Stock material or miscellaneous articles – Composite – Of polyamide
Reexamination Certificate
1999-09-07
2001-10-09
Kiliman, Leszek (Department: 1773)
Stock material or miscellaneous articles
Composite
Of polyamide
C428S425800, C428S476300, C264S173160, C264S173150, C264S173190
Reexamination Certificate
active
06299984
ABSTRACT:
The present invention refers to an improved, highly oriented and heat-shrinkable, multilayer thermoplastic film comprising a layer comprising a polyamide and an outer heat-sealing layer comprising a polyolefin, to a process for the manufacture thereof and to its use as a packaging material.
Orientation is a process whereby a plastic film or sheet is stretched in such a way to orient the polymeric chains of the plastic material in the direction of the force applied.
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 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.
Also, another very important contribution given by orientation to the end film properties resides in the introduction of a shrink feature. In fact, if the film obtained by orientation, 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.
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.
Polyamides are very widely employed in the manufacture of heat-shrinkable films. They can be employed i.a. as core layers of multi-layer films having a heat-sealable polyolefin layer, because of their gas-barrier properties. It is in fact possible to modulate the gas permeability of the end films by suitably selecting the type of polyamide of the core layer.
In general, oriented, heat-shrinkable films comprising a polyamide layer and an outer heat-sealing polyolefin layer, are obtained by a tubular orientation process. In said process a thick multi-layer tube is first extruded through a round die, quenched as rapidly as possible to prevent or slow down crystallization, reheated, e.g. by passing it into a hot water bath or an IR oven, and then stretched in the transverse direction (TD) by introducing into the tube an air pressure that expands the tube diameter to a sort of a bubble and in the machine direction (MD) by running the two sets of nip rolls that contain said bubble at a different speed.
By this method stretching in the two perpendicular directions, MD and TD, occurs simultaneously. It is thus possible to carry out the orientation step at a fairly low temperature, compatible with the presence of an outer polyolefin layer and with the requirement for a low temperature shrink for the end film.
The orientation ratios that can be applied with the tubular orientation processes are however limited and ratios up to about 3.5:1 are typically applied.
Higher stretching ratios could, in line of principle, be employed using flat extrusion and flat stretching. Flat stretching is generally done sequentially, i.e. the film is first stretched in the MD and then in the TD. The MD stretching is accomplished by drawing the heated sheet between sets of heated rolls with the downstream set moving at a higher speed. The TD stretching is on the other hand obtained by means of a tenter frame, a machine that consists of two continuous chains on which are mounted clamps gripping the two edges of the film and carrying it along as the chain is driven forward. The two chains gradually move part and as they do they draw the film in the TD between them.
Conventional stretching ratios for the flat, tenter frame orientation process are up to about 7:1 in MD and up to about 12:1 in TD.
Particularly in case of crystalline or partially crystalline polymers, sequential stretching may however present some problems as the first stretching step induces some polymer crystallization that increases the resistance of the film to further stretching, thus limiting the applicable stretching ratios or requiring more drastic conditions.
In the patent literature there are described heat-shrinkable films comprising a polyamide layer and a polyolefin outer layer obtained by tenter frame stretching, that however have been stretched to a limited stretching ratio (<2.2:1 in MD and >4:1 in TD in Japanese kokai 79/15981 (Derwent AN 79-20793B) or 3:1 in MD and 4:1 in TD in Japanese kokai 92/52137 (Derwent AN 92-117943)).
The use of high stretching temperatures, particularly for the transverse stretching, would help to increase the stretching ratios but these high temperatures would not be compatible with the presence of the polyolefin resin of the outer layer. Furthermore they would impair the shrink and mechanical properties of the end film as the higher the stretching temperature, the less oriented the product.
It has now been found that it is possible to obtain a highly oriented, heat-shrinkable, multi-layer film comprising a layer comprising a polyamide and an outer heat-sealing layer comprising a polyolefin, which film has been oriented at a stretching ratio ≧ about 3.0:1 in the machine direction and at a stretching ratio ≧ about 6.0:1 in the transverse direction, when the polyamide is a crystalline or partially crystalline co-polyamide with a T
g
≦100° C.
The highly oriented heat-shrinkable multi-layer film, thus obtainable, is characterized by a combination of good mechanical properties, very good shrink properties, good gas barrier properties and good sealability.
DEFINITIONS
As used herein, the term “film” is used in a generic sense to include a plastic web, regardless of whether it is a film or a sheet. Preferably, films of use in the present invention have a thickness of 150 &mgr;m or less, more preferably of from about 8 to about 120 &mgr;m, and even more preferably of from about 10 to about 90 &mgr;m.
The term “oriented” designates a multi-layer structure which has been stretched at a temperature—indicated as the “orientation temperature”—higher than the T
g
of each of the resins making up the structure and lower than the m.p. of at least one of said resins, and set by cooling while substantially retaining its stretched dimensions. As used herein the term “oriented” designates bi-axially oriented materials, i.e. materials wherein the stretching is carried out in two perpendicular directions, i.e. the machine or longitudinal direction (MD) as well as the transverse direction (TD). An “oriented” material will tend to return to its original unstretched (unextended) dimensions when heated to a temperature close to the orientation temperature (“heat-shrinkable”).
For the purposes of the present invention “heat-shrinkable” films are those films that shrink by at least 10% of their original dimensions, in each one of the machine and transverse directions, when heated to a temperature of 120° C. for 4 seconds. The quantitative determination of this “% Free Shrink” is carried out according to ASTM D2732, as set forth in the 1990 Annual Boo
Cryovac Inc.
Kiliman Leszek
Quatt Mark B.
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