Stock material or miscellaneous articles – Web or sheet containing structurally defined element or... – Adhesive outermost layer
Reexamination Certificate
2000-11-29
2002-09-17
Tarazano, D. Lawrence (Department: 1773)
Stock material or miscellaneous articles
Web or sheet containing structurally defined element or...
Adhesive outermost layer
C428S3550AC, C428S447000, C428S516000, C428S520000, C428S910000
Reexamination Certificate
active
06451426
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a thermoplastic film which is peelable and sealable. More specifically, the invention relates to a thermoplastic film with a separable layer sandwiched between an inner layer and a sealing layer which permits peeling of the inner layer from the seal layer.
BACKGROUND OF THE INVENTION
A peelable film structure is described in WO 96/04178 published on Feb. 15, 1996. The film structure comprises (a) a core layer comprising an olefin polymer and a heat sealable layer comprising a blend of low density polyethylene (LDPE) and a material incompatible with the LDPE, such as an olefin polymer or co- or terpolymer of ethylene, propylene or butene. The film structure can be heat sealed to a plastic container to form the lid of the container, or to itself to form a package. It is disclosed that when used with a plastic container, the film structure can be readily peeled from the container in order to open it.
In U.S. Pat. No. 5,358,792 a heat sealable composition is described comprising (a) from about 30 to about 70 weight percent of a low melting polymer comprising a very low density ethylene based copolymer defined by a density in the range of about 0.88 g/cm
3
to about 0.915 g/cm
3
, a melt index in the range of about 1.5 dg/min to about 7.5 dg/min, a molecular weight distribution (M
w
M
n
) no greater than about 3.5 and (b) from about 70 to about 30 weight percent of a propylene based polymer.
SUMMARY OF THE INVENTION
This invention relates to a sealable film comprising:
(a) an inner layer comprising an olefin polymer;
(b) a sealing layer; and
(c) a separable layer positioned between the inner layer and sealing layer, the separable layer comprising an ethylene-propylene copolymer or a blend of polyethylene and another olefin which forms an incompatible mixture or blend. The sealed film can be unsealed by causing the sealing layer to separate from the inner layer without substantially impacting the integrity of either the sealing layer or the inner layer. In one embodiment, the olefin which forms an incompatible blend with the polyethylene comprises an olefin polymer selected from the group consisting of an olefin homopolymer, copolymer or terpolymer. The invention further relates to a method of making the sealable and peelable film.
The sealable layer can be coated with a sealable coating, such as a heat sealable coating. Examples of the heat sealable coating are acrylic, ethylene-acrylic acid copolymer or polyvinylidene chloride.
It is an object of the invention to provide a thermoplastic film having at least three layers, including a seal layer, which is sealable to itself and other surfaces but which permits separation of the seal layer from at least one other layer of the film.
It is a feature of the invention to have a separable layer positioned to permit separation of the sealing layer from at least one other layer of the film.
It is an advantage of the invention that when a multilayer film comprising an inner layer and a sealing layer is sealed to itself, or another surface, the sealing layer can be separated from the other layers of the film.
DETAILED DESCRIPTION OF THE INVENTION
The invention relates to a sealable film comprising
(a) an inner layer comprising an olefin polymer;
(b) a sealing layer, typically a skin layer, on at least one side of the inner layer; and
(c) a separable layer positioned between the inner layer and the sealing layer, the separable layer comprising (1) an ethylene-propylene copolymer, specifically a block copolymer or (2) a blend of polyethylene and as an olefin which forms an incompatible blend or mixture with the polyethylene either (i) a polypropylene homopolymer or (ii) an ethylene-propylene copolymer, specifically a block copolymer. The polymers can be produced by any catalytic technique known in the art, e.g. Ziegler-Natta catalysis, metallocene catalysis, etc.
The inclusion of a separable layer between an inner layer and a sealable layer provides a sealable film which permits peeling of the inner layer from the sealable layer without substantially destroying either the inner or the sealable layers.
The separable layer consists of, typically, a material which, in combination with the sealable and inner layers of the film, provider a weak bond between itself and the inner layer and/or the sealable layer so that stress applied to the sealed film promotes destruction of the material of the separable layer and not the sealable or the inner layers. In another embodiment, the separable layer has less cohesive strength than either the sealable layer or the inner layer which causes the separable layer to give way before either the sealable layer or the inner layer. The term “peel material” is used herein to cover the class of film layer forming materials included in the separable layer that allow the sealing layer and inner layer of a composite film to be separated from each other under stress as essentially integral layers.
Usually, the peel material comprises at least one olefin polymer. Examples of the peel material include those olefin polymers selected from the group consisting of an ethylene-propylene block copolymer or a blend of various kinds of polyethylene and another olefin which forms an incompatible blend or mixture with the polyethylene, specifically either (i) a polypropylene homopolymer or (ii) an ethylene-propylene block copolymer. Typically, the polyethylene is selected from the group consisting of low density polyethylene, high density polyethylene, linear low density polyethylene, medium density polyethylene, metallocene-catalyzed versions of these polyethylenes and blends thereof. The term polyethylene includes homo-, co- or terpolymers which include ethylene. Comonomers can be C
3
-C
8
olefins. A suitable ultra low density polyethylene produced by metallocene catalysts can be defined as having a density in the range of about 0.88 g/cm
3
to about 0.915 g/cm
3
, a melt index in the range of about 1.5 dg/min to about 7.5 dg/min, and a melting point in the range of about 60° C. to about 115° C., measured as DSC peak T
m
.
Any layer of the film can include a propylene homopolymer component, the propylene homopolymer can be predominantly isotactic, syndiotactic or atactic and can include a blend of any of the foregoing forms of propylene homopolymer. Usually the (c)(2) separable layer comprises about 70 to 80 wt % isotactic polypropylene or about 70 to 80 wt % syndiotactic polypropylene based upon the entire weight of the olefin polymer of the separable layer.
Specific examples of commercially available materials which comprise the peel material are ethylene-propylene block copolymer sold by Himont as “8523”; linear low density polyethylene sold by Exxon as “LL3001”; a metallocene catalyzed plastomer ethylene-hexene copolymer sold by Exxon as “SLP 9045”; high density polyethylene sold by Oxychem as “M6211”; linear low density polyethylene sold by Chevron as “PE1019” and a metallocene catalyzed polyethylene sold by Exxon as “2009”, and a propylene homopolymer sold by Fina Oil and Chemical Company as “3371.” The ethylene-propylene block copolymer sold by Himont as “8523” is an example of a material that can function unblended as a separable layer.
The low density polyethylene can be defined by ethylene polymers having a density in the range of about 0.91 to 0.925 g/cm
3
. The high density polyethylene can be defined by ethylene polymers having a density in the range of about 0.941 to 0.965 g/cm
3
and even higher. The medium density polyethylene can be defined by ethylene polymers having a density in the range of about 0.926 to about 0.940 g/cm
3
.
The proportion of polyethylene in blends of the separable layer of (c)(2) varies over a wide range. Typically, this layer comprises polyethylene in an amount ranging from about 20 to about 80 wt %, based on the entire weight of the olefin polymer of the separable layer. More typically, the amount of polyethylene ranges from about 30 to about 70 wt % based on the entire weight of the olefin polymer of the separable layer.
When the separable la
Caputo Michael J.
Kong Dan-Chang
Liu Leland L.
Peet Robert G.
ExxonMobil Oil Corporation
James Rick F.
Santini Dennis P.
Tarazano D. Lawrence
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