Stock material or miscellaneous articles – Structurally defined web or sheet – Including components having same physical characteristic in...
Patent
1996-08-02
2000-08-22
Thibodeau, Paul
Stock material or miscellaneous articles
Structurally defined web or sheet
Including components having same physical characteristic in...
428 349, 428367, 428520, 428516, 428518, 428910, B32B 2732
Patent
active
061069345
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a multilayer, bioriented, heat shrinkable film, a method for the manufacture thereof and use thereof for packaging food products and consumer articles.
More particularly, the present invention relates to a multilayer, bioriented, heat shrinkable film extruded from a flat die and stretched both in machine direction and in cross-direction wherein at least one layer comprises at least one ethylene/alpha-olefin C.sub.4 -C.sub.12 copolymer.
BACKGROUND OF THE INVENTION
Multilayer heat shrinkable films have been known for a long time.
U.S. Pat. No. 4,532,189 (W.R. Grace & Co.) discloses a multilayer, heat shrinkable film comprising: a linear medium density polyethylene; weight, of an ethylene/propylene copolymer and from 10% to 30%, by weight, of a propylene homopolymer; 200.degree. F. of at least 12% and an average cross-direction free shrink at 200.degree. F. of at least 17%.
The core layer may also comprise other polymers such as, for example, ethylene/propylene copolymers, ethylene/vinyl acetate copolymers, ionomer resins and non-linear low density poly ethylenes.
Moreover, said film may also comprise two intermediate layers comprising a blend of approximately 90% by weight of an ethylene/vinyl acetate copolymer and approximately 10% of an ionomer resin.
U.S. Pat. No. 4,551,380 (W.R. Grace & Co.) discloses a multilayer heat shrinkable film comprising: density polyethylene; and density polyethylene, (2) a linear medium density polyethylene and (3) an ethylene/vinyl acetate copolymer. at least three times their original linear dimension in at least one direction, a film having the following homogeneous composition: least one C.sub.8 -C.sub.18 alpha-olefin, said copolymer having the following characteristics: determined by differential scanning calorimetry (DSC), the temperature difference between said regions being at least 15.degree. C.; and consisting of ethylene homopolymers and copolymers of ethylene with an ethylenically unsaturated comonomers, said polymer having only one crystallite melting point below 128.degree. C.; range defined by the two melting points of the crystallites of the ethylene/alpha-olefin copolymer of the above paragraph (A).
These films are manufactured by the well-known air bubble technique. An example of method and equipment of this technique is disclosed by U.S. Pat. No. 4,841,605.
However, the films obtained with this technique have the disadvantage of not having sufficiently uniform thickness and planarity. In fact the total thickness variation in said films is .+-.15% while, as regards planarity, it has defects consisting of deviations from a straight line (snaking) and sags. More particularly the average deviations from a straight line (snaking) is approximately 50 mm, whereas the average sag is approximately 35 mm.
In addition to the air bubble technique, stretching the films also by the so-called "tenter frame" technique is known. Examples of machines suitable for implementing this technique are disclosed by U.S. Pat. Nos. 3,148,409 and 3,201,826.
The tenter frame technique, also known as "flat orientation technology", consists of extruding a film-forming material through a flat die over a chill roll, preferably immersed in water, to chill the molten film.
In the case of multilayer films the various polymers or blends of polymers are generally coextruded by conventional techniques but, when only a few of the layers have to undergo special treatments, such as for example irradiation with fast electrons to induce cross-linking, only the layer or layers to be treated is extruded or are coextruded, the tape obtained in this way is subjected to the required treatment and then the remaining layers are extruded on the same.
The tape is then oriented, by stretching, in two separate and successive steps, although devices able to stretch the tape simultaneously in both directions are known (U.S. Pat. No. 3,148,409).
Generally stretching is performed first in machine direction (MD) and then in cross-direction (TD).
MD stretching
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Buongiorno Livio
Cerani Luca
Ciocca Paolo
Forloni Roberto
Patrick Ray E.
Cryovac Inc.
Quatt Mark B.
Tarazano D. Lawrence
Thibodeau Paul
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