Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Reexamination Certificate
2002-05-14
2004-06-15
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
C428S515000, C428S520000, C428S910000
Reexamination Certificate
active
06749948
ABSTRACT:
CROSS REFERENCE TO RELATED PATENT APPLICATION
The present patent application claims the right of priority under 35 U.S.C. §119 (a)-(d) of German Patent Application No. 10124209.3, filed May 18, 2001.
FIELD OF THE INVENTION
The present invention relates to biaxially stretched multilayer polyolefin films having a base layer, at least one intermediate layer, and at least one peelable outer layer, in which the intermediate layer is interposed between the base layer and the peelable outer layer. The present invention also relates to a method of preparing such multilayer films, in which the base and intermediate layer(s) are biaxially stretched, and the peelable outer layer(s) is monoaxially stretched.
BACKGROUND OF THE INVENTION
In stretched polyolefin films, in particular biaxially oriented polypropylene films, the mechanical strength of seal seams is often higher than that of the film itself, and therefore when a sealed pack is opened it is not only the seal seam which is broken apart. Typically, a tear propagates in an uncontrolled manner through the entire film upon opening. Peelable films are films which once sealed can be separated again mechanically without damaging or destroying the film itself. Peelable seal layers are typically required to have good sealing properties and also permit controlled opening of the pack. An important feature of this type of peelable seal layer is that this behaviour arises over a very large range of temperature of the sealing tools, and that the seal seam strength is very constant over this temperature range. The seal seam strength for stretched polyolefin films here must not be excessively high, since these films have very low tear propagation resistance and tear apart in an uncontrolled manner when subjected even to the most minor damage. Another important feature is the requirement of good film transparency, this being desirable for many applications in order to give the product complete or partial visibility and in some instances to provide good legibility of markings. An example of where transparency is desirable is with the packaging of compact discs.
Another important feature is good hot seal seam strength. This is necessary since, especially when producing bags on vertical tubular bag machines, directly after sealing of the basal seam the product to be packaged drops onto the basal seam while it is still hot. The resultant mechanical load placed upon the bag's basal seam, which is still hot, must not lead to any damage to the seam, nor indeed cause the bag to break open.
U.S. Pat. No. 4,666,778 discloses transparent, peelable films with high seal seam strength. The peelable layers have a thickness of 40 &mgr;m or above. Seal layers of this thickness are disadvantageous for oriented polyolefin films, since they make them unsuitable for many applications where thin films are required.
EP-A-0 692 373 and EP-A-0 781 652 describe opaque films with at least one peelable outer layer. However, a transparent film is desirable for many applications.
U.S. Pat. No. 5,500,265 discloses a peelable film. The coating is composed of an acrylic polymer or of a polyvinylidene chloride polymer. A disadvantage of this film is that the cut material cannot be recycled. The film is expensive to produce, since coating has to take place in a separate operation after film production, and often requires an additional adhesion promoter.
SUMMARY OF THE INVENTION
The object was therefore to provide a stretched multilayer polyolefin film with high transparency, which is peelable after sealing at various temperatures and thus has a wide peel range. The film also has a high hot seal seam strength and very constant seal seam strengths over this peel range.
In accordance with the present invention, there is provided a peelable, biaxially oriented, transparent multilayer polyolefin film, having at least three layers, comprising:
a) a transparent polyolefin base layer;
b) at least one peelable outer layer comprising one of,
(i) a polymer selected from at least one of LLDP, LDPE, MDPE, ethylene-vinyl acetate copolymer, ethylene-methyl acrylate copolymer and HDPE, and
(ii) a combination of at least one propylene co- or terpolymer and at least one ethylenic polymer; and
c) at least one intermediate layer comprising a propylene co- or terpolymer,
wherein said intermediate layer c) is interposed between said base layer a) and said peelable layer b).
Unless otherwise indicated, all numbers expressing quantities of ingredients, reaction conditions, etc. used in the specification and claims are to be under stood as modified in all instance by the term “about.”
DETAILED DESCRIPTION OF THE INVENTION
The film of the invention has at least three layers and encompasses, as substantive layers, always: the transparent base layer, which is preferably composed of polypropylene; at least one intermediate layer made from propylene co- or terpolymer; and at least one peelable outer layer, preferably outer layers on both sides, which may be identical or different. In the case of a single-side peelable outer layer the opposite outer layer is preferably a sealable outer layer. Among the numerous materials used for sealable layers, preference is given to the use of the polymers selected from at least one of: random propylene-ethylene copolymers; random propylene-1-olefin copolymers; and random propylene-ethylene-olefin terpolymers.
A particularly preferred propylene-ethylene copolymer for use in the optional sealable layer is characterized in that it contains from 1.0 to 10% by weight of ethylene. The density of the seal layer polymer is preferably in the range of 0.895 to 0.960 g/cm
3
, and, depending on the type, the crystalline melting point is preferably from 120 to 150° C.
In an embodiment of the present invention, the multilayer film has four layers, one of which is a sealable outer layer d). More particularly, in the four-layered film: the transparent polyolefin base layer a) is interposed between the sealable outer layer d) and the intermediate layer c); and the intermediate layer c) is interposed between the transparent polyolefin base layer a) and the peelable outer layer b). The structure of this four layer film is represented by the following:
|sealable layer (d)|base layer (a)|intermediate layer (c)|peelable layer (b)|
If the film of the invention has two peelable outer layers, the film also preferably has two intermediate layers, location of each of these being between the outer layers and the interior base layer. If the film of the invention has only one peelable outer layer, the film also typically has only one intermediate layer between the peelable outer layer and the base layer.
In an embodiment of the present invention, the multilayer film comprises five layers, in which two outer peelable layers (b), two intermediate layers (c) and one transparent base layer (a) are present. More particularly, in the five layered film: the transparent polyolefin base layer a) is interposed between two intermediate layers c); and each of the intermediate layers c) is interposed between a peelable outer layer b) and the transparent polyolefin base layer a). Each of the two separate intermediate layers c) may have the same or different compositions, and each of the two separate peelable outer layers b) may have the same or different compositions. The structure of this five layer is represented by the following, in which “int.” means intermediate, and “peel” means peelable:
|peel layer (b)|int. layer (c)|base layer (a)|int. layer (c)|peel layer (b)|
The overall thickness of the multilayer film of the present invention may vary within wide limits. In a preferred embodiment, the multilayer film has an overall thickness of from 10 to 100 &mgr;m, and more preferably from 15 to 40 &mgr;m. The thickness of the peelable outer layer(s) is preferably in the range from 0.5 to 8 &mgr;m. The thickness of the intermediate layer(s) made from propylene co- or terpolymer is preferably in the range from 0.5 to 5 &mgr;m.
The base lay
Schwarz Willi
Sperlich Bernd
Wagner Helmut
Wunderlich Hans
Foley & Larbner LLP
Nakarani D. S.
Wipak Walsrode GmbH & Co. KG
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