Polymeric films

Details Polymeric films Polymeric films

1996-08-20

1998-06-30

Lee, Helen

Stock material or miscellaneous articles

Web or sheet containing structurally defined element or...

Composite having voids in a component

4283199, 428354, 428516, B32B 2732

Patent

active

057731365

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention concerns polymeric films and more particularly polyolefin films which form peelable heat seals.
2. Description of Related Art
Heat sealable polyolefin films which form peelable heat seals have been proposed hitherto in which the films either part from each other or from a substrate to which they have been heat sealed, when the heat seal is pulled apart. An example of such a heat sealable film is described in WO93/17863, peelability resulting either from a relatively weak heat seal between the film and the substrate to which it has been attached or by delamination of a layer of the olefin polymer which forms the heat seal from other layers of which the film is constructed.
The disadvantage of providing a peelable seal which parts at the heat seal between a polyolefin film and a substrate to which it has been heat sealed is that the peelability can be unpredictable. If the heat seal layer of the film forming the peelable seal forms a particularly strong bond to the substrate to which it is heat sealed, peelability may be difficult, requiring a high force to separate the sealed surfaces. On the other hand, if the heat seal is too weak, the seal can be broken before being required, for example merely by the forces involved in handling packages including the heat seals.
Peelability resulting from delamination of a heat sealable polymer layer from other layers of the film would be expected to be more predictable. However, such delamination requires a low inter-ply bond strength between the heat-seal layer of the film and the remainder of the film, and this in turn tends to give rise to unpredictable results for the bond strength between the heat seal layer and substrates to which the film is heat sealed.
In addition to these problems with the peelability of these prior art films, they do not provide good tamper evidence.
Tamper evident films giving good evidence of tampering have, however, been proposed hitherto, these including a layer of polypropylene containing a relatively large amount of a particulate filler which induces microvoids during biaxial stretching of the film during their manufacture. Instead of such films parting from substrates to which they have been heat sealed or delaminating when their heat seals are pulled open, the layer including the microvoids ruptures and splits open, parts of the layer being left on both of the peeled apart surfaces. Such films tend to give predictable results since peelability is not a function of the ability of layers to be separated from substrates to which they have been heat sealed, they don't rely on low inter-ply bond strengths which can lead to weak heat seals to substrates, and they do show good evidence that the seals have been peeled, that is tamper evidence. However, such films have the inherent problem of being opaque as a result of the presence of the high number of microvoids.


SUMMARY OF THE INVENTION

According to the present invention there is provided multilayer polymeric film comprising a base layer of a propylene polymer, a non-voided intermediate layer on the base layer, and a heat sealable polymer layer on the intermediate layer, the non-voided intermediate layer having an internal cohesive strength less than the bond strength of the intermediate layer to either the base layer or the heat sealable layer and less than the internal cohesive strengths of either the base layer or the heat sealable layer.
It is generally preferred that the internal cohesive strength of the intermediate layer is less than that of other layers in the film, and particularly less than that of either the base layer or the heat seal layer to which it is attached, or the cohesive strength of the intermediate layer to either of these layers.


DETAILED DESCRIPTION OF THE INVENTION

The present invention provides heat sealable films which can be readily peeled. Furthermore, particularly when the films are substantially transparent, they can indicate where peeling of the films has occurred by an increase in

REFERENCES:
patent: 4615926 (1986-10-01), Hsu et al.
patent: 4700531 (1987-10-01), Hsu et al.
patent: 4704314 (1987-11-01), Hsu et al.
patent: 4881649 (1989-11-01), Hsu et al.
patent: 4882229 (1989-11-01), Hwo
patent: 5175035 (1992-12-01), Pinsolle et al.
patent: 5302442 (1994-04-01), O'Brien et al.

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