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 an intermediate 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/cm3 to about 0.915 g/cm3, a melt index in the range of about 1.5 dg/min to about 7.5 dg/min, a molecular weight distribution (Mw/Mn) 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, provides 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 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 C3-C8 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/cm3 to about 0.915 g/cm3 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 Tm.
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 Exxpol as “2009”.
The low density polyethylene can be defined by ethylene polymers having a density in the range of about 0.91 to 0.925 g/cm3. The high density polyethylene can be defined by ethylene polymers having a density in the range of about 0.941 to 0.965 g/cm3 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/cm3.
The proportion of polyethylene in the separable layer 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.
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 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. A useful commercially available polypropylene is sold by Fina as “3371”.
When the separable layer is ethylene-propylene block copolymer, the copolymer comprises about 3 to about 25 wt. % ethylene, based on the entire weight of the copolymer. The term “block” copolymer is used in the ordinary sense meaning a polymer whose molecule is made up of alternating sections of ethylene or polyethylene separated by sections of a different olefin such as propylene or polypropylene.
In one aspect of the invention, the separable layer is a blend of ethylene homopolymer or copolymer and another olefin homo- or copolymer. In this aspect, the separable layer combination is, typically selected from, but not limited to, the group consisting of (a) a blend of linear low density polyethylene and polypropylene; (b) a blend of high density polyethylene and polypropylene; (c) a blend of low density polyethylene and polypropylene; (d) a blend of metallocene catalyzed polyethylene and polypropylene; and (e) a blend of linear low density polyethylene and ethylene-propylene block copolymer. Any proportion of the two polymers is appropriate but choice of the blend proportion may be made to meet optical clarity objectives through routine experimentation.
The film structure includes an inner layer which is the innermost part of the film structure. The inner layer can be a single layer or a plurality of layers. Typically there is a core layer and one or more intermediate layers located next to the core, e.g. between the core layer and the separable layer. The inner layer, typically, comprises an olefin polymer such as polypropylene or polyethylene. In general, the polypropylene of the inner layer is predominantly isotactic, syndiotactic or atactic. However, it can comprise greater than about 80 wt. % of isotactic polypropylene based on the entire weight of the polypropylene of the inner layer or greater than about 80 wt. % of syndiotactic polypropylene, based on the entire weight of the polypropylene of the inner layer.
When the inner layer comprises polyethylene, the polyethylene is, typically, high density polyethylene. The term high density polyethylene generally includes polyethylenes ranging in density from about 0.94 to about 0.96 and over. A useful commercially available high density polyethylene is sold by Oxychem as “M6211”.
The inner layer can also comprise an ethylene co- or ter-polymer in which the remaining monomer constituent is a C3 to C4 monomer such as propylene and/or butene-1. A useful ethylene copolymer is an ethylene-propylene block copolymer which is commercially available from Himont as “8523”
The intermediate layer can include any olefin polymer material. Any material used in the core layer is especially appropriate. Typically, however, the intermediate layer is propylene homopolymer.
The inner layer can be clear or opaque.
The sealing layer is applied on the separable layer side of the film. The materials of the sealing layer are characterized by features which permit the separable layer to separate away when stress is applied to the film. Typical examples of materials which are suitable for use as the sealing layer are coextrudable materials which form a seal upon application of elevated temperatures and, at least slight, pressure. Examples of thermoplastic materials which can be used for the sealing layer include olefinic homo-, co- or terpolymers. The olefinic monomers can comprise 2 to 8 carbon atoms. Specific examples include ethylene-propylene random copolymer, ethylene-butene-1 copolymer, ethylene-propylene-butene-1 terpolymer, propylene-butene copolymer, low density polyethylene, linear low density polyethylene, very low density polyethylene, metallocene-catalyzed polyethylene, metallocene-catalyzed polymers known by the term plastomer, metallocene-catalyzed ethylene-hexene copolymer, metallocene-catalyzed ethylene-butene copolymer, metallocene-catalyzed ethylene-octene copolymer, ethylene-methacrylic acid copolymer, ethylene-vinyl acetate copolymer and ionomer resin. A blend of the foregoing materials is also contemplated such as a blend of the plastomer and ethylene-butene copolymer.
The metallocene-catyalyzed ethylene homo-, co- or terpolymers of the sealing layer can be the same as that of the separable layer provided that both layers are different and have separability.
A coating can be applied to the seal layer which facilitates sealing of the film to itself or to another film surface. Examples of suitable coatings include acrylic, polyvinylidene chloride, or ethylene-acrylic acid copolymer, among others.
The invention is also directed to a method of making a sealable film comprising the steps of (a) providing an inner olefin layer; (b) coextruding the inner olefin layer with a separable layer, the separable layer comprising at least one olefin polymer selected from the group consisting of an (1) ethylene-propylene block copolymer or (2) a blend of polyethylene and another olefin which is incompatible with the polyethylene, specifically, either (i) a polypropylene homopolymer or (ii) an ethylene-propylene copolymer; and (c) applying a sealable layer to a surface of the separable layer. This method can further comprise the step of coating the outer layer with a coating composition selected from the group consisting of acrylic, polyvinylidene chloride, and ethylene-acrylic acid polymers.
The thickness of the separable layer can be important. In general, depending upon the material of the separable layer, the separable layer can constitute at least about 20%, preferably about 30% to about 50% of the entire thickness of the film. For example, in a film of about 10 μm to about 80 μm, specifically about 15 μm to about 35 μm, and when the separable layer is, for example, ethylene block copolymer or a blend of ethylene polymer and polypropylene, the separable layer should range from about 5 μm to about 15 μm, specifically about 71 μm to about 12 μm.
One or more of the layers of the film, but typically an exposed surface layer, can include a small amount of a finely divided inorganic material which can impart antiblock characteristics to the structure and reduce the coefficient of friction. Contemplated finely divided inorganic materials include synthetic amorphous silica, such as silica, diatomaceous earth and clay. An effective amount of a slip agent can be employed. A typical slip agent is selected from the group consisting of particulate crosslinked hydrocarbyl-substituted polysiloxanes.
The resins of this invention are formed into multilayer films using film forming technology that is well known to those skilled in the art. The resins are, typically cast extruded or coextruded into a film using a flat die or blown extruded or coextruded using a tubular die. The films are then, usually, oriented either uniaxially or biaxially by known stretching techniques. The sealable layer can be applied by coextrusion or extrusion coating. In one embodiment of the invention, after orienting the film in the machine direction the sealable layer is extrusion coated onto the separable layer followed by orienting the film in the transverse direction.