Multilayer oxygen barrier packaging film

Details Multilayer oxygen barrier packaging film Multilayer oxygen barrier packaging film

1998-12-10

2001-04-24

Copenheaver, Blaine (Department: 1771)

Stock material or miscellaneous articles

Structurally defined web or sheet

Including components having same physical characteristic in...

C428S220000, C428S516000

Reexamination Certificate

active

06221470

ABSTRACT:

FIELD OF THE INVENTION
The present invention refers to a multilayer film endowed with good optical, mechanical, gas barrier, and heat sealability properties. The film is particularly suitable for packaging food products.
BACKGROUND OF THE INVENTION
Multilayer, thermoplastic films are being used for packaging various food and non food products because they protect the item itself from the environment during storage and distribution. Furthermore, for the end consumer, it is desirable to present the product packaged in a preferably transparent thermoplastic film that allows visible inspection of the package contents to help assure the quality of the product.
Optical characteristics are therefore often important for a thermoplastic film for packaging.
Other properties are also desirable, such as good mechanical properties that keep the package unaltered until it is offered to the customer on consumer.
A shrink feature can also be imparted to a thermoplastic film by orientation or stretching of the film, either mono-axially or biaxially, during film manufacture. This shrink feature allows the film to shrink or, if restrained, create shrink tension within the film upon exposure to heat. In a typical process, the thick structure which is extruded through either a round or a flat extrusion die is quickly quenched, then it is heated to a suitable temperature, called the orientation temperature, which is higher than the glass transition temperature (T
g
) of the resins used in the film itself but lower than the melting temperature (T
m
) of at least one of the resins, and stretched in either or both of the machine (longitudinal) and transverse directions.
For food packaging, it is often necessary that the film has oxygen barrier characteristics to delay or avoid product oxidation or degradation during its shelf-life.
Good heat sealability is sometimes required. It is essential, particularly for oxygen barrier films used in applications where the contained product is to be kept either under vacuum or under a modified atmosphere, that the seal(s) that close the package have adequate strength and, as a consequence thereof, that the package remains tight.
Several different materials have been used to decrease the oxygen permeability of thermoplastic films. Among these materials a very good gas barrier material is EVOH (ethylene/vinyl alcohol copolymer). Several “barrier” thermoplastic films comprising an EVOH layer are described in the patent literature.
In prior art films the different materials employed for the skin layers are suitably combined with the aim of improving as much as possible the film characteristics, particularly those characteristics that are needed for the specific intended applications. As an example, the use of a low density ethylene/alpha olefin copolymer provides for fair heat-sealability and remarkable oil resistance properties; the use of EVA (ethylene/vinyl acetate copolymer) improves the shrinkability and the sealability properties; the use of propylene homo- and/or copolymer increases the stiffness of the structure; etc.
It is however known that a resin that improves a specific property typically worsens other properties, and therefore the research efforts in this field tend to reach an optimum balance of these properties.
More particularly the film characteristics that still need to be improved, in a way that however should not negatively affect the other characteristics such as optical, mechanical, barrier and shrinkability properties, are the sealability properties.
It has now been found that it is possible to provide a film with optical and gas barrier properties at least comparable to those of known films containing EVOH, and having remarkably improved mechanical and heat-sealability properties, by using in the film sealing layer a blend of ethylene/alpha olefin copolymers of suitably selected different densities.
SUMMARY OF THE INVENTION
In one aspect, a multilayer thermoplastic film comprises a core layer comprising an ethylene/vinyl alcohol copolymer; two outer layers; and two adhesive layers each disposed between the core layer and a respective outer layer; wherein at least one of the outer layers comprises a blend of a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density of greater than 0.915 g/cm
3
and less than 0.925 g/cm
3
, a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density greater than or equal to 0.925 g/cm
3
, and a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density less than or equal to 0.915 g/cm
3
.
Only one of the outer layers (the outer layer that will be used as the sealing layer) needs to comprise a blend as defined above. The other outer layer can have a different composition and comprise a single polymer or a blend of a polymers typically selected from ethylene homo- and co-polymers, e.g. polyethylene, homogeneous or heterogeneous ethylene/alpha olefin copolymers, EVA, etc. In a preferred embodiment, however, both outer layers comprise a blend as defined above.
Films of the invention comprise at least five layers. Films with a higher number of layers, either symmetrical or unsymmetrical, are obtained when one or more additional layers are present between the adhesive layers and one or both of the outer layers, and/or between the core layer and one or both of the adhesive layers.
Preferably, the adhesive layers directly adhere to the core layer.
When, in said preferred embodiment, the surface of each of the adhesive layers that does not adhere to the core layer, directly adheres to the respective outer layer, the film will contain five layers.
Films can also contain a higher number of layers if one or more additional layers are positioned between the adhesive layers and the outer layers. As an example, a six or seven layer film can comprise additional layer(s) between the adhesive layers and the outer layers, the additional layers made with recycle material from the scrap of the same film, optionally blended with a compatibilizer.
In a second aspect, a multilayer thermoplastic film comprises a core layer comprising an ethylene/vinyl alcohol copolymer, the core layer having two major surfaces; two intermediate layers each comprising polyamide, each of which is directly adhered to one of the two major surfaces of the core layer; two outer layers; and two adhesive layers each disposed between a respective intermediate layer and a respective outer layer; wherein at least one of the outer layers comprises a blend of a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density of greater than 0.915 g/cm
3
and less than 0.925 g/cm
3
, a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density greater than or equal to 0.925 g/cm
3
, and a homogeneous or heterogeneous ethylene/alpha olefin copolymer having a density less than or equal to 0.915 g/cm
3
.
The film of this embodiment is thus a seven layer film, when each of the adhesive layers is directly adhered to a respective intermediate layer as well as to a respective outer layer. It may also comprise more than seven layers when additional layers, such as layers made with recycle material, are present between the adhesive layers and the outer layers.
DEFINITIONS
As used herein, the term:
“film” refers to a flat or tubular flexible structure of thermoplastic material having a thickness up to about 120 &mgr;m. Generally, for the purposes of the present invention, said structure will have a thickness of up to about 60 &mgr;m and typically up to about 35 &mgr;m;
“core layer” or “inner layer” refer to any film layer having its two principal surfaces adhered to other layers of the multilayer film;
“outer layer” or “skin layer” refers to any film layer of a multilayer film having only one of its principal surfaces directly adhered to another layer of the film;
“heat-sealing” or “heat-sealant” layer, as applied to multilayer films, refers to an outer layer which is involved in the sealing of the film to itself, to another film layer of the same or another film, and/or to another ar

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