Stock material or miscellaneous articles – Composite – Of addition polymer from unsaturated monomers
Patent
1998-07-02
2000-02-15
Copenheaver, Blaine
Stock material or miscellaneous articles
Composite
Of addition polymer from unsaturated monomers
428517, 428521, B32B 2708, B32B 2732
Patent
active
060250798
DESCRIPTION:
BRIEF SUMMARY
The present invention refers to a new heat-shrinkable multi-layer film with very good optical and mechanical properties, improved heat-shrinkability properties and sealability, particularly suitable for high-speed packaging of food and non food items.
Heat-shrinkable thermoplastic films have many useful applications in the packaging industry for food and non food products.
In general terms the packaging of food and non-food items by means of a 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.
These films should be used to both provide the package with an aestethically appealing appearance and guarantee that because of their good mechanical properties the packaged product would be protected from the environment.
Heat-shrinkable films are typically made by extruding or coextruding polymers from a melt into a thick film, followed by a quick quenching to prevent or delay crystallization and by orientation of the thick film by stretching it, either monoaxially or preferably biaxially, under temperature conditions 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 it will tend to shrink in seeking to recover its original dimensional state.
The polymers can be extruded or co-extruded through a round die giving a tubular thick film called "tape", that is immediately and quickly quenched by means of a water bath or cascade typically to about room temperature. Said tape is then heated at the orientation temperature and stretched biaxially, while at this temperature, e.g. by the so-called "trapped bubble" technique that uses internal gas pressure to expand the diameter of the tape to form a large "bubble" and advancing the expanded tube at a faster rate than the extrusion rate so as to obtain transverse and machine directions of orientation respectively. Usually the stretch is at least about 3.times. in each direction. The film is then cooled and rolled up in the cooled state so as to retain the property of heat-shrinkability. The orientation temperature range generally depends on the type of polymers employed and on the end use of the film. The orientation temperature used for the manufacture of heat-shrinkable films is in any case lower than the melting temperature of at least one polymer present in the film.
Alternatively heat-shrinkable films can be obtained by extruding the polymers through a flat die in the form of a sheet, and after a quenching step, heating the sheet to the orientation temperature and stretching it. Longitudinal orientation is generally obtained 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. Cross-wise or transversal orientation is generally done in a tenter frame where the edges of the sheet are grasped by clips carried by two continuous chains running on two tracks that move wider apart as they go along. In alternative to the above described sequential stretching, i.e. either longitudinal first and then transverse or vice-versa, the stretching may be simultaneous in both directions. The stretched film is then cooled and rolled up as usual. Also in the case of orientation by a tenter frame the stretch is usually at least about 3.times. in each directions, but higher ratios are also common.
Most commonly, however, in the case of tenter frame orientation, the process includes a step of so-called "heat-setting" or "fixing" where the film is heated while restrained from s
REFERENCES:
patent: 4390385 (1983-06-01), Ferguson et al.
patent: 5212009 (1993-05-01), Peiffer et al.
patent: 5372882 (1994-12-01), Peiffer et al.
patent: 5443895 (1995-08-01), Peiffer et al.
Ciocca Paolo
Forloni Roberto
Copenheaver Blaine
Cryovac Inc.
Quatt Mark B.
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