Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Reexamination Certificate
2001-06-13
2003-11-11
Nakarani, D. S. (Department: 1773)
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
C428S215000, C428S336000, C428S516000, C428S520000, C428S910000
Reexamination Certificate
active
06645609
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. 100 30 879.1, filed Jun. 23, 2000.
FIELD OF THE INVENTION
The present invention provides stretched polyolefin multilayered films with at least one peelable outer layer.
BACKGROUND OF THE INVENTION
Heat sealed joints in stretched polyolefin films, in particular biaxially oriented (i.e., stretched) polypropylene films, often have greater mechanical strength than the film itself, so that on opening a sealed package not only is the heat sealed joint ripped apart but the entire film tears in an uncrontrollable manner. For this reason, peelable heat sealing layers are used which, on the one hand, have good sealing properties and, on the other hand, facilitate controlled opening of a package. An important feature of this type of peelable heat sealing layer is that this behavior occurs over as wide a range of temperature as possible for the heat-sealing mould and that the strength of the heat sealed joint is as constant as possible over this temperature range. This means that the strength of the heat sealed joint in the case of stretched polyolefin films should not be too high because these films have a very low tear propagation resistance and tear in an uncontrolled manner when subjected to even a very small degree of damage.
U.S. Pat. No. 4,666,778 describes transparent, peelable films with a high heat sealed joint strength. The peelable layers have a thickness of 40 &mgr;m and more. Such thick heat sealed layers are a disadvantage for oriented polyolefin films because these are then used for many applications where the use of thin films is no longer appropriate.
EP-A-0 692 373 describes a peelable film with incompatible polymers in the peelable outer layer. However, this makes the film cloudy which is a disadvantage for many applications.
SUMMARY OF THE INVENTION
Thus, there is the object of providing a stretched polyolefin multilayered film which is peelable after being heat sealed at a variety of temperatures and therefore has a wide peeling range. In addition, the film must have as constant a heat sealed joint strength as possible over this peeling range.
In accordance with the present invention, there is provided a stretched multilayered film comprising:
(a) a polyolefin layer; and
(b) at least one peelable outer (or skin) layer,
wherein each peelable outer layer (b) independently is 0.5 to 5 &mgr;m thick, comprises a copolymer of vinyl acetate and ethylene, and has a vinyl acetate proportion of from 3 to 30 wt. %, based on the weight of said peelable outer layer (b). In an embodiment of the present invention, the peelable outer layer (b) is in contact with said polyolefin layer (a).
Other than in the operating examples, or where 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 according to the present invention is at least two-layered and always includes, as essential layers, the polyolefin layer (a) and at least one peelable outer layer (b), preferably two outer layers (one on each face of the polyolefin layer (a)), each of which may be identical or different in thickness and/or composition. In the case of a three-layered film and having a single outer layer (b) on one side, the layer located on the opposite face from this outer layer is preferably a heat sealable, non-peelable outer layer (c). From among the many materials which can be used for sealable, non-peelable layers, the following are preferably used in the present invention:
statistic propylene/ethylene copolymers
statistic propylene/olefin copolymers
statistic propylene/ethylene/olefin terpolymers
mixtures of two or three of the preceding polymers.
Particularly preferred as the heat sealable, non-peelable outer layer (c) is a propylene/ethylene copolymer characterised in that it contains 1.0 to 10 wt. % of ethylene, based on the weight of the propylene/ethylene copolymer. The density of the heat sealing layer polymers are preferably in the range 0.895 to 0.960 g/cm
3
, and the crystallite melting point, depending on the particular type, is in the range 125 to 148° C.
The total thickness of the multilayered film of the present invention may vary between wide limits. The multilayered film according to the invention typically has a total thickness of 5 to 200 &mgr;m, preferably 10 to 100 &mgr;m, and more preferably 15 to 40 &mgr;m. The thickness of each peelable outer layer (b) independently is preferably in the range 0.5 to 5 &mgr;m, in particular 0.6 to 2 &mgr;m, wherein the density and composition of each outer layer (b) applied to the two faces of the polyolefin layer (a) may be identical or different.
The polyolefin layer (a) preferably consists substantially or entirely of a polypropylene homopolymer. An isotactic polypropylene homopolymer with a density of 0.90 to 0.91 g/cm
3
and a melt flow index of 1 to 4 g/10 min at 230° C./21.6 N (according to DIN 53 735) is preferably used.
The multilayered film of the present invention may optionally contain lubricants and antistatic additives. Classes of lubricants that may be used include higher molecular weight aliphatic acid amides, higher molecular weight aliphatic esters, waxes and metal soaps and also polydimethylsiloxanes. The active amount of lubricants used is typically in the range 0.01 to 3 wt. %, and preferably 0.02 to 1 wt. %, based on the weight of the multilayered film. The addition of higher molecular weight aliphatic acid amides in the range 0.01 to 0.25 wt. %, based on the weight of the multilayered film, is particularly suitable. A particularly suitable aliphatic acid amide is erucic amide.
Preferred antistatic additives include, but are not limited to: alkali metal alkanesulfonates; polyether-modified (i.e., ethoxylated and/or propoxylated); polydiorganosiloxanes (e.g., polydialkylsiloxanes, polyalkylphenylsiloxanes and the like); and/or substantially straight-chain and saturated aliphatic tertiary amines with an aliphatic group having 10 to 20 carbon atoms, which are substituted with &ohgr;-hydroxy-(C
1
-C
4
)-alkyl groups. Of the tertiary amines, N,N-bis-(2-hydroxyethyl)-alkylamines with 10 to 20 carbon atoms, preferably 12 to 18 carbon atoms in the alkyl group are particularly suitable. The active amount of antistatic agent is typically in the range 0.05 to 0.5 wt. %, based on the weight of the multilayered film. Furthermore, glycerine monostearate is also suitable as an antistatic agent, and is typically used in an amount of 0.03 % to 0.5 wt. %, based on the weight of the multilayered film.
The peelable outer layer (b) and optional heat-sealable, non-peelable outer layer (c) each preferably and independently contain 0.02 to 1.0 wt. %, and particularly preferably 0.05 to 0.2 wt. %, of antiblocking agent, the wt. %'s being based on the weight of each outer layer. Suitable antiblocking agents include, but are not limited to: inorganic additives such as silicon dioxide, calcium carbonate, magnesium silicate, aluminum silicate, calcium phosphate and the like; and/or cross-linked polymer particles such as, cross-linked polymethacrylates or cross-linked polyalkylsiloxanes; and/or incompatible organic polymers, such as polyamides, polyacrylates, polymethacrylates, polyesters, polycarbonates and the like. Silicon dioxide is a preferred antiblocking additive. In the case of particulate antiblocking agents, the average particle size is typically between 1 and 10 &mgr;m, and in particular between 2 and 5 &mgr;m.
In a preferred embodiment of the present invention, the peelable outer layer (b) comprises a mixture of an ethylene/vinyl acetate copolymer and a second polymer (e.g., a polyethylene polymer), wherein the ethylene/vinyl acetate copolymer has a vinyl acetate content of 5 to 30 wt. %, based on the weight of the ethylene/vinyl acetate copolymer, and a melt flow index (MFI) of between
Schwarz Willi
Sperlich Bernd
Wagner Helmut
Wunderlich Hans
Akorli Godfried R.
Eyl Diderico van
Nakarani D. S.
Wolff Walsrode AG
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