Pucker resistant film and package

Details Pucker resistant film and package Pucker resistant film and package

2003-09-29

2004-12-21

Nolan, Sandra M. (Department: 1772)

Stock material or miscellaneous articles

Hollow or container type article

Shrinkable or shrunk

C428S035700, C428S213000, C428S516000, C428S518000, C428S520000

Reexamination Certificate

active

06833170

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to an improved flexible package with walls made up of multi-layer thin films. In particular, the package comprises a wall with a permeability characteristic that reduces absorption of substances associated with a packaged food product, and thereby prevents swelling and disfiguration of the package wall.
2. Description of Related Art
Packages made up of multi-layer flexible thin films are commonly used as food containers. The materials used to construct such films are generally chosen for their barrier properties, which act to prevent shelf life-reducing agents, such as oxygen, moisture, and light, from entering the bag. Different materials have different resistances to penetration of oxygen and moisture, so such variations are factors considered when the combination of films that make up a package is selected. It is also generally true that the cost of flexible films, and the containers they form, increases as the films' barrier characteristics improve. So, from a manufacturing perspective, it is desirable to maximize barrier characteristics, while minimizing cost.
As newer materials with improved barrier properties are developed, and package designs are improved to incorporate those developments, the shelf life of the food product in the improved package generally increases. One example of such an improvement is the use of ethyl vinyl alcohol (EVOH) as one of the layers making up a food container. Use of EVOH in food packages makes them more resistant to the penetration of oxygen, which in turn increases the shelf life of the product.
The various film layers incorporated into flexible food packages are often employed for purposes other than to prevent penetration of agents that directly affect product shelf life. For example, a layer may be incorporated to preserve another layer of the package wall. In this regard, ethyl vinyl alcohol (EVOH), an excellent barrier to oxygen, is often used as one of the layers making up a food package. However, its resistance to oxygen penetration is reduced if it is exposed to moisture. Additionally, moisture that could erode the oxygen resistance of EVOH is often present not only on the outside of a package, but, depending on the type of food product, may also be present on the product side of the package wall. Therefore, the package wall must often include barriers to moisture on both the outside and product side of an EVOH layer.
In addition to moisture, any oils and other substances associated with a food product also come in contact with the product side of a package wall. For example,
FIG. 1
is a cross-sectional view of an embodiment of the wall of a typical multi-layer prior art food package. The first flexible thin film
220
is on the product side
210
of the package wall
200
, and comprises a sealant film layer
222
and a core film layer
224
. Preferably, the core film layer
224
is a polyolefin film, such as a polypropylene (PP) or polyethylene terephthalate (PET), or metalized PP or metalized PET. The sealant layer
222
typically comprises a ter-polymer blend of polypropylene, polyethylene, and polybutene. The ratio of these polymers on a weight basis is typically 5% polyethylene, 2% polybutene, and the remainder polypropylene. The sealant layer
222
is on the product side
210
of the first film
220
. Thus, the sealant layer
222
comes into contact with the food product in the package, which may be chips, candies, or other snacks. Any other substances associated with the food product, such as preservatives, flavoring, moisture, or oils, also come into contact with the sealant layer
222
of the first film
220
.
The second film
230
typically includes an EVOH layer between protective polymer layers. Alternatively, the second film
230
may include weakly bonded polymer layers that prevent tearing of the package upon opening by the consumer. The third film
240
includes a PP or PET film layer
244
that is on the outside
250
of the package, and often includes a printed ink layer
242
, the characteristics of which depend on the application. When a printed ink layer
242
is included, the PP or PET film layer
244
is made transparent so that the ink layer
242
can be viewed.
One unforeseen consequence of improvements in barrier properties and the resultant extensions of product shelf life is excessive absorption by the package wall of substances associated with the packaged product, such as oils or moisture. Such substances come into direct contact with the inner surface of the package wall throughout the useful life of the package. Prior art packages made up of multi-layer thin films designed to possess barrier properties that prolong a product's shelf life did not anticipate absorption of oils from within the package that could become excessive and have destructive consequences. Over time, and especially at elevated temperatures, substances associated with a product that are in direct contact with the inner surface of a prior art package wall are absorbed by the thin films that make up the package wall. As a result, accumulation of the absorbed substances results in swelling of package walls, and blemishing of the package's appearance.
FIG. 2
illustrates a cross-sectional view of the disfigured appearance of a prior art package wall
300
due to absorption of substances, such as oils and/or moisture associated with a food product inside the package. As in
FIG. 1
, the first flexible thin film
10
comprises a sealant film layer
12
and core film layer
14
, which may be any of PP, PET, metalized PP or metalized PET film. In applications requiring a prohibitive oxygen barrier, a second flexible thin film
30
typically includes an EVOH layer
34
sandwiched between moisture-blocking polymer layers
32
,
36
. A third flexible thin film
40
, also a PP or PET film, is on the outside
50
of the package. Any liquid or semi-solid substances, such as oils and/or moisture, that are associated with a packaged product on the inside
20
of the package wall
300
may be absorbed by the wall
300
. In the case of oils, the oils penetrate the sealant layer
12
and accumulate in the first thin film
10
. Over time, and especially at elevated temperatures, the accumulation in the first thin film
10
causes it to swell. The swelling
15
is manifested in the appearance of the package as the surface
302
,
304
of the package wall
300
, on both the inside and outside of the package, develops a slightly uneven look, which may be described as dimpled or puckered. This undesirable disfiguration greatly reduces the shelf appearance, and aesthetic market value, of the package.
Therefore, an improved package design and method are needed that minimize the absorption of product oils in the walls of a package containing food. The improvement should provide optimum shelf appearance for a longer time period. At the same time, the cost associated with the improved package should be minimized and other package characteristics, such as product shelf-life enhancing properties, should be maintained.
SUMMARY OF THE INVENTION
The proposed invention comprises a flexible food package formed from multi-layer thin films, where a film layer having a permeability characteristic that reduces absorption of food product substances from within the package is incorporated into the package wall design. The permeability characteristic, also referred to herein as “reduced permeability” and “reduced permeability characteristic,” is achieved by removing either the polyethylene or polybutene component from the ter-polymer sealant layer (making it a co-polymer layer) prior to orientation. The thus altered sealant layer decreases the amount of amorphousness of the polymer upon orientation, thereby reducing its permeability to food substances such as oil. An alternative embodiment of the invention reduces the amount of polyethylene and/or polybutene in the terpolymer of the sealant layer, leading as well to a decrease in amorphousness upon orientation and a reduction

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