Plastic and nonmetallic article shaping or treating: processes – Forming continuous or indefinite length work – Layered – stratified traversely of length – or multiphase...
Reexamination Certificate
2001-08-16
2003-08-05
Nakarani, D. S. (Department: 1773)
Plastic and nonmetallic article shaping or treating: processes
Forming continuous or indefinite length work
Layered, stratified traversely of length, or multiphase...
C264S173120, C264S173140, C264S173190, C264S210100, C264S210200, C264S211120, C428S516000, C428S520000, C428S910000
Reexamination Certificate
active
06602455
ABSTRACT:
FIELD OF THE INVENTION
The present invention refers highly bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer film and to the process for the manufacture thereof.
More particularly the present invention refers to a heat-shrinkable, thermoplastic, multi-layer film comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer (EVOH), a first outer layer (B) comprising an ethylene homo- or co-polymer and a second outer layer (C), which may be equal to or different from the first outer layer (B), comprising an ethylene homo- or co-polymer, characterized in that said film has been bi-axially oriented at an orientation ratio in the longitudinal direction higher than 4:1, preferably higher than 4.5:1, even more preferably of at least 5:1, and at an orientation ratio in the cross-wise direction higher than 4:1, preferably higher than 4.5:1, even more preferably of at least 5:1.
BACKGROUND OF THE INVENTION
Bi-axially oriented, heat-shrinkable, thermoplastic, multi-layer films are films that have been oriented by stretching in two perpendicular directions, typically the longitudinal or machine direction (MD) and the transverse or crosswise direction (TD), at a temperature higher than the highest Tg of the resins making up the film layers and lower than the highest melting point of at least one polymer of the film layers, i.e. at a temperature where the resins, or at least some of the resins, are not in the molten state.
Bi-axially oriented, heat-shrinkable, thermoplastic films are made by extruding polymers from a melt into a thick sheet that is quickly quenched to prevent or delay polymer crystallization, and then oriented by stretching under temperature conditions, as indicated above, where molecular orientation of the film occurs and the film does not tear. Upon subsequent re-heating at a temperature close to the orientation temperature, the oriented, heat-shrinkable, film will tend to shrink in seeking to recover its original dimensional state. In fact, when the film, where the polymer molecules are aligned in the direction of the drawing force and locked into this configuration by cooling, is heated to a temperature close to the orientation one, mobility is restored in the polymer molecules and they relax back to the coil configuration, physically manifesting said relaxation with a shrink along the direction of the orientation.
Orientation brings out the maximum strength and stiffness inherent in the polymer system, thus increasing the tensile properties of the film.
Orientation also induces higher level of crystallinity so that properties like gas barrier properties are further enhanced in an oriented film.
In general orientation leads to a crystalline structure that scatters much less light than the crystalline domains formed in unoriented films and therefore orientation leads to generally superior optical properties.
Oriented, heat-shrinkable films are therefore widely appreciated and widely used in packaging, particularly in food packaging. In general terms the packaging of food and non-food items by means of an oriented, heat-shrinkable, thermoplastic film comprises configuring the heat-shrinkable packaging material, either partially or completely, around a product, removing excess air if necessary, sealing it to itself or to the rims of a support containing the product to be packaged or otherwise let the two edges of the packaging material to overlap and adhere to each other without heat-sealing them and thereafter exposing the package to a heat source thereby causing the heat-shrinkable film to shrink and conform with the contours of the packaged item or become tight between the rims to which it has been sealed.
Heat-shrinkable films are used to both provide the package with an aestethically appealing appearance and guarantee that the packaged product is protected from the environment.
Bi-axially oriented, heat-shrinkable multi-layer films comprising a core layer (A) comprising an ethylene-vinyl alcohol copolymer, a first outer layer (B) comprising an ethylene homo- or copolymer and a second outer layer (C) comprising an ethylene homo- or co-polymer, are known.
As an example; EP-A-141,555 discloses an oriented five layered film with a core layer of a blend of an ethylene-vinyl alcohol copolymer and a polyamide, two outer layers of a blend of ethylene-vinyl acetate and low density linear polyethylene, and two tie layers on the two surfaces of the core layer, adhering said surfaces of the core layer to a respective outer layer. EP-A-141,555 describes, as the most practical manner of extruding and orienting the film, the “double-bubble” technique, according to which the film is extruded downwardly as a tube formed by an annular die, quenched by a water cascade and a water bath, re-heated to the suitably selected orientation temperature, and then oriented by stretching. Stretching in the machine direction is carried out by two sets of rolls that are rotated in such a way so as to establish a linear rate differential therebetween, while the simultaneous orientation in the cross-wise or transversal direction is carried out by inflating the bubble trapped between the to nips of the rolls. Convenient orientation ratios there described are comprised between 2:1 and 4:1 in both directions.
EP-A-217,596 describes an oriented, heat-shrinkable cross-linked film having a core layer comprising an ethylene-vinyl alcohol copolymer, two outer layers comprising a blend of ethylene-vinyl acetate, low density linear polyethylene and medium density linear polyethylene, and two tie layers adhering the surfaces of the core layer to a respective outer layer. The process there described involves extrusion of a thick sheet in the form of a tube, cooling thereof, cross-linking by irradiation, re-heating to the suitably selected orientation temperature and orientation in a way similar to that described in EP-A-141,555. The orientation ratios described in the examples of EP-A-217,596 are about 3.5:1 in each direction.
WO-A-95/13,187 describes an alternative process for the manufacture of bi-axially oriented, heat-shrinkable multi-layer films, including those described in the above patents, having at least one layer comprising an ethylene-a-olefin copolymer and showing more than one melting point in DSC. Said process provides for the extrusion of the polymers through a flat die in the form of a sheet, and after a quenching step and an optional irradiation step, for the heating of the flat sheet to the orientation temperature and the stretching thereof first longitudinally, by running the sheet over at least two series of pull rolls wherein the second set runs at a higher speed than the first one, and then transversally, by grasping the edges of the sheet by clips carried by two continuous chains running on two tracks that move wider apart as they go along.
The films obtained by this latter method distinguish from those obtainable by the trapped bubble technique in the thickness variation, that is always lower than 10% and in the planarity, that is significantly more controlled.
While, in line of principle, higher stretching ratios could be employed using flat extrusion and flat orientation with respect to those obtainable with the trapped-bubble technique, an MD stretching ratio of 2.4 to 1 and a TD stretching ratio of 4.5 to 1 are reported in Example 26 of WO-A-95/13,187 for the manufacture of a film comprising a core layer of a blend of ethylene-vinyl alcohol and 20% by weight of a polyamide 6/12.
Ethylene-vinyl alcohol is in fact a highly crystalline polymer known to be difficult to orient. Particularly in the sequential stretching described in WO-A-95/13,187, the first orientation step induces some polymer crystallization that increases the resistance of the film to further stretching. Ethylene-vinyl alcohol is therefore typically admixed with a polyamide or other plasticizers in the orientation processes.
The use of high stretching temperatures, particularly for the transverse stretching, would help to increase the stretching ratios. As a matter of fact orientation of film
Cryovac Inc.
Nakarani D. S.
Quatt Mark B.
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