Stock material or miscellaneous articles – Hollow or container type article – Nonself-supporting tubular film or bag
Reexamination Certificate
2002-03-25
2004-05-11
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Hollow or container type article
Nonself-supporting tubular film or bag
C156S292000, C156S308400, C428S036600, C428S036700, C428S220000, C428S500000, C428S515000, C428S516000, C428S520000, C428S522000, C428S910000
Reexamination Certificate
active
06733851
ABSTRACT:
FIELD OF THE INVENTION
The present invention pertains to packaging articles having a heat seal, and to processes for making heat-sealed packaging articles.
BACKGROUND OF THE INVENTION
Packaging articles are often made by heat sealing a flexible film to itself or another article, such as another film, a tray, etc. It would be desirable to lower the heat sealing temperature of the seal layer of a film, so that the seal could be made more quickly, using less heat, while still obtaining a strong, hermetic seal. It would also be desirable to be able to produce a strong seal quickly at low temperature.
SUMMARY OF THE INVENTION
The present invention is directed to a packaging article made from a film having a heat seal layer which is capable of being heat sealed at low temperature to make a strong seal. The seal layer contains a blend of a highly branched homogeneous polymer with a semicrystalline polymer. This blend has been found to be capable of making strong heat seals at a temperature substantially below the seal temperature of the semicrystalline polymer by itself. As a result, the present invention provides an improvement in the heat sealing of films, and is particularly useful in packaging articles made by heat sealing a film.
As a first aspect, the present invention is directed to a packaging article comprising a hermetic heat seal of a multilayer film to itself or another component of the article. The multilayer film has a seal layer containing a blend of a highly branched homogeneous polymer and a semicrystalline polyolefin. The highly branched homogeneous polymer has at least 40 branches per 1000 methylene groups, has a Mw/Mn less than 3.0, and a density of less than 0.89 g/cc. The highly branched homogeneous polymer is present in an amount of from about 5 to 50 weight percent (preferably, 20 to 40 weight percent) based on layer weight. The semicrystalline polyolefin has a density of at least 0.90 g/cc, and is present in an amount of from about 50 to 95 weight percent (preferably, from 60 to 80 weight percent), based on total layer weight. The heat seal has a seal strength at 75° C. of at least 100 grams per centimeter. The seal strength is preferably from 100 to 2000 grams per centimeter, more preferably from 200 to 1000 grams per centimeter. Preferably, the seal has a strength of at least 200 grams per centimeter.
Preferably, the semicrystalline polyolefin has a density of at least 0.905 g/cc, more preferably at least 0.91 g/cc, and can even have a density of at least 0.92 g/cc.
Preferably, the highly branched homogeneous polymer has from 40 to 120 branches per 1000 methylene groups, more preferably, from 40 to 90 branches per 1000 methylene groups, and still more preferably, from 45 to 80 branches per 1000 methylene groups.
Preferably, the semicrystalline polyolefin comprises at least one member selected from the group consisting of very low density polyethylene, linear low density polyethylene, low density polyethylene, high density polyethylene, propylene homopolymer, propylene copolymer, linear homogeneous ethylene/alpha-olefin copolymer, homogeneous ethylene/alpha-olefin copolymer having long chain branching. The semicrystalline polyolefin is different from the highly branched homogeneous polymer in that the semicrystalline polymer, if branched, has less than 60 branches per 1000 methylene groups. Preferably, the highly branched homogeneous polymer is a homopolymer of ethylene.
In one embodiment, the multilayer film comprises an oxygen barrier layer. Preferably, such a multilayer film further comprises a tie layer between the oxygen barrier layer and the seal layer. Preferably, the heat seal layer is an inside layer of the packaging article, and the tie layer between the oxygen barrier layer and the seal layer is a first tie layer, with the multilayer film further comprising a second tie layer between the oxygen barrier layer and an outside layer.
In one embodiment, the film is heat-shrinkable. Preferably, such a film has a total free shrink, at 185° F., of at least 20 percent; preferably, from 20 to 120 percent; more preferably, from 20 to 80 percent. In another embodiment, the multilayer film has a total free shrink, at 185° F., of less than 10 percent.
Preferably, the film has a thickness of from about 0.3-15 mils; more preferably, from 0.5 to 10 mils; or 1.5 to 6 mils, or 1.5 to 5 mils, or 1.5 to 4 mils, or 2 to 4 mils.
As a second aspect, the present invention is directed to a process for making a packaging article. The process comprises heat sealing a multilayer film having a seal layer in accordance with the first aspect of the present invention. The heat seal is made by heating the seal layer to a temperature of from about 50° C. to 100° C. The heat seal has a seal strength of at least 75 grams per centimeter.
DETAILED DESCRIPTION OF THE INVENTION
Preparation of the highly branched homogeneous polymer (also sometimes referred to by those of skill in the art as a “hyperbranched homogeneous polymer”) used in the present invention is described in U.S. Pat. No. 5,880,241, to Brookhart et al, entitled “Olefin Polymers”, hereby incorporated in its entirety, by reference thereto. The polymers of Brookhart et al are disclosed as having a unique structure in terms of the branching in the polymer, with the number of branches, and to a certain extent the length of the branches, being determined by NMR spectroscopy. The amount of branching is expressed in terms of the number of branches per 1000 of the total methylene (—CH
2
—) groups in the polymer, with one exception. Methylene groups that are in an ester grouping, i.e.—CO
2
R, are not counted as part of the 1000 methylenes. Otherwise, the methylene groups include those in the main chain and in the branches.
As used herein, the phrase “highly branched homogeneous polymer” (i.e., “HBH polymer”) refers to single site catalyzed resin with a polymer architecture wherein there are at least 40 side branches from the main chain for every 1000 main chain carbons. While not wishing to be restricted to any single sub-architecture, it is possible that some of the side branches may themselves contain side branches, similar to dendritic, or “tree-like” structures. It is believed from observations of the performance of these highly branched homogeneous polymers, that the number and type of branching results in a macro-structure which assumes a somewhat spherical conformation. This substantially spherical topology of the polymer chain is believed to be responsible for unique flow properties, as a consequence of interaction with other polymer chains.
The HBH polymer useful in the present invention have greater than 40 side chain branches per 1000 carbon atoms, preferably at least 50 side chain branches per 1000 carbon atoms; preferably from about 60 to 140 side chain branches per 1000 carbon atoms. For every 100 side chain branches that are methyl, the HBH polymer has 1 to about 80 ethyl branches, 1 to about 20 propyl branches, 1 to about 50 butyl branches, 1 to about 20 amyl branches, and 1 to about 100 hexyl or longer branches.
More preferably, the HBH polyolefins useful in the present invention have from about 40 to 110 side chain branches per 1000 carbon atoms. For every methyl 100 side chain branches, preferably there are from 1 to about 20 ethyl branches, from 1 to about 10 propyl branches, from 1 to about 15 butyl branches, from 1 to about 10 amyl branches, and from 1 to about 30 hexyl or longer branches.
More preferably, the HBH polymer useful in the present invention have from about 50 to 100 side chain branches per 1000 carbon atoms. For every 100 branches that are methyl, preferably there are from about 2 to about 18 ethyl branches, from about 2 to about 8 propyl branches, from about 2 to about 12 butyl branches, from 2 to about 8 amyl branches, and from about 8 to about 30 hexyl or longer branches.
The HBH polymer is preferably a narrow molecular weight (Mw/Mn), single site catalyzed resins. The HBH polymer preferably has a molecular weight distribution less than 3, preferably less than 2.5. However, it is possible to
Cryovac Inc.
Hurley Jr. Rupert B.
Nakarani D. S.
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