Lidstock laminate

Details Lidstock laminate Lidstock laminate

2001-03-13

2003-09-30

Nolan, Sandra M. (Department: 1772)

Stock material or miscellaneous articles

Hollow or container type article

Shrinkable or shrunk

C206S461000, C206S471000, C428S036600, C428S036700, C428S200000, C428S201000, C428S204000, C428S205000, C428S212000, C428S213000, C428S349000, C428S514000, C428S518000, C428S522000

Reexamination Certificate

active

06627273

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates to a packaging film, and more particularly to a laminate useful as a lidstock for sealing a tray closed.
It is common in food packaging operations for a food product, such as fresh meat, to be placed on a tray, such as a thermoformed expanded polystyrene tray having a central depressed area and a surrounding peripheral flange. A thermoplastic film or laminate may then be positioned over the food and heat sealed to the peripheral flange to hermetically enclose the food product. In such arrangement, the thermoplastic film or laminate is the “lid” or “lidstock” and the tray is a “support member.”
It is important that the lidstock be capable of forming a strong, hermetic seal with the support member. This is true even where the sealing area of the tray may be exposed or contaminated with by-product (e.g., meat purge) from the packaged food. This is also true where, as is commonly the case, the support member is relatively rigid. Heat sealing a flexible lidstock to a rigid support member is more challenging than heat sealing the flexible lidstock to either another flexible film or laminate or to itself (for example, in a fin seal arrangement commonly used in vertical form-fill-seal operations).
To heat seal the lid to the support member, a heated bar engages the outside of the lid to compress it against the flange of the support member. In so doing, heat transfers from the heated bar to the outside of the lid, through the thickness of the lid, to the inside sealant layer of the lid, and to the flange of the support member. The resulting heat and compression causes the contacting surfaces of the lid and support member to become molten and to intermix with one another. The heating bar is then removed to allow the sealed area to cool and form a sealed bond.
The seal strength of the resulting sealed package may be determined by several methods. The support member may be pierced with an inflation needle and the interior of the sealed package may then be inflated until the lid or seal between the lid and support member fails. A higher internal inflation pressure at failure indicates a stronger seal strength. Alternatively, the sealed package may be placed in a vacuum chamber and subjected to decreasing external pressures until failure—a lower external pressure at failure indicating a stronger seal strength. Also, a representative sample of the seal may be cut from the sealed package (or formed separately) so that the lidstock may be pulled from the support member, for example, using an Instron tensile tester under specified conditions. A higher maximum force attained before failure indicates a stronger seal strength.
In all of these tests of seal strength, the failure mechanism may occur in one or more of several ways. In each case, the failure mode seeks a failure path requiring the least amount of force. For example, the bond between the lidstock and the support member may fail adhesively so that the lidstock simply peels away from the support member. Or, the lidstock may fail cohesively along a path cutting generally perpendicularly through one or more layers of the lidstock—and then fail adhesively along the interface between two layers of the lidstock. The failure path may combine an intricate path of cohesive and adhesive failures—all while the lidstock is being stretched by the applied force—to present a complicated failure mode.
The above discussion is made to establish that a weaker cohesive strength within a layer of the lidstock and/or a weaker adhesive bond strength between layers of the lidstock may weaken the seal strength of the sealed package. This is especially true where the seal strength failure mode is not simply the peeling of the lidstock from the support member by adhesive failure of the sealing bond between the lid and the support member.
A desirable lidstock provides gas (e.g., oxygen, carbon dioxide) barrier attributes sufficient to enhance the storage life of the packaged food. The barrier characteristics of the lidstock may have increased importance where the interior atmosphere of the package may be modified, for example, to decrease the concentration of oxygen from that of ambient air or to increase the concentration of oxygen and carbon dioxide from that of ambient air. For example, in packaging meat, the atmosphere in the sealed package may comprise about 80% by volume oxygen and about 20% by volume carbon dioxide in order to inhibit the growth of harmful microorganisms and extend the time period in which the meat retains its attractive red (“bloom”) coloration. Oxygen and carbon dioxide barrier attributes may be imparted to a film by incorporating, for example as a film layer, one or more resins having low permeability to oxygen. (Since carbon dioxide barrier properties generally correlate with oxygen barrier properties, only oxygen barrier properties are discussed in detail herein.)
It is not unusual for the inter-layer bond strengths associated with the incorporation of barrier resins or barrier layers into a lidstock to be weaker than the inter-layer bond strengths that would be present if the barrier resin or layer were absent. That is to say, the inter-layer bond strength between a barrier layer and an adjacent layer is usually the weakest inter-layer bond strength of a film. It is also possible that weaker inter-layer bond strengths may be associated with one or more “tie layers” that may accompany the use of a barrier layer. Although a tie layer may be inserted between the barrier layer and an otherwise adjacent film layer in order to improve the inter-layer bond adhesion, the resulting bond strength between the tie layer and its adjacent film layer may be less than the bond strength between the tie layer and its adjacent barrier layer. Accordingly, the tie layer may present the weakest inter-layer bond strength of the lidstock—and thus present the failure path during a seal strength test.
In order to produce packaged product at a fast (and therefore economical) rate, the lidstock should be capable of being quickly heat sealed to the support member. A lidstock that facilitates quick heat sealing is said to have good “sealability.”
It is also desirable for the lidstock to be printed. Such printing provides important information to the end-user of the packaged food—information such as the ingredients of the packaged food, the nutritional content, package opening instructions, food handling and preparation instructions, and food storage instructions. The printing may also provide a pleasing image and/or trademark or other advertising information to enhance the retail sale of the packaged product.
Such printed information may be placed on the outside surface of the lidstock. However, such surface printing is directly exposed to a heated bar during the heat seal operation that seals the lid to the support member. As a result, the surface printing may become smeared or otherwise degraded. A surface printing is also exposed to other physical abuses during distribution and display of the packaged product. Such abuse may also degrade the clarity and presentation of the printed image.
An existing lidstock laminate (set forth as Comparative 1 in the Examples section) has the general construction of A/B/C/D/C/B/A//E, where A comprises a linear low density polyethylene (“LLDPE”) sealant layer, B comprises an anhydride grafted LLDPE layer, C comprises a nylon blend polyamide layer, D comprises a nylon and an ethylene/vinyl alcohol copolymer (“EVOH”) blend oxygen barrier layer, and E comprises a biaxially oriented polypropylene (“BOPP”) layer. The double slashes “//” represent the lamination (i.e., adhesive lamination) interface. The E layer is reverse trap printed. Although this lid laminate performs well, there exists a demand for lidstock having improved seal strength and sealability, while maintaining good oxygen barrier and printability performance.
SUMMARY OF THE INVENTION
The present invention addresses one or more of the aforementioned problems.
A laminate useful as a lidstock comprises first and

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