Stock material or miscellaneous articles – Hollow or container type article – Flexible food casing
Reexamination Certificate
1999-09-13
2003-05-06
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Flexible food casing
C428S034900, C428S523000, C428S910000, C428S409000, C426S113000, C426S127000, C426S129000, C426S412000, C053S477000, C525S240000, C526S352000, C526S348100, C526S348200, C526S348400, C526S348500
Reexamination Certificate
active
06558760
ABSTRACT:
The present invention relates to a mono- or multi-layer thermoplastic film with improved sealability properties and to bags, pouches, and other containers made therefrom by sealing the film to itself. The invention also relates to the use of such film in packaging food products in which the packaged product is submerged in heated water or autoclaved or retorted for a period of time sufficient for pasteurising and/or cooking the packaged product, the package being essentially non degradable under such conditions.
The term “pasteurisable” as used herein is intended to refer to packaging material structurally capable of withstanding exposure to pasteurising conditions while containing a food product. Many food products require pasteurisation after they have been hermetically packaged to destroy harmful microbes which grow in the absence of air. Specific pasteurisation conditions tend to vary from country to country; however limiting conditions are probably submersion of the hermetically closed package in water at 95° C. for one hour. Thus, for a bag to be characterised as pasteurisable, structural integrity of the bag must be maintained during pasteurisation, i.e. the bag must have superior high temperature seal strength.
The term “cook-in” as used herein is intended to refer to packaging material structurally capable of withstanding exposure to cook-in time-temperature conditions while containing a food product. Cook-in packaged foods are essentially pre-packaged, pre-cooked foods that go directly to the consumer in that configuration which may be consumed with or without warming. Cook-in time-temperature conditions typically refer to a long slow cook, for example submerging the packaged product in water at 75-85° C. for four to six hours. Such cook-in time-temperature requirements are representative of institutional cooking requirements. Under such conditions, a packaging material properly characterised as cook-in will maintain seal integrity.
Preferably in both cases the packaging film is a solid-state oriented film, either mono-axially or bi-axially oriented, as solid-state orientation improves the mechanical properties of the end structure.
While said mono- or bi-axially oriented films can then be heat-set, preferably, for use in cook-in applications, these oriented films are not thermofixed and shrink under these cook-in conditions so as to form a tightly fitting package. Heat-shrinkable solid-state oriented films are therefore preferred.
Also, preferably the packaging film is a multilayer film comprising a gas-barrier layer.
Thermoplastic, heat-shrinkable, solid-state oriented films bearing a sealing layer of polyethylene or of an ethylene-&agr;-olefin copolymer, either alone or blended with one or more other resins compatible therewith, are known in the patent literature.
For instance Japanese kokai No. 58-82752 describes a gas-barrier heat-shrinkable film comprising a sealing layer of a copolymer of ethylene and an &agr;-olefin optionally blended with up to less than 80% of ethylene-vinyl acetate.
Japanese kokai No. 58-102762 teaches a heat-shrinkable multi-layer barrier film comprising at least one layer of a blend of three different specific resin and a surface layer of linear low density polyethylene (LLDPE).
U.S. Pat No. 4,424,243 describes a heat-shrinkable barrier film with outer layers of ethylene-&agr;-olefin copolymers and an intermediate ethylene-vinyl acetate copolymer layer interposed between the core barrier layer and at least one outer layer.
U.S. Pat. No. 4,456,646 teaches a heat-shrinkable film comprising a core barrier layer and both outer layers consisting of a blend of an ethylene-&agr;-olefin copolymer with 30 to 80% of ethylene-vinyl acetate copolymer.
These films are generally said to be oil and heat resistant. The tests used to evaluate said properties however provide for submerging a package in water with oil floating on the surface thereof at a temperature of 80-95° C. for up to 10 minutes. These conditions therefore are far from those that simulate the heat-treatment used in cook-in applications or for pasteurisation.
In all the above patents the multilayer films are obtained by either coextrusion or extrusion coating of the resins of the different layers to result in a primary thick tube (primary tape) that is quickly cooled to stop or quench the crystallisation of the polymers. The resulting quenched thick tube is thereafter reheated to the so-called orientation temperature and then biaxially stretched at this temperature by a tubular solid-state orientation process using a trapped bubble. In said solid-state orientation step the primary tape is stretched in the transverse direction (TD) by inflation with air pressure to give a bubble, and in the longitudinal or machine direction (MD) by the differential speed between the two sets of nip rolls that contain the bubble. The term “solid-state orientation” is used herein to describe the orientation process carried out at a temperature higher than the highest Tg of the resins making up the structure and lower than the highest melting point of at least one polymer, i.e. at a temperature where the resins, or at least some of the resins, are not in the molten state. “Solid-state orientation” is contrasted to “melt-state orientation”, i.e. a process, such as the hot blown one, where stretching takes place upon emergence of the molten resins from the die. For the sake of simplicity, in the following “orientation” and “oriented” have been used to mean “solid-state orientation” and “solid-state oriented” respectively.
While mono-layer polyethylene films cannot be oriented by a tubular orientation process unless they are irradiated before orientation, with multi-layer films, depending on the other layers of the structure and on the specific polymer used, irradiation may be not strictly necessary. It is however widely known in the art that with multi-layer films having a sealing layer comprising a polyethylene or an ethylene-&agr;-olefin copolymer, the primary tape comprising said layer is preferably submitted to an irradiation step before orientation to improve bubble stability.
Generally therefore, in the industrial manufacture of heat-shrinkable multi-layer films having a sealing innermost layer comprising a polyethylene or an ethylene-&agr;-olefin copolymer by a tubular process, either all the layers are coextruded, the tape is quenched, irradiated, reheated and oriented, or some of the layers, including the innermost layer that will become the sealing layer of the end packaging material, are extruded, the tape thus obtained is quenched, irradiated and coated by extrusion of the remaining layers thereon, quenched again, reheated and oriented.
Besides increasing bubble stability, irradiation is also known to improve the physical properties (e.g. strength) of the irradiated resins. In some cases therefore, for instance when a non oriented film is desired or when it would be possible to orient the tape even without irradiation, irradiation may be carried out in order to improve the physical properties of the end packaging film.
Irradiation of the sealing layer is however known to decrease heat-sealability of the film, particularly with polyethylene or ethylene-&agr;-olefin copolymers that are easily cross-linkable polymers.
When the film has to be used in the manufacture of pasteurisable containers or containers for cook-in applications that are highly demanding as to the seal strength, this represents a real problem.
It has now been discovered that it is possible to improve the sealability of mono- or multi-layer films having an irradiated sealing layer comprising a polyethylene and/or an ethylene-&agr;-olefin copolymer to such an extent that cook-in pasteurizable bags can be obtained therefrom by using for the sealing layer a polyethylene and/or an ethylene-&agr;-olefin copolymer with a density ≧0.915 g/cm
3
and submitting said sealing layer to a corona discharge treatment.
It has been found in fact that by submitting the irradiated sealing layer of a film, comprising a polyethylene and/or an ethyle
Paleari Mario
Spigaroli Romano
Cryovac Inc.
Ruble Daniel B.
Tarazano D. Lawrence
LandOfFree
Packaging film and containers made therefrom does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Packaging film and containers made therefrom, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Packaging film and containers made therefrom will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3004465