Stock material or miscellaneous articles – Hollow or container type article – Glass – ceramic – or sintered – fused – fired – or calcined metal...
Reexamination Certificate
2000-04-07
2002-08-20
Dixon, Merrick (Department: 1774)
Stock material or miscellaneous articles
Hollow or container type article
Glass, ceramic, or sintered, fused, fired, or calcined metal...
C428S034600, C428S034800, C428S035200, C428S035700, C428S035900
Reexamination Certificate
active
06436495
ABSTRACT:
FIELD OF THE INVENTION AND RELATED ART
The present invention relates to a multilayer film including a seal structure comprising a seal layer and a second seal layer, more specifically a multilayer film including a seal layer comprising principally a polyolefin produced by polymerization in the presence of a metallocene catalyst (hereinafter termed a “metallocene-catalyzed polyolefin”), which is excellent in film-formability, sealability and flexibility.
A metallocene-catalyzed polyolefin is generally excellent in sealability, hot tack and transparency but is liable to be accompanied with poor film-formability or film-processability. Moreover, while depending on the shape or form of a package, a flexible multilayer film formed therefrom is liable to have insufficient sealability, film strength and heat resistance.
Several packaging materials using metallocene-catalyzed polyolefins have been proposed. For example, Japanese Laid-Open Patent Application (JP-A) 6-8383 (Bekele) discloses a cook-in film having an improved seal strength, including: a seal layer comprising a polyamide blend, and a core layer and an outer layer each comprising a metallocene-catalyzed ethylene-butene copolymer, of which at least the seal layer is crosslinked. JP-A 7-1680 (Bekele) discloses a multilayer film showing a low oxygen permeability, including at least one oxygen-barrier layer comprising a blend of a terpolymer of monomers selected from ethylene, an acrylic acid ester, maleic acid and glycidyl methacrylate, and an ethylene-vinyl alcohol copolymer, and also a surface layer comprising a blend of metallocene-catalyzed polyolefin on a non-sealing side. JP-A 59-143639 (Oval) discloses a six-layered shrinkable film including a first layer (seal layer) of, e.g., propylene-ethylene random copolymer or ionomer resin having a higher softening point than a second layer (shrink layer) which has a thickness large enough to govern a shrinking temperature of the entire multilayer film by the shrinking temperature of the second layer and comprises, e.g., ethylene-vinyl acetate copolymer or linear low-density polyethylene of identical thickness, a third layer (adhesive layer) of modified polyethylene, a fourth layer (gas impermeable layer) of hydrolyzed ethylene-vinyl acetate copolymer, a fifth layer (adhesive layer) and a sixth layer (abrasion-resistant layer). JP-A 6-210810 (Lamesh) discloses a back-seamable multilayer film, which is a heat-shrinkable multilayer film including a heat-sealing layer comprising a blend of at least ca. 50wt. % of a propylene-based copolymer and a homogeneous ethylene-&agr;-olefin copolymer having a density of below ca. 0.90 g/cc. JP-A 6-320685 (Ohmori et al) discloses a polyolefin-based multilayer film having a gas-barrier layer, and polyolefin resin layers including innermost and outermost layers each comprising an ethylene-type copolymer having a molecular weight dispersion factor (weight-average
umber-average molecular weight ratio) of below 3 and an intermediate layer of ethylene-methacrylic acid copolymer. The multilayer film may be irradiated with electron rays from the surface layer. JP-A 6-166157 (Yoshii) discloses a multi-layer blown plastic container including a gas barrier layer and a layer of metallocene-catalyzed polyolefin resin having a molecular weight distribution factor of below 2.5.
As described above, a metallocene-catalyzed polyolefin has advantageous properties including excellent sealability, hot tack and transparency but is also accompanied with a difficulty of poor film formability. A major characteristic of a metallocene-catalyzed polyolefin is that it has low melt tension. Further, because of a narrow molecular weight distribution, a sufficient degree of molecular entanglement does not occur in a melting state, so that the metallocene-catalyzed polyolefin is liable to cause melt fracture during film forming or film processing to result in poor film surface properties and cause difficulties, such as unstable bubble formation in the inflation process and low draw resonance, whereby stable film forming or film processing becomes difficult. Regarding the sealability, a metallocene-catalyzed polyolefin can provide a large seal strength when formed in an ideal seal shape but generally shows a narrow seal condition or range, so that it is liable to cause a flow at a seal line and result in a lower strength because of a smaller thickness. The above difficulties may be alleviated to some extent, e.g., by introducing longchain branches onto a main chain of the metallocene-catalyzed polyolefin, but it is not sufficient in some cases.
As described above, it is difficult to obtain a flexible multilayer film excellent in film formability and sealability only by disposing a seal layer of a metallocene-catalyzed polyolefin, and further improvement is desired.
SUMMARY OF THE INVENTION
Accordingly, a principal object of the present invention is to provide a flexible multilayer film excellent in film formability or film processability and sealability.
According to the present invention, there his provided a multilayer film, including: a first seal layer comprising principally a metallocene-catalyzed polyolefin, and a second seal layer comprising a copolymer of at least one oxygen-containing monomer and ethylene; said copolymer having a crystal melting point lower than that of the metallocene-catalyzed polyolefin.
It is preferred that second seal layer has a thickness larger than that of the first seal layer. In case where any of the first seal layer and the second seal layer comprises a mixture of polymers, the above-mentioned crystal melting point relationship should be satisfied between the polymer species occupying the largest proportions in the first seal layer and the second seal layer respectively. The multilayer film may preferably have a secant modulus at 2.5% strain of 150-450 MPa as a whole.
These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiments of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
A metallocene catalyst used in production of the metallocene-catalyzed polyolefin in the present invention refers to a catalyst comprising a compound having a structure wherein a transition metal is sandwiched with a pair of unsaturated cyclic compounds, which may have various forms based on different species of metals and ligands, and may be used as polymerization catalysts for olefins, such as ethylene and propylene, and other vinyl monomers, such as styrene, in combination with specific promoters as desired. The metallocene catalysts are also called Kaminsky catalysts, Kaminsky-Sinn catalysts, single-site catalysts, or homogeneous catalysts.
A constrained geometry catalyst is a type of metallocene catalyst developed by Dow Chemical Company. The constrained geometry catalyst may provide ethylene-&agr;-olefin copolymers which may be classified as a substantially linear polyethylene resin having ca. 0.01-ca. 3, preferably ca. 0.1-ca. 1, more preferably ca. 0.05-ca. 1, long-chain branch(es) per 1000 carbon atoms. Because of long-chain branches each having ca. 6 or more carbon atoms selectively introduced into its molecular structure, the ethylene-&agr;-olefin copolymer may be provided with excellent physical properties and good formability or processability, and an example thereof is commercially available from Dow Chemical Company under a trade name of “Affinity” (including 1-octene as &agr;-olefin).
A characteristic of the metallocene catalyst is the homogeneity of active sites, thus resulting in a product polymer having enhanced homogeneity of molecular weight, molecular weight distribution, composition and compositional distribution. For example, a larger comonomer content provides a lower density, a lower melting point, and a film having improved strength and transparency but also lower heat-resistance and film processability. The metallocene-catalyzed polyolefin can contain comonomer units introduced uniformly into the main chain a
Kitada Ichiro
Nishimoto Yoshiharu
Tsukamoto Hajime
Dixon Merrick
Kureha Kagaku Kogyo Kabushiki Kaisha
Wenderoth Lind & Ponack LLP
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