Adhesive bonding and miscellaneous chemical manufacture – Methods – Surface bonding and/or assembly therefor
Reexamination Certificate
1997-02-07
2001-04-24
Ball, Michael W. (Department: 1733)
Adhesive bonding and miscellaneous chemical manufacture
Methods
Surface bonding and/or assembly therefor
C156S229000, C156S244240, C156S332000, C428S308400, C428S451000, C428S483000, C428S910000
Reexamination Certificate
active
06221191
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to multilayer polyolefin film structures and methods of making the same and, more particularly, the invention relates to a biaxially-oriented polypropylene film incorporating at least one polyester layer.
2. Description of Related Art
Biaxially-oriented polypropylene (BOPP) films are widely used in packaging because they have good stiffness, strength, optical properties (low haze and high gloss), and moisture barrier properties. Users of packaging films, particularly users of biaxially oriented polypropylene films, are continually seeking structures with improved printability, metallizing properties, and gas barrier. Because of their olefinic nature, typical BOPP constructions have low surface energy and require treatment (corona, flame, etc.) in order to be printed or metallized. Polyester is known to have high surface energy and possesses excellent printing and metallizing attributes. Additionally, polyester, both in clear and metallized structures offers improved gas barrier performance to BOPP films. This is especially true in the case of metallized polyester films which are in order of magnitude or more lower in oxygen transmission rate.
Although there has been a long felt need for BOPP films incorporating polyester layers, problems in obtaining acceptable optical and processability characteristics, adequate interply adhesion, and other properties have been encountered in previous attempts to produce polyester-containing BOPP structures. For example, U.S. Pat. No. 5,324,467 discloses a process for the preparation of an oriented multilayer laminate film having at least three layers, including polypropylene, a tie layer, and copolyester. The films are formed by combining the layers in the molten state, either in coextrusion, or in separate extrusions brought together outside the die, then subsequently cooling the film, orienting it uniaxially or biaxially, and heat setting to lock in the properties. A major problem in producing a structure according to this method on commercial scale equipment is the strong tendency of polyester to adhere to the heated metal rolls of the machine direction orientation section. This makes it difficult to achieve good optical properties free of visual defects and may also decrease other properties such as the seal initiation temperature.
U.S. Pat. No. 4,874,656 describes a multilayer laminate having a high mechanical resistance and an impermeability to gases and vapors. In the disclosed structures, a polyester layer is joined to a polypropylene layer after the polypropylene is biaxially-oriented, the polyester layer is quite thick (i.e., 12 to 24 microns), and the structure includes a layer of metallic foil and a layer of polyethylene. While joining a polyester layer to a BOPP layer after biaxial orientation is possible, this method is impractical for incorporating thin layers of polyester.
U.S. Pat. No. 4,924,525 also describes a structure wherein a polyester laminate is adhered to a BOPP film after the polypropylene is biaxially oriented, precluding the use of thin polyester layers in the final structure.
SUMMARY OF THE INVENTION
It is an object of the invention to overcome one or more of the problems described above.
Accordingly, the invention provides a biaxially-oriented polypropylene film incorporating a polyester layer, and a method of making the same.
The inventive multilayer film is prepared by the steps of forming a polypropylene core, orienting the core in a first direction, providing on at least one side thereof a multilayer outer film (cap layer) comprising at least one polyester layer and at least one tie layer interposed between the polyester layer and the core, and orienting the resulting multilayer film in a second direction transverse to the first direction.
Advantageously, the polyester layer contains sufficient silicone fluid to provide substantially uniform stretching characteristics.
In one embodiment of the invention, an outer surface of the multilayer film opposite the polyester layer is metallized. In another embodiment, a tie layer adhesive is an ethylene/carboxylic acid or anhydride copolymer or an ethylene/ester/carboxylic acid or anhydride terpolymer.
The invention further provides a white or colored biaxially oriented polypropylene film incorporating a polyester layer that is prepared by incorporating inorganic minerals in the polypropylene core layer.
If desired, an additive such as a pigment, dye, etc. may be incorporated into the multilayer film.
In yet another embodiment, the core layer comprises a polypropylene film coextruded between first and second polyolefin layers.
Further objects and advantages of the invention will be apparent to those skilled in the art from a review of the following detailed description taken in conjunction with the appended claims.
DETAILED DESCRIPTION OF THE INVENTION
The invention addresses various concerns of the prior art by providing a structure that positions the polyester at the outer layer of the film, thereby taking advantage of the improved printability and metallizing attributes of polyesters, also eliminates the difficulty of contacting polyester over the heated rolls of the machine direction orienter and, further, provides thin polyester layers that allow a structure of economic value to be produced. An additional benefit of the invention is that a broad range of polyester products, including amorphous homopolymer grades, may be included in the inventive film structures. This allows the designer a wide choice in making films with improved optical properties, printability, and metallizing attributes as well as stiffness and heat resistance.
The films of the present invention provide excellent barrier to flavors and aromas. Moreover, since the polyester layer of the inventive film is formed from an extruded high molecular weight polymer, there is no problem with loss of flavor or aroma barrier due to cracking or abrasion. Furthermore, BOPP films with an external polyester layer can be readily printed or metallized, or adhesively coated on the polyester surface. Also, the film surface opposite the polyester side can be metallized, leaving the polyester layer available for other modifications.
Generally, the polyester-containing BOPP film of the invention includes a core and a multilayer outer film (cap layer) or film adhered to at least one surface of the core. The multilayer BOPP film of the invention is prepared using interdraw coating or lamination techniques.
The BOPP core comprises a layer of polypropylene and, in one embodiment, further comprises a tie layer, as described below.
The cap layer is applied on one or both surfaces of the monoaxially oriented core layer. The cap layer comprises a layer of a polyester resin and at least one tie layer. The tie layer is disposed between the polyester layer and the core.
Additionally, the polyester layer contains a sufficient amount of silicone fluid as a processing aid to provide substantially uniform stretching characteristics to the polyester layer.
In a preferred embodiment, the cap layer contains a second tie layer comprising a polypropylene copolymer or terpolymer or linear ethylene polymer interposed between the adhesive tie layer and the core. In a variation on this embodiment, the polypropylene copolymer or terpolymer tie layer forms part of the core, and is positioned adjacent the polyolefin adhesive tie layer in the final structure. This tie layer may be oriented with the polypropylene layer of the core.
The multilayer BOPP film of the invention is prepared by the steps of forming and orienting the core in a first direction, providing the cap layer to at least one side of the monoaxially oriented core to form a multilayer film, and then orienting the resulting multilayer film in a second direction transverse (and preferably perpendicular) to the first direction to provide a biaxially-oriented multilayer film.
The biaxially-oriented multilayer film may then be subjected to a heat setting treatment to allow the film to crystallize. In a pref
Davis Alan M.
Lawrence John H.
Weng Tung-Yao
Ball Michael W.
Marshall O'Toole Gerstein Murray & Borun
QPF, L.L.C.
Tolin Michael A.
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