminate; and

(c) the second or other film layer of the laminate comprises a polymer selected from the group consisting of nylon, polypropylene, polyethylene, ionomers, ACR, ethylene vinyl acetate, polyethylene terephthalate, polystyrene, ethylene vinyl alcohol, polyvinylidene chloride, and combinations thereof.